Leg adjustments during running across visible and camouflaged incidental changes in ground level

Abstract

SUMMARYDuring running in a natural environment, humans must routinely negotiate varied and unpredictable changes in ground level. To prevent a fall, changes in ground level, especially those that are invisible, require a quick response of the movement system within a short time. For 11 subjects we investigated two consecutive contacts during running across visible (drop of 0, 5 and 10cm) and camouflaged (drop of 0 and 10cm) changes in ground level. For both situations, we found significant variances in their leg parameters and ground reaction forces (GRFs) during the perturbed second contact but also one step ahead, in the unperturbed first contact. At visible first contact, humans linearly adapt their GRF to lower and smooth their centre of mass. During the camouflaged situation, the GRF also decreased, but it seems that the runners anticipate a drop of approximately 5-10cm. The GRF increased with drop height during the visible perturbed second contact. At the camouflaged second contact, GRFs differed noticeably from the observed reaction when crossing a similar visible drop, whereas the contact time decreased and the initial impact peak increased. This increased impact can be interpreted as a purely mechanical contribution to cope with the event. Furthermore, we observed an increased angle of attack and leg length with drop height for both situations. This is in accordance with results observed in birds running over a track with an unexpected drop, and suggests that adaptations in swing leg retraction form part of the strategy for running across uneven ground.

Preparing the leg for ground contact in running: the contribution of feed-forward and visual feedback

“…Previous studies have shown that during running across visible and camouflaged drops in ground level, humans extend their ankle joint before touchdown (Müller et al, 2012a). With this study, we wanted to investigate the neuro-mechanical strategy responsible for the increased plantar flexion angles at touchdown.…”

“…Thus, changes in flight time will be small. In contrast, during running across camouflaged drops of −10 cm, it seems that runners anticipate a drop of about 5-10 cm (Müller et al, 2012a). The absence of the expected contact (CD10) results in increased flight time.…”

“…In preparation for such perturbations, humans flex their ankle joint angle (dorsiflexion) at touchdown for visible elevations in ground level (Grimmer et al, 2008; and extend their ankle joint angle (plantar flexion) for visible drops in ground level (Müller et al, 2012a). Interestingly, they also extend their ankle joint if the drop is invisible because of camouflage (Müller et al, 2012a).…”

“…Interestingly, they also extend their ankle joint if the drop is invisible because of camouflage (Müller et al, 2012a). In this study, we investigated the muscle activity and neuronal control strategy responsible for these findings with perturbation experiments.…”

“…As the plantar flexion before touchdown occurs in running across visual and camouflaged drops in ground level (Müller et al, 2012a), we hypothesize that a feed-forward strategy dominates the muscle activity of the lower leg muscles. Following the results of the previous studies Müller et al, 2012b), we expect that gastrocnemius pre-activation therefore increases while tibialis pre-activation remains unaltered for drops in ground level.…”