Complexity, Composition, and Control of Bipedal Balancing Movements as the Postural Control System Adapts to Unstable Support Surfaces or Altered Feet Positions.

The current project investigated the dynamics of postural movements and muscle activity during balancing with feet-together and feet-apart positions on different support surfaces (firm surface (FS), modified- and conventional balance boards). We hypothesized that movement complexity and muscle activation would increase with increased balance-task difficulty, and that differences in the composition and control of postural movements between bipedal wide- and narrow-based balancing would be observed in all surface conditions. We applied a principal component analysis (PCA) to decompose postural movement trajectories of 26 active-young adults into sets of movement components (principal movements; PMs). Three PCA-based variables were calculated for each PM: the cumulative relative variance as a measure of movement complexity; the relative explained variance as a measure of the composition of postural movements; and the PM-acceleration as a measure for the control of the movement components. The main results revealed that both movement complexity and muscle activity increased with increased balance-task difficulty, of which altering support surfaces yielded more and greater effects than changing feet positions. Only on the FS, different movement structures were observed between narrowed- and wide-based standing (p ≤ 0.016); whereas different control of PMs was observed on all surfaces (p < 0.05). Standing on the stable surface illustrated opposite control behaviors compared to balancing on both multiaxial-unstable surfaces. In summary, on stable surface, changing the feet position affected inter-segment coordination. On unstable surfaces, the postural control system appeared to maintain inter-segment coordination characteristics, while the adaptation was confined to the sensorimotor integration processes.