THE MUSCULOSKELETAL SYSTEM AND AGING PH 131 Physiology ­ Dr. Paul Michael Hernandez Group 3 Dela Cruz, Dean. Desquitado, Lyselle. Dimatatac, MJ. Escovidal, Karl. Espinosa, Elise. Esparagoza, Jan. Fernandez, Mia. Fernandez, Mon. Flores, Diony. Galapon, Trisha.

Objectives:

1. To discuss the physiological state of the musculoskeletal system at different life stages.

2. To discuss processes involved in the maintenance of the musculoskeletal system.

3. To identify measures in properly dealing with the aging musculoskeletal system.

DEVELOPMENT

Three types of muscles form in the embryo: skeletal, smooth, and cardiac. Skeletal muscles form

by the fusion of mononucleated myoblasts to form multinucleated myotubes. Smooth muscle develops

from mesodermal cells that migrate to and envelop the developing gastrointestinal tract and viscera.

Cardiac muscle develops from mesodermal cells that migrate to and envelop the developing heart while it

is still in the form of endocardial heart tubes (Tortora, 2009). Muscle satellite cells are also formed during

embryonic development, which persist in a quiescent state in the adult muscles, aiding in muscle growth

during exercise or in muscle repair.

Muscle development occurs in the embryo through the formation of myoblasts, which undergo

extensive proliferation to form terminally differentiated, postmitotic myocytes. Myocytes express Actin,

Myosin, and other contractile proteins and fuse to form contractile myofibrils (Schoenwolf et al., 2009).

Striated muscle development occurs in three waves: primary myogenesis, secondary myogenesis,

and postnatal growth. Prenatally, primary myogenesis occurs during the stage of the embryo and

secondary myogenesis occurs during the stage of the fetus lay down the muscular system. Postnatally,

satellite cells act in muscle growth in response to exercise or muscle damage (Schoenwolf et al., 2009).

Osteogenesis (Bone formation)

The formation of bone has three skeleton lineage namely the somites that generate the axial

skeleton, the lateral plate mesoderm that generates the limb skeleton,and the cranial neural crest that gives

rise to the branchial arch and craniofacial bones and cartilage (Gilbert 2000).

Osteogenesis involves two modes of ossification, intramembranous and endochondral

ossification.

In intramembranous ossification, neural crest­derived mesenchymal cells in the skull proliferate and

condense into compact nodules. Some of these cells develop into capillaries and others change their shape

to become osteoblasts, committed bone precursor cells. The osteoblasts secrete a collagen­proteoglycan

matrix that is able to bind calcium salts. Occasionally, osteoblasts become trapped in the calcified matrix

and become osteocytes. As calcification proceeds, bony spicules radiate out from the region where

ossification began. The entire region of calcified spicules becomes surrounded by compact mesenchymal

cells that form the periosteum. The cells on the inner surface of the periosteum also become osteoblasts

and deposit osteoid matrix parallel to that of the existing spicules. In this manner, many layers of bone are

formed (Gilbert 2000).

Endochondral ossification on the other hand involves the formation of cartilage tissue from

aggregated mesenchymal cells, and the subsequent replacement of cartilage tissue by bone. It can be

divided into five major stages. First, the mesenchymal cells are committed to become cartilage cells. After

their differentiation, the committed mesenchymal cells condense into compact nodules and differentiate

into chondrocytes. Then the chondrocytes proliferate rapidly to form the model for the bone. After this,

the chondrocytes stop dividing and increase their volume dramatically, becoming hypertrophic

chondrocytes. Lastly, the process is ended by the invasion of the cartilage model by blood vessels (Gilbert

2000).

Growth during infancy

As early as during infancy, it has to be ensured that the baby grows normally and healthy. The

first year after birth is very significant as it determines the overall wellness and health of the child for the

later years. As for the musculoskeletal system of a baby, it serves as the physical foundation of a baby’s

growth and development (Pikechiropractic, 2014).

