Compliant surfaces are often used in rehabilitation of ankle injuries and recently in general exercise programs;​ possibly to prevent future injuries. Although studies have reported increased postural sway during stance on compliant surfaces, it is unknown how movement strategies are affected. The central purpose of this series of studies was to gain insight into the differences in movement and control mechanisms during stance on compliant surfaces. The objectives were to identify movement synergies using principal component analysis, determine the relative changes in joint angles, muscle activity and center of pressure displacement, and investigate feed-forward and feed-back control systems.

Center of pressure trajectories, three-dimensional kinematics and electromyogram (EMG) activity were recorded from ten active male subjects aged between 20 and 30 who performed ten 30s trials of unipedal stance on each of solid, foam and an air-filled disc.

The first study used principal component analysis on three-dimensional joint angles from the supporting limb (ankle, knee, hip) and trunk to identify movement synergies. Results revealed a consistent temporal coupling between joints in a single functional synergy on the foam surface, with the first principal component explaining 89% of variance in joint angles. Ankle inversion/eversion was dominant in the first principal component on all surfaces, but its relative importance was more variable on the solid surface. The involvement of hip ab/adduction was important on the compliant surfaces but not on the solid surface. More than one functional synergy was evident during stance on the solid surface, with the second and third principal components being dominated by ankle inversion/eversion, but also with varying degrees of hip abduction/adduction. The synergies required to maintain postural stability were less variable on the compliant surfaces than on the solid surface.

The second study used cross-correlation analysis to describe relative timing of changes in joint angles, muscle activity and center of pressure displacement. The number of significant cross-correlations was greater for the compliant surfaces compared to the solid surface, and for the medial-lateral direction compared to the anterior-posterior direction. The results suggested that the postural system may have been constrained most in the least stable conditions (surface and direction).

Muscle activity preceded center of pressure displacement in both medial-lateral and anterior-posterior directions. Peroneus activity led medial center of pressure shifts by a greater amount on the air-filled disc (169 ms) than on the other surfaces (140ms), potentially to accommodate the delay in torque generation due to surface compliance. A similar shift was not observed in the medial gastrocnemius, which led anterior COP by 200-240 ms on each surface. The difference between medial-lateral and anterior-posterior responses could have been due to differences in torque generation, which are facilitated by a longer lever arm and larger muscles in the anterior-posterior direction. Muscles appeared to act in a feed-forward mechanism, and in some cases their latency was affected by surface compliance.

The third study used stabilogram diffusion analysis on the center of pressure trajectory to determine how feed-forward and feedback mechanisms were affected by surface compliance. The time interval when center of pressure displacements switched from persistence to anti-persistence (open-loop to closed-loop) was unaffected by surface, but the corresponding displacement increased with surface instability. Thus, there was a greater tendency for center of pressure to drift farther away from equilibrium on compliant surfaces, but not drift for longer.

Aggregate results suggested that postural control was more constrained in more unstable, situations, and control were implemented when required, consistent with the principle of "minimal intervention" that has been used to describe several kinematic tasks.