Tri-axial forces at the seat and backrest during whole-body vertical vibration

During exposure of seated subjects to vertical whole-body vibration, forces in the fore-and-aft, lateral and vertical directions at the seat and backrest have been measured. The responses at the seat have been compared with those measured previously on a seat without a backrest. Twelve male subjects were exposed to random vertical vibration in the frequency range 0.25–20 Hz. The subjects sat on a rigid seat with a rigid backrest and were exposed to a 16 different conditions: four vibration magnitudes (0.125, 0.25, 0.625, and 1.25 m s−2 r.m.s.) and four sitting postures (with varying thigh contact with the seat). Although the excitation was vertical, considerable dynamic forces were found in the fore-and-aft direction on both the seat and the backrest. In the vertical direction on the backrest, and in the lateral direction on the seat and the backrest, the forces were low. At both the seat and the backrest, forces in all directions showed a non-linear behaviour. The presence of the backrest modified the forces on the seat in both the vertical and fore-and-aft directions: in all four postures there was an increase in the resonance frequency of the apparent mass when using the backrest. The effect of the backrest on fore-and-aft forces on the seat depended on whether the feet were supported or not. The results show the importance of considering the backrest when studying the response of the human body to whole-body vertical vibration.