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Chapter 8
There are three types of muscle in the human body. The most
abundant type is called skeletal muscle because virtually all
these muscles are attached to the bones of the skeletal system. Skeletal muscle
makes up the more than 600 muscles in the body, most of which are close to the
surface of the body, between the integumentary system and the bones (Fig.
8.1). Many muscles bulge when they contract; therefore, they are visible
and can be felt as firm lumps under the skin. This chapter is concerned mainly
with skeletal muscle.
Cardiac muscle
is found exclusively in the heart (Chap. 4). Visceral muscle, or
smooth muscle, is found within organs in many body systems.
Main Functions for Homeostasis
The muscle system performs three functions that help
maintain homeostasis: movement, support, and heat production.
The movement produced by muscles allows a person to carry
out the last step in negative feedback systems: making an adjustment to a
change in conditions. Movement is used to get away from impending danger (e.g.,
fire, falling objects), escape from unfavorable conditions (e.g., intense
sunlight), and eliminate wastes and unwanted materials (e.g., carbon dioxide,
splinters).
Movement is also important in taking positive actions. It
allows a person to move toward, obtain, and use items and conditions that
promote the welfare of the body and quality of life. These needs include basic
physical needs (e.g., food, water, shelter) and other needs (e.g., social
interactions, recreational activities). Movement allows people to rearrange
their environment and construct and repair useful and decorative artifacts to
suit human requirements and desires.
The muscle system provides support when muscle contractions
prevent the movement of a part of the body. Support maintains proper positional
conditions of parts of the body so that they function well. For example, muscle
contractions can maintain an upright posture. This activity includes holding
the bones in place and preventing the protrusion of the organs in the lower
trunk. With proper posture, circulation is improved because blood vessels are
open rather than pinched shut, and respiration is assisted because the lungs
have room to inflate easily. Holding the head up positions the eyes for viewing
the surrounding environment.
Heat production is essential for maintaining a proper and
fairly stable body temperature because most people live in environments that
are cooler than normal body temperature. Therefore, the body is always losing
heat to the environment, just as any warm object or substance, such as warm
food, loses heat and becomes cool. However, if the body is allowed to become
cool, this will make the rate of its chemical reactions too slow to sustain
life functions (e.g., heartbeat, respiration, brain activities) and perform
effective negative feedback responses. Therefore, to prevent cooling of the
body, the amount of heat loss must be balanced by an equal amount of heat
production.
Heat is produced by many chemical reactions in the body, but
the muscle system is the main heat producer. One reason for this is that the
muscle system is one of the largest systems, usually accounting for one-third
or more of body mass. Second, the muscle system is one of the most active
systems in the body. When a person is awake but resting, this activity involves
steady muscle contractions (muscle tone) that help maintain posture. This system
is especially active and produces much more heat when a person is forcefully
contracting muscles during vigorous exercise. Still, the muscle system performs
many chemical reactions even when the muscles are relaxed; this is why a
sleeping person remains warm.
Age Changes versus Other Changes
The functioning of the muscle system depends on the nervous,
circulatory, and respiratory systems. As a result, many age-related changes in
this system derive from age-related changes in the other systems, which vary
greatly among individuals. Furthermore, alterations in exercise received by
muscles quickly and dramatically affect the muscle system. Though there is an
average age-related decline in exercise, changes in the total amount of
exercise of the body and of each muscle vary greatly both from time to time and
from one person to another. Two consequences of these variables are that they
add to the age-related increase in heterogeneity among people and make it quite
difficult to identify true age changes in the muscle system. Therefore, in this
chapter the causes of each age-related change will be noted.
The muscles are composed primarily of muscle cells, which
perform the three functions of the muscle system. Other materials in each
muscle include nerve cells, collagen and elastin fibers, fat, and blood vessels
(Fig.
8.2).
The main activity of muscle cells is contraction,
which produces both the force needed for movement and support and most of the
heat derived from the muscle system. Muscle cells have many specializations
that permit them to perform contraction.
Muscle cells are very long and thin, reaching lengths of up
to several centimeters. Usually, these cells are as long as the muscle in which
they are contained. Because of their shape, muscle cells are also called muscle
fibers (Fig.
8.2).
Cell Membrane
(Sarcolemma) The muscle cell membrane (sarcolemma)
is modified in three ways (Fig.
8.3a, Fig.
8.3b). First, the spot on the membrane that receives stimulatory messages
from a somatic motor neuron is highly convoluted. This modified area (motor
end plate) apparently provides more surface area and receptor molecules
to receive and respond to molecules of acetylcholine from the somatic motor
neuron. Second, the cell membrane can carry messages in the form of action
potentials, just as axons do. Third, the membrane has many penetrating
indentations (T tubules), which deliver action potentials deep
within the cell.
Myoglobin, Oxygen, and Energy The muscle cell
cytoplasm (sarcoplasm) contains a protein called myoglobin,
which is found only in muscle cells and causes muscles to appear red in color.
Myoglobin attracts oxygen from the blood into the muscle cells and stores
oxygen. As soon as a muscle cell uses some of the oxygen, its myoglobin quickly
attracts more (Fig.
8.4).
The muscle cell uses the oxygen to obtain energy from sugar
and other nutrient molecules. As long as the cell has enough oxygen, it can
obtain much energy from nutrients while producing only carbon dioxide (CO2)
and water as waste products. The CO2 and water are easily removed
from the cell and can be eliminated from the body by the respiratory and
urinary systems, respectively.
When a person engages in vigorous activity, the amount of
oxygen required to produce the energy needed by a muscle cell often rises above
the supply of oxygen to the cell. The cell can continue to work because some
energy can be obtained by breaking nutrients down partially. One of the main waste
products from this process is lactic acid (Fig.
