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Advanced Concepts of Strength & Conditioning

SportMetabolism

NCSF

CertifiedStrengthCoach

Chapter

6

Sport MetabolismWinning or losing in competitive sports has as much to do

with metabolic factors as it does with the physical attributes and

technical skills of the athletes. The loss of energy from the body

leads to deleterious effects on performance, and is often a

difference maker in the last stages of a race or second half of

a game. For this reason, strength and conditioning programs

should be constructed to exploit the metabolic systems for com-

petition, while considering proper nutritional support to ensure

adequate provisions are available for work and recovery.

The term bioenergetics essentially describes the body’s use of

energy following the process of nutrient consumption and absorp-

tion. The metabolic activity supporting both rest and active

conditions occur through chemical reactions that free energy from

nutrient sources into bodily systems. Unlike mechanical objects,

which have an on and off switch, living organisms need a constant

flow of energy to support the requirements of tissue. To further

complicate this matter, humans also require a variety of nutrients

for different purposes. Therefore, to manage the constant need

for energy and the variability of a system’s affinity for a particular

type of fuel, the body functions using an ongoing process of

energy transfer. The laws of thermodynamics encompass this

notion, as the first law states “energy cannot be created or

destroyed”. Assuming the universe is a closed-system, all energy

in it is reusable, either existing in a state of potential or being

transferred kinetically. In a closed-system, energy is constant

because it is locked in and infinitely recycled; while the total quan-

tity of energy in an open system, such as the human body, is

variable because energy can be added or removed. Due to the fact that all living organisms func-

tion as open systems, energy is constantly needed to replenish that which is being lost.

The second law of thermodynamics relates to the concept that during any energy exchange

there is a loss in order, or entropy, as it is scientifically referred. Whenever energy is transferred

in the body from one form to another, some of the energy is lost due to inefficiency (loss of

order). This makes it impossible for the body to harness all the energy in a given reaction. The

aerobic use of a six-carbon glucose molecule for instance, is roughly 35%; similar to the efficiency

of a light bulb. From a practical perspective, an athlete running a race is losing energy through

all biological mechanisms. Energy is lost as it is transferred to manage ongoing mechanical work,

heat dissipation, and metabolic homeostasis. Due to the fact that energy is constantly being lost

from the body, an athlete in a long race will need to add new energy to sustain their workload, or

they will be forced to slow down.

Energy-yielding nutrients are formed from the elements carbon, hydrogen, and oxygen, with

the addition of nitrogen to form protein. These chemical structures can be separated or recon-

figured to take on other forms of energy or become byproducts and substrates as elements are

removed during metabolism. Interestingly, the byproducts themselves reflect configurations of

the foundational elements so they too can be restructured with other elements to form new

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DEFINITIONS

Bioenergetics –

Describes the processes of energy usewithin the body

Entropy –

Energy disorder associated withchemical reactions within the body, thesystem loses order and efficiency isconsequently reduced

Chapter 6 NCSF Advanced Concepts of Strength & Conditioning

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energy. Forms of energy in the body may be mechanical, biochemical, or thermal depending on

the role it is fulfilling. For example, a fully-fueled athlete who participates in a competition will

lose a notable proportion of his or her stored energy by the end of the game, which is made evi-

dent by fatigue. But the body does not simply become drained from the mechanical activity used

to perform the sport’s movements; rather it loses energy through the metabolic mechanisms

necessary to keep it alive.

The sweat dripping off the athlete represents energy lost through heat-related mechanisms;

the increased respiration from high-intensity actions release gases, while the constant movement

during competition derives mechanical energy from inside the tissues; mobilizing it into a free

form. By the end of a two hour event an athlete may have lost over a thousand calories of energy

from the body. Energy that participates in any body function is always reduced by the amount

of free energy it creates. Therefore, energy in storage represents a whole value, and metabolic

processes reduce that value by the quantity freed into the system. Once free, the energy may

follow any number of paths. Carbons freed through glucose metabolism may connect with

oxygen (O2) and leave the body as carbon dioxide (CO2), or may connect with the nitrogen and

hydrogen (H+) liberated from amino acids to form urea; which will leave the body as urine.

Energy released into the environment can no longer be re-used to form energy within the body,

and is therefore lost, but as the laws of thermodynamics state, it is not destroyed. The CO2

breathed into the air will likely be recaptured by plants to form more usable energy such as

carbohydrates (CHO). Humans must constantly replace or regenerate energy due to continuous

loss. Performance outcomes are ultimately determined by the body’s ability to manage available

energy; without a constant energy supply the systems cannot function properly and performance

declines.

Free Energy and the Laws of Thermodynamics

The idea of free energy referenced in the prior text can be better understood when appliedthrough the laws of thermodynamics:

It is important to recognize that this equation attempts to represent what happens to free energy

when energy is transferred. Change in free energy (∆G) is related to three other factors: 1) total

internal energy within the system, or enthalpy (H), 2) absolute temperature (T) and 3) entropy

(S) or the loss in order. In essence, this formula aims to calculate available energy by taking the

initial amount of energy within the system and subtracting the energy that is lost due to disrup-

tion and inefficiency.

The change in free energy (∆G) during any reaction is derived from the difference between

the free energy released from the reaction (byproduct) and the amount of free energy used to

cause the reaction. From a training perspective, biochemical reactions either release energy for

direct use during exercise (ATP ↔ ADP + Pi + energy) or require energy in the process of

providing energy for exercise (PCr + ADP + energy↔ ATP + C). For a reaction to provide

energy, the free energy contained in the final molecules must be lower than the free energy in the

initial molecules – the difference represents the cost of the reaction (second law of thermodynam-

ics). This is where the concept of energy breakdown occurs, and in its process, how free energy

is released. It also explains why different energy sources may provide more or less energy at a

given time and provide different limits in total energy availability during a physical event.

DEFINITIONS

Enthalpy –

The sum of the internal energy of thesystem plus the product of the pressureof the gas in the system and its volume

NCSF Advanced Concepts of Strength & Conditioning Chapter 6

Sport Metabolism

The human body is not a closed system andtherefore lacks efficiency, leading to a constantdepletion of energy.


The concept of free energy is important toward understanding the reactions that occur

within sports metabolism because different reactions cause different outcomes. It is also impor-

tant to understand that the higher the metabolic rate associated with any given sport action, the

faster energy is depleted during its execution. In some cases, the metabolic requirements are so

demanding that only certain sources of energy can support the activity. This causes a rapid and

notable drain on local stores leading to fatigue. Maximal-speed sprints for instance, are too fast

to be supported by the oxidation of fat or proteins; therefore, only CHO can contribute to the

supportive metabolism. To put this into a sport example, pre-game glycogen storage can be

reduced by as much as 40-70% by the end of a soccer match due to the quantity of high-speed

activity that occurs over the 90-minute competition. Without adequate training and nutrition

to support the energy demands an athlete will slow to non-competitive speeds, potentially

contributing to a losing outcome.

Food and drink provides a variety of energy-yielding nutrients for the body, but when it

comes to using the energy for mechanical purposes there is only one exchange medium; the

breakdown of adenosine triphosphate (ATP). ATP is unique in that it is the only energy source

utilized by muscles within the contractile units (sarcomere). Therefore, ATP availability is

requisite to muscle activity. Energy-yielding nutrients hold the elements necessary to form ATP,

but they must be refined through reactions to provide the ATP to working tissue. This is con-

ceptually similar to providing fuel for a car. Crude oil must go through significant processing

before it becomes gasoline, which is the only usable form of fuel to drive the pistons in a gasoline

engine. Pouring crude or even refined oil in the gas tank would not work, even though the key

elements exist. This is the same reason proteins and lipids cannot be used for immediate energy

as they are still in a “crude” form.

ATP consists of an adenine molecule (a nitrogenous base), a sugar (ribose) and three

phosphate groups. The actual “energy” is held in the bonds that form its molecular structure.

When the phosphate bonds break (as a result of their negative charge and their tendency to repel

each other), energy is released.

ATP reaction:

The splitting of the ATP molecule frees energy, resulting in one molecule of adenosine

diphosphate (ADP), an inorganic phosphate (Pi), and roughly 7.3 kcal of energy. This does not

mean the body losses energy at this rate with ATP metabolism, but rather it releases energy into

the system during each reaction. The arrows in the formula above imply this reaction is

reversible, allowing for ATP to be metabolized and resynthesized. This is important because

ATP storage in muscle represents a relatively small quantity of energy at any given time. Relative

to the full spectrum of the metabolic pathways, it is important to understand that while ATP

will serve as the medium for reactions that drive sports, immediately accessible stores of ATP in

the muscle provide only enough energy for a single maximal action. Therefore, any energy

expenditure used to perform additional work will require the resynthesis of ATP via rephospho-

rylation – being the remanufacturing of ATP from energy-yielding compounds through various

metabolic pathways.

Sport Metabolism

The breaking of an ATP molecule frees energyresulting in one molecule of adenosinediphosphate (ADP), an inorganic phosphate (Pi)and roughly 7.3 kcal of energy released.

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The Metabolic Pathways

The small quantities of ATP stored in muscle tissue necessitate an ongoing process of

remanufacturing to support static or dynamic muscle function. The more immediate the avail-

ability of ATP, the higher the potential force output from muscle contractions. This presents

relevant consideration for programming because the type of energy used indicates the absolute

force potential of the work. It also determines the length of necessary recovery between repeat

actions requiring the same force. The more reactions needed to produce ATP, the lower the force

associated with the contraction. Stored ATP requires a single reaction and therefore provides

maximal force, whereas the multiple reactions required for lipid metabolism to form ATP can

only support the force needed for an activity such as jogging. This is partially related to the

interactions between the energy pathways and select muscle fiber affinity for a particular path-

way. Fast-twitch fibers are optimized with phosphagen-derived pathways, whereas slow-twitch

fibers adapt more efficiently to the aerobic production of ATP from the breakdown of sugar and

fats – a much longer process requiring more reactions.

Each energy pathway is characterized by the specific metabolic substrate used to produce

ATP as well as its explicit purpose. The type of substrate and the dynamics of how ATP is derived

through its reactions provide for three metabolically-controlled performance factors. The path-

way determines 1) the associated availability of energy at a given time, 2) the quantity of potential

force produced, and 3) the duration of time needed for rest between repeat actions.

The immediate energy system is normally referred to as the phosphagen system due to the

interaction of phosphagen-driven fuel to support short duration, intense work. The system

reflects energy derived from stored ATP and phosphocreatine (PCr). Once stored ATP is

exhausted (≤3 seconds), new ATP is formed from stored or end-product elements within the cell

to support high-force actions lasting ≤15 seconds. Phosphagen stores in the muscle provide rapid

and powerful energy but have very limited support times. The body

accounts for this limitation using a second anaerobic system to maintain

ongoing work called the glycolytic pathway or glycolysis if referring to the

reaction. Sugar stored in muscle as glycogen provides anaerobic energy

for intermediate-length work demands lasting roughly 90 seconds. As a

consequence of the increased number of reactions needed to produce ATP

in this pathway, total force production declines. Sugar metabolism how-

ever, better accounts for sustaining force when compared to the phosphagen

system due to the higher provisional storage in muscle tissue. The higher

storage of sugar-based fuel provides for greater total work capacity at

intensities reaching moderate-high levels. This energy can sustain the work

necessary to perform an 800-m run.

Sport Metabolism

Figure 6.1 Practical Applications of the Metabolic Systems

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If the body is forced to continue activity for a prolonged period of time, energy support

shifts towards a preference for aerobic metabolism. Aerobic metabolism provides energy for very

long periods of sustained work but at an even greater reduction in force potential. This signifi-

cant drop in force explains why most athletes do not train using the aerobic pathway as it does

not align with actual sports demands.

