Changes in Muscle

After about age 30, the number and size of muscle fibers progressively decrease, resulting in a decrease in skeletal muscle mass and thus lean body mass. This process is termed sarcopenia. Age-related factors contributing to sarcopenia may include reduced levels of exercise and physical activity; a loss of motor units, possibly beginning during middle age; and reduced skeletal muscle protein synthesis. Other factors that may contribute in some cases include a relative deficiency of anabolic hormones, such as growth hormone (GH), insulin-like growth factor I (IGF-I), testosterone, and perhaps dehydroepiandrosterone (DHEA). GH stimulates production of IGF-1, which may mediate the anabolic effect of GH. GH levels decrease with aging. In young adults, resistance exercises increase GH and IGF-1 levels. In the elderly, the increase in GH and IGF-1 is attenuated. In the elderly, exogenous GH, when combined with exercise, increases muscle mass more than exercise alone but does not seem to meaningfully increase muscle strength.

In healthy young people, 30% of body weight is muscle, 20% is adipose tissue, and 10% is bone. Muscle accounts for 50% of lean body mass and about 50% of the total amount of body nitrogen. By age 75, about ½ the muscle mass has disappeared; 15% of body weight is muscle, 40% is adipose tissue, and 8% is bone.

The faster-contracting type II muscle fibers participate in sudden powerful muscle contractions, whereas the slower-contracting type I fibers function to maintain posture and to perform rhythmic, endurance-type exercises. Type II fibers decrease to a greater extent than do type I fibers. Maximum isometric contraction force decreases about 20% by age 50 and about 50% by age 70. Despite this, healthy elderly people can usually easily climb stairs, rise from a squatting position, walk along a straight line, hop on either foot, and perform typical activities of daily living.

Elderly people whose mobility is restricted, particularly those with acute illness or who are bedridden, lose muscle mass and strength (deconditioning). The rate of loss is greatest in the antigravity muscles--those used to sit up, stand up, and pull up--which are essential for performing activities of daily living. Up to 1.5%/day of muscle mass can be lost; for 1 day of absolute bed rest, up to 2 wk of reconditioning may be necessary to return to baseline function.

Patients with sarcopenia and insufficient dietary protein require nutritional support; those with sarcopenia and hormonal deficiencies may require hormonal supplementation. Hospitalized elderly people, especially those who are bedridden, require early physical therapy and individualized exercise regimens. In the elderly, the beneficial effects of exercise are usually specific to the training activity. For example, if the goal is to improve throwing a ball, then strengthening the muscles involved in throwing is necessary; however, to develop maximum function, the exercise should involve actually throwing a ball.

In addition to the debilitated, exercise is helpful in all elderly people. More studies are needed to determine the optimal frequency, intensity, duration, and types of exercise.

This topic was last updated May 2005.