8.4), which tends to accumulate in muscle cells and causes them to become
acidic. A result of lactic acid accumulation is weakening of the muscle cell's
contractions. The affected person experiences fatigue in the forms of muscle
weakness and muscle pain. The person also feels out of breath.
If activity decreases, the circulatory and respiratory
systems can again deliver oxygen to the muscle cell faster than oxygen is
consumed. The extra oxygen is used to complete the breakdown of lactic acid
into CO2 and water; this not only eliminates the lactic acid but
also makes a large amount of energy available to the muscle cell. The affected
person's sensation of fatigue subsides and he or she may claim, "I have
caught my breath." The oxygen used to eliminate the lactic acid produced
by vigorous exercise is called the oxygen debt.
Much of what has just been said is also true of cardiac
muscle cells. For example, when cardiac or skeletal muscle cells accumulate
lactic acid, they become weak. However, unlike cardiac muscle cells, skeletal
muscle cells are rarely seriously injured or killed by lactic acid. These cells
can continue to work with lactic acid present as long as the acid concentration
does not become too severe. Still, exceedingly high levels of lactic acid will
prevent skeletal muscle cells from contracting.
Contraction The membranes of the endoplasmic reticulum
within muscle cells are arranged in the form of lacy tubes that extend over the
length of the cell (Fig. 8.3). These membranes are called sarcoplasmic
reticulum and regulate the movement of calcium ions needed for
contraction.
The region in the cell surrounded by each tube of
sarcoplasmic reticulum contains an array of tiny fibers called myofilaments.
Clusters of thick myofilaments alternate with clusters of thin myofilaments (Fig.
8.5). The alternating clusters overlap to form units called sarcomeres,
which extend from one end of the cell to the other like links in a chain. Each
chain of sarcomeres is surrounded by sarcoplasmic reticulum and is called a myofibril.
When the cell is stimulated and action potentials pass over
the sarcolemma, the sarcoplasmic reticulum releases calcium, which causes the
thick myofilaments to pull on the thin myofilaments and slide farther among
them. The pulling and sliding cause the muscle cell to become shorter;
contraction has occurred. The contraction applies a pulling force to the bone
or other structure to which the muscle is attached, and the structure is either
moved or held in place.
Recall that energy for contraction comes from the breakdown
of nutrient molecules. Only some of the energy released from these molecules is
converted into movement of the myofilaments; the remainder is converted into
heat. This is why muscles produce so much heat when they contract.
Types of muscle cells The proportions of
muscle cell components are different among muscle cells, so the cells have
different characteristics. Type I fibers contract more slowly and
can work longer before becoming fatigued. Type IIA fibers
contract more quickly and resist becoming fatigued, also. Type IIB fibers
contract quickly, but they become fatigued quickly. Type II fibers are intermediate between Type IIA
and Type IIB. Type IIA and Type IIB fibers are most important for fast and
powerful movements. Different muscles have different combinations of these
types of muscle cells, and the combinations change gradually during adulthood.
Internal Components As muscle cells age, the convolutions in the
motor end plate decrease and the sarcolemma becomes smoother. The resulting
decrease in surface area diminishes the ability of the muscle cell to be
stimulated by the motor neuron. Other changes in the sarcolemma cause the
action potentials that lead to contraction to become weaker, slower, and more
irregular. Because of the changes in the action potentials, the cell takes
longer to begin to contract and is less able to recover from one contraction
and prepare for the next. Age-related slowing of calcium release and retrieval
by the sarcoplasmic reticulum contribute to these effects.
The large-scale results of these cellular age changes
include a longer time to respond when a person wants to move suddenly and a
diminished ability to perform rapidly repeated movements such as playing fast
music on a piano. Muscle composed of aging cells also has a weaker maximum
strength when used for activities requiring rapid and very strong contractions,
such as grasping a handrail to stop a fall.
Another change in aging muscle cells is a decrease in the
substances used to supply energy for contraction (ATP, creatine phosphate,
glycogen). Much of this change seems to be caused by a decrease in exercise
rather than by aging. Lack of exercise also seems to cause most of the decrease
in the enzymes that extract energy from nutrients. There is even a decrease in
the number and size of mitochondria, which perform most of the energy
extraction. Many remaining mitochondria have been damaged, so they are less
efficient and produce more free radicals (*FRs). Some muscle cells seem to
accumulate damaged mitochondria and become sources of *FR damage to surrounding
cells. All these changes leave the cells with less energy, especially for tasks
requiring a prolonged effort.
The final substantive change inside muscle cells is a
decrease in the number of sarcomeres within the myofibrils. This tends to cause
the cells and the muscles they compose to become shorter and have a reduced
distance through which they can move. The affected person experiences stiffness
and diminished freedom of movement. The loss of sarcomeres also reduces the
strength of the cells and muscles.
Cell Thickness Since muscle cells that get little exercise
lose parts of their internal components, they decrease in thickness. This
shrinkage is prevalent among the elderly because of the general reduction in
physical activity as people age. Regularly exercised muscles show little change
in cell thickness until age 70 or beyond. Even then, there is only a slight
thinning of cells in muscles that receive plenty of exercise. Therefore,
reduction in exercise rather than aging is the main cause of muscle cell
thinning and much of the consequent decrease in muscle thickness and strength
that usually accompanies advancing age.
Cell Number Most of the decrease in the thickness of
muscles with aging is caused by the death of muscle cells. Up to half the
muscle cells in a muscle may be lost by late old age. This loss occurs in
exercised muscles and in muscles receiving little use. Lost muscle cells are
not replaced by new ones because except in very unusual circumstances, adult
muscle cells cannot form new muscle cells.
