Effects of voluntary leg muscle contractions on cardiac output while standing.

To clarify the contribution of voluntary muscle contractions on the cardiovascular variables of humans, we measured stroke volume (SV), heart rate (HR), and cardiac output (CO) by impedance plethysmography in young males maintaining an upright standing position with 1) no voluntary muscle contractions owing to as little movement as possible (control condition), 2) voluntary muscle contractions in the calves along with a forward sway of the body, and 3) muscle contractions throughout the legs along with a backward sway. Muscle contraction in the calves did not provoke any significant increase in either SV or CO. Voluntary contractions throughout the legs, on the other hand, caused a reduction in SV and CO. From these observations we may conclude that contractions along with forward or backward swaying dose not enhance venous return unless the abdominal muscle is also contracting and unless the contractions are synchronous to one-minute wave in body fluid volume change.