Movement of the upper-body of seated subjects exposed to vertical whole-body vibration at the principal resonance frequency

The dynamic responses of eight male subjects exposed to vertical whole-body vibration have been measured at eight locations of the body in three directions within the sagittal plane: in the vertical, fore-and-aft and pitch axes. The motions were measured on the body surface at the first, fifth and tenth thoracic vertebra (T1, T5, T10), at the first, third and fifth lumbar vertebra (L1, L3, L5) and at the pelvis (the posterior-superior iliac spine), and were corrected so as to estimate the motions of the skeleton. The head motion was measured with a bite-bar. The force at the seat surface was also measured. The subjects were exposed to vertical random vibration in the frequency range from 0.5-20 Hz at a magnitude of 1.0 ms(-2) r.m.s. The movement of the upper-body at the principal resonance frequency of the driving-point apparent mass is illustrated by using the transmissibilities from seat vertical vibration to vertical and fore-and-aft vibration at the eight locations on the body. A bending of the lumbar spine, and probably the lowest thoracic spine, possibly coupled with a rocking motion of the upper thoracic spine about the lower thoracic spine, appeared to be dominant. A small bending along the full length of thoracic spine was also found. Pitch motion of the pelvis, possibly accompanied by longitudinal and shear deformations of the tissue underneath the pelvis, was found to occur near the resonance frequency range, but did not appear to make a principal contribution to the resonance observed in the apparent mass. Any significant axial motions along the spine occurred at higher frequencies. (C) 1998 Academic Press.