Type II fibers become thinner and are lost faster than Type
I fibers. The ratios of loss are different for different muscles. Some muscles
may lose Type II fibers more than twice as fast as they lose Type I fibers.
Type IIB fibers are lost faster than Type IIA fibers. Some of the loss of Type
II fibers may be from conversion to Type I fibers. Most age-related decreases
in strength and speed result from thinning and loss of Type II fibers.
In muscles receiving much regular strenuous exercise, the
space left by the lost cells may be largely filled by the remaining cells. This
occurs because muscle cells pulling against heavy loads on a regular basis
adapt by synthesizing more internal components. The additional components
increase the thickness and strength of these cells, which encroach on the
vacant areas. As a result, the decline in thickness and strength of exercised
muscles is slow.
Aging muscles that receive little strenuous exercise have
the spaces left by lost cells filled with fibrous tissue and fat. Such muscles
become thinner and considerably weaker as time passes.
Cell Repair Though muscle cells are unable to reproduce,
they can repair themselves after an injury. One common cause of injury is
contracting against a load much heavier than that normally encountered by
muscle cells. This type of injury can be sustained when a person who normally
lifts objects weighing less than 30 pounds tries to support a 60-pound object.
A muscle containing muscle cells injured by an excessive
force, such as lifting a heavy object, is weakened and causes the sensations of
muscle soreness and stiffness. If the muscle is rested, the injured muscle
cells will repair themselves within a few days and the soreness and stiffness
will subside. As was mentioned above, the cells will adapt to the heavy demands
previously placed on them by becoming thicker and stronger. They will then be
more resistant to injury caused by excessive loads. For muscle cells receiving
regular strenuous exercise, the ability to repair injury and recover from such
weakness and soreness is not altered by aging.
It is uncertain whether muscle cells that receive little
exercise can repair themselves as quickly as exercised muscle cells do. Still,
muscle cells in exercised or unused muscles retain the ability to adapt to
heavier loads by manufacturing internal components. Thus, the thickness and strength
of muscles can be increased by strenuous exercise regardless of age. However,
muscle cells in older individuals make the compensatory increase in thickness
more slowly.
Recall from Chap. 6 that skeletal muscle cells are
stimulated to contract by nerve cells called somatic motor neurons.
The axon from each motor neuron branches as it passes through its muscle. Some
motor neurons have only a few branches, while others have several hundred.
Each branch from a motor neuron axon ends on a muscle cell,
and each muscle cell receives a branch from only one motor neuron (Fig.
8.6a). Thus, the muscle cells in a muscle are organized into groups, with
all the cells in each group being controlled by one motor neuron. The
combination of one motor neuron and all the muscle cells it controls is the
functional unit of the muscle and is called a motor unit.
When an impulse travels down a
motor neuron, it passes along every branch of its axon. Therefore, every muscle
cell in the motor unit is stimulated to contract; it is not possible to cause
only some of these cells to contract.
The strength of each contraction is determined by which
motor units and how many motor units are activated at a given time. (Fig.
8.6b). Since more varied combinations of numbers of muscle cells can be
selected in muscles with small motor units, a person has more control over the
amount of strength provided by each contraction in such muscles (e.g., finger
muscles). It is more difficult to select precise levels of strength from
muscles with large motor units because the muscle cells contract in larger
groups (e.g., thigh muscles). Thus, a person has more control in controlling
and modulating the strength of finger muscle contractions than large thigh muscle
contractions. The difference in the degree of control is similar to the
difference between the ability to pay an exact amount when one has many
one-dollar bills and small change and the ability to pay any exact amount when
one has only bills of large denominations.
Since motor units change in many ways as people get older,
the functioning of a muscle also changes. Some of these changes and their
consequences were described in Chap. 6.
One change is an exponential decrease in the number of motor
neurons. The loss may reach 50 percent by age 60. This is the main reason for
the decrease in the number of muscle cells because a muscle cell degenerates
and dies if it does not receive stimulation from a motor neuron. As more motor
neurons and their muscle cells are lost, the maximum strength of contraction
the muscle can produce diminishes.
Fortunately, many surviving motor neurons produce additional
axon branches that connect to the orphaned muscle cells. These adopted muscle
cells survive and function. This compensatory process helps slow the decline in
the strength of the muscle. Note, however, that the size of the remaining motor
units increases. This means that there is a decrease in control of the strength
of each contraction. This may be one reason people have a reduced ability for
fine movements as age increases. Also, Type II fibers are often
"adopted" by motor neurons from Type I fibers. This alteration speeds
up the conversion of Type II fibers to Type I fibers.
A second age change is a slowing in the passage of impulses
to muscle cells. There is a variable amount of slowing among the motor neurons
controlling a muscle. As mentioned in Chap. 6, three alterations in the overall
contraction of the muscle result. First, it takes longer for the muscle to
reach its peak strength of contraction. Second, the peak amount of strength is
lower. Third, the entire contraction takes more time. These alterations further
reduce the maximum amount of strength a muscle can produce and make it more
difficult to perform very quick movements.
Another change in motor neurons is a decrease in the
frequency with which impulses are sent to the muscle. Normally, a motor neuron
sends a volley of impulses in rapid succession so that the muscle cells
contract rapidly. A rapid series of contractions—incomplete tetany—provides
a fairly smooth and strong contraction that can be maintained for a long time.
Since age changes in muscle cell action potentials decrease the frequency at
which muscle cells can contract, reducing the frequency of neuron impulses may
be compensatory. Sending impulses faster than the muscle cells can respond
would be wasteful of neuron energy and neurotransmitter materials.
