The effects of foot position and orientation on inter- and intra-foot coordination in standing postures: a frequency domain PCA analysis.

We investigated the effect of foot position and foot orientation on asymmetrical body weight loading and the inter- and intra-foot coordination dynamics of standing postures. The participants were instructed to stand with the feet side-by-side and in staggered and tandem positions with the right foot oriented at different angles (30, 60 and 90°). The results showed that the participants naturally loaded more on their left foot when positioned with their right foot forward. As the orientation of the right foot was changed from 90 to 30°, they loaded a significantly large proportion of their body weight on their left foot and the degree of coordination of the 4 COP time series increased. Foot position, constrains both the area of the base of support and the loading of the feet, played an important role differentiating the contribution of each COP to the postural control system. In particular, when postural stance was challenged by the limitation of the base of support, the COPs in the unstable plane (inter-foot coordination) showed larger factor weightings. In contrast, when standing posture was not challenged by the base of support boundary, the COPs of the more loaded foot (intra-foot coordination) dominated foot coordination in postural control. These findings show that the mechanical constraints of foot position and orientation interact to channel different patterns of inter- and intra-foot coordination dynamics of standing postures.