Physical Activity and Optimal Health: The Challenge to Epidemiology

Abstract

Epidemiologists seek associations between environmental factors, lifestyle influences and human health; they use current modifications of a series of guidelines enunciated by Bradford Hill to assess the hypothesis that observed associations are causal in nature. We now have a long list of medical conditions where physical activity has been suggested as having a beneficial influence in prevention and/or treatment. Questionnaire evaluations of such claims have been hampered by the limited reliability and validity of self-reports. The introduction of pedometer/accelerometers and other objective monitors has facilitated the determination of causality, allowing investigators to study the effects of clearly specified types, intensities, frequencies and durations of physical activity. Nevertheless, further improvement of monitoring devices is needed in order that epidemiologists can capture the full range of activities typical of children and younger adults. Objective monitoring does not support the hypothesis that a minimum intensity of physical effort is needed for health benefit; indeed, in sedentary individuals the largest improvements in health are often seen with quite small increases of habitual activity. There is no obvious threshold of response, but for many medical conditions available data suggests a ceiling of benefit, with no apparent gains of health once habitual activity attains a specified upper limit. Causality can never be totally proven, but objective data allows the inference that multiple health benefits will stem from moderate daily physical activity; the evidence is sufficiently strong that people of all ages should be urged to adopt such behaviour.