None of the various methods used to measure habitual physical activity over days, weeks, or years in the general population have yet proven entirely satisfactory. A major problem is that no "gold standard" exists for the validation of various questionnaires, logs, or diaries that can be used in large sample population studies. Attempts have been made to accurately measure the activity profile by using heart rate or various motion sensors or accelerometers, but each approach has had significant limitations. The availability of new solid state recording techniques and computer-based analytic and display procedures now makes it possible to simultaneously record heart rate and body movement continuously for days and to combine the analysis of these data using customized software. Preliminary evaluation of this concept of simultaneous recording and analysis of heart rate and body motion via movement sensors on an arm and leg were conducted in 19 men. Subjects performed a variety of exercises in the laboratory during which heart rate, leg motion, arm motion, and oxygen uptake were recorded. Various issues regarding the prediction of energy expenditure from heart rate and body movement independently and in combination were evaluated. The results demonstrate that the accuracy of estimating oxygen uptake during a wide range of activities is improved when individualized heart rate--oxygen uptake regressions are used and heart rate and body movement are analyzed simultaneously rather that separately.