Subjective Responses to Hand-arm Vibration: Implications for Frequency-weighting and Gender Differences

Exposure to vibration transmitted to the hands and arms may increase the risk of developing chronic disorders. Current ISO standards regarding exposure are based in part upon assumptions regarding the relationship between threshold measurements and subjective experiences of vibration at higher intensity levels. This study investigated this relationship by comparing absolute threshold levels at four frequencies (8, 31.5, 63, and 125 Hz) in 20 participants (10 males, 10 females) with perceived intensity and discomfort ratings made at four intensity levels for each frequency. The results indicate that intensity ratings do decrease with increasing frequency, but that the current ISO weightings underestimate the actual perceived intensity. The effect was more pronounced in females who had higher intensity ratings than males. Studies of hand-arm vibration have tended to focus on the physiological effects variations of the stimulus have on the human receiver. These studies have shown quite conclusively that exposure to vibration can lead to neurological, vascular, and oesteoarticular disorders (see Griffin, 1980; Gemne, Lundström, & Hansson, 1993 for reviews); collectively known as hand-arm vibration syndrome (HAVS). Assessment of vibration severity, as outlined by ISO 5349(1986), is based on the expected occurrence of vibration-induced white finger. This disorder is considered to be highly associated with, if not a precursor to, the other disorders that are seen in HAVS. Central to the ISO standards regarding the measurement and evaluation of human exposure to hand-transmitted vibration is the use of weights to compensate for the assumed importance of different frequencies in causing injury to the hand. The ISO standard for weighting frequencies in hand-arm vibration is built on the assumption that threshold differences between frequencies results in differential effects on perceived intensity the higher the frequency, the less affect stimulus intensity will have on perceived intensity. Several authors have pointed out shortcomings of the ISO Standards and in particular, the usefulness of the frequency-weightings. In reviewing epidemiological studies of the relation between exposure to hand-transmitted vibration and disorders of the hand-arm system, Bovenzi (2000) concluded that the current weightings may be unsuitable for preventing some types of vibration injury at some vibration levels. Griffin (2000) has pointed out that the frequency weighting is not well supported by epidemiological or experimental studies. Griffin (1980) points to two studies by Miwa (1967, 1968) as influential in shaping the frequency-weighting used in the ISO standard. The first study (Miwa, 1967) determined equal sensation contours on the hand for vertical and horizontal sinusoidal vibration. The second study (Miwa, 1968) determined maximal thresholds for perceived unpleasantness and tolerance for sinusoidal vibration. Griffin's (2000, 1980) primary critique has not been directed at the studies per se, but rather, that the standards adopted by the ISO did not accurately reflect the results found by Miwa and the standards extrapolated the results beyond the low and medium frequencies that were studied. However, closer inspection of the studies leads to even further questions. For example, in both studies subjects held their hands ". . .pushed to the vibration table and joints of hand and elbow were bent at right angle with each other. . ." (Miwa, 1967, p. 216). This was done to prevent transmission of vibration from hand to head, but at the same time it exaggerates the effect of vibration on the hand (at least compared to the normal situation where the vibration is free to travel up the arm). Additionally, while subjects were in most cases provided with feedback about the force of their hand pressing against the platform, it is likely that the amount of force varied widely during the stimulus presentations (some of which were as long as 10 min). In the study presented here, we take a closer look at the relationship between thresholds and perceptions of hand-arm vibration at higher intensity levels. We are primarily interested in three aspects: Does perceived intensity and discomfort ratings decrease with increasing frequency as thresholds do? Can individual threshold levels be used to predict the growth of perceived intensity and discomfort? And lastly, do the ISO recommended frequency-weights accurately predict perceived intensity ratings? Additionally, we will investigate how these aspects of hand-arm vibration affect both men and women. The number of women exposed to occupational hand-arm vibration is large and has been increasing. Recent studies (see Bylund, 1998 for a review) indicate that women may be more susceptible to HAVS and are at the very least underrepresented in the literature.