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Originally, a baby’s body framework consists mainly of cartilage and 300 bones. However, as the

baby ages, the cartilage elements undergo ossification in order to produce a stronger framework for the

body. On the other hand, adults have less number of bones than baby’s (206 bones as compared to 300),

because separate small bones fuse together in order to form bigger and stronger bones that are more fitting

for adults (Pikechiropractic, 2014).

An example of bones that fuse together is the cranium. At birth, the cranium is comprised of

several separate pieces in order to change shape as it passes through the mother’s birth canal. The baby’s

cranium has two soft spots, known as the fontanelles. These are gaps formed between the plates of the

cranium. By the end of the baby’s first year after birth, these gaps would have already closed. This

process is called molding (Pikechiropractic, 2014).

In an infant’s life, there are five milestones that indicate major musculoskeletal developments.

First would be the lifting and supporting of the head on their own. This is often called head control.

Around one month after birth, the baby should be able to lift his head slightly. At around 4 months, the

baby should be able to keep his head upright when held in a sitting position. By around 6 months of age,

the baby should be able to control and support his head totally (Pikechiropractic, 2014)

The second milestone that occurs is the ability of the baby to roll over. The belly­to­back flip

happens at around 4 months of age, and is mastered fully by the age of 5 or 6 months. However, the

back­top­front flip happens at a later time as it requires more neck and arm strength (Pikechiropractic,

2014).

Third, most babies are able to remain in an upright position by the age of 4 to 7 months. By the

time they are 8 months old, they should be able to keep upright without any assistance. Meanwhile, the

fourth milestone that usually occurs in an infant’s life is his ability to crawl. This skill usually develops at

the age of 7 to 10 months, and this is also the time when babies try all kinds of approaches in order to

move from one place to another (Pikechiropractic, 2014).

Lastly, the fifth milestone that an infant undergoes through is walking. The baby’s first step

would most likely happen at around 9 to 12 months. By the time the baby reaches his 14th or 15th month,

he would have to be able to walk on his own. However, since some babies later develop later than the

other babies, it is still quite normal to be able to walk at the 16th or 17th month (Pikechiropractic, 2014).

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During puberty, this is when a person, yet again, undergo significant changes in his body,

especially in the aspect of bones and muscles. First, this is the time when peak bone mass is achieved.

Usually, the end of one’s puberty stage would also indicate the end of his bone growth. Aside from a

person’s diet and lifestyle, bone growth may also be remarkably affected by the increase in production of

sex hormones. It is believed that a girl’s first menstruation or a boy’s circumcision is responsible for

his/her growth spurt (Skeletal Development, n.d.). But really, the reason behind this occurrence is the

increase in sex hormone production. It is by mere coincidence that a girl’s first menstruation or a boy’s

circumcision is done during a person’s puberty stage.

MAINTENANCE AND REPAIR

Muscles get their energy based on the situation that the muscles are in. Whenever we use them to

produce a low to moderate level of force, we utilize aerobic respiration. Aerobic respiration requires

oxygen and is able to produce about 36­38 ATP molecules from a single molecule of glucose. This

process is very efficient, and can go on as long as the muscle receives adequate amounts of oxygen and

glucose to make them contract.

On the other hand, whenever we use our muscles to produce a high level of force, they contract so

much that the blood carrying the oxygen cannot enter the muscle. This results into a condition wherein the

muscle creates energy through lactic acid fermentation, a form of anaerobic respiration. Compared to

aerobic respiration, this process is much less efficient ­ mainly because only 2 ATP molecules are

produced for each molecule of glucose.

Our muscle fibers contain numerous important energy molecules. One of which is Myoglobin, a

red pigment that contains iron and stores oxygen. The oxygen from this pigment allows the muscle to

continue aerobic respiration in the absence of oxygen. Another chemical involved in muscle maintenance

is creatine phosphate. This chemical donates its phosphate group to ADP to turn it back into ATP to

provide additional energy to the muscle. Lastly, glycogen which is found in the muscle fibers and stores

energy. It is a large macromolecule made of many linked glucoses. Muscles break off these glucoses from

the glycogen molecules during activity. This process provides the muscles an internal fuel supply.

