Sensory Integration for Postural Control during Stance on Compliant Surfaces
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Background. Both Sides Up (BOSU) balls are often used in sports rehabilitation as a form of `somatosensory training'. Memory foam is often used in neurologic rehabilitation as a method of disrupting the somatosensory systems' contributions to balance in order to enhance dependence on vision and vestibular input. Objectives. The purposes of this study were to quantify how much young adults depended on vision (via response to moving dots projected on a screen) and on somatosensory cues (via their response to bilateral brief Achilles tendon vibration) when standing on different surfaces. Methods. The responses of 30 healthy young adults (HEALTHY) were compared to the responses of 10 young adults with history of repeated ankle sprains (SPRAIN) when participants were standing on a firm surface, memory foam and a BOSU ball. The primary measures of visual dependence were: the ratio of body-sway amplitude at the visual frequency over all other sway peaks (Visual Amplitude Fraction [VAF]), and the estimated frequency of the peak of body-sway (EFP) amplitude. To quantify response to vibration we used traditional (Center of Pressure excursion) and nonlinear measures (Complexity Index). Results. In both groups, VAFs were significantly higher on the BOSU compared to floor or foam (P<0.001). The difference for EFP was significant (P<0.001) with clear matching of the stimulation frequency only on the BOSU. The excursion response to vibration was significantly different between the surfaces (P<0.001).We observed an increase in excursion when vibration was introduced on the floor, a smaller increase on the foam and a decrease on the BOSU. An increase in Complexity Index with vibration differed significantly between surfaces (P<0.001) and was larger on the foam and BOSU compared to floor. SPRAIN showed similar trends with overall decreased response to vibration compared to HEALTHY. Limitations. Our SPRAIN sample was small and heterogeneous. Training effects were not measured. Conclusions. Young adults with and without a history of ankle sprains strongly responded to the visual stimuli and appeared to use cues from vibration to stabilize themselves when standing on BOSU. The findings suggest that visual dependence increases and somatosensory integration decreases in stance on challenging compliant surfaces.