Effect of force magnitude of touch on the components of postural sway.

Background: Lightly touching the tip of the index finger on an external surface reduces the postural sway during upright standing due to the additional somatosensory information provided by the touch to the postural control system. But when the individuals apply more force, it provides more mechanical support. However, because most of the studies investigated only two levels of force, whether the control mechanisms of postural sway are affected by the different force levels was unknown.

Research Question: To examine the influence of the magnitude of force (up to 1, 2, 4, 6 or 8 N) applied to the touch bar on the mechanisms used to control the postural sway during quiet standing with eyes open or closed.

Methods: Ten young right-handed adults stood for 35 s on a force platform, with feet apart, while touched a rigid bar with different force levels with eyes open or closed. The amplitude and velocity of the center of pressure and its components, Rambling and Trembling trajectories, respectively, related to more supraspinal and spinal control mechanisms, were assessed.

Results: The touch reduced all trajectories, mainly of the Rambling component and with closed eyes. There was a floor effect of the touch force as amplitudes and velocities were minimal at 4 N of force.

Significance: The component of postural sway under the supraspinal neural control is more affected by different force magnitudes applied to the touch bar.