This study investigated the biodynamic responses of the seated body to two-axis vibration applied along the fore-aft and vertical directions in the 0.5 to 20 Hz range, independently and simultaneously, using 9 adult male subjects. The measurements were performed with the subjects seated with and without the hands and back supports under two different magnitudes of vibration. The apparent mass responses distributed over the seat pan and the back support are determined in order to better understand the dynamic interactions of the body with the back support under single as well as dual-axis vibration. The results suggest strong influences of the back and hand supports, and considerable cross-axis apparent mass along the vertical axis. Only minimal coupling effects of dual axis vibration, however, could be observed, although coupled sagittal plane motions were perceived by the subjects. Using the linear system theory, the total response along each axis was also computed from the direct and cross-axis responses to individual axes vibration, which emphasized contributions due to cross-axis response and thus the coupling effects of multi-axis vibration.

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