Speed and Constraint Modulate the Mechanistic Coupling between Kinetic and Spatio-Temporal Gait Symmetry

Human walking adapts to task demands within constraints imposed by limb mechanics. Altering these constraints via injury, neurological impairment or external restriction reduces the set of feasible gait solutions, yet the impact of this on gait symmetries remains poorly understood. We analysed gait in non-impaired adults walking with and without a unilateral knee constraint across multiple speeds and cadences. Using linear mixed-effects models, we quantified the mechanistic coupling between spatiotemporal symmetries and propulsive impulse symmetry, speed, cadence, and trailing limb angle. During unconstrained walking, propulsion asymmetry showed weak associations with spatiotemporal symmetry and substantial inter-individual variability, consistent with high motor abundance where diverse strategies yield similar outcomes. In contrast, the constraint markedly reduced this solution space: step length symmetry became tightly coupled to propulsion asymmetry, which alone accounted for over 50% of the explained variance. Under constraint, propulsion acted as the primary mechanism for regulating step length, whereas step time symmetry shifted to a new baseline but remained tightly regulated across participants. These findings suggest that emulated hemiparetic gait reflects adaptive walking patterns emerging within a reduced gait solution space, supporting interventions targeting propulsive asymmetry as a promising strategy for rehabilitation.