The Impact of ankle range of motion on lower-limb gait biomechanics

Movement at the ankle joint serves several purposes during unimpaired locomotion, including stabilization and propulsion. While the healthy ankle can achieve 71°range of plantar/dorsiflexion, only about 30°is used in level-ground walking. Little is known about how gait is affected by isolated limitation of ankle range of motion (RoM) using end-range hard stops. To study this, we developed a simple exoskeleton that restricts ankle range of motion and evaluated the impact of this exoskeleton during level-ground treadmill walking at two speeds, in ten young adults with no ankle pathology. We found that even with 30°range of motion (±15°), significant hard-stop contact occurred compared to when there was no restricted motion, and that individuals did not adjust their gait to avoid contact in this condition. Hard-stop contact time was greater for conditions with less permitted motion, but we did not find significant differences in global kinematic asymmetry at any joint except the ankle, and only in comparisons of other conditions against simulated fusion ($p<0.02$ for all comparisons); providing even ±10°at the ankle did not lead us to observe significant differences in kinematic asymmetry. We also observed significant changes to ankle positive work at RoMs lower than ±15°($p<0.03$ for all comparisons), but we did not observe significant changes in net work over the gait cycle at any other joint of the lower limb in any conditions except simulated fusion. These results suggest that hard-stop contact does not disturb gait enough for users to adapt to gait strategies that avoided contact, and that ankle range of motion can be restricted in level-ground walking without causing significant differences in gait kinematics and joint work.