Physical inactivity, which has generally been studied as a precursor or an outcome of disease and disability, is the central focus of this review. The general perspective of this chapter is informed by two fundamental assumptions. First, for most persons 50 years of age and older, increasing age is not a cause of physical inactivity.13 As noted by Berger,1 current research, especially by Smith,40 suggests that 50 percent of the decline frequently attributed to physiological aging is, in reality, disuse atrophy resulting from inactivity in an industrialized world. Second, the ability to remain physically active underpins the ability to perform the activities of daily living.

This chapter considers physical activity separately from disease and disability. Research on the activities of daily living has been most closely associated with measuring detriments to the performance of specific, essential physical activities, caused by disease and disability; consequently, a discussion of such research falls outside the focus of this chapter. Therapeutic exercise prescribed for a specific disability or injury also will not be addressed in this chapter. Rather, what is known of the benefits of physical activity in general and how such activity can be encouraged and maintained during the "second 50" will guide the discussion that follows.

BURDEN

Prevalence

One measure of physical inactivity is sedentariness, which is defined as either no physical activity or physical activity less than three times per week and/or physical activity of less than 20 minutes per occasion.27 The Centers for Disease Control reported the prevalence of a sedentary lifestyle in selected states using data from its Behavioral Risk Factor Surveillance System. The results of a 1985 telephone survey of 25,221 respondents were analyzed, and approximately 55 percent of the respondents were classified as sedentary. Being sedentary was more common among women than men and increased with age (Figure 13-1).

To estimate the percentage of individuals in the United States who maintained appropriate levels of physical activity as specified by the 1990 PHS objectives, Caspersen and colleagues analyzed data from the National Health Interview Survey conducted in 1985. They developed detailed scoring procedures using intensity codes for each activity and determined that almost one-third of the population between the ages of 45 and 64 and close to half of the over-65 age group, were sedentary. Table 13-1 shows their data for leisure-time



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