ABSTRACT
As for any other activity, the maintenance of human bi-pedal posture implies that the numerous degrees of freedom involved must be coordinated. According to the neuromuscular approach, advocated by Nashner and collaborators (e.g., Nashner & McCollum, 1985), the maintenance of stance relies on two preferred postural modes of coordination: In the ankle mode, the position of the center of mass (Cm) is adjusted through rotation of the body about the ankles without motion of the hips; In the hip mode, the Cm is maintained above the feet by rotating the trunk relative to the legs (i.e., by bending at the hips). In this approach, segmental coordination is thought to be a consequence of more basic patterns of coordination among muscles, which are, in turn, dependent upon neural computations. As an alternative, we propose a topological analysis of multi-segment postural coordination (Bardy et al. , 1997; Marin, this volume). We suggest that stance is maintained through collective functional units of action that emerge in a self-organizing fashion from the interaction between task-based, body-based, and environment-based constraints (e.g., Riccio & Stoff regen, 1988). In two experiments, we manipulated body-based (the height of the center of mass, He; and the effective length of the feet, FLe) and task-based (the required amplitude of head movement, Ah) constraints and measured the relative phase between ankle and hip joints (~h-a) during a supra-postural head tracking task. Under the assumption that ~h-a is a collective variable suitable for describing hip-ankle coordination modes, we predicted that the coordination modes to be observed would reveal the presence of only a few attractive values of ~h-a (notably 00 inphase and 1800 anti-phase) and that the interaction between He, Fle, and Ah would determine the coordination mode adopted in each particular condition.
