8.8: Muscle System Performance - Aspects and Changes

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.

Skill

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.

Stamina

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.