Apparent mass of the seated human body during vertical vibration in the frequency range 2-100 Hz.

We studied the apparent mass during vertical whole-body vibration in the frequency range 2-100 Hz at four magnitudes (sinusoidal sweep signals, 1.0, 1.5, 2.0 and 2.5 ms r.m.s.) in 12 males and 12 females with upright and relaxed sitting postures. The first two peaks of apparent mass decreased with increasing vibration magnitude with both postures. The non-linearity characteristics became obscured at the two largest magnitudes and were less transparent with relaxed sitting posture. The peak frequencies and the normalised apparent masses were similar between males and females with both postures. The standardised three degrees-of-freedom parametric model with modified parameters was proposed to predict well the apparent mass of seated human body during vertical vibration in the frequency range 2-100 Hz and in the magnitude range 1.0-2.5 ms r.m.s. This study shows the frequency-dependence and magnitude-dependence of biodynamic responses in the frequency range of 2-100 Hz. The magnitude of apparent mass at frequencies above 20 Hz may not be negligible. The proposed 3 DOF model with modified parameters would help with understanding and developing the human-seat system. WBV: whole-body vibration; DOF: degrees-of-freedom; CMIFs: complex mode indicator functions.