Muscle fatigue is a condition wherein the muscles run out of energy during respiration, aerobic or

anaerobic. In this condition, the muscle quickly tires out and is not able to contract. When the muscle

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experiences fatigue, it has little to no oxygen, glucose, or ATP. Instead it has many waste products, like

ADP and lactic acid. In order to replace the oxygen that was stored in the myoglobin in the muscle fiber,

the body must intake extra oxygen after exertion. This will also power the aerobic respiration that will

rebuild the energy supplies of the cell. Extra oxygen that the body must intake to restore the muscle cells

into their resting state is referred to as oxygen debt (recovery oxygen uptake). This would explain why a

person feels out of breath for a few minutes after doing a strenuous activity. The body is trying to restore

itself to its normal state.

The maintenance of bones can be summarized into two processes: bone modeling and remodeling. Bone

modeling is the formation of new bone at one particular site and the optional removal of an old bone in

another site. The formation of new bone is called, deposition and is handled by osteoblasts. The process

of removing old bone, is called resorption and is carried out by osteoclasts. This is typically done during

adolescence and early adulthood, because this is where we start to adapt to the environment. Bone

modeling drastically changes the size and shape of the bone.

In bone remodelling, a small amount of bone on the surface of trabeculae or the interior of the cortex is

replaced. Bone resorption and deposition both occur at the same site. It is normally the dominant process

in bone maintenance, happening throughout life. Does not drastically change the bone size or shape. Bone

remodeling happens so that our bodies may fine­tune the bones and thus further adapt to the environment.

This is also done for calcium homeostasis.

DETERIORATION

AGING

"The ageing process is of course a biological reality which has its own dynamic, largely beyond human

control... active contribution is no longer possible." ­ Gorman, 2000

Aging is part of the natural process that the human body undergoes. It is the natural degradation

of bodily functions that comes as one grows older. It is characterized by a generally weaker physique due

to a decrease in muscle volume, increase in fatty deposits, and brittleness of bones.Despite being a

natural process, aging proceeds at different rates for different people as there are some factors which may

hasten it. These are: Lifestyle, Stress, Excessive exposure to harsh environments, and Humoral

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Factors.

Lifestyle refers to the way a person lives his/her life. It is the most choice­oriented factor amongst

the four. A person who chooses to live a sedentary lifestyle lacks exercise, preventing his/her muscles

from improving and being worked up thus, giving it less strength and making it much more prone to the

effects of aging. An unhealthy diet prevents the body from receiving the proper nutrients, and may

potentially increase the amount of fat in the body. Bad habits and vices such as drugs, alcohol and

smoking may cause complications within the body that may ultimately lead to diseases, which in turn,

leads to an increase rate of body degradation. (McLaughin, 2013)

Stress is the reaction of the body to a stimulus that disturbs its state of equilibrium. More often

than not, stress triggers reactions that promote the “wear and tear” of organs in order to make the body

work faster and more efficiently for the type of situation it is in. The more stressed someone is, the more

worn and torn the body might end up. Too much stress will affect the organs in such a way that they will

not function properly. Additionally, in order to cope with the loss of energy that stress brings, one is

stimulated to eat more, increasing the fatty deposits in the body. Stress may also be responsible for

making someone develop a bad posture, and may weaken someone’s immunity. (McLaughin, 2013)

Excessive exposure to harsh environments refers to too much exposure to some elements

around us such as sunlight, heat, cold, and radiation which may in turn have varying degrees of adverse

effects to the body. Too much sunlight will make one suffer the effects of being too exposed to UV rays.

