Experimental modal analysis of the seated human body during whole-body vibration: Effect of vibration direction and magnitude

In this paper, modal properties of the seated human body were identified from measured transmissibilities to the head, chest, lumbar spine L3, pelvis, hips and thighs with single-axis translational vibration; how they were affected by vibration direction and magnitude was quantitatively compared for the first time. To this end, eight subjects were exposed to fore-aft, lateral and vertical single-axis vibrations with three magnitudes at 0.4, 0.8 and 1.2 m/s² r.m.s., respectively. Three and five vibration modes were identified with fore-aft vibration and with vertical vibration in the sagittal plane, respectively, while three modes identified with lateral vibration were in the coronal plane. As the vibration magnitude increased from 0.4 to 1.2 m/s² r.m.s., the modal frequencies of the modes at 1.2, 2.2, 2.9 and 6.2 Hz, which contained the pitch, lateral movements, fore-aft movements and vertical movements of the upper body, respectively, were reduced to 1.0, 1.8, 2.3 and 5.8 Hz significantly, and the damping ratios of the modes identified at 0.8, 5.2 and 10.9 Hz were changed. High similarity was observed between the modal shapes identified with different vibration magnitudes.