Effect of whole body vibration on the postural control of the spine in sitting.

Stability is defined by the ability to return to the initial (or unperturbed) state following a perturbation and hence can be assessed by quantifying the post-perturbation response. This response may be divided into two phases: an initial passive response phase, dependent upon both the steady state of the system and the system's intrinsic mechanical properties; and a recovery phase, dependent upon active control and reflexes. These two phases overlap and interact with each other. Whole body vibration (WBV) is assumed to influence neuro-sensory functions and perhaps both response stages. The current study observed the effect of WBV on several novel response factors that quantify the two phases in response to an external perturbation. The results indicate a significant effect of vibration exposure on: (1) the normalized maximum distance traveled by center of pressure (COP) from the neutral seated posture, and (2) the normalized time to maximum distance (τ), such that B and τ increased after WBV exposure and decreased after sitting without WBV. These changes may be indicative of passive visco-elastic changes caused by WBV exposure on the spinal tissues which has been indicated as a creep deformation of tissues post-exposure. This change may make the spine vulnerable to injury. Similar trends were noticed in the variables calculated from center of mass data.