Postural sway was assessed [via center of pressure (COP) 95% elliptical area (EA), path length (PL), normalized path length (PL(n)) and sample entropy (SEn)] in four conditions of bipedal upright stance [compliant (Foam) vs. non-compliant (Hard) with eyes-open (EO) vs. eyes-closed (EC)] prior to, and immediately following, a six-week balance training intervention in a group of healthy adults (N = 26). The intervention was comprised of nine exercises progressed in difficulty based on the subjective assessments of individual competency. Results showed that EA and PL were increased, while PL(n) and SEn were decreased, in EC and Foam stance conditions (collapsed across effects of balance training). Interpretations were that restricted vision and a compliant surface represented constraints to postural control that caused increases in the amount (PL) and area (EA) of sway, but decreases in its coordinative twisting/turning (PL(n)) and temporal complexity (SEn). It was argued that these changes might represent compensatory adaptations in effort to maintain postural control given the demands of the imposed constraints. Balance training caused no change to EA, but did result in decreased PL, PL(n), and SEn for stance conditions performed on the Foam (either EO or EC). These changes were interpreted to reflect improved postural control, potentially through the learned adoption of a more deterministic postural control strategy that is uniquely defined by the constraints imposed on upright stance by the compliant surface.

Muscle fatigue has been shown to result in early onset of anticipatory postural adjustments (APAs) relative to those produced in a non-fatigued state. This adaptation is thought to reflect an attempt to preserve postural stability during a focal movement performed in a fatigued state. It remains unclear, however, whether this adaptation is of central (e.g., central nervous system motor command) or peripheral (e.g., muscle contractile properties), origin. One way to confirm that this adaptation is centrally driven is to identify fatigued-induced early APA onsets in non-fatigued muscles. In this study, APAs were obtained using a rapid bilateral reaching maneuver and recorded via surface electromyography before and after conditions of rest (n = 25) or fatigue (n = 25). Fatigue was generated using isokinetic exercise of the right leg. Results showed that fatigue-induced early APA onsets occurred in fatigued and non-fatigued muscles, confirming that fatigue-induced early APA onset is a centrally mediated adaptation.

No abstract

These results provide insight to the developing postural strategy necessary for maintaining upright stance within the dynamics created by interactions with an ultra-compliant surface. Participants showed sensitivity to surface properties, moving both more slowly and with a more regular movement pattern; suggesting that they were able to develop a more feed-forward approach to the maintenance of balance by exploiting task constraints.