Other Nerve-Muscle
Interactions
Several other changes in the nervous system alter the
operation of muscles as people age. Recall from Chap. 6 that age changes in
sensory neurons, synapses used by reflex pathways, and other areas of the
central nervous system involved in controlling voluntary movements all affect
adversely the ability of the muscle system to maintain homeostasis and the
quality of life.
Muscle cells depend on the circulatory system to supply
oxygen, nutrients, and other needed materials and to remove wastes. Service of
muscle cells diminishes somewhat even when no disease of the circulatory system
is present. Exactly how much of this decrease is due to age changes and how
much is due to a reduction in exercise is not known.
Reasons for the reduced ability of the circulatory system to
meet the needs of muscle cells include the decrease in the density of
capillaries among muscle cells and age changes in capillary structure. These
changes may contribute to the decline in the maximum rate of working and the
faster onset of muscle fatigue as people advance in age. These and other age
changes in and diseases of the circulatory system that can affect the muscle
system were discussed in Chap. 4.
The many changes at the cellular and microscopic levels in
the muscle system combine to reduce the thickness of each muscle and therefore
the total amount of muscle mass. Serious loss of muscle mass is called sarcopenia.
On the average, sarcopenia begins during the third decade. The rate of loss is
low at first, but the rate increases with age, rising quickly after age 50.
Muscle mass may decrease as much as 50 percent by age 80. This increasingly
rapid loss seems to be due primarily to the decline in physical activity that
usually accompanies advancing age. Most of the loss of muscle mass and
thickness is due to the loss of muscle cells rather than to thinning of the
cells.
The decline in muscle mass produces several effects, one of
which is a decline in muscle strength. This loss of strength is related to the
total thickness of a muscle since the amount of strength per unit of
cross-sectional area of muscle cells remains fairly stable regardless of age.
However, the reduction in muscle strength that accompanies aging is only
partially due to thinning of the muscles. Other important factors include
changes in muscle cell structure and functioning and increases in fat and
fibrous material among the muscle cells. Changes in factors outside the muscle
system (e.g., nervous system, joints, motivation) also play an important role
in the decline in strength with age.
In general, strength peaks during the third decade and
declines little during the fourth decade. Age-related decrease in strength
becomes more rapid and significant during the fifth decade. The decline in
strength becomes faster as age increases after that. However, the decline in
muscle strength varies considerably from person to person and from muscle to
muscle. There is variation with respect to the age at which a substantial
reduction in strength can first be detected and the rate at which strength
declines afterward. Muscles used for quick strong contractions show a greater
decline in strength than do muscles used to maintain posture or perform other
actions requiring long-lasting mild contractions.
It seems that the most important reason for heterogeneity in
loss of strength is the increased variability in the amount of strenuous exercise
performed regularly by each person and each muscle. For example, individuals
whose daily routines include gripping objects or tools lose grip strength
slowly, but these individuals may have fairly rapid loss of leg strength if
their activities include little use of the legs.
The amount of strength lost over a period of years may
impair an individual's ability to carry out ordinary activities such as
shopping, gardening, cleaning, climbing stairs, and breathing heavily during
exertion. It becomes increasingly difficult to continue in certain lines of
employment, such as those requiring lifting or moving heavy loads. It may be
necessary to forsake strenuous recreational activities such as sailing. Still,
many aging individuals can tolerate declining strength by using methods
requiring less brute strength, substituting power tools and appliances for
muscle power, and enlisting aid from others.
The unevenness in loss of strength among different muscles
creates an additional problem in the form of reduced coordination. This occurs
because the balance in strength among the muscles used to perform an action is
altered. An important effect of dwindling strength and decline in coordination
is an increase in the risk of falling. Reduced and unbalanced muscle strength
also modifies posture. Detrimental outcomes from deteriorating posture may
include biological effects (e.g., restricted ability to inhale, impingement of
bones on nerves), social and psychological effects of altered appearance, and
economic effects from the need to obtain different clothing or furniture.
The reduction in muscle mass accompanying aging can have
effects other than changes in strength. A change in body proportions can have
social, psychological, and economic consequences for the reasons noted above
related to altered posture. Another effect is the need to modify one's diet.
With less muscle mass, there is a decrease in the basal metabolic rate and a
consequent decrease in the amount of calories needed
per day. Though the diet consumed by most aging people should contain fewer
calories, it should be richer in protein to maintain the remaining muscle mass
while preventing an undesirable gain in weight. The declining metabolic rate,
along with a relative decrease in the proportion of body mass composed of lean
muscle, also necessitates adjustments in the doses of medications.
For more details about sarcopenia
and aging of muscle, go to https://www.biologyofhumanaging.com/Sarcopenia/Sarcopenia_indx.htm
(Suggestion:
Chap 08 - 180-1-2)
Reaction
Time and Speed of Movement
All the changes in the muscle system already mentioned,
combined with aging in the nervous, circulatory, respiratory, and skeletal
systems, lead to other noticeable alterations in the actions produced by
muscles. One is an increase in the time needed to begin a voluntary motion in
response to a stimulus (reaction time). For example, it takes
longer for a driver to move his or her foot from the gas pedal to the brake
when a traffic signal turns red. Most of the increase in reaction time is
caused by slowing of the processing of impulses in the central nervous system.
Note that by definition, reaction time ends when the person
begins to move. The time from the beginning of a motion to the end of that
motion also increases with age. This second alteration, a decrease in the speed
of movement, is caused by decreasing muscle strength.
