[Measurement and assessment of habitual physical activity in epidemiological studies]

Epidemiological study and laboratory evidence show that physical activity protects against the development and progression of chronic diseases. The physiological and biomechanical principles of the assessment of physical activity are ambiguous, energy expenditure calculations are complex, and numerous difficulties can be encountered in developing simple field techniques for assessing habitual activity. The greatest obstacle to validating field methods of assessing habitual physical activity or energy expenditure in humans has been the lack of adequate comparison criteria for the technique. Physical activity can be described as having four dimensions: duration (minutes/hours), frequency (time per week/per month), intensity (rate of energy expenditure) and circumstances or purpose of the activity. Methodology incorporating questionnaires/interviews are relatively inexpensive, and at present are the only methods feasible for large population studies. Specific activities can be identified in conjunction with frequency and duration. The procedure does not influence subjects' activities to the extent that can occur with observation or diary keeping. The limitations of questionnaires/interviews are the actual definitions and interpretations of the term "physical activity", despite the attempts of interviewers to provide a clear definition. Subjects do not necessarily recall their activities accurately; they may tend to overestimate time or intensity. A self-administered questionnaire must be suited to respondents' ages and education levels. Detailed questionnaires/interviews place a considerable burden on subjects.