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

The fore-and-aft, lateral and vertical forces on a seat and a backrest have been investigated with 12 male subjects exposed to random fore-and-aft whole-body vibration (0.25-10 Hz) at four vibration magnitudes (0.125, 0.25, 0.625, and 1.25 m s(-2) rms). Subjects sat in each of four sitting postures having varying foot heights, so as to produce differing thigh contact with the seat. The fore-and-aft forces on the seat depended on whether the feet were supported on a footrest: peaks were found at two frequencies when the feet were not supported, compared to only one peak when the feet were supported. The fore-and-aft forces at the backrest were high, with their peak magnitudes correlated with subject mass. Vertical forces were high on the seat but not on the backrest. Lateral forces were relatively low on both the seat and the backrest. In all directions, forces on the seat and the backrest showed a nonlinear behaviour. In comparison with a previous study undertaken with no backrest, it was found that the backrest reduced forces on the seat at low frequencies (in the fore-and-aft and vertical directions) but increased these forces at high frequencies. (C) 2004 Elsevier Ltd. All rights reserved.