Effect of backrest and torso twist on the apparent mass of the seated body exposed to vertical vibration.

Occupational exposure to whole-body vibration is often combined with a requirement to perform twisting actions. This paper reports a study where the effect of twisting on the biomechanical response of the seated person was investigated. Twelve male subjects were exposed to vertical random whole-body vibration at 0.4 m/s2 r.m.s. Each subject sat in four different postures: 'back-on', 'back-off', 'twist' (where subjects were required to twist the torso by 90 degrees) and 'move' (where subjects were required to performing a moving task with extended arms). Similar apparent masses were measured for the 'back-on', 'back-off' and 'twist' conditions, where a peak occurred at about 6 Hz. For the 'move' condition, the peak in the apparent mass was attenuated indicating a different biomechanical response in this posture. The 6 Hz peak in fore-and-aft cross-axis apparent mass was eliminated in the 'move' condition. It is suggested that the change in biomechanical response is due to either the extended arms acting as a passive vibration absorber or that the twisting action interferes with the usual acceleration-muscle feedback system. Further work will be required to test these hypotheses.