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

EMG-informed musculoskeletal simulations successfully replicated experimental whole-body metabolic cost trends for the majority of participants.

• Simulations replicated trends for 8 out of 10 participants (r > 0.7, p < 0.05)
• The study combined experimental data from ten healthy adults walking on a treadmill under various exosuit conditions with EMG-informed musculoskeletal simulations
• Various exosuit conditions included different stiffness levels and configurations of the BATEX device

Overall net metabolic cost reductions from the BATEX exosuit were primarily driven by significant savings during the swing phase of gait.

• Simulation results showed comparable overall net reductions primarily driven by significant savings during the swing phase
• The study analyzed cost fluctuations across the gait cycle to identify the timing of metabolic effects
• This finding was consistent across the simulated conditions

Combining the rectus femoris (RF) and hamstring (HAM) springs resulted in complex, non-additive interactions on metabolic cost.

• The BATEX features a spring parallel to the rectus femoris (RF) and a spring parallel to the hamstring (HAM)
• When RF and HAM springs were used together, their combined effect was not simply the sum of individual effects
• This non-additive interaction was identified through comparing single-spring versus combined-spring configurations

The two spring types (RF and HAM) operated through distinct muscle-level energy-saving mechanisms.

• The RF spring was associated with reduced muscle activation as its primary energy-saving mechanism
• The HAM spring was associated with reduced fiber velocity as its primary energy-saving mechanism
• These distinct mechanisms were identified through muscle-tendon energetic analysis in the musculoskeletal simulations

The exosuit produced both targeted effects on hip/knee muscles and non-targeted effects on ankle and hip abductor/adductor muscles.

• Targeted effects were identified on hip and knee flexors and extensors, which are the muscles the device was designed to assist
• Non-targeted effects were observed on ankle muscles and hip abductor/adductor muscles
• These non-targeted effects represent unintended biomechanical consequences of wearing the passive exosuit

Positive responders (PR) and negative responders (NR) to the exosuit showed markedly different muscle-level responses to the same device setup.

• Participants were categorized into positive responders (those experiencing reductions in metabolic cost) and negative responders (those experiencing increases in metabolic cost)
• Muscle-level responses differed markedly between PR and NR groups despite using the same device configuration
• The differing responses between groups motivated the authors to advocate for personalization of exosuit parameters

The BATEX is a passive biarticular thigh exosuit designed to assist hip and knee flexion/extension through springs parallel to the rectus femoris and hamstring muscles.

• The device features a spring parallel to the rectus femoris (RF) and a spring parallel to the hamstring (HAM)
• The exosuit assists hip and knee flexion/extension simultaneously due to its biarticular nature
• The device was tested under various stiffness and configuration conditions in ten healthy adults walking on a treadmill