Evaluation of responses to broad-band whole-body vibration

Abstract

The experiment was aimed at investigating the human response to different modes, frequencies and intensities of whole-body vibration (WBV), in order to check the evaluation procedures currently recommended. Six male seated subjects were exposed to sinusoidal (SIN) and octave-band-wide vibration (OWV) in the z axis with the frequencies or centre frequencies, respectively, of 2,4, 8 and 16 Hz at two intensity levels (except for 2 Hz), in accordance with the frequency weighting of ISO 2631 (ISO 1978 a). The 14 exposure conditions were compared by means of a slightly modified, complete paired comparison, the total number of exposures amounting to 1044. Subjective judgements of the severity of WBV, annoyance and the ability to control a constant sitting posture were obtained along with the bioelectrical activity of trunk muscles, transmissibility and impedance. An integral assessment of the exposures was rendered possible by the complex evaluation of different human responses. OWV and SIN with identical a_zw r.m.s. values (ISO 1978 a) produced almost identical effects. The results clearly speak in favour of the weighting procedure. This procedure was also supported by an additional pilot study with two-octave-band-wide vibration. The superiority of the weighting procedure suggests lower limits for broad-band vibration than those recommended at present (ISO 1978 a). Human response to WBV in the range near 4 Hz was more pronounced than that of equivalent exposures with other frequencies. Generally, higher intensities induced stronger effects. The biomechanical data exhibited a non-linearity for the WBV levels of intensity investigated. The patterns of myoelectric and biomechanical reactions depended on both anatomical and exposure conditions. The individual responses in discriminating the exposure conditions significantly agreed, but the extent of agreement between the individual responses varied for the effects investigated.