Estimates of mechanical work and energy transfers: demonstration of a rigid body power model of the recovery leg in gait.

Many studies concerning the mechanical work and efficiency of human motion have used models based on segmental energy. It has been shown theoretically that such work estimates may be in error due to offsetting compensations in power sources underlying the energy profiles. Further, mechanical energy transfers calculated from these energy models have been interpreted as metabolic energy-saving mechanisms. This paper examines the use of mechanical power analysis to calculate work and energy transfer estimates, using the motion of the recovery leg in walking and running for one subject as a demonstrative example. Work and energy transfer estimates from both energy and power models are compared and contrasted. The energy model underestimates the work of the recovery leg in both walking (54% of power model estimate) and running (38%), due to muscle powers at joints opposing each other in energy generation and absorption. Energy transfers calculated with energy models are shown to suffer the same problem of offsetting power sources. In contrast, the power model identifies four energy transfer mechanisms (pendulum, whip, tendon, and joint force transfers), which contribute to energy change within the leg in varying amounts. For the recovery leg, the joint force and whip transfer mechanisms have the greatest magnitude, while the pendulum and tendon transfers are much smaller. These energy transfers can be observed on a time-varying basis throughout a motion sequence and illustrate differences in energy distribution between walking and running. These power-based transfers are discussed in terms of their nature regarding metabolic energy cost and mechanical energy distribution within a multisegmented system. It is suggested that the work and energy transfers calculated from the power analysis are more accurate than those calculated from mechanical energy models and are more useful for understanding performance.