Both the increase in reaction time and the decrease in speed
of movement make the performance of rapid movements difficult. As can be seen in
the example of driving, these changes increase certain risks. There is also an
increased risk of falling. The probability of sustaining greater injury from a
fall also rises because it takes longer to grasp a handrail or piece of
furniture or to change body position to break or cushion the fall.
Longer reaction times and slower movements also make it more
difficult to perform rapidly repeated movements such as those used in playing
fast music or dancing. The effects of these changes become greater when individuals
attempt more complex or less familiar movements. As with declining strength,
changes in reaction time and speed of movement occur faster as age increases.
A third aspect of muscle activity that changes with age is
skill in performing tasks. Though changes in reaction time and speed of
movement have profound adverse effects on a person's skill in performing novel
activities, they have much less of an impact on activities that have been
performed routinely for many years. Skill in well-practiced actions can even
improve with age if repetition of the movements involved continues.
Practice also reduces the frequency of errors in performing
an intended movement and selecting sequences of movements to complete
complicated tasks. New strategies are formulated, and the efficiency of energy
use improves with practice. Therefore, experienced older individuals may
perform better than do younger individuals in activities requiring both
strength and speed.
The advantage of experience can be overshadowed by a gradual
drop in stamina. Stamina may be defined as the ability to perform
vigorous activity continuously for more than a few seconds. The effect of
dwindling stamina on overall muscle system performance is proportionately
greater than is the effect of the age-related decrease in speed of movement.
Stamina declines faster as age increases.
The decrease in stamina is manifested in four ways. First,
there is a decline in the maximum rate at which vigorous activities can be
performed. For example, the maximum speed at which a bicycle can be ridden
diminishes. Second, the length of time such activities can be performed without
stopping to rest becomes shorter. This decrease in endurance is
evident whether a person is working as fast as possible or at a rate somewhat
lower than the maximum rate. As will be explained below, important causes of
the reduction in endurance include a more rapid accumulation of lactic acid in
muscles and a faster and more intense onset of discomfort at a given rate of
vigorous activity.
The third indication of reduced stamina is a lengthening of
the time needed to recover after ending an activity such as running. For
example, it may take longer for respiration and heart rate to return to resting
values. One reason for the increase in recovery time is the faster accumulation
of lactic acid caused in part by a decline in the efficiency of movement.
Another factor is a slowing in the rate at which the heat produced by muscle
contraction is released from the body.
The fourth indication of dwindling stamina is a rise in
muscle stiffness and soreness experienced hours or days after a vigorous
activity has ended. Lactic acid buildup also seems to be a main reason for this
indication.
The decline in the maximum rate of performing physical
activity has been studied intensively. Therefore, this age-related change will
be discussed in detail below.
*VO2max The maximum rate at which a person can use
muscles to perform an activity is commonly determined by measuring the rate at
which that person uses oxygen while engaging in an activity at the fastest rate
possible. The maximum rate of working is expressed as the *VO2max. A
person's *VO2max is the amount of oxygen used per kilogram of body
weight per minute while a person is exercising at the fastest rate attainable.
Exercises commonly used for determining *VO2max include riding a
stationary exercise bike and walking or running on a treadmill. *VO2max is also called aerobic
capacity.
*VO2max declines with age. The decline begins at
about age 20 for men and about age 35 for women. These are average values,
however. As with many other age-related changes, there is great variability
among individuals of the same age in regard to actual *VO2max values
and the rate of decline in *VO2max values.
A main reason for differences in the levels and rates of
change of *VO2max
is variation in the amount of exercise a person gets. For example, the *VO2max
for people who have a rather sedentary lifestyle drops about twice as fast as
does the *VO2max of individuals whose jobs, home lives, and
recreational activities include large amounts of physical activity. Also, *VO2max
begins to decline faster when a person's activity decreases. By contrast, when
a person's participation in regular vigorous exercise increases, the decline
may be delayed and become slower or even be temporarily reversed. Still, some
reduction in *VO2max
eventually occurs in all people, including individuals who engage in highly
demanding physical activities throughout life. When vigorous physical training continues, *VO2max
declines 5 percent per decade.
Much of the decline in *VO2max is due to the
age-related decrease in total muscle mass combined with a relative increase in
the proportion of body fat. The rate of oxygen consumption of each kilogram of
muscle may be the same despite age.
Many other factors seem to contribute to the decline in *VO2max.
One factor is a reduction in the ability of muscles to extract oxygen from
blood. Other factors include changes and diseases that limit the functioning of
the circulatory, respiratory, and skeletal systems. A person may be affected by
more than one factor, and many people are affected by most or all of them.
Therefore, it is extremely difficult to identify how much of the decline in *VO2max
is due to aging of the muscle system rather than to other factors.
Consequences of Lowered *VO2max Since *VO2max
is an indicator of the maximum rate at which a person can perform activities, a
small decline means a drop in the maximum rate at which a person can run, climb
stairs, and carry out other vigorous activities. Individuals with lowered
values tend to stop physical activities sooner because of the discomfort such
activities induce. As *VO2max decreases further, limitations in less
demanding activities, such as walking briskly, become evident. When very low
values are reached, individuals may have trouble walking slowly or even getting
up from a chair or bed.
Since a substantial decline in *VO2max adversely
affects the performance of all types of physical activity, it can reduce a
person's effectiveness and participation in occupational, recreational, and
social activities. When *VO2max becomes very low, the performance of
ordinary daily activities needed to maintain a person becomes difficult or
impossible. Examples include shopping, dressing, and bathing. Serious losses in
the sense of independence and other negative psychological consequences often
develop. Undesirable alterations in one area can cause detrimental effects in
other areas, leading to a synergistic spiral of decline.
