Factors influencing vibration sense thresholds used to assess occupational exposures to hand transmitted vibration

The effects of various conditions, including temporary threshold shifts (TTS) induced by exposure to vibration on vibration sense thresholds, have been investigated. The vibration sense thresholds of five subjects were measured on the middle fingertip of the left hand. A contactor with a diameter of 7 mm was surrounded by three alternative plates with holes of different sizes. The contact force was controlled at either 1 N, 2 N, or 3 N. For the TTS test, the left hand was exposed to vibration at 20 ms-2 rms for five minutes. The frequencies of both the exposure to vibration and the vibration threshold test were in the range 16 Hz to 500 Hz. Using a surround around the contactor greatly reduced the vibration sense threshold at 16 Hz and 31.5 Hz but increased the threshold at 125 Hz, 250 Hz, and 500 Hz. An effect of contact force was seen only at the higher frequencies; larger contact forces led to lower thresholds at 125 Hz, 250 Hz, and 500 Hz. As temperature of the finger skin decreased, the vibration thresholds increased, with the changes at higher frequencies greater than those at lower frequencies. The TTS at 16 Hz and 31.5 Hz measured 0.5 minutes after exposure to vibration (TTS0.5) were highest after exposures to vibration at lower frequencies. The TTS0.5 at 63 Hz was similar after exposure to all frequencies. The TTS0.5 values at 125 Hz, 250 Hz, and 500 Hz were highest after exposures to vibration at 125 Hz and 250 Hz. It was apparent that the physiological characteristics of vibration sensation at low and high frequencies differed significantly. These findings suggest that two representative frequencies can be used when evaluation the neurological effects of occupational exposures to vibration by means of vibration sense thresholds.