Changes in leg and joint coordination during locomotor adaptation in amputees and able-bodied controls

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Selgrade, Brian Paul
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Activities of daily life require humans to locomote in unfamiliar environments. We respond to these new environments through adaptation, a gradual change in movement parameters in response to a sensory error caused by altered environmental conditions. I investigated changes in coordination at the joint and leg level as subjects adapted to split-belt treadmill walking and altered visual feedback in hopping. As subjects adapted to increase leg force, they preferentially reduced deviations in joint torque that affected leg force. Once peak leg force reached a steady level, subjects reduced all joint torque deviations, regardless of relevance to leg force, suggesting that when subjects achieved the task goal, they switched from a minimal intervention strategy to a total noise reduction strategy. As subjects adapt to split-belt walking, they reduce hip work and shift to doing more ankle work in the step-to-step transition. Because ankle work in the step-to-step transition is more efficient, this ankle timing strategy likely contributes to the reduction in metabolic power during split-belt walking. Both amputees and controls gradually adapted step length symmetry in split-belt walking, demonstrating an aftereffect when the split-belt condition was removed. This result is consistent with previous studies of intact subjects and indicates that interlimb coordination is changed using feedforward control. Subjects also adapt to split-belt walking by moving farther backward in single support on the fast belt and less backward on the slow belt. This center of mass displacement strategy persists in amputees and controls, when the split-belt condition is introduced gradually or suddenly, and no matter which belt the prosthetic foot is on. This work suggests that mechanical changes that improve efficiency underlie the reduction in metabolic power during split-belt walking adaptation.
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