Interaction of support surface stability and Achilles tendon vibration during a postural adaptation task.

Orchestration of sensory-motor information and 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 a remarkable ability to adapt to both long and short term challenges. To explore the interaction between support surface stability and Achilles tendon vibration during a period of adaptation we used both a linear measure and a non-linear measure derived from center-of-pressure (COP) data. An equilibrium score (ES), based upon peak amplitude of anterior-posterior sway towards theoretical limits of stability was the linear measure used to assess postural performance. We observed early effects of vibration on postural stability, depending on support characteristics. Participants were able to decrease sway with extended practice over days, independent of support surface stability. Approximate entropy analysis of COP data provided additional information about control adaptation processes.