Simulated Exoskeletons with Coupled Degrees-of-Freedom Reduce the Metabolic Cost of Walking

Exoskeletons that assist multiple joints could reduce the metabolic cost of walking beyond reductions achieved with single-joint devices. However, assisting many degrees-of-freedom can require multiple actuators with complicated controllers and long experiment times to tune these controllers. Coupled assistance, where the same control signal is applied to multiple joints, may help simplify exoskeleton design but has not been tested extensively. In this study, we used direct collocation optimal control and a lower-limb musculoskeletal model to simulate five different exoskeleton control strategies to assist multiple joints during walking. We simulated strategies where the torque provided at each joint was controlled independently or coupled between joints. We found that coupled assistance was able to provide similar savings in whole-body metabolic power consumption compared to independent assistance. Our results may help device designers create exoskeletons that achieve good metabolic savings while requiring fewer actuators and less experimental testing to create effective assistance strategies.