Adaptive locomotion: Foot strike pattern and limb mechanical stiffness while running over an obstacle

Previous studies of level running suggest runners adjust foot strike to control leg stiffness. This study aimed to determine how runners adjusted mechanical stiffness and foot strike prior to, during, and after a drop in surface height. Ten healthy subjects (5 male, 5 female; 24.32 ± 5.0 years) were video recorded as they ran on an outdoor path with a single drop in surface height (12.5 cm). Foot strike was recorded, while subject velocity, duty factor (DF), normalized maximum ground reaction force (GRF_bw), vertical hip displacement (Δy), leg compression (ΔL), vertical (K_vert) and leg stiffness (K_leg), touchdown (TD) and takeoff angle (TO), and flight (T_f) and contact time (T_c) were calculated. Compared to the step before the drop, T_f, GRF_bw, K_vert, K_leg, and TO increased, while T_c, DF, Δy, ΔL, and TD decreased in the step after the drop. Across trials, runners had either consistent or variable foot strike patterns. Runners using a consistent pattern most often shifted from rear to fore-foot strike in the steps before and after the drop, while those with a variable pattern showed less dramatic shifts. All parameters, except TD, were significantly different (p < 0.04) based on foot strike pattern, and comparisons between steps before and after the drop (except TD) were significantly different (p < 0.004). Runners with a variable foot strike pattern experienced smaller shifts within mechanical parameters when traveling over the drop, suggesting these runners may be able to stabilize limb mechanics on interrupted surfaces.