For the purposes of learning, these energy systems are typically presented in a manner that

seems arbitrary and absolute in terms of energy contribution related to time. In reality, they co -

exist in a much more cooperative and fluid state. It is important to comprehend that these

“metabolic” time frames are not concrete, but instead reflect a continuum (or overlap) of energy

production to support various physiological actions occurring at the same time. This concept

will be detailed later in the chapter. The reason each energy system is normally described as an

independent variable, often expressed by time and intensity, is the characteristics of each pathway

have particular nuances. Each system demonstrates unique aspects related to the efficiency of

energy-availability and the consequent contribution to work, expressed by force, during a

particular activity.

Realistically, all energy pathways work synergistically to drive human movement. A soccer

player, for instance, jogging down the field may break into a sprint toward the ball then perform

a maximal leap to a header resulting in a shot on goal. In this case the athlete is using all meta-

bolic pathways together. Dissecting each part by contribution is inconsequential; having the

support of each part though is paramount to success. Additionally, each metabolic pathway

represents provisional storage, which may vary based on the daily consumption of energy-yielding

nutrients and physical condition of the athlete. These facts underscore the relevance of both

specific conditioning by sport and adequate nutritional support for physical work and recovery.

Immediate (ATP Stores)

Textbooks commonly cite that immediate stores of ATP provide provisions for a 1RM effort,

or roughly three seconds of maximal work. The extremely short duration of tension is not

enough to provide significant adaptations, so most programs do not use single-repetition lifts.

Normally, 1RM efforts are only used during assessments, such as when testing Olympic cleans or

measuring vertical jump height. If a maximal effort is performed, recovery of depleted ATP stores

requires about 90 seconds for rephosphorylation. Some coaches challenge this notion suggesting

90 seconds is not enough time to recover from a single-repetition, maximal lift. In this instance,

the thought process is correct but out of factual context. Repeated maximal vertical jump meas-

Sport Metabolism

Figure 6.2

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ures can be obtained accurately with a 90 second recovery, but one must consider the time-

under-tension and total mass employed for the movement. When a strength assessment such as

a maximal squat is used the conceptual factors change. A 1RM back squat requires a lifter to

remove the bar from the rack position, step to an appropriate stance, and eccentrically lower the

bar to the transition point before reversing the process. For a maximal effort, this takes more

than three seconds and uses many muscle groups. Therefore it is not reflective of the point that

stored ATP serves a single maximal effort, but rather the whole phosphagen system supports the

act – explaining why the recovery may be as long as five minutes before the effort can be repeated

successfully.

During the process of recovery from one of the aforementioned actions, ADP and an inor-

ganic phosphate use energy to re-bond into a new ATP molecule through rephosphorylation.

Rephosphorylation is used to manage energy because ATP is a very heavy molecule; prompting

the body to forgo storage in exchange for remanufacturing the product. Conceptually, it would

seem that rephosphorylating ATP and PCr back to initial stores would provide for long periods

of high-force work since a recovery period allows for renewed energy. The problem though, is

the process requires energy for reformation due to the negatively-charged bonds. This causes an

ongoing depletion of energy during system use, and ultimately fatigue over time. If muscle was

a closed energy system high-intensity work could continue for long periods, but since it is an open

system, phosphagen-based work usually totals 5-10 minutes of time-under-tension.

Immediate (Creatine Phosphate)

When the phosphagen system is considered, PCr more often provides the primary energy

to support high-intensity movements consistent with sports training. Creatine is an amino acid

produced by the human body which can be synthesized into PCr. The system synthesizes ATP

when PCr donates its phosphate group to ADP (PCr + ADP + energy ↔ ATP + Cr) to become

simply creatine. The depletion of PCr associated with muscle work requires between 2-5 minutes

for the process of rephosphorylation to replenish stores. The reason there is a range rather than

a single value is several factors affect recovery including 1) an athlete’s efficiency (based on train-

ing in the system), 2) total storage, 3) the duration and intensity of the action, and 4) available

oxygen. Since the phosphagen system supports most relevant sport outcomes, an athlete’s

exposure to the system often predicts efficiency in the recovery process.

A detailed examination of the phosphagen system through literary review indicates that

there are three reactions that comprise the resultant energy outcome. However, only the creatine

kinase and adenylate kinase reactions actually produce ATP. These reactions are named according

to the enzyme that catalyzes them for energy. The two reactions serve different purposes: the

creatine kinase reaction is important as it creates greater quantities of ATP, whereas the adenylate

kinase reaction serves as a signaling mechanism when PCr becomes depleted. During ongoing

intense work the body recognizes energy loss from the phosphagen system due to an increase in

adenylate kinase reactions. This signals the body to increase ATP production from the glycolytic

system. During this process some ADP is split to produce AMP which is used by enzymes to

initiate glycogenolysis (the breakdown of glycogen) and the production of an activated form of

glucose called glucose-6-phosphate (G6P). G6P provides the energy for continued exercise via

glycolysis.

In many texts, the phosphagen and glycolytic systems are described independently, but in

reality they are cooperative and interactive. This explains why time-under-tension, workloads

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and recovery periods are codependent for adaptations. When the phosphagen system is signifi-

cantly depleted using heavy, low-repetition actions (3RM), a longer-duration recovery is needed.

But, when that same exercise is performed at 6RM intensity the recovery shifts to a shorter-

duration requirement. This is due to 1) less PCr contribution and 2) the interaction of PCr and

glucose to fuel the muscle tissue. Ultimately, recovery time is based on the total requirements for

PCr rephosphorylation and the environmental levels of byproduct in the cell at the end of an

exercise set. The more an athlete is exposed to this system interaction, using the correct amount

of tension and metabolic stress, the more he or she will improve. In a study evaluating elite

sprinters at 40-, 60-, 80- and 100-m distances, those with better capacity to breakdown PCr

(marked by higher enzyme concentrations) achieved a higher maximal speed and were able to

maintain that speed for a longer period of time [1].

Once resting measures of PCr are depleted, they must be regenerated by creatine kinase (ATP

+ Cr → PCr + ADP + H+). Therefore, the ability of an athlete to recuperate PCr stores following

bursts of maximal efforts is central to their ability to repeat performance over an event’s entirety.

This ability is a crucial element in performance outcomes, particularly in explosive sports such

as American football, rugby, basketball, tennis, hockey and soccer. Research concerning how to

most effectively expedite PCr system recovery is not easy to conduct; resulting in limited appli-

cable findings in the literature. The available information indicates that heavy loading with high

velocity seems to be the most effective technique for increasing creatine kinase and adenylate

kinase activity. This makes sense if one again looks at the relationship between sport metabolism

and motor unit physiology. Heavier loading and ballistic actions recruit mostly larger and faster

motor units. This recruitment pattern preferentially recruits fast-twitch fibers, which have a par-

ticular affinity for the immediate energy system. Thus, it is no surprise that the average PCr

content at rest in type II fibers is about 20% higher than in type I fibers and these values can

increase with training [2].

The text has thus far supported an emphasis on the immediate system, but simply focusing

on improved phosphagen efficiency would not be optimal for overall sport performance. There

have been estimates that during a 10-second maximal sprint, the energy provided by PCr is lower

than previously thought [3]. While somewhat controversial, it is clear that athletes can improve

their PCr system through fast, heavily-loaded movements, but also by improving their oxidative

capacity through sprint-based training. This was made evident from a clinical trial employing

intense, 30-second repeat sprints on a Wingate. The resistance used for the high-speed sprints

was 7.5% body weight for males, and 6.5% bodyweight for females, respectively. Results indicated

an average 14% improvement in PCr recovery time following the six training sessions [4]. The

important note here is that the rest periods used were relatively long at 4 minutes. This duration

is in line with energy system specificity of the immediate system, explaining its impact on effi-

ciency. It also explains why some forms of high-intensity training (HIT) have failed to replicate

these results in the PCr system due to insufficient recovery. Conceptually though, maximal

improvements lie in the interactions of the system due to the dynamics of recovery. This suggests

a relational load-velocity–recovery matrix is most effective for programming compared to any

independent focus on the training. The use of heavy loading combined with high-speed ballistic

actions and sprint-based training, using appropriate rest intervals, will positively influence the

recovery-rate of the phosphagen system and improve energy availability.

Sport Metabolism

DEFINITIONS

High Intensity Training –

A style of training which uses minimalrest periods to greatly challenge thelactate tolerance system

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Intermediate (Glycolysis)

When exercise or sport-related tension begins to encroach upon the 10-second mark, the

adenylate kinase reaction described previously signals the need for energy support via the

glycolytic system. In response, enzymatic activity increases the rate of ATP production from

glucose and glycogen. Glycogen is simply the storage form of sugar; attaching glucose to water

to retain energy in muscles and the liver. Glycolysis can be generally thought of as the process

by which one molecule of glucose is broken down into two molecules of pyruvate, while

simultaneously synthesizing two molecules of ATP. An important molecule for energy transport

called nicotinamide adenine dinucleotide (NAD) is also activated, and helps in supporting the

energy continuum. NAD accepts H+ to form NADH, which can later facilitate ongoing reactions.

Compared to the immediate system, glycolysis is a relatively slower process in terms of ATP

synthesis as it requires 10 reactions; however it is still much faster than aerobic metabolism. See

Table 6.1 for details on the ten reactions included in the glycolytic pathway.

The glycolytic reactions can be divided into two parts. The first part is a preparation stage

that actually involves investing ATP energy to produce additional units in the second phase. The

energy invested is used by the first reaction to trap the glucose inside the muscle cell by

phosphorylating it into G6P. G6P has more energy-potential than glucose itself, but cannot leave

the cell in the same manner as glucose. The second reaction changes the form of glucose into

fructose. The third reaction uses another ATP in a new phosphorylation that generates fructose

1,6-bisphosphate. This is catalyzed by an important enzyme called phosphofructokinase. In the

fourth reaction the process is split in two. The fructose 1,6-bisphosphate, containing six carbon

atoms, is fragmented down into two three carbon atoms, so that all subsequent reactions may be

thought of as occurring twice. The second stage refers to reactions that produce a positive net

balance of ATP. In reaction seven, ATP is produced by the phosphoglycerate kinase enzyme, and

by reaction 10, pyruvate kinase finally produces pyruvate and a surplus of useable ATP. At the

Sport Metabolism

DEFINITIONS

Pyruvate –

An energy substrate that results as anend product of sugar metabolism

ReactionNumber Reaction Enzyme

1 Glucose + ATP → glucose 6-phosphate +ADP + H+ Hexokinase

2 Glucose 6-phosphate ⇋ fructose 6-phosphate Phosphoglucose isomerase

3 Fructose 6-phosphate + ATP → fructose 1,6-bisphosphate +ADP + H+ Phosphofructokinase

4 Fructose 1,6-bisphosphate ⇋ dihydroxyacetone phosphate + glyceraldehyde 3-phosphate Aldolase

5 Dihydroxyacetone phosphate ⇋ glyceraldehyde 3-phosphate Triose phosphate isomerase

6 Glyceraldehyde 3-phosphate +Pi + NAD+ ⇋ 1,3-bisphosphoglycerate + NADH + H+ Glyceraldehyde 3-phosphate dehydrogenase

7 1,3-Bisphosphoglycerate + ADP ⇋ 3-phosphoglycerate +ATP Phosphoglycerate kinase

8 3-Phosphoglycerate ⇋ 2-phosphoglycerate Phosphoglycerate mutase

9 2-Phosphoglycerate ⇋ phosphoenolpyruvate + H2O Enolase

10 Phosphoenolpyruvate + ADP + H+ → pyruvate + ATP Pyruvate kinase

Table 6.1 Enzymatic Reactions of Glycolysis

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end there is a balance of two ATP molecules gained since 4 were produced but two were invested

in the preparation phase.

