More isn't always better: Higher exoskeleton torque does not consistently improve balance performance

Wearable exoskeletons are a promising tool for augmenting balance and reducing fall risk. Recent work suggests that active ankle exoskeletons need to act faster than the human to improve reactive balance control. However, the magnitude of exoskeleton torque that is best for improving reactive balance remains unknown. Drawing from the optimal torque for minimizing metabolic expenditure, we hypothesized that reactive balance would improve with increased exoskeleton torque. Participants wearing bilateral ankle exoskeletons were instructed to maintain standing balance during 15 cm backward support-surface perturbations. Three exoskeleton plantarflexion torque conditions were tested: NO (Off), LOW (15Nm), or HIGH (30Nm). LOW torque improved balance performance compared to NO torque (p < 0.001), with a 7 ± 3 % decrease in peak center of mass (CoM) displacement. Although HIGH torque caused a 9 ± 11 % decrease in peak CoM displacement compared to NO torque (p = 0.12), it was not significant due to high intersubject variability. Whereas LOW torque decreased peak CoM displacement in all (range: -0.2 to -1.6 cm), HIGH torque only decreased it in some (range:1.2 to -2.6 cm). Balance ability, quantified by the narrowing beam test, accounted for 37 % of the variance (R² = 0.37, Adjusted R² = 0.29) in the change in CoM displacement from LOW to HIGH torque (p = 0.06), while this relationship didn't meet conventional statistical significance, likely due to the small sample size, it suggests that higher levels of exoskeleton torque may hinder balance performance in individuals with better balance ability. Taken together, more exoskeleton torque is not always better for balance, highlighting a potential need to personalize exoskeleton torque for balance augmentation.