Invariant ankle moment patterns when walking with and without a robotic ankle exoskeleton

Area of Science:

• Biomechanics
• Robotics
• Human Motor Control

Background:

• Understanding human adaptation to powered exoskeletons is crucial for developing effective lower limb robotic devices.
• Robotic exoskeletons offer assistance but require users to adapt their motor control strategies.

Purpose of the Study:

• To quantify joint moments during adaptation to a powered ankle exoskeleton in healthy individuals.
• To investigate whether the duration of motor adaptation depends on the level of robotic assistance provided.

Main Methods:

• Eleven healthy subjects walked on a split-belt treadmill with a pneumatically powered ankle exoskeleton controlled by soleus electromyography (EMG).
• Subjects underwent two 30-minute walking sessions to adapt to the exoskeleton's plantar flexor torque assistance.

Main Results:

• Subjects significantly reduced soleus EMG by ~36% and achieved ankle joint moment patterns similar to unassisted gait.
• Ankle kinematic patterns during exoskeleton use differed significantly from unassisted gait.
• Adaptation to higher levels of robotic assistance took longer, with some subjects not reaching a steady-state gait pattern within the study period.

Conclusions:

• Humans prioritize replicating joint moment patterns over kinematic patterns when adapting to powered ankle exoskeletons.
• The magnitude of robotic assistance directly influences the time course of motor adaptation during exoskeleton use.