The efficiency of the system is limited by the amount of reactions involved in the manu -

facturing process, as well as the number of compounds involved. As mentioned earlier, the

glycolytic system’s advantage lies in its storage potential. Whereas phosphagen storage is limited,

difficult to manipulate, and requires replenishment; glycogen storage is much more significant

and does not require replenishment to repeat force production. This allows for shorter recovery

periods. Another added benefit with storing energy prior to competition, rather than attempting

to fuel it during the event, is glycogen converts directly into G6P. Therefore, the body does not

need to invest extra ATP in the process, as seen with free glucose. The use of glycogen over glucose

represents a 50% increase in total energy production because the body saves on the initial

investment. ATP production from glycogen increases from two (2) to three (3) ATP. Due to this

efficiency differential, the body will prefer glycogen as its primary fuel for glycolysis; underscoring

the need for coaches to monitor pre- and post-activity nutrition.

The human body normally stores around 400-450 grams of glycogen, of which about 25%

is maintained in the liver to preserve adequate blood glucose levels. While still a viable option to

support the demands of energy for exercise and sports, it is particularly sensitive to the needs of

the central nervous system (CNS). This fact provides both positive and negative implications.

On the positive side, higher hepatic (liver) glycogen levels aid in the maintenance of blood glucose

during an event and help prevent fatigue. On the negative side, the liver is sensitive to the needs

of the CNS; therefore, when liver glycogen content drops too low (<55% capacity), central fatigue

occurs at a rapid rate. This negative consequence is often a result of poor pre-activity glycogen

levels, performance of high-intensity actions and prolonged exercise duration.

Many athletes associate the glycolytic energy system with higher levels of discomfort, and

many blame lactic acid as the instigator. Lactic acid is actually not the problem; it is a lack of

oxygen and an accumulation of hydrogen protons that create the metabolic issue. During

anaerobic efforts, the enzyme lactate dehydrogenase allows the body to produce lactate from the

lactic acid or pyruvate released during glycolysis. This actually improves efficiency, as lactate is

a metabolic transport vehicle that removes pyruvate from the system, therefore limiting the

saturation rate of the product that would normally slow down the process. Additionally, lactate

regenerates an inactivated version of the NAD carrier, which allows it to receive energy from

glycolysis. If an inactivated version of NAD is not present to receive energy in the form of

electrons, glycolysis will not be efficient. Lactate is also a hydrogen proton acceptor and lowers

acidity, buffering the internal system. It can be converted into glucose through the hepatic process

of gluconeogenesis to increase glucose availability, and can also be readily used by metabolic

tissues and the CNS for energy. During exercise, lactate spares valuable glucose and glycogen as

muscle and organ cells have the capacity to use its energy without the need to convert it into

glucose first. Lactate is an important player in protein-sparing, as the increased efficiency in

glucose management makes it unnecessary for the liver to use protein products to form glucose.

This suggests that training in the glycolytic system is also relevant to improving energy during

competition while limiting fatigue.

Longer Duration (Aerobic)

It has been illustrated that anaerobic metabolism is not compartmentalized, but actually an

interaction of several pathways working together to supply energy to working tissues. Aerobic

metabolism is often assumed to function independently as its own system, but it too works as

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Sport Metabolism

part of the continuum. Aerobic metabolism simply reflects another shift in energy support; and

in the same way the immediate system signals the intermediate system to increase contribution,

the intermediate system signals the aerobic system to help support ongoing work. All cells need

oxygen, and without adequate supply they become dysfunctional. During anaerobic metabolism

no oxygen is needed to make fuel, but the cells still require oxygen to manage ongoing metabolism.

In aerobic metabolism, oxygen directly functions to form energy within the mitochondria of the

cell. The associated processes are referred to as mitochondrial respiration, where energy substrates

are metabolized in the presence of oxygen.

The anaerobic system produces pyruvate as a byproduct of glycolysis – which is now used

by the aerobic system to form acetyl CoA in the Krebs cycle. Also referred as the citric acid cycle,

the Krebs cycle represents a process of numerous molecular exchanges involving carbons,

hydrogen, and oxygen to produce energy. For example, glucose’s molecular chain represents

C6H12O6. When the glucose molecule is metabolized via mitochondrial respiration, the main

constituents are removed and 36 ATP remain as an end product. Since components of the glucose

molecule were removed during this process some energy is lost, while other components are

migrated to a different process to produce more energy. Due to the fact that aerobic metabolism

requires oxygen, it functions as a “pay as you go” system which enables the reactions to create a

stable environment during steady-state work. Unlike anaerobic glycolysis, there exist no

extraneous metabolites, such as hydrogen protons, that inhibit the process by affecting the cell’s

environment. Therefore, the act of producing force is ongoing and does not require a rest period.

It could be argued that while at steady state, low tension training could continue as long as energy

is available to keep the body moving. This explains the ultra-marathon, covering distances of

50-100 miles, as well as the need for refueling during distance race events.

Earlier text has stated that anaerobic athletes should not use aerobic training for improve -

ments in oxygen efficiency. The reason for this recommendation is oxygen is not just a fuel, but

a fuel-mediator. During anaerobic actions oxygen helps maintain proficiency in the anaerobic

systems. For instance, even well-trained anaerobic athletes experience decreases in tissue pH

during training, as lactate levels increase in working muscle and spill into the blood. This

chemical overflow triggers an increase in respiration through a signaling effect in attempts to

augment oxygen supply to working muscle cells. If an athlete has a higher oxygen capacity, they

will more efficiently buffer the anaerobic byproducts. The increased oxygen levels also promote

pyruvate clearance as it can be converted into Acetyl CoA, instead of lactate, and be used

aerobically. This substrate removal allows for improved glycolysis and increases the rate of ATP

production via mitochondrial respiration. This explains why a soccer player can tolerate higher

intensities for longer periods of time (aerobic) during the match, but also explains why they can

repeat high speed (anaerobic) sprints due to energy system interaction.

To this point sugar metabolism has rightfully been the focus of athletic-driven fuel; but lipids

and proteins can also be used in the aerobic system. Although they offer limited contribution

during team sports, endurance athletes must remain cognizant that both protein and fat represent

viable energy sources during prolonged, continuous work. In fact, an endurance athlete may use

as much as 15% of total energy from protein contribution via the breakdown of amino acids.

Lipids are even more important for endurance athletes, as higher fat oxidation rates support

glycogen-sparing. This will be explored in chapter 17. The detailed functions of the system

essentially involve the citric acid cycle and the electron transport chain. It is relevant to under -

stand though that lipid and protein products do not support higher-intensity work. Lipid use

for instance, is optimized below 70% VO2max (~62-65%).

DEFINITIONS

Mitochondria –

An intracellular organelle responsible for generating most of the ATP requiredduring cellular operations

Krebs cycle –

A series of enzymatic reactions thatoccur in the mitochondria, involvingoxidative metabolism of acetyl com -pounds, which produce high-energyphosphate compounds for cellularenergy

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Adaptations stemming from aerobic training are represented in both the cardiopulmonary

and muscular systems. An increase in stroke volume and cardiac output, both of which are critical

in improving maximal oxygen use, occur as the heart improves in conduction signaling and

metabolic efficiency. Training stress will also trigger architectural changes in the left ventricle

leading to desirable hypertrophy and increased contractile force capacity. Often overlooked is

the fact that training to improve cardiovascular conditioning reduces systolic blood pressure and

relative heart rate responses; consequently lowering rate-pressure product (RPP). By reducing

the RPP, the muscles of the cardiopulmonary system do less work, which spares energy over the

duration of the competition. When an athlete can exert the same force at lower cardiovascular

demands, the energy saved from cardiopulmonary function can be used by skeletal tissues for

improved endurance. This benefit is added at the same time the skeletal muscles increase

in capillary density and fiber efficiency for better oxygen management. A major structural

adaptation is an increase in the expression of mitochondria within muscle fibers.

These adaptations demonstrate relevant consideration for sports training, but the method

by which the adaptations are reached is a sensitive issue. Due to the permanent inverse relation -

ship between motor unit power capacity and aerobic efficiency, it is important not to improve

one at the expense of the other. More specifically, if a higher aerobic training time is employed

using low-to-moderate intensities while also training for strength and power development, any

improvements in the anaerobic aspects will be proportionally negated by improvements in the

aerobic system. This is common of aerobic training used for baseball pitchers, boxers and tennis

players. Steady-state training as a means for conditioning anaerobic athletes is generally

counterintuitive to maximal performance benefit. Conditioning drills need to be performed at

sport-speed, and the rest intervals need to be short enough to allow for adequate metabolic stress.

The emphasis is placed on system interaction, not simply aerobic efficiency. Combined or

concurrent system interaction aimed at strength, power and anaerobic metabolic endurance

(relative to the sport) is the best option for most team sport athletes.

A recently published example of this concept employed professional soccer players from

two categories; a conditioning group and a control. Both groups engaged in regular soccer

participation, but the treatment group also performed an 8-week resistance training program.

The program consisted of two concurrent anaerobic training sessions per week; which included

4 sets of 6 maximal-effort repetitions of high pulls, jump squats, bench press, back squat and

pull-up exercises with interval training using 16 maximum-intensity sprints; lasting 15 seconds

with a 1:1 work to rest ratio. Those that participated in the concurrent sessions not only experi -

enced greater performance improvements in vertical jump height as well as 10- and 30-m sprints

speeds, but also in the Yo-Yo intermittent recovery test. Most importantly, these athletes increased

their maximal aerobic speed, which is a major factor for success in soccer [5].

The Continuum Concept

Energy systems interact more than they function in a segregated fashion. Therefore, it is

more practical to view them by duration and intensity versus simply segments of time, particularly

when considering the overlap of metabolic process between them as intensities increase

(continuum concept). The first step to using the energy systems for programming for specific

adaptations is to realize that during any given activity the respective energy systems interact in

attempts to create metabolic homeostasis. Therefore the interactive relationship should be

emphasized when conditioning athletes.

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To accomplish this goal there are three major factors to take into consideration:

1) The training tenure and current condition of the athlete or team – An untrained or

novice athlete will not have foundational system aspects necessary to support high

quantities of work, particularly when high-tension is employed. Detriments in training

performance will be associated with both neuromuscular and metabolic issues;

essentially a coach cannot bypass pre-requisite adaptations by training harder or longer.

2) The volume of the work performed and total recovery – Athletes maintain limited

provisions of energy at a given time. Therefore, activities must be balanced across

phosphagen-driven acts and those supported by glycolysis. Adding volume with or

without changes in frequency requires consideration for proper nutrition and recovery

to account for residual fatigue. If the body is pushed beyond its energy support it will

suffer from fatigue; and consequently, muscle damage.

3) The end-product and rest interval relationship –When creating work-to-rest relation -

ships a coach must consider the athlete’s efficiency in the specific energy system as well

as the total energy system interaction. This is based on the intensity, each action’s

duration, and the rest interval used between sets. Following every sprint there are

residual metabolic byproducts (residuals) in the system. If partial recovery is used, the

athlete’s condition for the next set = residuals + new byproduct. By the fifth set of

sprints (with partial recovery); the athlete’s condition would be = 4x residuals + new

byproduct. This suggests an athlete cannot go from 0-100%, or maximally, over the

course of a workout because the system does not start at zero each time. Rest intervals

should be programmed by residual tolerance with sport-specific purpose and be

adjusted to maintain desired velocities.

