Effect of muscle tension on non-linearities in the apparent masses of seated subjects exposed to vertical whole-body vibration

In subjects exposed to whole-body vibration, the cause of non-linear dynamic characteristics with changes in vibration magnitude is not understood. The effect of muscle tension on the non-linearity in apparent mass has been investigated in this study. Eight seated male subjects were exposed to random and sinusoidal vertical vibration at five magnitudes (035–14 m/s2 r.m.s.). The random vibration was presented for 60 s over the frequency range 20–20 Hz; the sinusoidal vibration was presented for 10 s at five frequencies (315, 40, 50, 63 and 80 Hz). Three sitting conditions were adopted such that, in two conditions, muscle tension in the buttocks and the abdomen was controlled. It was assumed that, in these two conditions, involuntary changes in muscle tension would be minimized. The force and acceleration at the seat surface were used to obtain apparent masses of subjects. With both sinusoidal and random vibration, there was statistical support for the hypothesis that non-linear characteristics were less clear when muscle tension in the buttocks and the abdomen was controlled. With increases in the magnitude of random vibration from 035 to 14 m/s2 r.m.s., the apparent mass resonance frequency decreased from 525 to 425 Hz with normal muscle tension, from 50 to 438 Hz with the buttocks muscles tensed, and from 513 to 45 Hz with the abdominal muscles tensed. Involuntary changes in muscle tension during whole-body vibration may be partly responsible for non-linear biodynamic responses