Figure 1 Estimates of the prevalence (%) of the U.S. population with selected risk factors for coronary heart disease and the population from England. In both studies, a sedentary lifestyle was taken as 'no physical activity' or irregular physical activity (i.e., fewer than three times per week and/or less than 20 minutes persession). (From Killoran AJ, Fentem P, and Caspersen C (eds.) (1994) Moving On. International Perspectives on Promoting Physical Activity. London: Health Education Authority, with permission.)

A common weakness of such studies is that they often rely on a single measurement of fitness or activity at baseline, with subsequent follow-up for mortality within the cohort. With such a design, it is difficult to discount the possibility that genetic or other confounding factors are influential in the observed relationship between physical activity/ fitness and mortality. A further weakness in single baseline studies is that subsequent changes in activity/ fitness during the follow-up are not monitored, even though they may affect the observed relationships due to the phenomenon of 'regression to the mean.'

Some prospective studies have overcome these deficiencies by examining the effects of changes in physical activity and fitness on mortality. One study reported on the relationship of changes in physical activity and other lifestyle characteristics to CHD mortality in 10269 alumni of Harvard University. Changes in lifestyle over an 11- to 15-year period were evaluated on the basis of questionnaire

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