8.7: Changes in Muscle Mass

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.

Effects of Mass on Strength

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.

Other Effects

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)