['Dynamic calf muscle exercise of different intensities was performed at 90 degrees knee-joint flexion. To obtain different hydrostatic pressures and calf volumes, all exercise tests were conducted both in prone and supine position after 10 min of rest in the respective positions. Severe prone exercise was associated with increased anaerobic metabolism, reduced exercise tolerance time, and pain sensation. Despite these differences, maximal heart rates were nearly identical in both positions. During light to moderate exercise, heart rate was significantly increased in the prone position. In a steady-state exercise test this difference developed gradually during the first 10 min of exercise. Neither changes in muscle metabolism nor baroreceptor influence or pain sensation could account for this effect.'] Do local fluid losses enhance heart rate drives in light to moderate exercise?