Too much heat may cause burns, dehydration and wrinkling of the skin. Too much cold may cause

dehydration, and in extreme cases, frostbite­ a condition which may potentially make someone lose a

body part. Exposure to too much radiation may lead to radiation sickness, which may, in turn, lead to

cancer. (McLaugin, 2013)

Humoral Factors refer to factors transported by the blood such as hormones. Hormones are

chemicals produced by the brain which help regulate the amount of different minerals of the body. Some

hormones which help maintain the musculoskeletal system are (a) Parathyroid Hormone which regulates

calcium content in blood by extracting calcium from the bone when there is a lack of it in blood, it

increases in age and may eventually lead to weaker bones and osteoporosis; (b) Estrogen counteracts the

effects of the parathyroid hormone and minimizes calcium loss of the bone, it decreases with age, thus,

increasing the possibility of osteoporosis; (c) Testosterone is involved in the development of muscle bulk

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and strength, and increases growth hormone production, it decreases with age, thus, lessening muscle bulk

and strength, and (d) Growth hormone which promotes increased muscle mass and skeletal growth, it

naturally decreases with age thus, making it harder for someone to have increased muscle mass. (Hurd,

2014)

Deterioration of the skeletal system

Maximum bone density is achieved, on average, at the age of 25­30 years old, after which, a

gradual decrease in bone density can be observed. This decrease is more evident in females than males,

losing about 53% as compared to 18% in males. This is further accelerated by the fact that women tend to

have smaller bones and have lower peak bone mass.

In addition, ageing leads to a decrease in bone density as the rate of deposition is now lower than

the rate of resorption. This is, however, affected by several factors:

Calcium and Vitamin D Deficiency. Calcium is the one of the main minerals used by the body

for normal function. For one, it is used to strengthen bones. However, the small intestine can only absorb

a total of 30% of the daily required intake (A.D.A.M., 2014). This can lead to a deficiency of the said

mineral. Vitamin D is important in the absorption of calcium for body consumption. Usual sources of this

vitamin include exposure to sunlight, leafy and green vegetables, and milk. Lack of vitamin D can affect

the amount of calcium in the body, thereby promoting calcium resorption leading to lower bone density.

Moreover, lactose intolerant individuals are more likely to have calcium deficiency as they are unable to

efficiently digest dairy which are good sources of said of both Calcium and Vitamin D (Office of the

Surgeon General, 2004)

Decrease in Reproductive hormones. This is more evident in women as it is in men. It can be .

observed that a sudden drop in bone density in women after they have gone through menopause. This

means that a decrease in the production of sex hormones, in this case estrogen, can lead to a decrease in

bone density. Menopause can also elevate levels of Follicle secreting hormones (FSH) which promotes

osteoclastic activity and bone perforation (Demontiero, et. al., n.d.)

Estrogen stimulates an increase in the production of transforming growth factor B and

Osteoprotegrin which are essential in the decrease of osteoclastic activity. Moreover, this hormone can

stimulate both bone formation and procollagen synthesis, which is essential in the maintenance of bone

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density. Consequently, a lack of estrogen can increase osteoclastic activity whilst decreasing bone

formation. Males also exhibit this pattern as estrogen is present in their body as a derivative of their main

sex hormone. Moreover, testosterone is vital in development of the bones because it increases the levels

of the growth hormone and releases neurotransmitters that stimulate tissue growth. (Demontiero, et al,

n.d.).

Sedentary Lifestyle. Physical activity have direct effects on bone density as it is a determinant of

the density the bone needs in order to sustain pressure applied to it. Therefore, a decrease in physical

activity promotes osteoclastic activity within that site, thereby decreasing bone density. Conversely, an

increase in physical activity can lead to an increase in bone density as the area is subject to more

mechanical force. (Office of the Surgeon General, 2004)

Endocrine and Gastrointestinal Pathology. Several diseases of the endocrine and digestive

system can affect bone resorption, and to a greater extent, bone density. Hyperprolactinemia,

Hyperparathyrodism and Hyperthyroidism, conditions that are caused by the increase in the production of

prolactin, parathormone and thyroid hormones respectively, all cause a decrease in bone density due to an

increase in osteoclastic activity (Dhanwal, D, n.d.). A condition wherein the body tends to produce

copious amounts of calcium, referred to as hypercalcemia, can lead to an excess in the excretion of

calcium, which in turn, thins out the bone and poses more risk for fracture. Also, an overproduction of the

gastrointestinal­derived serotonin, can lead to the inhibition of bone formation (Clarke & Khosla, n.d.).