Most programmatic difficulties are associated

with controlling all the factors at play. For coaches

to determine proper balance (intensity/duration =

recovery schematics) they must account for each

factor. Specifically, what affect each factor plays

on the recovery demand as well as how long the

recovery must be to 1) drive the energy-specific

adaptation, 2) allow for optimal movement tech -

nique, and 3) permit work at the desired velocity.

High speed, low load, short duration: 20 m shuttle

(5 sec) – byproduct: low ADP/H+; recovery required

is 1:4-6 due to the low percentage of energy used

High speed, moderate load, short duration: 7 tuck

jumps, loaded 10% BW (6 sec) – byproduct: low-

moderate ADP/H+; recovery required is 1:5-7

Moderate-high speed, high load, short duration: sled drive loaded 30% BW (6 sec) – byproduct:

moderate ADP/H+; recovery required is 1:7-9

High speed, high load, short duration: Olympic cleans 3RM (10 sec) – byproduct: high ADP and

moderate H+; recovery required is 1:12-15

In the prior examples all of the activities were performed using the phosphagen system, but

Sport Metabolism

Figure 6.3 The Energy Continuum and Power Output

Wells, G. D., Selvadurai, H., & Tein, I. (2009). Bioenergetic provision of energy for muscular activity. Paediatric respiratory reviews, 10(3), 83-90.

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the necessary recovery varied for each. Factors specific to the load, velocity, time-under-tension,

total muscle mass employed and the resultant rest interval all affect relative performance.

Therefore, assigning a rest interval simply based on the duration of the activity is flawed. A 20

m shuttle lasting five (5) seconds would not justify a full recovery of 120 seconds because the

action will not fully drain the system. The Olympic cleans involved resistance of a much higher

load, and therefore used much more energy; thereby warranting a recovery 4-6 times longer than

the shuttle run even though both were maximal-effort, phosphagen-driven activities.

This concept becomes even more complicated when systems overlap. Consider the follow -

ing: a high-speed activity, such as eight (8) repeat plyometric tuck jumps, will be supported mainly

by phosphagen system because the duration of time needed to complete the act will be less than

10 seconds. A back squat, on the other hand, would likely take about 25 seconds to complete the

equivalent eight (8) repetitions using 80%1RM. Here, PCr stores overlap with glycolysis to

support the activity because it takes a longer period of time to complete. Interestingly, the tuck

jumps will warrant 30 seconds recovery to repeat the performance, whereas the back squat will

require a rest interval of about 90 seconds. To some, this may not make sense because the tuck

jumps were performed using the phosphagen system, which normally is associated with at least

120-300 second rest periods; while the squats were performed using the glycolytic system, which

usually warrants 30-90 seconds for recovery. Again it is the relative intensity, time-under-tension,

total energy demands and byproduct residuals that determine the rest interval – not simply the

amount of time it took to complete the act. Many practitioners fail to realize the practicality of

the energy systems. The duration of the rest must always be consistent with the quantified use

of available fuel, the time it takes to clear limiting by-products from the environment associated

with metabolic disruption, and the relative condition of the physiological systems.

There is a distinct difference in the need for recovery when comparing actions driven by the

glycolytic and PCr systems because the resultant fatigue is based on two different effects. Anaerobic

glycolysis represents the breakdown of sugar from circulating glucose and stored glycogen in

muscle, so the byproduct includes H+ protons and limited ADP residuals. The phosphagen system

on the other hand results in ADP and limited H+ protons residuals. Additionally, sugar is stored

in higher concentrations than phosphagen in the muscle tissue, and therefore is not depleted to a

point of inhibition. Due to the fact that sugar metabolism produces ADP at a slower rate when

compared to the PCr system, the body does not have to wait for

rephosphorylation during recovery between glycolytic-driven actions

as it does with phosphogen-driven work. Instead, it must remove the

H+ and related byproducts from the environment to continue to

produce force. With ongoing work, H+ accumulates in both the cell

and blood, promoting a progressively acidic environment. This acid

accumulation lowers cellular and blood pH levels which inhibits

enzyme activity and “turns off” the energy system – but not due to a

lack of available energy.

Athletes with the most experience working in the phosphagen

and glycolytic pathways with recovery matched for proper adap -

tations will benefit from two perspectives: 1) a prolonged higher-

intensity work capacity and 2) they will require shorter periods

of recovery between sets or activities on the field. This identifies

why training experience and specific stress exposure are so impor -

tant in athletic conditioning for sport. A body unaccustomed to

Sport Metabolism

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Sport Metabolism

work will not manage relative byproducts or heat efficiently, and will therefore shut down

prematurely. Likewise, rest intervals must be aligned in accordance with the energy demands or

the metabolic system will not properly acclimate to the situation. Too long of a rest interval will

reduce the interaction of systems while too short of a rest interval will compromise the

phosphagen system’s contributions to force and velocity. In order to achieve maximal athletic

performance, one must understand the fundamentals of each metabolic pathway and how a

continuum exists between them.

Sport-Specific MetabolismRegardless of a sport’s metabolic preference, all team sports require energy

from each of the aforementioned systems. This is a relevant concept as different

sports are better suited for different energy systems, but no team sport depends

exclusively on a single system. Coaches must therefore attempt to determine the

balance or distribution of energy requirements and the respective relationship

between systems for each sport using evidence-based criteria. From a sport-specificity

perspective, an analysis of the energy system’s contributions should be reflected

accordingly in the training. A common error is to emphasize the aerobic or glycolytic

pathway during conditioning, but analysis of most team sports suggest that event-

determining outcomes mainly occur in less than 10 seconds and at the highest

velocities. For example, the usual duration of a rally in volleyball lasts 6-10 seconds,

during which there is a need for quick lateral movements and explosive jumps [6].

Following the energy continuum, approximately 50% of the energy would be

provided by the PCr system, 35% by glycolysis, with only 15% support from the

aerobic system. Tennis players face a similar situation; while matches can go on for hours, the

sport actions per point last only seconds. A specialist in the 200-m distance swim (efforts up to

two minutes) on the other hand, could expect 4% of the energy to come from the PCr system,

46% from glycolytic contribution, and 50% from aerobic support. Extending the swim effort to

10 minutes causes the relative contribution of the systems to adjust to 1% PCr, 9% glycolytic

and 90% aerobic [7].

Figure 6.4

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Depending on the nature of the activity, some sports may require a varied combination of

system support as different demands are applied throughout an event. For example, a soccer

player will experience moments of low-intensity activity (1/3 of game time is spent walking) as

well as several series of vigorous sprints. In this situation, the system predominance during a

game is aerobic, but the game-changing events all occur at the highest speeds; reflecting very high

force demands. Therefore, the PCr system will be the most relevant system for success in soccer,

but acknowledgement and attention must still be paid to the other areas of the game to prevent

fatigue-related performance decline. At this point, the interaction of conditioning becomes most

relevant. In most team sports, the phosphogen system should be exploited using ballistic training,

sprints and interplay; with rest intervals determined based on the sport’s aerobic system demands.

Fundamental Applications of the Neuromuscular SystemMotor units serve as the functional component of muscle. They are comprised of a nerve

and a homogenous group of attached muscle fibers. When innervated by the nerve all of the

attached muscle fibers contract simultaneously to create force. Interestingly, while all motor units

need energy to produce force, all motor units do not use energy in a uniform manner. Rather,

they demonstrate preferences to particular energy pathways based on structural and enzymatic

distinctions found in muscle cells. These inherent differences are seen through a motor units’

expression of force, velocity, and endurance. Due to the fact that all the fibers of a single motor

unit have consistent characteristics, when innervated they all function in the same manner. From

this perspective the structural and metabolic properties of the motor unit is commonly referred

to as the fiber type.

Fiber types can be differentiated by several characteristics, but mainly by their ability

to use energy. The enzyme concentration within the myofilament structures, more

specifically myosin, heavily determine whether the contraction speed is fast or slow. The

more ATPase found in the myosin, the faster the ability of the muscle fiber to contract.

Since there are many isoforms and hybrids of myosin, most classifications differentiate

two types of fibers; fast twitch (type II) and slow twitch (type I). Fast-twitch fibers may

be further divided into Types IIA and IIX. The reason for the secondary classification of

fast-twitch fibers is the force-energy system specificity associated with each. Analyzing

fibers by these distinctions identifies an important relationship between the fiber

characteristics and its alignment with a particular sport. This is partly where genetics

connect to performance measures as certain fibers are better suited for certain sports

situations. Essentially, elite athletes in a particular sport have the best composition of fibers

for the relative demands of the competition. This identifies why a 100-m sprinter does

not also compete in the 800-m event, and explains the very limited crossover of athletes

between professional sports.

Athletes at the highest level of competition have a genetic make-up that aligns with the

specific demands of a sport. If the demands require the fastest movements for short periods, as

seen in the Olympic 100-m race, the athlete will need a preferential distribution of fast-twitch

fibers. If the race is extended to 5000 m the athlete would need a better distribution of slow-

twitch fibers to complement the endurance requirements. It would be difficult for an athlete to

successfully compete in both race distances because an inverse relationship exists between fibers;

Sport Metabolism

The more ATPase found in the myosin, the faster the ability of the muscle fiber to contract.

193


when fibers produce high force they fatigue rapidly, whereas fibers with the best endurance

produce limited force.

Fast-twitch, type IIX fibers are highly excitable and fire rapidly due to a well-developed

sarcoplasmic reticulum (SR) and a high concentration of ATPase. They produce roughly 50

grams of force per motor unit and demonstrate the largest diameter by cross sectional measures.

The performance limitation to type IIX fibers lies in their fatigue rate. This identifies why a

percentage of these fibers migrate toward type IIA characteristic as an adaptation in response to

training. Initial and ongoing training promotes force endurance within fast-twitch fibers;

demonstrating that fiber characteristics change to accommodate applied stresses. When the

fastest-twitch fibers are recruited for high-force activation they fatigue in roughly five (5) minutes

of accumulated time-under-tension – to a level that no longer warrants training in that system.

This explains why phosphagen-supported training intensities cannot be sustained for long

durations. Type IIA fibers serve an intermediate role for both high force and endurance. At peak

activation, each motor unit produces approximately 30 grams of force. The firing rate is slightly

lower due to a reduced SR complexity and myosin efficiency, with absolute force production

limited by differences in total circumference size (compared to IIX fibers). The uniqueness of

the type IIA fiber is its ability to share responsibilities. The high-force contribution from the

fiber fatigues at approximately six (6) minutes under significant tension. However, unlike type

IIX fibers, IIA fibers do not fully exhaust to a compromised condition. Instead, they shift to a

lower force capacity. Motor units fatigued from high-force contribution will drop output to

about four (4) grams of force. Type IIA motor units can maintain force at this rate for extended

periods of time to aid Type I fibers in endurance activities. For this reason, IIA fibers are referred

to as a fast-twitch, fatigue-resistant or fast-twitch oxidative fibers, due to their capacity to function

aerobically. Type I fibers represent the slow twitch category. The slow twitch is a result of a less

complex SR and reduced myosin capacity. Interestingly, slow-twitch fibers are the first fiber

recruited due to a low threshold for excitation, but contribute the lowest levels of force; about

five (5) grams per motor unit. They are the smallest of the fibers (by circumference) and do not

demonstrate a significant capacity for hypertrophy or force with resistance training, but they

are resilient. Slow-twitch fibers rely mostly on mitochondrial respiration, whereas Type II fibers

are more capable of utilizing the PCr and glycolytic systems.

It is important to note a few concepts related to

generalizing in this classification system. When

examining an entire muscle, a continuum of muscle

fiber types can be identified. Therefore, no muscle

is characterized by only type I or type II fibers.

