Nonlinear subjective and dynamic responses of seated subjects exposed to horizontal whole-body vibration

The effect of the magnitude of fore-and-aft and lateral vibration on the subjective and mechanical responses of seated subjects has been investigated experimentally using simultaneous measurements of relative discomfort and apparent mass. Twelve male subjects were exposed to sinusoidal vibration at nine frequencies (between 1.6 and 10 Hz) at four magnitudes (in the range 0.125–1.0 m s−2 r.m.s.) in both horizontal directions (fore-and-aft and lateral). The method of magnitude estimation was used to estimate discomfort relative to that caused by a 4 Hz reference vibration in the same axis. The apparent mass was calculated from the acceleration and the applied force so as to quantify the mechanical response of the body. With each direction of excitation, the apparent mass was normalised by dividing it by the apparent mass obtained at 4 Hz, so that the mechanical responses could be compared with the subjective responses.The relative discomfort and the normalised apparent mass were similarly affected by the frequency and magnitude of vibration, with significant correlations between the relative discomfort and the normalised apparent mass. The results indicate that the discomfort caused by horizontal whole-body vibration is associated with the apparent mass in a frequency range where motion of the whole body is dominant. In this frequency range, the nonlinear subjective responses may be attributed, at least in part, to the nonlinear dynamic responses to horizontal whole-body vibration.