Aging Skeletal Muscle: Physiologic Changes and the Effects of Training

According to the 1996 US Census Bureau population projections (middle series), during the period from 1995 through 2030, the percentage of the American population that is 65 years of age or older and 85 years of age or older will increase by 107% and 133%, respectively. In contrast, the percentage of those under 65 years of age will increase by only 21%.1 In addition, the percentages of older people with disabilities in activities of daily living and of older people requiring institutionalization for disabilities are expected to remain similar to current levels over the next 30 years, although these statistics vary by ethnicity.1 This means that the number of people requiring institutionalization for disabilities will increase substantially. Based on these data, the physiologic changes that occur in skeletal muscle as a result of aging and the effects of exercise on aging skeletal muscle are going to be of increased importance to physical therapists. The purpose of this update is to discuss the effects of aging on skeletal muscle and to discuss how exercise affects aging skeletal muscle. As humans age, the force-generating capacity (strength) of their skeletal muscles is reduced.2,3 As a result, many older people experience difficulty in performing their activities of daily living.2 Recent research indicates that the observed loss of force production in older people is primarily to the result of muscle atrophy and alterations in the percentage of contractile tissue within muscle3–6 rather than deficits in muscle activation (motor unit [MU] recruitment and firing rates).7–9 ### Effects on Muscle Fiber Number and Size Skeletal muscle cross-sectional area (CSA) decreases with age (Fig. 1).3,4,6 This phenomenon, which is referred to as sarcopenia, can be the result of a reduction in fiber size, fiber number, or a combination …