In addition, chronic use of corticosteriods can lead to decrease in bone formation and bone

density as it promotes the cell death of osteoblasts (Romas, n.d.)

Deterioration of the muscular system

Sarcopenia

Sarcopenia is a common consequence of normal aging that describes a gradual loss of muscle

mass and strength that may begin at the age of 30. This continues on together with ageing and by the age

of 80, muscle strength would have decreased by 50% (Marieb, 2013). Sarcopenia may be attributed to the

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reduction of fiber number, fiber size or a combination of the two. The cause of this is multifactorial with

reduced physical activity as a key factor (Marcell, 2003).

For healthy young adults, under equilibrium conditions, the degradation and synthesis of skeletal

muscle is a balanced and dynamic process.Small imbalances between degradation and synthesis

accumulated over several years result to the significant loss in muscle mass (Marcell, 2003).

Muscle cell changes

As muscles age, smoothening of sarcolemma in the muscle end plate decreases surface area and

diminishes stimulation of muscle cells. Thus, cells contract slower and become decreasingly able to

recover from a contraction and to ready itself for the next one. The slowing of calcium release and

retrieval of the sarcoplasmic reticulum with increasing age also affects this. A decreased number of

sarcomeres shortens the muscle, causing a reduced distance for moving which would then affect

flexibility(DiGiovanna, 1994).

Denervation and MU remodeling

There is a loss of motor axons due to the normal aging process, with the loss of alpha­motor

neurons being greatest among type II muscle fibers. These muscles either then become reinnervated by

sprouts of other axons within the vicinity, which are commonly from type I fibers, or denervates and

ultimately disappears (Marcell, 2003).

This may explain why Type II (fast­twitch) fibers decline with increasing age whereas Type I

(slow­twitch) fibers are seemingly resistant to age­related atrophy until the ages of 70­80. The size of

remaining motor units (MU) increase with reinnervation causing a decrease in controlling the strength of

each contraction, thus affecting fine movements (DiGiovanna, 1994).

Protein Synthesis

Declines in protein synthesis are involved in reduced protein mass but not all muscle proteins

display changes in synthesis rates. Separation of muscle proteins showed that Myosin Heavy Chain

(MHC) synthesis rates are less for the middle aged and old aged. MHC being a key contractile protein, its

reduction would lead to a decline in locomotor function and muscle strength.. As we age we do not lose

the ability to metabolize proteins, we lose ability to synthesize our own proteins and this is influenced by

hormonal changes (Williams, Higgins, & Lewek, 2002).

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Hormones

Reduced levels of circulating anabolic hormones such as somatotropin and testosterone, that

decline from middle age onwards, also affect muscle ageing. There is also an increased insulin resistance

with age that may contribute to muscle deterioration through the inhibition of the nitric oxide cascade that

is responsible for the absorption of amino acids in protein synthesis. In like manner, cortisol increases

with age and is a potent stimulus to protein catabolism (Knight, 2008).

Replacement of active muscle

Active muscle fibers are progressively replaced by collagen­rich, non­contractile fibrous tissue.

There is also an Increased deposition of fats with decreased lean muscle tissue, decreasing the force

production capability (Knight, 2008).

As mentioned earlier, muscle ageing is a multifactorial process and so it is affected by numerous

other processes in the body with not one of them claiming to be the sole reason for the changes. Capillary

structure and decrease in capillary density in muscles also affect muscle ageing. There is also an

increasing accumulation of lipofuscin (Knight, 2008).

The proper functioning of the muscular system is also dependent on other systems that change

with age such as the nervous, circulatory and respiratory systems (DiGiovanna, 1994).

Tendon Stiffening

Tendons are cord like tissues in the body that attaches muscles to bones. As one ages, tendons

may become stiff, causing someone to have more strain when it moving. The stiffening of tendons is

primarily caused by loss of water due to the decrease of proteoglycan( a mineral which is a component of

the ground substance of the tissue and helps in retainment of water content). Another cause would be the

loss of elastin that comes with age. Elastin is a mineral which makes the tissue more flexible and elastic, a

decrease of it will cause the tendons to be less flexible and elastic. (Leadbetter, et al. 2005)

Addressing the Aging Musculoskeletal System

Regular Physical Activity

Decline in physical activity increases the age­associated changes in the musculoskeletal system.

