Simulations predict that alterations in balance control contribute to the higher cost of walking with a prosthesis

Abstract

The high metabolic cost of walking in individuals with a transtibial amputation is often attributed to reduced ankle propulsion. However, the cost of stabilizing walking may also increase after amputation, potentially explaining why restoring ankle propulsion does not always normalize metabolic cost. Because the contributions of propulsion and stabilization are difficult to dissociate experimentally, we used simulations to probe the effects of transtibial amputation and different prostheses on walking cost. We simulated gait using a conceptual planar torque-driven model. Leg joint torques were controlled by feedforward and linear, time-varying full state feedback. We computed feedforward torques associated with propulsion and feedback gains for stabilizing walking that minimized expected effort (sum of torques squared) in the presence of sensorimotor noise while imposing stability. We simulated walking for an intact model and three walkers with a prosthesis: a passive, a feedforward-controlled active, and a feedforward and local feedback-controlled active prosthesis. We solved a deterministic approximation of the resultant stochastic optimal control problems for different levels of sensorimotor noise. Feedforward and expected feedback torques were higher for the passive prosthesis walker than for the intact walker. An active prosthesis restored total biological effort (i.e., effort of all but the prosthesis joint) to the level of the intact walker, but this effort was generated by fewer biological joints. Biological feedback effort was higher in prosthesis walkers than in the intact walker. Adding local feedback to an active prosthesis reduced biological feedback effort, but only at low noise levels. Our results suggest that walking with passive prosthesis increases effort related to both propulsion and stabilization. While active prostheses can support propulsion, local feedback control only marginally reduced stabilization-related effort, suggesting that controllers require sensory information from the rest of the body to fully restore the metabolic cost of walking. Author summary Amputation of the lower leg substantially increases the energy cost of walking, reducing mobility. Loss of ankle propulsion is often considered the main cause, motivating the development of active prostheses that generate propulsion. However, restoring propulsion does not consistently reduce the energy cost of walking, suggesting other contributing factors. Stabilizing walking also requires energy, and recent studies suggest that the required energy may be increased after amputation. Because an amputation simultaneously affects propulsion and balance, experimentally disentangling contributions of reduced ankle propulsion and altered balance control to increased energetic cost is hard. We therefore used simulations to study how sensorimotor deficits due to amputation affect energy costs of propulsion and balance control. Our simulations suggest that the high energy cost of walking with a passive prosthesis results from increased effort for both propulsion and balance. While an active prosthesis can restore effort required for propulsion, it does not restore effort required for balance control, even when supporting balance with local feedback of prosthetic ankle kinematics. These findings suggest that the limited ability of existing active prostheses to reduce walking energy cost may - at least partially - be due to the inability to reduce the energy cost for balance control.