Tri-axial transmissibility to the head and spine of seated human subjects exposed to fore-and-aft whole-body vibration.

Previous studies have quantified the biodynamic responses to vibration with more focus on vertical vibration than horizontal vibration. This study reports the transmissibility to the head and spine measured under whole-body fore-and-aft vibration. Sixteen seated male subjects were exposed to sinusoidal fore-and-aft vibration with magnitudes 0.311-2.426 ms r.m.s. and frequency range 2-6 Hz. The fore-and-aft (Txx), lateral (Txy) and vertical (Txz) transmissibilities to the head, three locations on the thoracic spine (T1, T8, T12) and L4 were measured. Txx, Txy and Txz showed high inter-subject variability at all locations. A peak in the range 2-2.4 Hz was evident at all locations indicating a whole-body resonance in this frequency range. Txy peak was smallest at T8 and greatest at the head with medians of 0.15 and 0.46, respectively. Txx peak was smallest at L4 and greatest at the head with medians of 0.65 and 2, respectively. Txz peak was smallest at T8 and greatest at the head with medians of 0.58 and 1.3, respectively. At T12 and L4 and at frequencies below 4 Hz, Txz was as high as or higher than Txx. At low frequencies, Txx decreased with moving down the spine while an opposite trend was found at high frequencies. Txz decreased with moving up the spine from L4 to T8. Txz at T1, however, was higher than that at T8, possibly influenced by the high motion of the head. The results are useful for developing models that help better understanding of human response to horizontal vibration.