More isn't always better: Too much exoskeleton torque can disrupt balance

Area of Science:

• Biomechanics and Robotics
• Human Augmentation
• Rehabilitation Engineering

Background:

• Wearable exoskeletons show potential for enhancing balance and mitigating fall risks.
• Active ankle exoskeletons require faster-than-human actuation for optimal reactive balance control.
• The ideal exoskeleton torque magnitude for improving reactive balance remains undetermined.

Purpose of the Study:

• To investigate the effect of different exoskeleton plantarflexion torque levels on reactive balance control during support-surface perturbations.
• To determine if increasing exoskeleton torque enhances balance performance and reduces center of mass (CoM) displacement.
• To explore the relationship between exoskeleton torque, balance ability, and CoM displacement.

Main Methods:

• Participants with bilateral ankle exoskeletons performed standing balance tasks during backward support-surface perturbations.
• Three torque conditions were tested: no torque (Off), LOW (15Nm), and HIGH (30Nm) plantarflexion.
• Peak CoM displacement was measured, and balance ability was assessed using the narrowing beam test.

Main Results:

• LOW exoskeleton torque significantly reduced peak CoM displacement (7±3%) compared to no torque (p <0.001).
• HIGH torque showed a non-significant reduction in peak CoM displacement (9±11%) compared to no torque (p =0.12) due to high inter-subject variability.
• The effect of torque on CoM displacement varied, with higher torque potentially hindering balance in individuals with better baseline balance ability.

Conclusions:

• Exoskeleton torque magnitude is critical for balance augmentation; more torque does not universally improve reactive balance.
• Personalized exoskeleton torque settings may be necessary to optimize balance performance and avoid hindering individuals with superior balance.
• Future research should focus on individualized torque optimization for wearable balance assistance devices.