As mentioned previously, the decline in *VO2max
can be slowed or even reversed when an adult of any age begins a program of
exercise or includes vigorous activity in his or her daily life. Individuals
with relatively high *VO2max values need to engage in activities
with high intensity and frequency to derive beneficial alterations in *VO2max.
People whose *VO2max is fairly low can slow the decline or increase
this parameter with less strenuous activities. For many, substituting muscle
power for convenience can achieve real gains. For example, parking farther from
stores and walking to reach them or climbing stairs rather than using an
elevator can significantly increase a person's amount of exercise.
Many age-related changes in the muscle system are caused or
greatly increased by a decrease in physical activity. Conversely, many of these
adverse changes can be greatly slowed or even negated by continuing to engage
in regular exercise. It is also possible to delay, slow, reduce, or prevent
many undesirable changes and diseases in other body systems by living a
physically active lifestyle. In general and within
reasonable limits, the more exercise a person gets, the greater the benefits.
Many effects from maintaining a high level of physical
activity are listed in Table 8.1.
Muscle Mass Besides retaining the strength to perform
both heavy and ordinary tasks, maintaining muscle mass helps stabilize body
proportions. It also reduces detrimental changes in the ability of the hormone
insulin to regulate blood sugar and certain metabolic activities in the body
(Chap. 14). The effects of ongoing exercise on the nervous system help slow
both the increase in reaction time and the decline in speed of movement.
*VO2max
The impact of ongoing exercise on slowing
the decline in *VO2max is so great that very active elderly people
have values equal to or greater than those of sedentary individuals of about
age 30. However, no amount of exercise can completely stop the decrease in *VO2max
as age increases. Therefore, younger people who exercise will have values
greater than those of older individuals who get in the same amount of exercise.
In considering the beneficial effects of years of physical
activity, it is important to realize that these benefits are obtained only by
persons who continue to lead active lives. People who are very active or
athletic during youth but then become sedentary for many years lose most of the
benefit they acquired in their previously active lives.
Getting regular exercise throughout life has been shown to
increase life expectancy, possibly because exercise reduces the risks of
certain causes of death. Exercise has not been shown to increase maximum
longevity. Finally, exercise undoubtedly improves the quality of life.
Long-term exercise enables an individual to participate more fully and with
greater pleasure in many more life activities. Years of regular exercise also
markedly reduce the risk of developing many disabling diseases. Those who
exercise and still develop a disease are often less affected.
Starting or Increasing Exercise
Clearly, elderly individuals who have been involved in
vigorous physical activity throughout their lives benefit from such a
lifestyle. Young people who adopt active lifestyles can expect to reap the same
benefits when they become elderly. Furthermore, people of any age who have
lived sedentary lives and begin to get exercise and those who have been getting
only low or moderate amounts of exercise for many years and increase their
exercise can improve their well-being. We will now examine outcomes in older
people who begin vigorous exercise or substantially increase their level of
physical activity.
Many effects on older people who begin or increase physical
activity are listed in Table 8.2.
Circulatory System The rise in maximum cardiac output is
evident within a few days to weeks of initiating an exercise program. The more
intense the exercise program, the sooner a significant increase in maximum
cardiac output appears. This rise begins to be reversed within days of ending
the exercise program. The final maximum cardiac output of those leaving an
exercise program will be about the same as that which existed when the exercise
program began. Altering blood lipoprotein levels requires a decrease in body
fat along with the effects of the exercise.
Respiratory System There is disagreement about whether
increasing exercise increases respiratory volumes and speed of airflow, but
long-term participation in exercise programs slows the decline in respiratory
functioning. Therefore, in the long run elderly individuals who exercise will
eventually have better respiratory system functioning than they will if they
remain sedentary.
The respiratory system changes caused by a proper exercise
program are of special importance to persons who have chronic obstructive
pulmonary diseases (COPDs) such as chronic bronchitis and emphysema.
Nervous System The mechanisms by which strenuous exercise
increases strength in older people are different from those in younger people.
At younger ages the increase in strength from training with heavy weights is
caused almost exclusively by thickening of the muscle cells rather than
alterations in the nervous system. Perhaps the change in mechanisms for
increasing strength is a way the aging body partially compensates for a
decreased ability of the muscle cells to adapt to lifting or moving heavy
loads.
Muscle System The gain in strength achieved by older
individuals is proportionately the same as that which younger
adults attain with the same type of exercise. For example, consider an
older person who has had little exercise for many years and a younger adult who
has had the same level of activity. The older person will not be as strong as
the younger adult because the older adult has been losing strength for a longer
period. If both individuals begin an exercise program designed to double the
strength of an adult, both will end the program with twice the strength they
had when they started. Of course, since the younger person was stronger at the
start of the program, he or she will be the stronger person at the end.
However, an older person who participates in such a program can become stronger
than a younger adult who remains sedentary.
Older individuals whose exercise is not strenuous enough to
cause an increase in strength still benefit because they at least have a slower
decline in strength. Therefore, after long-term involvement in physical
activity these individuals will be stronger than they would have been if they
had remained sedentary. They will also have retained more total muscle mass.
Keeping a high muscle mass helps the functioning of insulin.
Alterations in *VO2max caused by increased
exercise are similar in three ways to exercise-induced changes in strength.
First, the increase in
*VO2max attained by an older person is proportionately
the same as that achieved by a younger adult who starts with the same
capability and participates in the same exercise program. Second, elderly
people who increase their physical activity enough can develop values that are
greater than those of much younger adults who remain sedentary. Third, elderly
people whose increase in exercise is not enough to produce an increase in *VO2max
will still benefit because even small increases in physical activity slow the
decline in *VO2max. Therefore, these individuals will eventually
have a greater *VO2max than they would have had if they had remained
sedentary.
