Runners adjust leg stiffness for their first step on a new running surface

Human runners adjust the stiffness of their stance leg to accommodate surfa ce stiffness during steady state running. This adjustment allows runners to maintain similar center of mass movement (e.g., ground contact time and st ride frequency) regardless of surface stiffness. When runners encounter abr upt transitions in the running surface, they must either make a rapid adjus tment or allow the change in the surface stiffness to disrupt their running mechanics. Our goal was to determine how quickly runners adjust leg stiffn ess when they encounter an abrupt but expected change in surface stiffness that they have encountered previously. Six human subjects ran at 3 m s(-1) on a rubber track with two types of rubber surfaces: a compliant "soft" sur face (k(surf) = 21.3 kN m(-1)) and a non-compliant "hard" surface (k(surf) = 533 kN m(-1)). We found that runners completely adjusted leg stiffness fo r their first step on the new surface after the transition. For example, ru nners decreased leg stiffness by 29% between the last step on the soft surf ace and the first step on the hard surface (from 10.7 kN m(-1) to 7.6 kN m( -1), respectively). As a result, the Vertical displacement of the center of mass during stance( similar to 7 cm) did not change at the transition desp ite a reduction in surface compression from 6 cm to less than 0.25 cm. By r apidly adjusting leg stiffness, each runner made a smooth transition betwee n surfaces so that the path of the center of mass was unaffected by the cha nge in surface stiffness. (C) 1999 Elsevier Science Ltd. All rights reserve d.