Step-to-Step Ankle Inversion/Eversion Torque Modulation Can Reduce Effort Associated with Balance

Abstract

Below-knee amputation is associated with higher energy expenditure during walking, partially due to difficulty maintaining balance. We previously found that once-per-step push-off work control can reduce balance-related effort, both in simulation and in experiments with human participants. Simulations also suggested that changing ankle inversion/eversion torque on each step, in response to changes in body state, could assist with balance. In this study, we investigated the effects of ankle inversion/eversion torque modulation on balance-related effort among amputees (N = 5) using a multi-actuated ankle-foot prosthesis emulator. In stabilizing conditions, changes in ankle inversion/eversion torque were applied so as to counteract deviations in side-to-side center-of-mass acceleration at the moment of intact-limb toe off; higher acceleration toward the prosthetic limb resulted in a corrective ankle inversion torque during the ensuing stance phase. Destabilizing controllers had the opposite effect, and a zero gain controller made no changes to the nominal inversion/eversion torque. To separate the balance-related effects of step-to-step control from the potential effects of changes in average mechanics, average ankle inversion/eversion torque and prosthesis work were held constant across conditions. High-gain stabilizing control lowered metabolic cost by 13% compared to the zero gain controller (p = 0.05). We then investigated individual responses to subject-specific stabilizing controllers following an enforced exploration period. Four of five participants experienced reduced metabolic rate compared to the zero gain controller (−15, −14, −11, −6, and +4%) an average reduction of 9% (p = 0.05). Average prosthesis mechanics were unchanged across all conditions, suggesting that improvements in energy economy might have come from changes in step-to-step corrections related to balance. Step-to-step modulation of inversion/eversion torque could be used in new, active ankle-foot prostheses to reduce walking effort associated with maintaining balance.

Control of human gait stability through foot placement

“…Later studies showed that CoP shifts are also used after perturbations [ 13 , 19 ]. In a recent study, Kim & Collins [ 115 ] showed that in amputees, the effort associated with balance (e.g. energetic cost) could be reduced by appropriate control (inversion/eversion torque) of a robotic prosthesis, further highlighting the importance of stance leg control.…”

Shortcomings of human-in-the-loop optimization for an ankle-foot prosthesis: a case series

“…However, we tested five different control architectures, four vastly different from each other. Both the neuromuscular and the balance controller have been successfully used in the past to reduce the metabolic cost of walking with an active prostheses below that of walking with a passive prosthesis 18,20 . In addition, the time-based torque controller is similar to one that has been successfully used to reduce the metabolic cost of walking with exoskeletons 24 .…”

“…The second approach focuses on balance assistance in the form of ankle inversion or eversion torque in response to changes in center of mass velocity. Implementing such control in an ankle-foot prosthesis emulator led to reductions in energy expenditure during walking by 9%, when compared to walking with passive prostheses 20 . Although these results are promising, simulations demonstrate that active prostheses have more potential to mitigate problems faced by individuals with amputation, with the possibility of reducing the metabolic cost of walking to more than 70% less than unimpaired walking 21 .…”

“…1A); (2) a neuromuscular controller that emulated biological components of the muscle-tendon complex and has been previously used to reduce the metabolic cost of walking with an active prosthesis 19 (Fig. 2A); (3) a balance controller that provided ankle inversion/eversion torque based on deviation of the user's lateral center of mass velocity and has been previously used to reduce the metabolic cost of walking with a prosthesis emulator 20 (Fig. 3A); and (4) a time-based torque controller with similar control architecture to that used to reduce the metabolic cost of walking with exoskeletons ( Fig.…”

Dynamic Balancing Responses in Unilateral Transtibial Amputees Following Transversal Plane Perturbations During Slow Treadmill Walking Differ Considerably for Amputated and Nonamputated Side

“…Development of active prosthesis focused mainly on the aspects of improving propulsion on the impaired limb but did not explicitly consider dynamic balancing aspects, particularly not in the context of coping with unexpected perturbations (Windrich et al, 2016). Few studies investigated effects of stiffness properties of the prosthetic leg on balance in the sagittal (Major et al, 2016) and frontal plane (Kim and Collins, 2017). They showed that appropriate control of stiffness in the arti cial ankle joint may improve balance in terms of reduced variability and asymmetry in spatio-temporal parameters during unperturbed walking at self-selected speed.…”