Human response of vertical and pitch motion to vertical vibration on whole body according to sitting posture

When a whole body is exposed to vertical vibration, the body's asymmetric shape affects the response to translational and rotational motion. The degree of asymmetry of a body on a seat is affected by posture. In our study, sixteen male subjects sitting on a seat were used to obtain a response to vertical vibration over a frequency range of 3 to 40 Hz. Two kinds of magnitude of the vibration at each frequency were applied (0.224 m/s(2) and 0.708 m/s(2) RMS). Without a backrest, three kinds of sitting postures (average thigh contact PI, supported thigh contact P2, and minimum thigh contact P3) were set by adjusting the height of the footrest and by using an inclined seat pan. The vertical and rotational responses were measured using a force plate mounted on a rigid seat. The apparent eccentric mass (AEM) is defined in this study as rotational response of body to vertical whole body vibration exposure. In the result, the AEM of P2 posture was bigger than that of P1 and P3 posture, especially in a frequency range of 20 to 40 Hz where idle vibration of the passenger vehicle exists. The apparent mass (AM) was even changed by the three kinds of sitting posture, but the difference was not as much as in the case of the AEM. The bigger difference of the AEM is assumed that the sitting posture mainly affects the asymmetry of the fore-and-aft direction, which is more strongly correlated with the rotational pitch response.