OBJECTIVESTo investigate the dependency of temporary threshold shift of vibratoty sensation (TTS(v)) in fingertip on hand-arm vibration exposure period.

METHODSSix healthy students were instructed to grip a vibrating or nonvibrating handle in the experimental room. The gripping force was 40 N. The vibratory sensation threshold at 125 Hz was measured before and after the exposure in the exposed middle fingertip. The exposure vibration was vertical and the 1/3 octave-band vibration with had a central frequency of 200 Hz and an intensity of 39.2 m/s(2). The exposure periods were 8, 15, 30, 60, 120, 240 and 600 s. TTS(v,t) was evaluated as the difference in vibratory sensation threshold between immediately before and t seconds after the exposure.

RESULTSTTS(v) recovered exponentially as in several previous studies and its use enabled us to estimate the time constant and TTS(v,0). TTS(v,0) with vibration exposure was significantly larger than that without it. The regression analysis of the relationship between vibration exposure period (T) and TTS(v,0) (T) for each subject confirmed the good fit of the equation TTS(v,0)(T)=B(0)+B(1) (*)Log(10)(T), where B(0) and B(1) are the calculated constants (adjusted R(2)=0.56-0.87). The time constants did not show such a clear dose effect relationship of exposure period as TTS(v,0).

CONCLUSIONThe dependency of TTS(v,0) on vibration exposure period was asymptotically proportional to the logarithm of gripping period. To more quantitatively confirm the relationship of the time constants for recovering time course of TTS(v), it may be necessary to improve the measurement method for TTS(v).