A balancing act: Passive ankle exoskeletons alter neuromuscular function and postural stability during balance

Spring-loaded passive ankle exoskeletons have the potential to modulate neuromuscular function during standing balance by altering the mechanical demands placed on the muscle-tendon units spanning the ankle joint. This study investigated the effect of exoskeleton assistance on postural stability and neuromuscular function across balance conditions of varying difficulty. We hypothesised that assistance would improve postural stability and reduce ankle plantar flexor muscle activity during balance tasks, although these benefits may vary with balance task complexity. Participants performed standing balance tasks of varying difficulty with and without rotational stiffness provided by spring-loaded ankle exoskeletons. Balance tasks included (i) quiet standing with eyes open (EO), (ii) eyes closed (EC), and (iii) reduced base of support (RBOS). We recorded centre of pressure (COP)-based postural sway measures, lower limb joint kinematics, muscle activation data from the medial (MG) and lateral gastrocnemii (LG), soleus (SOL), and tibialis anterior (TA), and MG fascicle behaviour. Exoskeleton assistance reduced LG and SOL activation but also altered joint kinematics and coordination. Ankle-based adjustments were most evident with assistance during simpler balance tasks (EO and EC), whereas assistance during the most challenging task (RBOS) was associated with increases in COP sway velocity, COP variability and TA activation. Our results suggest that despite the potential energetic benefits from offloading the plantar flexors, passive ankle exoskeletons may induce unintended effects that hinder the control of standing balance, particularly during more challenging conditions. Further work is needed to determine the mechanisms underlying these effects.