Muscle Cells and Contraction

mitochondria

contractile filaments

sarcoplasmic reticulum

Muscle Types

• Type I - “Red”, “Slow”–Contracts relatively slowly

–Many mitochondria

–Good blood supply

• Type IIb - “White”, Fast”–Contracts relatively rapidly

– Few mitochondria

–Poor blood supply

–Packed full of contractile filaments

Gentle exercise

• Type I muscles recruited– good blood supply, heaps of mitochondria

• Most readily available fuel is glucose–Glucose transporters move to cell surface

glucose

BLOOD MUSCLE

glucose glucose

Transporters

Consequences of Glucose Usage

• Muscle glucose uptake leads to a decrease in blood glucose concentration

• Need to keep blood glucose constant!–Small change in blood glucose causes...

insulin glucagon

Effects of Low Insulin and High Glucagon

–Stimulation of glycogen breakdown in liver

–Stimulation of fat breakdown in white adipose tissue

FAT

fatty acids

GLYCOGEN

glucose

fatty acidsglucose

WATLIVER

Glucose Recycling

• Glucose stores (glycogen) are limited–Cannot convert fatty acids into glucose

• GLUCOSE CONSERVATION AND RECYCLING– Fatty acids substitute for glucose as a fuel

– Fatty acids prevent glucose from being wastefully oxidised

Fatty Acid and Glucose Oxidation

glucose

acetyl CoA

CO2

pyruvate

fatty acids

GLYCOLYSIS

FATTY ACID OXIDATION

Fatty Acid Oxidation inhibits Glucose Oxidation

glucose

acetyl CoA

CO2

pyruvate

fatty acids

GLYCOLYSIS

FATTY ACID OXIDATION

lactate

to liver

inhibits

WAT

LIVER

Summary of Gentle Exercise

– Initially, glucose is used

– Then fatty acids take over and glucose is recycled

FAT

fatty acids fatty acids CO2

GLYCOGEN

glucose

glucoselactate

lactate

CO2

MUSCLE

Moderate Exercise

• As the pace increases, the rate of fatty acid utilisation increases, but....– The enzymes that catalyse fatty acid oxidation

soon reach their maximum capacity

–During running, fatty acid oxidation alone is not sufficient to maintain ATP production

– Inhibition on glucose oxidation is removed

• Glucose oxidation occurs!• Less glucose recycling

• Liver glycogen stores depleted faster

Summary of Moderate Exercise

WAT

LIVER

FAT

fatty acids fatty acids CO2

GLYCOGEN

glucose

glucoselactate

lactate MUSCLE

Strenuous Exercise–As the intensity of the exercise increases

further, muscle glycogen is broken down.

WAT

LIVER

FAT

fatty acids fatty acids CO2

GLYCOGEN

glucose

glucose

lactate MUSCLE

GLYCOGEN

Glycogen Depletion during a Marathon Race

0

25

50

75

100

0.0 0.5 1.0 1.5 2.0 2.5

Time (hours)

Gly

cog

en

(m

g/g

)

slower

faster

Why Glycogen is Important

• When glycogen has run out, only fatty acid oxidation can be used for ATP generation

• Power output is lower when using only fatty acids

• “Hitting the Wall”

• Cannot sprint if there’s no glycogen

Strategies

• Start the event with more glycogen than your competitors

• Spare the glycogen by making more use of fatty acids

• Use fatty acids sooner so less glycogen is used in the early stages

Glycogen Supercompensation

–After extensive depletion, glycogen resynthesis overshoots.

0

25

50

75

100

0 1 2 3 4 5

Time (days)

pre-exercise level

Glycogen Loading - Classical

0

25

50

75

100

0 1 2 3 4 5

Time (days)

HiFAT HiCHO

– Interrupts training!

–Potentially dangerous

–Uncomfortable

–Character building!

Glycogen Loading - Tapered

0

25

50

75

100

0 1 2 3 4 5

Time (days)

• Fits well into normal training

• No dangerous full glycogen depletion.

always High Carbohydrate

Glycogen Sparing

• Increase the use of fatty acids–Carnitine helps fatty acids enter

mitochondria

– Training increases the activity of fatty acid oxidation enzymes

• Start fatty acid release from White Adipose Tissue early–Strong cup of coffee!

–NOT glucose drinks BEFORE an event

insulin glucagon = inhibition of fatty acid release

Fitness

• Better cardiovascular system

• Increased vascularisation of muscles– better oxygen supply

– better fuel supply (especially fatty acids)

• More mitochondria–Higher capacity to burn fatty acids

• Conversion of Type IIb to Type I?

Sprinting

• Uses Type IIb muscles–Poor blood supply

–Packed full of contractile filaments

– Few mitochondria

–VERY rapid consumption of ATP

• Fuel selection problem– Fatty acids?

» oxygen supply, mitochondria

–Blood Glucose?

» transporter recruitment, blood supply

Anaerobic Glycogen Utilisation

• Inefficient and incomplete

• High turnover

• Accumulation of lactate

• Takes time to stimulate glycogen breakdown

GLYCOGEN

pyruvate

lactateacetyl CoA

GLYCOLYSISADP

ATP

to blood stream

Buying time with Creatine Phosphate

• Less than 5 seconds supply of creatine phosphate–Enough to get glycogen mobilisation going

• Adrenaline stimulates massive glycogen breakdown.

creatine phosphate + ADP ATP + creatine

Fatigue

• Role of lactate??

• Acidity interferes with many processes– Enzymes involved in glycolysis

– Contractile process

– Calcium movements

• All very controversial!– But one things for sure: very low pH and running

out of glycogen are definitely bad news

– OK, so that was TWO things…

Fatigue 2

• Use pH buffers?–Bicarbonate widely used

–Certainly works but check with doctor first!

• Does glycogen ever run out in a sprint?–Not in short events (100 m)

– Important in longer events and multi-heat competitions

Does Glycogen run out during Sprinting?

• Have to ensure adequate glycogen resynthesis between races

• Rapidly absorbed carbohydrate

0

25

50

75

100

0 1 2 3 4 5

Time (hours)

depleted

Sprint Training

• Increased power output– Increased cell size

–More contractile filaments

–STEROIDS!

• Co-ordination

• Conversion of Type I to Type IIb?

• Adequate glycogen stores

Lessons

• Muscle Contraction–Muscle Types

• Getting energy from fat and carbohydrate–No work, no energy consumption!

– Increase work, increased oxidation of fuel

• Fuel mix during....–Walking, Jogging, Running and Sprinting

• Training effects

How the Fuel Mix ChangesLIGHT INTENSE

MODERATE

Fatty acids CO2

Fatty acids CO2

Fatty acids CO2

Glucose CO2

Fatty acids CO2

Glucose CO2

Fatty acids CO2

Glucose CO2

Glycogen CO2

SPRINTING

Glycogen lactate

Glycogen lactate

Creatine P creatine

Take Home Message!

• Glycogen is an important fuel to all athletes–Using fatty acids quickly and copiously

reduces the use of valuable glycogen

• Carbohydrate intake has to be high to allow adequate glycogen synthesis