The response of the seated human to sinusoidal vibration and impact.

Low back pain has been shown to occur more frequently among vehicle drivers than in representative control groups. Thus the response of the human to vibration and impact is of interest. This study investigated the response of the spine to both impact and sinusoidal excitation in either a relaxed or erect seated posture. The sinusoidal testing apparatus used was a resonating system consisting of two parallel wooden beams, simply supported, and the impact testing apparatus a bearing-guided, spring-suspended platform, struck from below. Ten subjects (5 males, 5 females) were evaluated using both methods. Transfer functions were compared at 2-4 Hz, 4-8 Hz and 8-16 Hz intervals using a sign test. Although in 24 comparisons of either test method (vibration or impact) or posture (erect or relaxed) where eleven showed differences significant at the p less than .05 level, only 2 out of 24 comparisons were the differences distinct enough to be significant (at the p less than .01 level). Both of these latter differences were due to test method while the subjects were sitting erect. In those instances where there were no significant differences due to test method, the impact method may be a viable replacement for the vibration test method. Where the levels of significance are higher (p less than .01 or p less than .05), further study of the magnitude of the differences is indicated and may reveal further insight into the seated individual as a system.