Comparison of mechanical energy expenditure of joint moments and muscle forces during human locomotion.

The mechanical energy expenditures (MEEs) of two human lower extremity models with different sources of mechanical energy - (1) muscles and (2) joint moments - were compared theoretically. Sources of mechanical energy producing movement of Model 1 were eight muscle, three of which were two-joint muscles. Sources of mechanical energy producing movement of Model 2 were net moments at its joints. These sources of mechanical energy were substituted by 11 one-joint muscles, with the assumption that antagonistic muscles did not produce force. Because of this assumption, summed MEE of all joint moments and all one-joint muscles of Model 2 were the same. It was shown that during the same movement the model with two-joint muscles could spend less mechanical energy than the model without two-joint muscles. This economy of mechanical energy realized by two-joint muscles was possible if (i) signs of the muscle powers which were produced by the two-joint muscle at both joints were opposite, (ii) moments produced by that muscle at each of the two joints had the same direction as the net joint moments at these joints, and (iii) muscles crossing these two joints from the opposite side did not produce force. Realization of these three conditions during human locomotion was checked experimentally. Electrical activity of eight lower extremity muscles of ten subjects was measured during treadmill walking and running. Based on this information, the periods where the muscles produce force were estimated. Moments and their power at joints of the lower extremity of two subjects performing walking and running were calculated using kinematics and ground reaction force measurements, and an inverse dynamics approach. It was shown that MEE of models with different sources of mechanical energy appeared to be different during certain periods of the swing phase. However, the magnitude of this difference was probably relatively small.