Mechanical and energetic consequences of reduced ankle plantar-flexion in human walking

Abstract

The human ankle produces a large burst of 'push-off' mechanical power late in the stance phase of walking, reduction of which leads to considerably poorer energy economy. It is, however, uncertain whether the energetic penalty results from poorer efficiency when the other leg joints substitute for the ankle's push-off work, or from a higher overall demand for work due to some fundamental feature of push-off. Here, we showthat greatermetabolic energy expenditure is indeed explained by a greater demand for work. This is predicted by a simple model of walking on pendulum-like legs, because proper push-off reduces collision losses from the leading leg. We tested this by experimentally restricting ankle push-off bilaterally in healthy adults (N=8) walking on a treadmill at 1.4 m s-1, using ankle-foot orthoses with steel cables limiting motion. These produced up to ∼50% reduction in ankle pushoff powerandwork, resulting in up to∼50%greater netmetabolic power expenditure towalk at the same speed. For each 1 J reduction in ankle work, we observed 0.6 J more dissipative collision work by the other leg, 1.3 J more positivework fromthe leg joints overall, and 3.94 J more metabolic energy expended. Loss of ankle push-off required more positivework elsewhere tomaintainwalking speed; this additional work was performed by the knee, apparently at reasonably high efficiency. Ankle push-off may contribute to walking economy by reducing dissipative collision losses and thus overall work demand.