The effects of ankle exoskeleton assistance on propulsive capacity in younger adult walking

Propulsive capacity - the maximum ability to generate forward propulsion during push-off in walking - is an important determinant of walking performance. Accordingly, an insufficient ankle push-off can limit physical activity and community accessibility. Wearable ankle exoskeletons (EXOs) have emerged as a strategy to augment ankle push-off, but most prior work has focused on metabolic cost or habitual walking mechanics, providing limited insight into how EXO assistance influences propulsive capacity in walking. Here, we used a treadmill-based propulsive stress test to determine how powered ankle EXO assistance affects propulsive capacity and its biomechanical determinants in healthy younger adults. Fifteen participants completed walking trials with four EXO assistance levels (none, low, medium, high) while a horizontal impeding force progressively increased until task failure. Propulsive capacity was defined as the maximum achievable impeding force. Group-level analyses revealed no main effect of assistance level on propulsive capacity or most biomechanical outcomes. However, exploratory within-subject analyses indicated that 12/15 participants improved propulsive stress test performance with assistance. Comparisons between unassisted walking and participants' best-performing assistance condition revealed significant increases in propulsive capacity and anterior ground reaction force (GRF) impulse without increases in peak biological ankle moment (A_Mom), trailing limb angle (TLA), or calf excitation. Improvements in propulsive capacity were associated with larger anterior GRF, which were jointly explained by those in A_Mom and TLA. These results highlight the importance of a capacity-based, within-subject framework to evaluate how ankle assistance influences maximal propulsive performance and provide a mechanistic foundation for future studies in populations with insufficient propulsion.