SUMMARY: During 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.

While running on uneven ground, humans are able to negotiate visible but also camouflaged changes in ground level. Previous studies have shown that the leg kinematics before touch down change with ground level. The present study experimentally investigates the contributions of visual perception (visual feedback), proprioceptive feedback, and feed-forward patterns to the muscle activity responsible for these adaptations. The activity of three bilateral lower limb muscles (M. gastrocnemius medialis, M. tibialis anterior and M. vastus medialis) of nine healthy subjects was recorded during running across visible (drop of 0, -5 and -10 cm) and camouflaged changes in ground level (drop of 0 and -10 cm). The results reveal that at touchdown with longer flight time M. tibialis anterior activation decreases and M. vastus medialis activation increases purely by feed-forward driven (flight time-dependent) muscle activation patterns, while M. gastrocnemius medialis activation increase is additionally influenced by visual feedback. Thus, feed-forward driven muscle activation patterns are sufficient to explain the experimentally observed adjustments of the leg at touchdown.