Subjective reactions to whole-body vibration of short duration

Abstract An important problem in the evaluation of whole-body vibration is how shocks are to be included in a comprehensive measure of the vibration load on man. This calls for knowledge of how the effect of the vibration changes as a function of its duration. This was the purpose of three experiments in which subjects were exposed to vertical sinusoidal vibration of varying duration. The vibrations were each presented together with a vibration of the same frequency but with a constant duration. The subjects task was to adjust one of the vibrations until both gave rise to the same discomfort towards the end of the vibration. Experiment I dealt with durations between 0·1 and 4 seconds of a 31·5 Hz vibration with intensities of either 1·1 or 2·3 m/s2. In Experiment II the same vibrations were used but the investigated interval was 1–128 seconds. These two experiments showed that the subjective effects, expressed as log m/s2, increases linearly with log exposure duration up to 3–4 seconds. After this “critical time” the effect continues to increase, but at a much slower rate. In Experiment III the 31·5 Hz vibration was compared to one of 6·3 Hz with respect to the increase of the effect between 0·2 and 117 seconds. The 31·5 Hz vibration yielded the same growth function as in Experiment II, whereas the effect of the 6·3 Hz vibration increased linearly during the whole interval and thus showed no indication of a “critical time”. Furthermore, this low frequency vibration showed a slower growth rate. The results are compared to those of two earlier studies in which partly different conclusions were drawn.