Locomotor patterns change over time during walking on an uneven surface

During walking, uneven surfaces impose new demands for controlling balance and forward progression at each step. It is unknown to what extent walking may be refined given an amount of stride-to-stride unpredictability at the distal level. Here, we explored the effects of an uneven terrain surface on whole-body locomotor dynamics immediately following exposure and after a familiarization period. Eleven young, unimpaired adults walked for 12 min on flat and uneven terrain treadmills. The whole-body center of mass excursion range (COM_exc) and peak velocity (COM_vel), step length and width were estimated. On first exposure to uneven terrain, we saw significant increases in medial-lateral COM_exc and lateral COM_vel, and in the variability of COM_exc, COM_vel and foot placement in both anterior-posterior and medial-lateral directions. Increases in step width and decreases in step length supported the immediate adoption of a cautious, restrictive solution on uneven terrain. After familiarization, step length increased and the variability of anterior-posterior COM_vel and step length reduced, while step width and lateral COM_vel reduced, alluding to a refinement of movement and a reduction of conservative strategies over time. However, the variability of medial-lateral COM_exc and lateral COM_vel increased, consistent with the release of previously constrained degrees of freedom. Despite this increase in variability, a strong relationship between step width and medial-lateral center of mass movement was maintained. Our results indicate that movement strategies of unimpaired adults when walking on uneven terrain can evolve over time with longer exposure to the surface.