Simulating Muscle-Level Energetic Cost When Humans Walk With a Passive Biarticular Thigh Exosuit

Passive exosuits offer potential for reducing the metabolic cost of walking, but evaluating their effectiveness is challenging due to difficulties in measuring internal biomechanics and muscle-level energetics. This study investigates the metabolic effects of the BATEX, a passive biarticular thigh exosuit assisting hip and knee flexion/extension. The exosuit features a spring parallel to the rectus femoris (RF) and a spring parallel to the hamstring (HAM). We combined experimental data from ten healthy adults walking on a treadmill under various exosuit conditions (different stiffness and configurations) with EMG-informed musculoskeletal simulations to estimate muscle-level behavior. The simulations aimed to replicate experimental metabolic measures, analyze cost fluctuations across the gait cycle, and elucidate muscle-tendon energetic responses, comparing individuals experiencing reductions (positive responders, PR) versus increases (negative responders, NR) in metabolic cost. Simulation results successfully replicated experimental whole-body metabolic cost trends for most participants (8/10, ${r} \gt {0}.{7}$ , ${p}\lt {0}.{05}$ ), showing comparable overall net reductions primarily driven by significant savings during the swing phase. Combining RF and HAM springs resulted in complex, non-additive interactions. Muscle-level analysis identified both targeted effects on hip/knee flexors/extensors and non-targeted effects on ankle and hip ab/adductor muscles, with responses differing markedly between PR and NR groups. Distinct energy-saving mechanisms were associated with each spring type (RF: reduced activation; HAM: reduced fiber velocity). The differing responses of the PR and NR groups to the same device setup motivate personalization, and our muscle-level insights offer a foundation for future device refinement.