Powered ankle-foot prosthesis for the improvement of amputee walking economy

Abstract

The human ankle provides a significant amount of net positive work during the stance period of walking, especially at moderate to fast walking speeds. On the contrary, conventional ankle-foot prostheses are completely passive during stance, and consequently, cannot provide net positive work. Clinical studies indicate that transtibial amputees using conventional prostheses exhibit higher gait metabolic rates as compared to intact individuals. Researchers believe the main cause for the observed increase in metabolism is due to the inability of conventional prostheses to provide net positive work at terminal stance in walking. This objective of this thesis is to evaluate the hypothesis that a powered ankle-foot prosthesis, capable of providing active mechanical power at terminal stance, can improve amputee metabolic walking economy compared to a conventional passive-elastic prosthesis. To test the hypothesis, a powered prosthesis is designed and built that comprises a unidirectional spring, configured in parallel with a force-controllable actuator with series elasticity. The prosthesis is controlled to mimic human ankle walking behavior, in particular, the power generation characteristics observed in normal human walking. The rate of oxygen consumption is measured as a determinant of metabolic rate on three unilateral transtibial amputees walking at self-selected speeds. The initial clinical evaluation shows that the powered prosthesis improves amputee metabolic economy from 7% to 20% compared to the conventional passive-elastic prostheses (Flex-Foot Ceterus and Freedom Innovations Sierra), even though the powered system is twofold heavier than the conventional devices. These results support the proposed hypothesis and also suggest a promising direction for further advancement of ankle-foot prosthesis.