Evidence of sensory error threshold in triggering locomotor adaptations in humans

Changing body biomechanics or external conditions trigger neural adaptations to optimize motor behavior. While the adaptations appear continuous to minimize movement errors, not all errors necessarily initiate sensorimotor adaptations. The locomotor system exhibits resilience to change and rehabilitation. This suggests the presence of an error threshold to trigger the adaptation mechanism. Here, we imposed kinematic and kinetic constraints of stepping using a passive orthosis and real-time feedback about limb loading. We aimed to manipulate stepping asymmetry and explore the interplay between adaptation and locomotor error magnitude. Uninjured healthy adults were tested in three locomotor conditions: unconstrained, constrained, and washout unconstrained walking. Surprisingly, kinematic asymmetry alone did not induce persistent adaptation. However, the addition of asymmetric interlimb loading triggered the expected adaptation. Our finding suggests that uninjured locomotor systems can cope with a specific range of kinematic asymmetries without initiating persistent adaptations that lead to aftereffects, and that loading may be a key variable for evoking the adaptation. The presence of an error threshold may mitigate possible disruption of vital motor functions and contribute to locomotor adaptation during walking. These insights elucidate the mechanism of neural plasticity and have implications for rehabilitation.