There is an important difference between the effects of
exercise on alterations in conditions such as blood lipoproteins, body
composition, percent body fat, functioning of insulin, and strength and the
effects on alterations in *VO2max. Though vigorous activity is
needed to effect substantial changes in the first five parameters, for very
sedentary older people even low levels of easy activities such as walking can
substantially increase *VO2max. The resulting improvements can
restore the ability of very sedentary elderly individuals with extremely low *VO2max
values to perform the ordinary activities of daily living.
All the exercise-induced alterations in the nervous and
muscle systems just described combine to produce several other benefits in the
elderly. These benefits include the perception that less effort is needed to
perform demanding tasks; improved mood and sense of well-being; improved social
interactions; and increased independence.
Skeletal System Aging of the skeletal system raises the risk
of sustaining fractures by causing demineralization of bones and reducing the
ease of movement and range of motion of joints. Some forms of the joint disease
called arthritis exaggerate these changes.
No one knows the best exercise program for slowing or reversing
bone demineralization caused by aging or osteoporosis; different programs may
be effective for different individuals. Also, different individuals can
tolerate different amounts and types of exercise. The causes of these
differences include physical condition, presence of diseases, lifestyle, and
motivation.
The possible benefits of slowing bone demineralization and
deterioration of joint functioning through a large increase in strenuous or
vigorous physical activity must be weighed against the added risk of injury.
Some more common problems include increased risk of fracture of the bones in
the spinal column caused by lifting or holding heavy loads, increased risk of
fracturing hip, leg, or arm bones by falling, traumatic injury to the bones, muscles,
ligaments, and tendons in the lower legs from walking or running on hard
surfaces or with improper footwear, and injury to the joints from excessive
movements or forces, including impact forces.
Endocrine System Exercise leading to a decrease in body fat
significantly improves glucose tolerance and insulin sensitivity in elderly
people who have a reduced glucose tolerance or non-insulin-dependent diabetes
mellitus (NIDDM). Individuals who improve their glucose tolerance and insulin
sensitivity have a substantially reduced risk of developing the complications
associated with diabetes.
These beneficial effects of exercise begin to develop within
days or even hours after a person increases the level of vigorous physical
activity. Improvement increases as body fat decreases. However, the beneficial
effects of the exercise begin to dwindle within a few days of ending
involvement in the exercise program. Therefore, to sustain the benefits of
exercise, a person with reduced glucose tolerance or NIDDM must engage in the
exercise at least once every three days.
By contrast, individuals with type I (insulin-dependent)
diabetes mellitus (IDDM) have very unstable blood sugar levels. Therefore, the
amount of exercise they get must be carefully monitored and adjusted according
to factors such as the severity of the disease, diet, and insulin treatments.
Other Effects The effects of increasing exercise mentioned
up to this point are related to specific body systems, but elderly people who
increase their exercise seem to benefit in many other ways. These other
benefits include helping to maintain normal body weight by improving nutrition
and using more calories; increasing independence by generally slowing the onset
of disability and physical limitations; and helping to enhance psychological
health by improving mood and sense of well-being while reducing boredom,
anxiety, and stress.
The physical and psychological effects of exercising also
increase older individuals' ability to remain productive and economically
self-sufficient. Their social situation is bolstered by the additional social
interactions obtained through exercise programs and through an enhanced ability
to participate in other activities in the community. Therefore, while
increasing exercise has not been shown to lengthen life, it dramatically
improves the quality of life for older individuals.
Having reviewed how exercise benefits the elderly, we will
now consider information and suggestions that have been found important in
achieving these benefits.
First, as with other mechanisms that maintain homeostasis,
adjustments made in body systems to each form of exercise represent attempts to
minimize or prevent disturbances in internal conditions. Furthermore,
adjustments and improvements made by the body are specific to the demands
placed on it. For example, if conditions in leg muscles are significantly
disturbed by lifting heavy loads, those muscles will become stronger and
therefore will be less affected when the loads are lifted a few days later. By
contrast, if conditions in leg muscles are disturbed by an activity involving
many repeated actions that do not require much strength, such as walking
briskly for a long distance, the adjustments in the body will increase stamina
for walking but will have little effect on muscle strength.
The first step in preparing to increase exercise is to
establish specific goals. Then activities can be selected that will cause the
body to make the adjustments needed to achieve those goals. For example, if an
increase in the range of motion of the arms is desired, activities requiring
movement of the arms over wide angles can be selected. If increasing the grip
strength of the hands is a goal, activities using strong grasping should be
undertaken. As more goals are identified, a greater number and variety of
exercises or activities must be employed.
In a more general way, if exercise is being used to improve
the functioning of the circulatory and respiratory systems, a number of
activities demanding faster blood flow and increased respiration can produce
the desired results. Examples include walking or riding a bicycle at a fast pace,
running, and swimming.
When one is deciding on exercises, attention must be paid to
the condition of the person who is participating in the exercises. Careful
attention to an elderly person's physical condition is particularly important
because of the higher incidence of disease and the increased heterogeneity
among older people. At this point, a qualified professional should perform a
physical examination and evaluation of the participant and the information
obtained should be used to determine the appropriateness of the anticipated
activities. At least one follow-up examination and evaluation should be
performed several days or a few weeks after the individual has taken up the new
level of activity. Data from the first and later examinations and evaluations
should be used to determine what changes are occurring because of the increased
exercise and to suggest improvements in the activities.
