A new technique for the calculation of the energy stored, dissipated, and recovered in different ankle-foot prostheses

Previous research reported calculation of mechanical power of ankle-foot devices using the dot product of the ankle moment times the ankle angular velocity. Unfortunately, there are two errors in this analysis technique. The biomechanical model used assumed a rigid foot articulating around the ankle and there was no accounting for energy storage or dissipation and recovery in the viscoelastic material of the cosmetic cover. The first purpose of this paper is to propose a rigorous technique for the calculation of the net energy efficiency that could be used for any articulated or nonarticulated ankle-foot prosthesis. The second purpose is to quantify the amount of energy stored or dissipated and then recovered in order to discriminate between different ankle-foot prostheses. The SACH, Seattle, Flex-foot, and Golden-Ankle ankle-foot prostheses; were evaluated on the same amputee, while walking at his natural cadence. The power entering and leaving the distal end of the prosthetic leg was calculated as the sum of the translational (force-velocity product) and rotational (moment-angular velocity product). All ankle-foot prostheses showed the same distal power pattern. After initial contact, a large energy storage was observed in the cushioned heel, which was followed by some energy recovery. Then, during mid and late stance, another period of storage or dissipation and recovery was observed. Even the SACH foot should be considered an energy storing foot prosthesis since it´s cosmetic material was seen to be capable of recovering energy. The balance between the rate of change of foot mechanical energy and the foot powers showed that the new technique takes into account the energy stored or dissipated and then recovered within the compliant material and flexing keel. The new analysis technique can be used by prosthetic designers to assess any type of ankle-foot prostheses. Criteria for ankle-foot prosthesis selection should include, not only the net mechanical efficiency for both rearfoot and forefoot sections, but also, the total energy recovered by the ankle-foot prostheses