Likewise, individual muscle cells may be hybrids

(e.g., I/IIA, IIA/IIX, I/IIA/IIX); adding further

diversity to recruitment dynamics. Finally, even if a

particular fiber was identified as type I or type II, or

a specific hybrid (i.e., I/IIA), it should be understood that differences exist among muscle types

in the myosin heavy chains from molecule to molecule. This supports the role of adaptations;

as all skeletal muscle tissue has the capacity to change to accommodate new stresses applied to it.

The genetic factor lies in the initial distribution of muscle fiber types, the proficiency of an

individual’s nervous systems, how quickly and to what magnitude the athlete can adapt, and the

inherent potential for change within the athlete.

The ability to change an athlete’s distribution of slow or fast twitch fibers does not seem

Sport Metabolism

DEFINITIONS

Sarcoplasmic reticulum –

A tubular network that surrounds each individual myofibril and acts as a storage site for calcium within skeletal muscle

Table 6.2 Muscle Fiber Force Contribution

Fiber type Force per unit Fatigue rate

Type II X 50 grams of force per motor unit 5 minute fatigue rate

Type II A 30 grams of force per motor unit 6 minute fatigue rate4 grams of force per motor unit Fatigue resistant

Type I 5 grams of force per motor unit Fatigue resistant

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possible as findings do not support a transition from slow twitch to fast twitch as an adaptation

response. Rather, research suggests the most common adaptation involves characteristic changes

in the muscle fiber architecture and cross-sectional area [8]. This adaptation provides for greater

production of force via parallel fibers within the muscle. Essentially, when a muscle fiber grows

in size it produces more force. Therefore, since genetic manipulation is limited, it is better to

understand these factors as useful in gauging performance potential(s) only. A coach can be

most impactful by adjusting training efforts to account for an athlete’s natural limitations. As

mentioned earlier, there exist specific fiber compositions and efficiencies that best serve different

sports based on their characteristics, but those concerns are better addressed by recruiting the

right athlete for the right position. For example, long-distance runners have shown a higher

percentage of type I fibers in the gastrocnemius (71.1%) compared to middle-distance and

recreational runners at 56.3% and 59.8%, respectively [9]. Essentially at the elite level, the sport

will pick its athletes, not the other way around.

Metabolic Implications for Training

The characteristics that define the performance of skeletal muscle fibers also delineate the

specific training necessary to exploit the fibers’ adaptation potential. With this in mind, the

volume within exercise programs can be tailored to specific adaptation responses. The first step

is matching the energy system and fiber recruitment preferences with the volume of work. Fast-

twitch fibers thrive in the phosphagen system but fatigue within minutes of accumulated

near-maximal tension. This suggests efforts aimed at adaptations in the immediate system require

high intensities with limited volume and appropriate rest intervals. Emphasis should be placed

on maximal loading (or speed) and technique for movement proficiency. Attempting to prolong

periods of heavy work (>85%1RM or maximal power) is counterintuitive to the adaptations.

This is where many coaches make programming mistakes. Based on the fatigue rates of fast-

twitch fibers, total work attainable through the phosphagen energy system will equate to less than

5-10 minutes of total time-under-tension. This should be measured by time of contraction.

Longer durations of tension at this intensity are not beneficial, and commonly lead to uninten -

tional overtraining syndrome.

When the intensity is reduced to 70-85%1RM (or velocity is reduced), time-under-tension

may double as metabolic support shifts to the glycolytic pathway. As a consequence of reduced

Sport Metabolism

Table 6.3 The Biochemical Characteristics of Muscle Fiber Types

Muscle Fiber Characteristic Type I Type IIA Type IIX

Capillary density 1.0 0.8 0.6

Mitochondrial density 1.0 0.7 0.4

Myoglobin content 1.0 0.6 0.3

Phosphorylase content 1.0 2.1 3.1

Glycogen content 1.0 1.3 1.5

Triglyceride content 1.0 0.4 0.2

Phosphocreatine content 1.0 1.2 1.2

Myosin ATPase activity 1.0 >2.0 >2.0

Phosphofructokinase activity 1.0 1.8 2.3

Adapted from: Sport Nutrition: An Introduction to Energy Production and Performance, 2nd edition;Jeukendrup & Gleeson

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loading during work sets, training regimens aimed at moderate to moderately-high intensity

work increase to as much as 20-30 minutes of time-under-tension. The actual amount of time

is determined by the load used to create the volume, particularly if combined with phosphagen-

based activities. A traditional hypertrophy workout for instance, may use 10-12 exercises with

3-4 sets of 8-12 repetitions. At approximately 30-40 seconds per set, the time-under-tension will

range from 15-30 minutes (depending on the schematic). Conditioning (tension) periods will

be even less. Attempting to increase or maximize volume for the purpose of simply doing more

work will most often lead to residual muscle fatigue, poor training technique and compromised

recovery. Adaptations that promote sport performance from anaerobic training are derived from

the details of the program, not on maximal training volume. It is important to realize fitness

training is different than sports training as it attempts to prolong activity to burn calories, whereas

sports performance enhancement is specific to adaptations in an attempt to spare calories through

improved efficiency.

As acknowledged earlier, aerobic training is not commonly used during team sport condi -

tioning. Endurance sports however, rely on aerobic metabolism for success. Here adaptations

continue to reflect the metabolic- and tension-specificity for motor unit recruitment. The

distances covered by most endurance sport athletes require prolonged durations of continuous

activity. Interestingly though, training for endurance events should also be based on the inter -

action of the anaerobic and aerobic systems. Among elite endurance athletes, VO2max is not the

primary factor separating the winners from the losers; it is normally the anaerobic system that

makes the difference. Those individuals with both an efficient aerobic and anaerobic system

usually dominate the race. Anaerobic training emphasizes high-intensity intervals for up to 20-

30 minutes to increase demands placed upon fast-twitch fibers; whereas aerobic adaptations such

as slow-twitch fiber hypertrophy and improved lipid oxidation are derived from long, slow

distance training for 60-90 minutes (or more). Endurance athletes must use the metabolic

pathways to adapt both slow- and fast-twitch fibers to promote the desired performance benefits.

Gradation of Contractile Force

It was stated earlier that within a motor unit a distinct number of muscle fibers are con-

nected to each nerve which signals information to each fiber using what are referred to as action

potentials. An action potential starts as an electrical impulse that runs along the nerve. When

DEFINITIONS

Action potential –

A wave-like change in the electricalproperties of a cell membrane, electricalsignaling causes a chemical message thatforces the muscle cell to contract

Figure 6.5


Sport Metabolism

it reaches the neuromuscular junction the electrical impulse is converted and relayed via

chemical signaling to all of the fibers innervated by that motor neuron. This process of

events is known as the all-or-none principle. When considering force gradation, it

is important to understand that contrary to the individual fibers within a motor unit

which all fire at the same time; the individual motor units throughout an entire muscle

are not innately synchronized. Since gradation of force, or tension, has the capacity to

vary at different points, sports specificity becomes more relevant for improvements in key

areas. In order to execute sport movements successfully, the athlete must precisely adjust

the tension within the musculature to regulate specific actions. If the athlete activates too

many fibers or not enough; the action may falter or fail. Furthermore, if the recruitment

pattern is not organized, the movements of associated bodily segments lose force effi-

ciency. This includes syncing motor unit recruitment so slow-twitch fibers, which

stabilize motion segments, precede fast-twitch fibers to maximize force transfer across the

segment. Essentially, if the motor units fire at different times, or they lack proper

sequencing or synchronization, the precision needed within an action will not occur.

What often separates the good athletes from the great athletes lies in the measures of intel-

ligence and coordination within nerves.

The muscular system has the capacity for incredible force output, but that capacity lies

mainly in potential. Untrained females for instance, have been observed lifting extremely heavy

objects in efforts to save their child. Certainly, adrenaline is the driving force behind this remark-

able feat, but the fact remains that the individual’s musculature was capable of producing forces

well beyond the tissues measurable 1RM.

This observational data suggests if the nerv-

ous system is able to recruit the correct

quantity of motor units with precise timing

– it can far exceed its normal performance.

The ability of a strength coach to extract as

much of this potential as possible is the key

to success. A caveat to this concept is the

training must be aimed at applicable force

(sports proficiency), not just weightlifting

force.

Motor Unit Firing Rate, Recruitment and Synchronicity

Muscle force (and consequently movement control) is based on motor unit firing rates,

recruitment and synchronicity; all of which are trainable components of the nervous system.

However, there seems to be differences in adaptation responses when comparing small and large

muscle groups. Adaptations to firing frequency are more easily attained in smaller muscles,

whereas improved motor unit recruitment seems to be more pronounced in larger muscles.

This is likely due to the ratio of muscle fibers recruited per motor neuron in the respective areas

as well as the specificity of the tasks they perform. The thigh musculature for instance, may

have a ratio of 1:3000, whereas the calf may have a ratio 1:1800; very small muscles within the

hand are further refined to a ratio of 1:100. Where recruitment and firing rate seem to have

some adaptation specificity, factors related to improved synchronicity seem to be based on motor

rehearsal and can be improved to an equal extent in most small and large muscles, respectively.

In regards to motor unit firing, research has indicated that a change in firing rate occurs in

An action potential starts as an electrical impulse that runsalong the nerve to the neuromuscular junction; at whichtime the electrical impulse is converted and relayed viachemical signaling to all of the fibers innervated by thatmotor neuron.

All-or-None Principle

DEFINITIONS

Neuromuscular junction –

Physiological structure that allows anaction potential from a nerve toinnervate a muscle via an electrical tochemical conversion

All-or-none principle –

A muscle cell either fully contracts, ornot at all; if a nerve impulse is at leaststrong enough to depolarize the cell, itwill contract regardless of the absolutestrength of the impulse

196

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Sport Metabolism

response to adjustments in the threshold stimulus. The reduction in electrical threshold allows

for an earlier release of signaling, and consequently a more rapid firing rate. Additionally, firing

rate has been identified as a key component in the development of important performance

mechanisms such as the rate of force development (RFD) or “explosiveness”, which represents

one of the best predictors of athletic ability [10,11,12,13]. The most effective means for achieving a

reduced firing threshold is the performance of ballistic training. Researchers have found an

increase in the percentage of motor unit firing doublets (two motor unit signals that occur at

very short intervals) following 12 weeks of movement-specific power training. Specifically, firing

doublets increased from 5.2% to 32.7% [13]. The best results in terms of firing frequencies are

usually detected when the athlete is beginning his or her training process. This explains, in part,

the improvement rates noted in the initial stages of training compared to adaptation responses

that occur later on in the athlete’s tenure. An untrained athlete will see significant improvements

in strength and power during the first 4-6 weeks of training as motor units improve their firing

efficiency. In addition, the firing patterns learned can be environmentally specific. For example,

elite sprinters demonstrate maximum motor unit signaling frequency at the start of a 100-meter

race, as a fast start is paramount to success [14]. This supports the rationale

of sport-specificity in training to develop neural signaling in a manner con-

sistent with the sport.

The size principle has been used to describe the activation pattern

of motor units. Smaller motor units that innervate type I fibers will be

activated before the larger, more powerful motor units that innervate and

excite the type IIA and IIX fibers. The size principle comes into play as a

result of the different thresholds of each unit. The smaller units have lower

thresholds, thus requiring lower electrical levels of activation than those

required by larger motor units. This is likely due to the need to stabilize a

segment before loading it and to spare energy by using smaller units when-

ever possible such as to support postural requirements. Attempts have been

made to promote recruitment adjustments, but studies suggest that the size

principle remains unaltered even after periods of power training [13]. How-

ever, thresholds for recruitment of high-power motor units can be reduced with ballistic and

plyometric training; demonstrating the important role of power exercises in explosive movement

development. Training programs which properly emphasize velocities and loading can optimize

recruitment-rate specificity. A combination of ballistic training and compound exercises using

heavy loads seems to best exploit this adaptation response.