Once appropriate activities have been selected and the
condition of the participant has been ascertained, decisions about the
intensity and the length of exercise can be made. The time allotted for
exercise should include time for warming up and cooling down. The frequency
with which the activity will be performed can also be established. A healthy
person should exercise at least 30 minutes each session, with sessions
occurring at least every three days.
Generally, starting a program of exercise with a fairly low
level of intensity and a short duration of activity is best, especially for
very sedentary or frail individuals, who can achieve substantial benefits from
relatively low levels of exercise. Also, such individuals are more likely to
sustain injuries or other adverse effects from a sudden increase in physical activity.
Beginning with low levels of exercise also helps prevent negative attitudes by
minimizing the discomfort caused by an increase in exercise.
Consideration might also be given to the number of weeks or
months during which the participant expects to perform the activity. Exercise
programs lasting only a few weeks produce little benefit, while those lasting
several months or longer yield significantly better results. Longer-lasting
programs are especially important for older people since the rate of improvement
caused by exercise decreases with age. Sustained participation in the exercise
program is aided by using positive feedback and other motivational strategies,
such as combining exercise sessions with social activities. Since the exercise
program should last for an extended time, it is important to provide for proper
nutrition.
As the exercise program continues, the intensity or duration
of each exercise or the frequency with which it is performed each week should
be increased gradually so that the participant continues to improve. If the
exercise is not increased, the participant's level of physical fitness will
soon stabilize, and boredom may become a problem if there is no variation.
Furthermore, the psychological benefits of exercise become most apparent once a
high intensity and greater frequency have been achieved.
Although the benefits are directly proportional to the
intensity, duration, and frequency of exercise, care must be taken not to
exceed reasonable limits. As levels of exercise increase, so do the risks of
overheating, being physically injured, and developing complications from
existing diseases.
Very strenuous activities present a special danger to those
with atherosclerosis because people tend to hold their breath while pulling or
pushing with great force. Blood pressure rises to a very high level during such
maneuvers, placing a great burden on the heart and arteries. A heart attack, a
stroke, or damage to the retina or vitreous humor can result. When the straining
ends, there is a sudden drop in blood pressure, placing additional burdens on
the heart and sometimes causing dizziness, fainting, and falling. These
problems can be largely avoided by minimizing exercises requiring great
strength and maximizing activities involving free movement of parts of the
body.
When one is discussing exercise, focusing on activities
whose primary purpose is exercise is easy (e.g., aerobics, weight lifting,
jogging). Using such activities and the many available exercise machines and
devices provide means of carefully regulating the amount of exercise obtained
and measuring physical status and improvements in physical fitness.
While some individuals enjoy such purposeful exercise,
others find it unpleasant, expensive, or unavailable. These individuals can
still obtain plenty of beneficial exercise through activities with other
primary purposes. Examples include recreational activities such as dancing,
sports, and hiking and activities related to occupations requiring physical
work. Activities performed in caring for one's home and family, such as
gardening and mowing a lawn, shopping, and doing laundry, can provide
opportunities to get healthful exercise. Choosing to walk or climb stairs
rather than riding can add substantially to the amount of beneficial exercise
obtained.
Achieving the benefits of increasing exercise often involves
nothing more than substituting muscle power for motor power. What may be needed
first, however, is replacement of the notion that using minimal physical effort
means living well with the realization that only through regular physical
exercise can an older person achieve "the good life."
Driving accidents increase in numbers and in rates as the
age of the drivers increase. For example, elderly drivers have twice as many
accidents per mile compared with younger drivers. Elder drivers who have
accidents are more likely to sustain serious or fatal injury than are younger
drivers. At the same time, the number and proportion of elder drivers are
increasing, and they will continue to increase at faster rates for the next few
decades. By 2024, drivers over age 64 will make up 25 percent of all drivers.
Potential accident situations often require making quick and
coordinated responses in new situations. Therefore, elder drivers can be very
safe drivers until they meet surprising or complicated situations that demand
quick reactions in unfamiliar situations. Examples of problematic situations
for elders are intersections. In such demanding situations, age changes in
muscle strength, speed, reaction time, and coordination contribute to an
age-related decrease in driving ability. Age changes and age-related
abnormalities in other body systems also contribute to reduced driving ability.
These changes and abnormalities are described in other chapters.
Neurological parameters that change very little with normal
aging include implicit memory of driving skills and making automatic
coordinated responses. Other age-related changes that do not have a major
impact on driving ability in elders include modest decrements in vision and in
hearing. The small effects on driving safety among elders from decrements in
vision and in hearing may be due to elders avoiding driving at times and
conditions where these decrements are important (e.g., nighttime, bad weather,
heavy traffic, high speeds, time pressures). In general, overall cognitive
ability has little to do with driving skills until cognitive abilities become
severely reduced. Therefore, people in early stages of dementia can still be
good drivers.
Important neurological factors that limit driving ability
for elders include age-related decreases in avoiding distraction; changing
attention quickly; responding quickly in unfamiliar situations; noticing,
accurately identifying and responding to sudden changes in the visual field;
noticing and responding to a novel change in the environment; and distinguishing
between important and unimportant items in the visual field.
Driving is very important to elders for many reasons
including mobility; independence; a sense of self-efficacy; and a sense of
self-worth. Loss of driving often causes major psychological, social, and
economic problems for elders. Demands to provide alternative means of
transportation increase (e.g., family, friends, community groups, private
companies, governmental agencies).
Recommendations that can accommodate these diverse factors
include providing reliable, practical and regular tests for evaluating elder
drivers; providing education and training to maintain or improve elders'
driving skills and safety; and developing alternative transportation for elders
as they give up or lose their driving privileges.
©
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Copyright 2020: Augustine G. DiGiovanna, Ph.D.,
Salisbury University, Maryland
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