Objective. Gait asymmetry is a common adaptation observed in lower-extremity amputees, but the underlying mechanisms\nthat explain this gait behavior remain unclear for amputees that use above-knee prostheses. Our objective was to develop\na working hypothesis to explain chronic stepping asymmetry in otherwise healthy amputees that use above-knee\nprostheses. Methods. Two amputees (both through-knee; one with microprocessor knee, one with hydraulic knee) and\nfourteen control subjects participated. 3D kinematics and kinetics were acquired at normal, fast, and slow walking speeds.\nData were analyzed for the push-off and collision limbs during a double support phase. We examined gait parameters to\nidentify the stepping asymmetry then examined the external work rate (centre of mass) and internal (joint) power profiles\nto formulate a working hypothesis to mechanistically explain the observed stepping asymmetry. Results. Stepping\nasymmetry at all three gait speeds in amputees was characterized by increased stance phase duration of the intact limb\nversus relatively normal stance phase duration for the prosthesis limb. The prosthesis limb contributed very little to\npositive and negative work during the double support phase of gait. To compensate, the intact leg at heel strike first\nprovided aid to the deficient prosthetic ankle/foot during its push-off by doing positive work with the intact knee, which\ncaused a delayed stance phase pattern. The resulting delay in toe-off of the intact limb then facilitated the energy transfer\nfrom the more robust intact push-off limb to the weaker colliding prosthesis side. This strategy was observed for both\namputees. Conclusions. There is a sound scientific rationale for a mechanistic hypothesis that stepping asymmetry in\namputee participants is a result of a motor adaptation that is both facilitating the lower-leg trajectory enforced by the\nprosthesis while compensating for the lack of work done by the prosthesis, the cost of which is increased energy\nexpenditure of the intact knee and both hips.
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