We compared the relative ability of continuous accelerometric, electromyographic (EMG), and hydrostatic posture measurements to diseriminate tasks involving variations in motor activity and posture and to predict heart rate (HR) variability. EMG was a more sensitive measure than accelerometry in differentiating the tasks. How-ever, accelerometry and EMG explained comparable amounts of HR variance. The hydrostatic posture was a stable measure that elearly differentiated postures and explained a significant amount of HR variance but less than accelerometry or EMG. Accelerometric and EMG measures of motor activity used either alone or in combination with the hydrostatic posture are valuable in discriminating activities and in controlling for the effects of motor activity and posture on HR during ambulatory measurement.