Inter-segmental coordination: motor pattern in humans stepping over an obstacle with mechanical ankle joint friction

This study examined the influence of a mechanical perturbation of the ankle joint on obstacle avoidance pattern. A decoupled control between the distal joint and the combined (hip-knee) proximal joints was observed according to the task requirement. In this context, a greater mechanical friction at the ankle should be compensated at this joint (local compensation) or alternatively, by regulating more combined proximal joints (knee and/or hip). The leading limb inter-segmental coordination was evaluated in both no constraint and constraint conditions in calculating ranges of motion (ROM), moments of force and powers (from heel-off to obstacle) at the ankle, knee and hip joints. Electromyographic activities were also analyzed. With the constraint, the dorsiflexor moment and the tibialis anterior activity remained unchanged while both ROM and power bursts (absorbed and generated) decreased. The hip and knee ROM remain invariant. At heel-off the absorption by hip extensors decreased and the forthcoming generation by knee flexors increased in the constraint condition. To quantify the inter-joint coordination, principal component analysis was used and indicated a high level of inter-joint coupling (synergy) that decreased with the constraint (i.e. less inter-joint coupling). At the ankle joint, the results suggest that the central command was the same in both conditions thus, not be adapted. At both the hip and knee joints, a combined joints modulation occurred to overcome additional friction.