Fast-Twitch Recruitment Patterns

When analyzing recruitment factors it is important to understand that specific motor units

need to be trained using speeds, power levels, and patterns that mimic the dynamics of the sport.

For instance, if a given sport demands quick explosive power, training should involve similarly

characteristic movements that require recruitment of the fastest-twitch fibers in order to reduce

motor unit thresholds. For concurrent improvements in firing synchronicity, it is important that

both the movement patterns and training speeds remain consistent with the speed and type of

sport actions. When training employs actions that resemble the sport requirements in whole or

in part, at the proper velocities, improved firing synchronicity has been noted. Analysis has

shown that improvements occur in the connections between the motor neurons, which results

in enhanced coordination and an increased rate of force production [15].

DEFINITIONS

Size principle –

Tension on muscle promotes progressiverecruitment of fibers based on motorunit size and activation threshold;smaller motor units that innervate Type I fibers are activated before larger,more powerful motor units thatinnervate Type II fibers

If a given sport demands quick explosive power,training should involve movements that requirerecruitment of the fastest-twitch fibers so thattheir motor unit thresholds can be reduced.

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Sport Metabolism

Considering these concepts, it is important to provide appropriate balance between the

resistance and speed used during training for sport. There are two developmental considerations

here; (1) ensuring adequate prime mover strength, strength-balance across movement segments

and stability across the skeleton, and (2) ensuring load applications properly serve the force-

velocity curve. These suggest faster speed is more important than heavier loading. Expectedly,

the two most common errors in promoting motor unit adaptations for sport are the performance

of ballistic training without musculoskeletal balance, and excess loading at the expense of move-

ment velocity. Therefore, phasically-appropriate resistance assigned to compound and ballistic

movements is extremely relevant. A key element in the program is to understand that when too

much weight is employed during ballistic and plyometric exercises – the benefits of the motor

unit adaptations will be relatively reduced. Common examples include weighted jumps using

heavy loaded vests or excessive dumbbell loads, overloading drag or sled training, and using high

heights for depth jumps. When these types of activities are properly employed, jump resistance

should be less than 12% of total bodyweight, loads used for drag training should not have any

notable effects on running biomechanics (3-8% BW) and depth jumps should occur at heights

that allow for the shortest amortization phase possible (maximally 30 inches – less than 18 inches

is recommended). The force-velocity curve should be referenced for appropriate load and

velocity by training phase. A progressive model in training loads and speeds should logically

reflect a neuromuscular path towards consistent game speed. Additionally, when deciding on

traditional weight training volumes, a strength coach must be sure to consider adequate quantities

of athletic-based ballistic training. This is exemplified by the fact that greater improvements in

vertical jump height were found among NCAA Division I volleyball players following ballistic

jump squat training using loads at 30, 60, and 80% 1RM, when compared to traditional heavy

(squat) training [16]. Later chapters will proportionate concurrent strength training and intensity-

specific power training in a progressive model.

Increased Motor Unit RecruitmentThe execution speed of exercises may also be reduced to promote specific adaptations.

During the earlier phases of a training cycle a coach may emphasize muscle tension via slow-

velocity training (tempos) for certain benefits. A recent study showed that the performance of

leg extensions using a relatively light load (30% of 1RM), executed at a slow tempo of six (6)

seconds of concentric work and six (6) seconds of eccentric work, produced a greater increase in

muscle protein production than when executed at a fast tempo of one (1) second up and one (1)

second down [17]. While not surprising for its “bodybuilding effects” on muscle growth, the

stimulus also increased the number of motor units recruited. Bodybuilders have long identified

that as time-under-tension increases, motor unit fatigue triggers greater motor unit recruitment

and anabolic responses. This has application for both hypertrophy and strength as a result of

fiber cross-sectional improvements.

Activities aimed at improving sport performance will not involve twelve-second repetitions

that activate isolated muscle groups, but evidence related to the time-tension relationship on

motor unit recruitment holds merit. Hypertrophy-endurance and hypertrophy-strength phases

can both exploit this condition by utilizing athletic lifting techniques such as squats, deadlifts,

presses and related compound movements to increase motor unit recruitment in muscles used

for sport training and performance. Sport differences will determine the specific emphasis on

muscle growth, but it is important to recognize that improvements in muscle force production

DEFINITIONS

Amortization phase –

Also known as the transition or contactphase; constitutes the period of timebetween the concentric and eccentricphases of a plyometric exercise whereinthe stretch-shortening cycle is exploitedto maximize power production

Bodybuilders have long identified that as timeunder tension increases, motor unit fatiguetriggers greater fiber recruitment.

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are associated with 1) an increased cross-sectional area of associated muscle fibers and 2) the

recruitment of more motor units as a result of increased time-under-tension. Both are beneficial

to performance because these newly-recruited motor units will become more active;

consequently increasing force production as more fibers activate during work. A

caveat to the aforementioned is when programming hypertrophy-based training, as

the strength coach must be cognizant of morphological considerations for the sport.

Athletes carrying additional mass tend to be slower and require more oxygen for the

same task when compared with a lighter counterpart. Training for muscle mass

should not negate ballistic and/or athletic-based training nor affect metabolic

conditioning.

It is evident that phases of training should be balanced and present varied

methods of stress for desired results. It should be understood however that multiple

variables can be applied simultaneously. This suggests the use of appropriate velocities

throughout all stages of a periodized training program; including the hypertrophy

phase when more emphasis is usually placed on motor unit recruitment. When

constructing training programs using multiple adaptation goals per exercise bout,

operations should be ordered based on the energy system-tension relationship.

Since most programs are aimed at multiple objectives the order will reflect the energy

system first, and then the speed of the movement second. Therefore phosphagen-

driven power exercises (<5 reps) precede phosphagen-based strength exercises (<5

reps), which precede phosphagen/glycolytic strength exercises (<10 reps). Essentially,

nerves are trained before muscle, both on the field and in the weight room; neural

adaptation training will precede metabolic conditioning.

Potential of Motor Unit Force Output – Program AttentionThere are two neurophysiological aspects of muscle fibers that may help promote the desired

training effect when attempting to develop power. The first is post- activation potentiation

(PAP). PAP describes the improvement in muscle performance (e.g., power and force) resulting

from a previous, high-intensity stimuli. Pre-activation elicits a PAP response when performed

within 12 minutes of the prior exercise. Essentially, the acute contractile history of motor units

can have a positive effect on future actions due to changes in neural excitation. These effects

are commonly targeted during training via the use of complex sets. A complex set describes a

set couple where a heavy resistance exercise (>85% 1RM) is followed by an explosive activity.

Numerous studies have concluded that complex sets, (sometimes referred to as contrast sets),

such as heavy squats followed by rapid jump squats, are effective for improving performance vari-

ables such as jump height through the benefits of PAP during the execution of the explosive

component. It is important to note that the load of the initial exercise is the key aspect in pro-

moting an optimal PAP response. The second consideration lies in the lengthening of muscle

tissue and its affect on motor unit activation. When muscle moves through a full ROM, twitch

force increases; consequently reducing the fatigability of the motor unit by lowering localized

force production. This occurs through an increased number of contractions [18]. In addition to

total muscle activation, full ROM contractions promote cross-fascial excitation and improve force

coupling potential. This identifies that the use of very heavy loads moved over a short duration

is not as beneficial as heavy loading across a full ROM. Unilateral exercises can optimally exploit

this benefit.

Figure 6.6 Proper Training Stress Order

DEFINITIONS

Post-activation potentiation –

Post-movement excitation in thenervous system which enhances powerand force output during performance ofa subsequent movement using the sametissue

Complex set –

Set couple where a heavy resistanceexercise (>85% 1RM) is followed by anexplosive activity (e.g., heavy squats andbox jumps)

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Practical Implications of Motor Unit Metabolism and PhysiologyProper exercise prescription for elite athletes should address three key elements connected

with the motor unit and metabolic adaptations including the total force produced, the maximal

velocity of movement, and the time-tension relationship. Due to the fact most sports require

repeat bursts of high-force and high-velocities; phosphagen-derived energy becomes the priority

system. Research has demonstrated that PCr recovery is slowed when the system is placed under

hypoxic conditions, and improved when oxygen is available during recovery. These findings

suggest two things: 1) optimal benefits within the PCr system occur with adequate recovery, and

2) athletes who develop efficient cardiovascular proficiency will experience higher neuromuscular

performance in the PCr system due to greater oxygen availability [19]. This concept was tested

with hockey, tennis, basketball, handball and football players using work intervals with varying

periods of recovery. Researchers found that interval training with short rest periods compromised

phosphagen-recovery and the resultant adaptations, whereas intervals with longer rest periods

demonstrated higher PCr efficiency and contributed to improved PCr synthesis rates after five

weeks of HIT[20]. These findings are relevant because these adaptations are necessary for repeating

high force in sports situations.

The evidence suggests if an athlete wants to effectively produce, reproduce, or sustain high

force they must comply with proper tension-time/recovery relationships during training. An

important concept to understand is recovery is dictated by the level of resistance and volume of

muscle employed to move it and not simply the time to complete the activity. For example, an

experienced athlete performing four Olympic cleans using 87.5%1RM will require two or more

minutes of recovery before repeating the set, whereas an athlete performing the same exercise

and repetition schematic using 65%1RM will need only 90 seconds of recovery to duplicate the

effort. The difference in recovery is associated with the depletion of PCr during the set. In the

case of competitive weightlifting, recovery from maximal capacity lifts are sometimes five minutes.

But in sports training recovery of this magnitude is unrealistic, and of little consequence, as

strength and conditioning focuses on improving sports performance not weightlifting.

HIT has become a popular technique to train multiple systems simultaneously. The issue

with HIT as an all-encompassing model is the short rest intervals cause hypoxia – which impedes

phosphagen system function. Therefore, even though HIT circuits may include lifts normally

associated with PCr adaptations (such as the Olympic lifts), the training does not provide the

athlete with the same benefits. Repeat power output associated with the PCr system is improved

among athletes who perform phosphagen-supported training using the correct work-rest

relationships. Olympic lifts and related PCr-driven activities should be employed independently

and not be used in circuit-based training. HIT training is designed for metabolic conditioning

using moderate-force applications. It is an ideal system of training to enhance the body’s ability

to deal with high-intensity, intermittent work associated with glycogen-supported actions.

Unlike the phosphagen system, training for improvements in glycolytic capacity does not

warrant long recovery periods for efficient adaptations. The recovery should reflect the metabolic

byproducts, which in the case of glycogen-supported activities is H+. Coaches should always

base the rest interval on the amount of ADP and H+ in the system after each training set. When

intensities drop below 80% of load capacity, ADP rephosphorylation becomes reduced; and

subsequently, so do the rest periods. Training the glycolytic system does not require exhausting

the system at terminal levels of time to be affective either. Typically, intervals of 10, 15, 30, 45 or

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60 seconds are used with the shortest, tolerable rest period. The term tolerable reflects repeat -

ability; in the glycolytic system, recovery can be minimized to the point that allows for repeat

performance with proper movement efficiency at the desired velocity. If the technique or velocity

is compromised due to residual byproducts, the rest interval should be lengthened to accom -

modate improved movement quality. The glycolytic system can also be pushed for longer periods

of time (anaerobic capacity is generally 90 seconds), but it would have to reflect a defined purpose

in conditioning. The primary decisions around load, time, and recovery will be dependent on

either 1) improving the system’s efficiency with particular applications or 2) increasing the

athlete’s buffering capacity when using the system for sport activity. Once more, the rest period

is of key importance; greater increases in glycolytic enzymes and buffering have been measured

when high-intensity intervals are separated by appropriate recovery. Rest intervals that are too

short are far less beneficial.

