Biomechanical modifications of running when approaching and jumping over an obstacle

The present work focused on running when leaping over an obstacle, ranging from a little barrier to a high hurdle, in recreational runners (RR) and trained athletes (TA) and at different speeds. Four studies were performed from a global to a segmental approach. Experiments were realised on a 33 m long running track equipped with a 13 m long force plate. Runners were filmed by a high-speed camera. Results show that the velocity of progression increases from the fourth to the second step before the barrier, depending on the obstacle height, approaching velocity and level of expertise of the runner (RR vs TA). Moreover, we founded that the bouncing mechanism of running is modified during the two steps immediately preceding the barrier. During the contact period two steps before the barrier, a decrease of the torsional stiffness at the level of the knee and of the ankle induces a decrease of the leg-spring stiffness (kleg). Consequently the centre of mass of the runner (COM) is accelerated forward while remaining lower compared to steady-state running. During the last step before the barrier, the runner places the foot, from which he takes off, well in front of the body. During the contact, the increase of the torsional stiffness at the level of the knee and of the ankle induces an increase of the kleg. Consequently the COM is decelerated forward and elevated. During this phase, the leg-spring stores elastic energy and changes the direction of the velocity vector to release this energy in a vertical direction. The same strategy is used by TA and RR to cross an obstacle. However, TA optimise the mechanism of jumping by reducing the loss in the velocity of progression when crossing the obstacle and by limiting the vertical trajectory of the COM.