Interaction of Support Surface Stability and Achilles Tendon Vibration During a Postural Adaptation Task

Adaptation to internal or external, acute or chronic changes is one of the fundamental features of human postural control. The postural control system is challenged on a daily basis and displays the ability to adapt to both short- and long-term challenges. Little is known about the role of external factors like support-surface conditions on the characteristics and time course of adaptation. In order to investigate adaptation over the course of several trials and days, 18 healthy college students performed a postural task on either a fixed or an unstable (sway-referenced) support-surface. After an initial baseline test, participants performed 27 trials of the postural task over the course of three days including sensory perturbation. During these test sessions, bi-pedal Achilles tendon vibration was applied to perturb proprioceptive input emerging from the lower legs. For subsequent data analysis, we derived a linear and a non-linear measure from center-of-pressure (COP) data of all trials. An Equilibrium Score (EQ), based upon peak amplitude of anterior-posterior sway towards theoretical limits of stability was used to assess postural performance. Approximate entropy (ApEn), a measure of signal complexity, was computed from anterior-posterior COP data to investigate potential, subtle motor control changes. We observed immediate effects of vibration on postural stability, independent of support characteristics. Participants were able to decrease postural sway with extended practice over days, but changes differed according to support-surface conditions. ApEn data indicated specific immediate and longer-term adaptation processes, potentially due to sensory reweighting and postural strategy changes associated with each support-surface condition.

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Our data suggests that sway amplitude in a challenging postural task can be reduced with practice, but the underlying motor control processes differ, depending on support-surface characteristics. ApEn analysis could be valuable for motor adaptation studies, due to detection of control changes not revealed by EQ alone. Future research should investigate underlying neurophysiological mechanisms of postural strategy changes and adaptation, and their reflection in ApEn.