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 ene rgy producing movement of Model 1 were eight muscles, three of which w ere two-joint muscles. Sources of mechanical energy producing movement of Model 2 were net moments at its joints. These sources of mechanica l energy were substituted by 11 one-joint muscles, with the assumption that antagonistic muscles did not produce force. Because of this assu mption, summed MEE of all joint moments and all one-joint muscles of M odel 2 were the same. It was shown that during the same movement the m odel with two-joint muscles could spend less mechanical energy than th e model without two-joint muscles. This economy of mechanical energy r ealized by two-joint muscles was possible if (i) signs of the muscle p owers which were produced by the two-joint muscle at both joints were opposite,(ii) moments produced by that muscle at each of the two joint s had the same direction as the net joint moments at these joints, and (iii) muscles crossing these two joints from the opposite side did no t produce force. Realization of these three conditions during human lo comotion was checked experimentally. Electrical activity of eight lowe r extremity muscles of ten subjects was measured during treadmill walk ing and running. Based on this information, the periods where the musc les produce force were estimated. Moments and their power at joints of the lower extremity of two subjects performing walking and running we re calculated using kinematics and ground reaction force measurements, and an inverse dynamics approach. It was shown that MEE of models wit h different sources of mechanical energy appeared to be different duri ng certain periods of the swing phase. However, the magnitude of this difference was probably relatively small.