Physical activity is a behavior that is characterized by any bodily movement that results in an increase in energy expenditure above resting levels (Caspersen, 1989). This behavior has been linked to a reduced risk of premature mortality (Haskell, 1994) and several diseases such as heart disease, certain types of cancer, noninsulin dependent diabetes mellitus, hypertension, and osteoporosis (Haskell, 1994). However, it is not clear how much physical activity is required to reduce risk for these diseases.

There are a number of risk factors that have been linked to increased health risk, particularly for coronary heart disease. Hyperlipidemia, hypertension, smoking, and obesity have all received widespread attention in the research literature as well as in the popular press as increasing one's risk for coronary heart disease. All of these'risk factors' are relatively easy to quantify either at home or through routine medical examination procedures. In addition, these measures exhibit a fairly high degree of stability day-to-day. However, assessment of physical activity behavior is much more difficult to characterize as it is based on individual habits and practices that vary considerably day-to-day. Often self-report measures are used to assess physical activity and are commonly subject to recall errors and inaccurate perception of one's activity behavior. With the current public health emphasis on the exposure (ie, accumulating 30 minutes of moderate intensity physical activity), it becomes imperative that we use more objective methods for quantifying this be-