Direction-Dependent Adaptation of Dynamic Gait Stability Following Waist-Pull Perturbations.

Balance recovery during an unexpected disturbance is a complex motor task, where part of the variability depends on the type of the perturbation itself. Despite of this, little is known to what extent adaptation mechanisms to repeated perturbations are dependent on the direction and the amplitude of the applied disturbances. Here, we used a modified version of the Active Tethered Pelvic Assist Device (A-TPAD) to apply unexpected force-controlled multidirectional waist-pull perturbations while subjects were walking. Healthy young subjects were divided into two groups and were exposed to a single training session. Each group received perturbations of different amplitudes along the Medio-Lateral (ML) or the Antero-Posterior (AP) direction. Dynamic stability was determined in both the AP and ML directions in terms of base of support (BoS) and margin of stability (MoS). Results showed: 1) an adaptation of the balance recovery reactions only for perturbations delivered along the AP directions; 2) aftereffects able to modify the control of stability during the post-training session of which type and extent depends on the direction of the perturbations previously applied; and 3) a directional and amplitude effect on the dynamic stability at the end of the balance recovery reactions.