The actions that occur in the anaerobic systems are often the event-deciding plays for most

sports, but aerobic capacity remains vital to intermittent sports such as soccer, basketball,

hockey and rugby. Repeat sprint ability and high-velocity actions are sustained by oxygen-

supported recovery. Likewise, sports that may be performed over prolonged periods of time,

such as tennis, require adequate aerobic efficiency to spare energy. This being said, aerobic

benefits should be derived from anaerobic training because the improvements in sports come

from the interaction of the systems – not simply elevated oxygen consumption. An important

concept here is aerobic metabolism, while integral from a support perspective, can be detrimental

to force if trained as an energy system preference. Coaches must carefully manage conditioning

so aerobic metabolism functions as an interactive system and does not compromise anaerobic

functions. Over-emphasizing aerobic training will cause deleterious effects on force and power

output due to recruitment and morphological changes in the motor units. Therefore, team sport

athletes should not be conditioned using long-duration, steady-state training. By training with

anaerobic intervals (shorter work bouts with higher rest periods), problems that affect force

production and power including motor unit recruitment, firing asynchronicity and negative

enzyme changes can be properly accounted for in the program. Using the glycolytic system to

promote aerobic conditioning requires higher-intensity activity with work-to-rest ratios of 1:1

to 1:6. The intensities achievable through anaerobic energy systems promote improvements in

stroke volume and capillary density as well as enzymatic adjustments that more closely mimic

sport conditions. This allows the athlete to transport oxygen into muscle cells more efficiently;

improving sprint recovery and longer-duration play at higher intensities.

The Balance of Physiological Disruption and RecoveryThe concept of training quality over quantity extends to pre-exercise

nutrition and post-exercise recovery. It is important to recognize that the

quality of an exercise bout is dependent on relative fluid and energy stores

in the body at the initiation of training. Performing while in a fatigued state

can have injurious effects and result in muscle damage if the intensity and

duration is too aggressive or prolonged. Exaggerated levels of strain are

often the result of accumulative stress from improper doses of loading, poor

management in the quantity and type of tension (eccentric vs. concentric

volume) and inadequate recovery. Athletes are at an elevated risk for system

Event-deciding plays during competition areoften anaerobically-driven, but a high aerobiccapacity remains vital to intermittent sports suchas soccer, basketball, hockey and rugby.

The combination of significant contractile forces, inadequate recovery, andmetabolic disruption can create a deleterious environment.

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strain and injury during higher volumes of training, such as those seen in the hypertrophy and

strength phases of periodized programs. These higher-volume phases often cause strain-

associated muscle fatigue. As stated earlier, “more is better” is a flawed concept when applied to

loading conditions, particularly when neuromuscular stress is acutely or chronically too high.

Special consideration should be applied to programs simultaneously employing plyometrics,

ballistics, resisted sprints, and heavy compound exercises. Coaches should plan bouts based on

recovery, and during longer training cycles, plan for periods of unloading.

Training volume is not the only consideration when analyzing the risk for muscle damage.

The collective stress associated with muscle tension and inadequate recovery becomes more of

an issue when combined with glycogen depletion, high levels of reactive oxygen species (ROS)

associated with hypoxic states, and inadequate nutrition (including calcium, sodium and

potassium intake). It is commonly multi-factorial stress that promotes or exacerbates muscle

damage. Therefore, exercise selection and volumes should be considerately applied and balanced

with recovery measures; including sound nutritional strategies. To this point glycogen-loading

and adequate hydration are collectively associated with reductions in tissue damage when training

volumes are matched. A checks and balance approach should be used to evaluate and edit

programs that have become overzealous in any particular area. Stress is often accumulative, so

coaches should evaluate all associated risks for overtraining (e.g., sleep quality, glycogen stores,

hydration, heart rate variability), rather than just the few considered to be the most detrimental

(i.e., high volume, eccentric work).

Training Threshold & Recovery

The idea of using thresholds as a guide for training and recovery is by no means novel, but

in some cases the nomenclature can create confusion in application. For this reason, thresholds

should be regarded from two points of view; one is conceptual and the other is operational. From

a conceptual perspective, there are two ventilatory thresholds (VT1 and VT2) and two metabolic

thresholds (lactate threshold; LT1 and LT2) [21]. While clear physiological boundaries or

“thresholds” are nearly impossible to determine; these thresholds represent the beginning and

the end of the aerobic-anaerobic transition. Essentially, it should be considered a metabolic shift,

as no metabolic pathway is ever completely shut down. Similarly to a car, the body can downshift

to gain power, and upshift to gain efficiency. Lower gears of a car use the most fuel to produce

the most force, and consequently produce the most exhaust. Similarly, the higher the intensity

experienced by the body the more metabolic exhaust (lactate). The concentration of metabolic

“exhaust” triggers a corresponding response to maintain homeostasis. These metabolic changes

in the system reflect the lactate thresholds, whereas the ventilatory responses to those conditions

represent the ventilatory thresholds. VT1 is demonstrative of increased ventilation to mediate

lactate in a process of buffering. VT2 is characterized by a loss of lactate steady-state resulting in

lactate build-up, metabolic acidosis and a disproportional rise in ventilation.

Key elements of chronic muscle strain:

Excess loading without adequate recovery

Repeated isolated loading

High volume eccentric contractions

Excessive hip-trunk extension

Poor lifting technique – load over quality movement

Overall high volume of physical activity

Lack of proper nutrition and hydration

Inadequate sleep

DEFINITIONS

Reactive oxygen species –

Chemically-reactive moleculescontaining oxygen ions and peroxides;are a natural byproduct of oxygenmetabolism and serve important roles in cell signaling and homeostasis

DEFINITIONS

Lactate threshold –

Describes a work intensity where therate of lactate accumulation matcheslactate removal; any further increasewill cause excessive lactate accumu -lation and force the participant toreduce the workload or stop within agiven period of time due to acuteacidosis

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The concept of anaerobic lactic capacity is more accurate than lactate threshold when used

for program design. When considering lactate metabolism, glycolysis is not very efficient and

results in greater H+ production than when sugar is used for aerobic metabolism. Glycolysis allows

for higher-intensity work, but at the same time creates increased quantities of byproduct. As a

result, the entire ATP production process slows down, acidity increases and muscles fatigue.

Previous text has identified the inability of enzymes to function in acidic environments. Therefore,

unmanaged byproducts like H+ limits enzyme activity to a point that promotes muscle fatigue, or

forced volitional failure. An individual in better metabolic shape, which combines all energy

systems, is able to produce more force for longer periods of time and repeat the actions with limited

rest. This represents the lay concept of being “in shape”. The unification of more efficient energy

systems reduces deleterious metabolic products in the environment, spares energy, and increases

repeated force capabilities. Therefore, repeated force is just as much metabolic as it is neuro -

muscular in nature; supporting a balance within strength and conditioning methodology.

Metabolic Conditioning

If an athlete needs to function at higher blood lactate levels for improved performance

metabolic conditioning must: 1) impart a stress that increases lactate production to higher levels

and 2) control rest intervals to force improvements in buffering capacity from appropriate levels

of exposure. In terms of training, this can be done in both the weight room and on the field. In

the weight room, anaerobic capacity is usually pushed at an intensity range of 50-70%1RM using

12-25 reps. The volume is high at 30 to 45 sets in a training session, with short rest intervals of

up to 30 seconds. Sprinting, rowing or cycling at maximum capacity for 30 seconds to 1 minute

has also been shown to increase anaerobic lactic capacity. Since Type IIA and IIX fibers produce

more lactate, higher activation of these fibers will result in enhanced adaptations. This is

accomplished, by maximizing movement velocity over specific distances. Select track distances,

weighted locomotion drills (including stair work), loaded/unloaded ballistic work, and novel

activities such as battle rope, kettlebell or sledgehammer drills can serve this purpose. The use

of supersets, tri-sets, metabolic circuits and on-field stations function well for improving

anaerobic capacity as long as the rest interval is appropriately short. Due to the high fatigue rate,

selected exercises should be uncomplicated to promote technique adherence. For obvious

reasons, Olympic lifts should not be used for these purposes. When speed, agility, and quickness

(SAQ) activities are used for conditioning, neural work involving finite change of direction (COD)

drills or agilities should precede gross COD/agilities to reflect fatigue-based technique adherence.

When employing on-field/court conditioning, multidirectional drills should be used over linear

drills as they enhance lactate production over a broader range of musculature; further promoting

fatigue consistent with sport applications. Changes in the center of mass (directionally and

vertically) cause dramatic increases in the activity of postural muscles and global stabilizers to

create a much better overall conditioning approach compared to linear running alone.

Types of FatigueFatigue is often defined as a general reduction in motor unit force output, but has impli -

cations in the psycho-emotional state of an athlete as well. This suggests that fatigue is not simply

metabolic in nature but also impacts the nervous system. Fatigue cannot be viewed as a general

concept in athletics as it is sport specific; factors related to successful competitive performance

are different for every sport and impart different issues during competition. For example, aFatigue is not simply metabolic in nature but alsoimpacts the nervous system.

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football coach may be concerned with decreased muscular force and power in the defensive line

during the fourth quarter of a game, whereas a golfer might be more negatively affected by a

decline in technique and finite skill performance at the closing round of a three-day tournament.

When addressing fatigue, it is important to understand the root cause of the problem, which

suggests an analysis of both metabolic and neural factors. The neuromuscular junction (NMJ)

is the physiological formation that allows the electrical signal (action potential) from the nerve

to innervate the muscle cell via a chemical conversion. Central (neurological) fatigue refers to

the reduction in all actions that occur on the neuron side of the NMJ, while peripheral fatigue

relates to those that impact the muscle contractile function.

Intense training and inadequate recovery can negatively impact the nervous side of the

contractile system. This combination can reduce brain motor signaling due to poor neuro trans -

mitter function and immune cell activity from high-stress exposure. Brain activity has also been

shown to be affected by ammonia accumulation from protein breakdown in the liver when sugar

supply is inadequate. These factors fatigue the CNS; affecting both the generation of action

potentials as well as mental sharpness during training/competition. At the muscle cell level the

NMJ also experiences fatigue. This can cause a slowdown in electrical-chemical transmission;

impeding force proficiency. Likewise, acute changes associated with high-tension training occur

at the NMJ; further slowing the contraction rate. From a practical sense however, it is important

to recognize central fatigue is most often associated with total-body depletion of carbohydrates

(glycogen) common of intense training without adequate nutritional support [22].

Peripheral fatigue, or muscle fatigue, refers to aspects that affect the metabolic properties

of tissue, and consequently the function of actin and myosin in the sarcomere. These include

reduced energy availability from immediate and intermediate sources, energy production

inhibition from byproduct accumulation, and muscle contraction limitations due to acidosis and

dehydration. When the cell experiences acute dysfunction due to pH changes that cause acidosis,

it is termed acute peripheral fatigue. The word acute is used because the environment can be

buffered to re-initiate work capacity during the training bout. When the fatigue occurs due to

lack of energy provisions in the cell, from either low pre-exercise stores or localized depletion of

glycogen in the muscle from ongoing work, it is termed general peripheral fatigue. General

peripheral fatigue does not improve with an acute rest interval because sugar is no longer available

to support the force demands. At this point, ongoing training could be viewed as detrimental

and therefore the exercise bout should be ended.

The key factors that need to be collectively controlled to prevent or delay fatigue are the

initial carbohydrate storage, the intensity and duration of activity, the recruitment patterns

involved in the movements and the work-to-rest ratio. Each of these factors is controllable and

therefore should be accounted for in the program. Coaches should ensure that athletes are 1)

adequately fueled and hydrated, 2) adequately exposed to metabolic-specific stress, and 3)

perform sport-specific movements at the intensity and durations associated with the sport.