These changes can be delayed or prevented by living a physically active lifestyle. Elderly people who are

sedentary can greatly benefit from exercise (Seeley, Regan, & Russo, 2014). Aerobic activities and

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strength training programs improve musculoskeletal performance (Tortora & Derrickson, 2009). During

these activities, muscles push and tug against bones, making them both stronger (National Institute of

Child Health and Human Development [NICHD], 2014). The muscle fibers damaged during workout are

replaced by the body and the repaired myofibrils grow in thickness and number (muscle hypertrophy).

Moderate activity and weight­bearing exercises (e.g. walking, jogging, hiking, dancing, etc.) help build

bone mass and enhance neuromuscular function. Resistance exercises (strength or weight training) can

help build bone strength and muscle mass (Marieb & Hoehn, 2013). However, risk of injury associated

with the increase of strenuous physical activity must be assessed.

Maintaining a high level of physical activity throughout life has several positive effects to the

muscular system. It slows the decline in energy molecules (ATP, creatine phosphate, glycogen), oxidative

enzymes, blood supply, speed of movement, stamina, endurance, Vo2max (maximal uptake of oxygen),

and increase in overall muscle size and strength. Effects of exercise on the skeletal system include slower

decline in bone minerals and decreased risk of fractures and osteoporosis. For those people who are just

starting to engage in regular physical activity, the benefits are almost similar, so it is never too late to start

working out (DiGiovanna, 1994).

Healthy Diet

Eating a well­balanced diet can do wonders to the growing bones and muscle. Make sure to get

enough calcium as the bones are increasing in density. It greatly reduces the risk of fractures (Marieb &

Hoehn, 2013). Phosphorus, including smaller amounts of magnesium, fluoride, and manganese are also

needed in bone remodeling (Tortora & Derrickson, 2009). Some sources of calcium are milk (and other

dairy products), leafy greens, fruits, legumes, and seafood.

Vitamin D, which is naturally made by the body when the skin is exposed to the sun, is needed by

the body to absorb calcium from food into the blood. Other essential vitamins, such as vitamin A

(stimulates activity of osteoblasts), vitamin C (necessary for collagen synthesis), vitamin K and B12

(needed for synthesis of bone proteins) are also important (Tortora & Derrickson, 2009). Fish oils, fatty

fish, mushrooms, beef liver, cheese, egg yolk, fruits, and vegetables are possible sources of these

vitamins.

Protein requirement for adults is higher than for younger people. However, too much animal

proteins can cause loss of bone and muscle mass (Brink, 2007). Make sure to get protein from a variety of

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sources such as seafood, white and lean meat, dairy, eggs, beans, soy, peas, quinoa and nuts.

Stick to minimally processed foods (fruits, vegetables, whole grains, seeds, etc.) and prevent

processed foods. High intake of carbonated beverages, sugar, and alcohol promote poor bone

mineralization. They extract minerals from bone, decreasing its density (Marieb & Hoehn, 2013). Fruits

and vegetables, on the other hand, contains the vitamins and minerals need for healthy bone and muscle

growth.

Other Tips to Slow Down Musculoskeletal Aging

Hormones affect bones and muscle as people age. Annual blood testing to track hormone levels is

recommended, especially to the elderly (Brink, 2007). Some medications and hormone replacement

therapies can be helpful to those who can afford it.

Those who strive to maintain healthy bones and muscle must consider some major lifestyle

changes. Make great efforts to stop smoking. According to the American Academy of Orthopaedic

Surgeons (2010), smoking decreases blood supply to the bones, as well as calcium absorption. Nicotine

also reduces the production of osteoblasts. It speeds up estrogen breakdown, making the bones more

fragile. Moreover, it causes decrease in oxygen uptake, so less is available for muscle use. Also, avoid

stress as much as possible. Any damage to the other body systems due to stress can, in one way or

another, affect the musculoskeletal system.

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