Changes in spine length during and after seated whole-body vibration.

The authors examined the relation between exposure to seated whole-body vibration (WBV) and an increase in the loss of height of the spine over and above normal diurnal changes. A device for measuring sitting height is described (SD, 1.4 mm for repeated measures). The mean change in body height (diurnal reduction) during two normal days in five men aged 23 to 25 years was 10.6 mm, (SD, 3.2 mm). On the third day, the change in sitting height was measured before and after vertical vibration (5 Hz with a peak-to-peak amplitude of 3 mm, and peak acceleration less than 2 m/s2) and again at the end of the day. The mean reduction in sitting height over the half hour of vibration exposure was 9.0 mm versus less than 1 mm for the control condition. The mean height loss over the third day (the day of 30 minutes of vibration exposure) was only 3.6 mm (compared with 10.6 mm lost over a control day with no vibration exposure). Hence, exposure to vibration increased the creep response in all subjects during exposure but, at the end of the day, there was a recovery in height, such that subjects were taller at the end of the day of vibration exposure. It is hypothesized that this "rebound" effect is due to an inflammatory response in the spine.