These have collectively demonstrated athletic improvements in reaction time, agility and repeat

sprint endurance throughout the duration of competition. By suppressing peripheral fatigue,

athletes perform sharper movements and experience a reduced risk for injury in the final quarter

or period of a game. Additionally, athletes who are also exposed to concurrent heavy compound

lifts and ballistic training are better able to repeat high-level contractile force when compared

to athletes who have not experienced the same preparatory stress. Appropriate balance of sport-

specific power and conditioning work aimed at exploiting the PCr and glycolytic systems will

DEFINITIONS

Acute peripheral fatigue –

Occurs when cells experiencedysfunction due to a metabolicreduction in pH; acid limits enzymeactivity requiring buffering compoundsbefore work can be re-initiated

General peripheral fatigue –

Occurs with a lack of energy in the celldue to low pre-exercise stores orlocalized depletion of glycogen; acuterest intervals will not help as sugar is nolonger available

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allow the body to endure higher tension for a longer period of time. When the environmental

stress is familiar, albeit internal or external, it is much easier for the body to deal with in a

successful manner.

Muscle DamageMuscle damage occurs as a result of excess physical stress placed on muscle fibers and

metabolic stress experienced during training and competition. As mentioned earlier, the

meta bolic contributions to muscle damage include insufficient energy storage, inefficient manage -

ment of byproducts due to inadequate recovery, depleted provisional states, and premature

exposure to progressive training volumes. The end result of each is exercise-induced muscle

weakness and damage. An added concern is insufficient fluid balance to maintain the metabolic

environment as cells cannot function without proper cellular fluid volume. The combination of

problems leading to muscle damage are most commonly associated with low oxygen levels, low

carbohydrate availability and insufficient metabolic water (electrolyte balance); all of which are

controllable factors.

Metabolic disruptions leading to muscle damage during exercise are associated with a

number of compounding factors. However, most problems seem to be tied to inadequate oxygen

availability from intense conditions. As mentioned previously, exercise creates hypoxic conditions

as a result of reduced oxygen supply to tissues. This leads to the proliferation of ROS, or free

radicals, as detailed earlier. These chemically-reactive molecules are a natural byproduct of

oxygen metabolism and serve important roles in cell signaling and homeostasis. ROS also play

an integral role in the inflammatory process leading to muscle adaptations, and their presence

promotes positive responses in skeletal muscle healing when training and recovery is properly

matched. However, during times of very high stress, such as heavy training in high heat, ROS

levels can increase dramatically; resulting in significant damage to cellular structures. Research

has shown that ROS production is associated with both intense aerobic and anaerobic work. In

fact, ROS have been directly associated with excessive muscle damage after resistance exercise [23].

Hypoxia and ROS have a direct effect on all metabolic systems; however, two mechanisms

seem to be most important. The first surrounds the fact that exercise during acute hypoxia

increases muscle and blood lactate levels more than during exercise with normal oxygen supply.

This means that hypoxia causes a greater than normal decrease in pH; thereby increasing potential

muscle damage. The second mechanism involves enzymes, as oxygen deprivation can cause

a number of enzymes to become less efficient. Glycogen phosphorylase, the enzyme which

promotes glycogenolysis in working muscle, has been shown to slow down in strained environ -

ments. Creatine kinase activity has also been found to be altered. In fact, rhabdomyolysis, or

the breakdown of injured muscle tissue and the release of myoglobin into circulation, is usually

diagnosed through creatine kinase serum levels.

It has been established that requisite levels of glycolytic conditioning is necessary to manage

hypoxic conditions and improve buffering during intense training. However, adaptations take

time and should be progressive. If athletes are exposed to excessive exercise durations or inten -

sities without adequate recovery in attempts to increase adaptation rates, they may be injured or

experience acute rhabdomyolysis. This is also true in trained athletes, even if temperature and

hydration are accounted for during the training. Although numerous documented cases indicate

hydration levels and temperature as key factors, this is not always the case with rhabdomyolysis.

Metabolic disruptions leading to muscle damageduring exercise are associated with a number ofcompounding factors. However, most problemsseem to be tied to inadequate oxygen availability.

DEFINITIONS

Rhabdomyolysis –

A potentially life-threatening issue;involves the breakdown of injuredmuscle and the release of myoglobin intocirculation following extreme metabolicand mechanical stress

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For instance, a healthy, well-trained 18-year-old NCAA Division 1-A football player was diagnosed

with rhabdomyolysis after executing a training program that included 10 sets of 30 squats and

Romanian deadlifts with one-minute rest periods, using two 40 lbs dumbbells [24]. This case was

important because it showed that rhabdomyolysis is not only a risk with excessive heat and

dehydration, but also aggressive training plans. Empirical evidence also implicates excessive

eccentric loading and high-volume training without physical readiness as risk factors for

rhabdomyolysis.

Role of Anabolic HormonesFundamentally, hormones are chemical communicators that either cause or inhibit cellular

actions. These biochemical regulators are released from specialized glands in order to change the

action of cells and tissues for growth, maintenance and repair, or function to maintain metabolic

homeostasis. When considered for training purposes, the anabolic hormones including growth

hormone (hGH), testosterone and insulin-like growth factor (IGF-1) as well as the adrenal

hormones epinephrine and cortisol warrant attention. The type of program employed deter -

mines the associated response, which is generally tied to the adaptation. This occurs because

each hormone has an affinity for a particular type of stress. For instance, when oxygen is needed

adrenal hormones trigger an increase in heart excitation to increase supply, whereas when sugar

regulation becomes relevant hGH plays a greater role.

During exercise, hormones function as messengers that provide a specific directive. Chem -

ical messages are sent to specific receptors in tissue to provide information for an action. These

messages can be clear and well-received, or incomplete; consequently impacting the effectiveness

of the response. The endocrine system has the ability to adapt to chronic stress; suggesting

training can cause both an improvement in the signaling and message, as well as the tissue’s

affinity for the message and response. This partially explains differences between novice and

experienced athletes performing the same program.

In the case of anabolic hormones, most studies show adaptations are greatest when training

requires the use of more muscle mass and a high number of motor units. The total mass recruited

for these types of exercises promotes increased concentration of circulating hormone, which has

an impact on the whole body. When experienced weightlifters performed unaccus tomed leg

training exercises in addition to their normal training in a given study, they experienced a 27%

greater increase in their biceps brachii strength compared to those who continued to only train

their upper body [25]. The increased activation of total mass and greater loading associated with

lower body, compound exercises improves the overall hormonal response and adaptations across

the body. This outcome is demonstrative of the relationship of mass and load on circulating

anabolic hormones as well as the adaptation effects of hormone on trained tissue.

Growth Hormone (hGH)

Growth hormone is highly regarded for sport metabolism and has been used as an ergogenic

aid by both athletes and non-athletes alike. Somatotropin, or hGH, is vital as it initiates growth

in body tissues, increases cell size and potentiates mitosis (cell division). The regenerative func-

tions of the hormone are also imperative to recovery from fatigue and injuries. Additionally,

growth hormone is involved in metabolic functions and energy provision; suggesting it is both

an anabolic and metabolic hormone.

DEFINITIONS

Insulin-like growth factor 1 (IGF-1) –

Currently considered to be a central“signaling” hormone that initiatesmuscle growth following resistanceexercise; it can also directly signal cellsto reproduce and grow

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Growth hormone is involved to some degree during most exercise regimens, but is especially

partial to higher-stress environments. Maximal strength training programs (PCr/glycolytic) con-

ducted at 75-95% 1RM, hypertrophy training programs (glycolytic) at 70-85% 1RM, and power

training either at 30-50% 1RM (glycolytic) or 60-95% 1RM (PCr) have all showed positive effects

on hGH activation when rest intervals are appropriately applied. In addition, it appears that

muscular endurance at 50-70% 1RM has minimal but positive effects on the production of

growth hormone when short rest intervals are used. In sports that benefit from additional mus-

cle mass, hGH is relevant but its response is very specialized. Growth hormone serves as a trigger

for important liver hormones, particularly IGF-1, when rep-load-recovery time is properly bal-

anced. The stress causes a reduction in blood pH and high lactate levels to stimulate cortisol and

epinephrine release from the adrenal glands. A humoral (hormone to hormone) pathway with

hGH relays signaling to increase anabolic activity from the liver which causes the release of IGF-

1. The ideal recipe seems to be 8-12 repetitions, using 75-80% 1RM, with 30-60 second rest

intervals within a high-volume training program. Bodybuilders will also employ these tactics

with muscle isolation to further fuel localized muscle recruitment and inflammation for increases

in size.

Testosterone

Testosterone serves several functions at the genetic level among males and is produced in

lesser quantities in females. In addition to sexual functions and development, testosterone is

intimately tied to specific utilities in the tissue and overall health in males. In females, testosterone

serves as a homeostatic hormone in balance with estrogen. Testosterone is most cited for its

impressive affects on growth and metabolic enhancements; stimulating increased mass with

subsequent reductions in fat. Individuals that supplement testosterone experience muscle growth,

improvements in body composition, and enhanced recovery rates. Natural methods to increase

testosterone levels seem to occur in response to large muscle activation with high loads, as both

the back squat and deadlift yield desirable responses in blood concentration during bouts of heavy

strength training. Testosterone release is also consistent with the aforementioned training stress

that stimulates hGH. While women generally demonstrate about 1/10th the concentration of the

average male, positive testosterone responses following resistance exercise has been identified.

A study examining 47 women, with an average age of 22 years, showed that total testosterone

increased by 25% following 6 sets of 10 maximal effort squats separated by a 2-minute rest

period [26]. Testosterone levels have also been associated with performance variations in master’s

level athletes. Expectedly, athletes with higher testosterone levels outperformed their age-

equivalent counterparts. Due to reductions in the testosterone precursor DHEA during

andro pause, male athletes over 40 require modifications in training to promote higher circulating

concentrations.

Insulin-like Growth Factor (IGF-1)

IGF-1 is currently considered to be a central “signaling” hormone that initiates muscle

growth following resistance exercise. The term “signaling” refers to IGF’s capacity to generate

action from other molecules that ultimately result in the stimulation of muscle protein synthesis.

It can also directly signal cells to reproduce and grow. Due to the apparent role of IGF-I up-

regulation in muscle remodeling, IGF-1 mRNA (or mechano-growth factor (MGF)), is considered

a key element in mass gains. MGF is almost exclusively released from trained tissue which may

explain why mass gains in training-tenured athletes are greater than in novice athletes. Therefore,

Natural methods to increase testosterone levelsseem to occur in response to large muscleactivation with high loads as both the back squatand deadlift yield impressive responses in bloodconcentration associated with heavy strengthtraining.

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it appears that muscle growth depends on the presence of MGF. IGF-1 is clearly elicited by

hypertrophy-type training and is fairly exclusive to the glycolytic system. As mentioned pre -

viously, multiple-sets of moderate-high intensity with shortened rest intervals seems to be most

effective technique. The goal is to appropriately push the glycolytic pathway to its tension-time

limit. The rest period selected is very important for hormonal activation because it is the

metabolic byproducts that ultimately trigger the cascade of events leading to IGF-1 release. This

underscores the importance of tracking rest intervals with a stop watch. High-volume training

with rest periods of 30-60 seconds promotes the desired blood chemistry to trigger the signaling

response needed to stimulate anabolic hormones. Higher loads with longer rest intervals do not

create the same concentration of IGF-1 and consequently yield reduced signaling for mass gains.

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Sport Metabolism

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