of a number of authors, however, are roughly analogous to the three descriptive categories (i.e., impairments, disabilities, and handicaps) used in that classification system. Therefore, the discussion below is organized around these categories.

The ability of skeletal muscle to perform aerobically for sustained periods of time is influenced by two primary factors: the delivery of oxygen by the cardiovascular system and oxygen consumption by mitochondria in the tissue. Reductions in the oxidative capacity of tissue and reduced oxygen delivery, which often accompany the aging process, support the proposal that aging muscle has impaired ability.7 Yet the precise causes of muscle impairment in the elderly are uncertain.7 Muscle power and strength increase until the third decade; they then plateau and begin to decline in the middle years. With advanced age and senescence, further reductions have been observed. Cross-sectional research using muscle biopsies of healthy older adults indicates that impairments can be attributed to reduced muscle mass as a result, primarily, of a loss of muscle fiber. There are, however, other factors that need further study, including muscle fiber area and changes with age in the ratio of Type I and Type II muscle. Longitudinal studies and more comprehensive

FIGURE 13-3 Age-specific death rates from all causes in Harvard Alumni Study over 16 years of follow-up, according to physical activity levels in kilocalories per week.

Source: Adapted from R. S. Paffenberger, R. T. Hyde, A. L. Wing, and C. C. Hsieh, "Physical Activity, All-Cause Mortality and Longevity of College Alumni," New England Journal of Medicine, Vol. 314, pp. 605-613, 1986.

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