Individuals with a vestibular‐related disorder use a somatosensory‐dominant strategy for postural orientation after inclined stance

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Postural control incorporates postural orientation and equilibrium.1 The visual, somatosensory, and vestibular systems are critical for establishing a sensorimotor set for postural control. Set refers to the state of readiness of the sensory systems to respond or adapt to changes in task demands.2 To achieve this, set must continually be adjusted by reconfiguring the integration of the senses so that postural orientation and equilibrium are appropriately matched to the task at hand, be it standing, walking, reaching, or other activities of daily living. Failure to do so results in ineffective postural control which may lead to incoordination or loss of balance.
Recent research has sought to determine how these sensory systems combine in different proportions to impact postural control. Kluzik et al.3 sought to determine the effects of standing on an inclined surface on postural control with eyes closed. Upon returning to a horizontal surface, a continuum of responses across subjects ranged from a large body lean aftereffect in the direction of the incline to an upright alignment. A significant finding was that the subjects were consistent in their response over months. The authors concluded that individuals have a consistent and coherent method to integrate the somatosensory and vestibular systems to adapt their posture after standing on the inclined stance. They hypothesized that the range of responses may be due to how each subject proportioned the contributions of the vestibular and somatosensory systems. Those who had larger amplitudes of body lean were hypothesized to integrate the somatosensory system to a greater degree. It was as if they were attempting to maintain the sensorimotor set by reproducing the ankle angle which they experienced while standing on the incline. On the other hand, those who remained standing upright were thought to rely more on vestibular inputs, as the vestibular system was not manipulated.
A follow‐up study found that standing on a steeper incline induced a correspondingly greater lean aftereffect. In addition, when the lower extremities were constrained to remain vertical in the post‐incline period, the upper body still leaned. These results support the hypothesis of an adaptive sensorimotor set for postural orientation which reflects the relative integration of vestibular and somatosensory inputs.4
An additional study showed that subjects immediately adopted an upright stance when they were allowed to open their eyes for 20 seconds during the post‐incline period. When the subject closed their eyes again, they immediately resumed the leaning, suggesting that visual inputs did not significantly reorient the sensorimotor set from the somatosensory manipulation.6
The vestibular system integrates information from head acceleration and infers the force of gravity and forces generated from and by the body to supply the CNS with critical information regarding spatial orientation. Vestibular disorders can affect an individual's sense of movement, visual stability, and ability to maintain balance.7 This may result in a form of sensorimotor compensation in which the somatosensory system exerts a greater amount of influence on postural control.10 Here, we tested the hypothesis that individuals with a vestibular‐related disorder will produce a somatosensory‐dominant response following prolonged stance on an inclined surface, that is, they will display the lean aftereffect as opposed to standing upright. Such novel findings may enlighten postural rehabilitation protocols and specify a more targeted approach to treat vestibular‐related disorders more effectively.
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