Effects of vibration frequency and postural changes on human responses to seated whole-body vibration exposure.

The present investigation evaluated the effects of changes in pelvic orientation and vibration frequency on the seated human's response to wholebody vibration (WBV). Seat-to-trunk and seat-to-head acceleration transmissibility, peak-to-peak pelvic motion and erector spinae EMG and mean erector spinae EMG was collected across three pelvic orientations (9 degrees anterior pelvic tilt, neutral pelvis, and 9 degrees posterior pelvic tilt) and frequencies ranging from 4.5 to 16 Hz. Subjects included 30 healthy males between the ages of 18 and 35. Ensemble averages, two vibration cycles in length, were produced for each subject within each frequency-pelvic orientation combination. Group ensemble averages within each frequency-pelvic orientation combination were then compared using ANOVA. Changes in pelvic orientation produced significant differences in acceleration transmissibility, pelvic motion, and erector spinae EMG. At frequencies below 6 Hz, acceleration transmissibility at the head and pelvic motion were significantly greater in the posterior pelvic orientation than in the other two. At frequencies above 6 Hz, acceleration transmissibility at the head and trunk were significantly greater in the anterior pelvic orientation than in the other two. Peak-to-peak EMG responses were similar across all pelvic orientations at frequencies below 6 Hz. However, above 6 Hz, the response was significantly greater in the anterior pelvic orientation than in the other two. Thus, vibration frequency and pelvic orientation were shown to have significant interactive effects on the seated human's response to WBV. These interactive effects need to be considered when determining appropriate vibration exposure guidelines.