SUBJECTIVE RESPONSES TO HAND-ARM VIBRATION: IMPLICATIONS FOR FREQUENCY-WEIGHTING AND GENDER DIFFERENCES

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1 SUBJECTIVE RESPONSES TO HAND-ARM VIBRATION: IMPLICATIONS FOR FREQUENCY-WEIGHTING AND GENDER DIFFERENCES Greg Neely, Lage Burström, and Magdelana Johansson Swedish National Institute for Working Life Abstract 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 (, 31.5, 3, and 15 Hz) in participants (1 males, 1 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, 19; Gemne, Lundström, & Hansson, 1993 for reviews); collectively known as hand-arm vibration syndrome (HAVS). Assessment of vibration severity, as outlined by ISO 539(19), 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 () concluded that the current weightings may be unsuitable for preventing some types of vibration injury at some vibration levels. Griffin () has pointed out that the frequency weighting is not well supported by epidemiological or experimental studies. Griffin (19) points to two studies by Miwa (197, 19) as influential in shaping the frequency-weighting used in the ISO standard. The first study (Miwa, 197) determined equal sensation contours on the hand for vertical and horizontal sinusoidal vibration. The second study (Miwa, 19) determined maximal thresholds for perceived unpleasantness and tolerance for sinusoidal vibration. Griffin's (, 19) primary critique has not been directed at the studies per se, but rather, that the standards adopted by the ISO did not

2 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, 197, p. 1). 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 1 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, 199 for a review) indicate that women may be more susceptible to HAVS and are at the very least underrepresented in the literature. Participants Methods Twenty university students, 1 males and 1 females, volunteered to participate in the experiment. All participants reported being in good health. Participants received 5 SEK (approximately 5 USD) for their cooperation. Materials Hand-arm vibration stimuli consisted of signals: acceleration levels (.1,.5, 1. and 5 m/s) presented at four frequencies (, 31.5, 3 and 15 Hz). Signals were generated with a Ling Dynamic System vibrator controlled by an Instant Replay 3 signal generator and a Sentac PA9 amplifier. Each signal was presented for 1 sec, including one second ramps at the beginning and end of each presentation. A round, cm diameter, wooden platform was mounted on the vibrator for delivery of the signals to the hand. Subjective ratings were made using the Borg CR-1 Scale (Borg, 19). The CR-1 scale consists of a numerical scale from (Nothing at all) to 1 (extremely strong) with nine verbal anchors placed on the number scale in an approximately logarithmic fashion. The scale is used by first finding the verbal label which best fits the experience and then using the number scale to make adjustments to the rating. "Absolute maximum" is labeled above 1 and not assigned any specific number in order to avoid ceiling effects. Procedure Each session began with vibration threshold determinations for, 31.5, 3, and 15 Hz. Thresholds were determined using a staircase method where the subject placed their pointer

3 finger on a stylus and controlled stimulus intensity changes (ascending/descending) by pressing a button with the other hand. Subjective intensity measurements were collected in a following session where all 1 combinations of frequencies and accelerations were presented in four blocks where each block contained one repetition of each stimulus. Stimulus order was randomized over both blocks and subjects. Results Average thresholds for the four frequency levels are presented in Table 1. A mixed design ANOVA using Gender (male, female) and the repeated measure Frequency (, 31.5, 3, 15 Hz) revealed no significant gender differences. However, there was a significant Frequency difference (F= 3., p <.1). Post-hoc t-tests with a Bonferroni adjustment for multiple comparisons revealed no significant differences for the thresholds at the 3 and 15 Hz frequencies. All other comparisons were significantly different at the.5 level. Average perceived intensity and discomfort ratings for each of the acceleration levels and frequencies are presented in Figure 1. Individual exponents calculated from power functions from the ratings of perceived intensity and discomfort were obtained. Mixed-design ANOVAs were calculated on the exponents using the variables Gender (male, female) and the repeated measure of Frequency (, 31.5, 3, and 15 Hz). For perceived intensity, The main effect of Frequency (F 3,5 =., p <.1) and the interaction effect Gender X Frequency (F 3,5 = 3,, p =.1) were significant and the main effect of Gender was nearly significant (F 1,1 =., P =.). The interaction effect can be seen in Figure, while there is no difference between exponents at Hz, females have consistently higher exponents at the other levels. For perceived discomfort, only the main effect of Frequency was significant (F 3,5 = 7., p <.1). The overall pattern of average exponent size compared to frequency (see Figure 3) was similar to that seen with perceived intensity. Post-hoc t-tests with a Bonferroni adjustment for multiple comparisons revealed that exponents at 3 Hz were significantly lower (p<.5) than exponents at both and 31.5 Hz. Regression analyses revealed that thresholds were poor predictor of both exponents and ratings of perceived intensity. At best, thresholds at 3Hz could account for 15% of the variance in exponents for perceived intensity at 3 Hz (Y =.1X -., R =.15, p =.9). Typically, however thresholds -1% of the variance in exponents or ratings. Table 1. Average absolute thresholds (and standard error) in db at the four frequencies. Hz 31.5 Hz 3 Hz 15 Hz Thresholds (N=) 1 (.95) (1.) 1.9 (1.3) 1. (1.5)

4 1 Hz Hz Hz 1 15 Hz Females, Perceived Intensity Females, Perceived Discomfort Males, Perceived Intensity Males, Perceived Discomfort Figure 1. Average ratings of perceived intensity and discomfort using the Borg CR-1 scale for both males (n=1) and females (n=1) at the four frequencies. 1 Average Exponent,,,, Females Males EHz Hz 31.5 Hz 3 Hz 15 Hz Figure. Average exponents for perceived intensity at the four frequencies for males (n=1) and females (n=1).

5 1 Average Exponent,,,, Hz 31,5 Hz 3 Hz 15 Hz Figure 3. Average exponents for perceived discomfort at the four frequencies (N=). From the raw data collected at Hz, predicted values for perceived intensity at the three highest frequencies were calculated as would be predicted by the ISO frequency weights. The resulting scores are compared in Figure with the actual ratings. As can be seen, the ISO determined scores underestimated the actual ratings. This effect is particularly clear at the highest intensity levels for the females. Discussion The results from this study indicate that the relationship between threshold values for hand-arm vibration and perceptions of vibration at higher intensity levels may not be as straightforward as the ISO standards assume. of both intensity and discomfort both declined as frequency increased (as did the growth functions). of perceived intensity and discomfort were on average higher for females than for males at all for frequencies. For both males and females, discomfort ratings were on average similar in size to intensity ratings. However, significant gender effects were found only for the growth functions of the perceived intensity, not the perceived discomfort. This indicates that there is more individual variability in discomfort ratings. Thresholds decreased with frequency as well. However, at an individual level, thresholds were poor predictors of perceived intensity. More problematic, however, is the relationship between the actual ratings and the predicted ratings using the ISO frequency-weights. It is clear from the results presented in Figure that predicted values were much lower than the obtained values. In particular, the ratings of females were grossly underestimated at the higher intensity levels. This underestimation could put workers at risk for HAVS because the current frequency weights are used in evaluating hand-held tools. The results seen here indicate that the aforementioned criticisms of the frequencyweightings described in ISO 539 are warranted. Further research is needed to identify weights that more accurately represent the subjective effects of hand-arm vibration.

6 1 Females (n=1) 1 Males (n=1) Actual Actual 1 Predicted 1 Predicted 1 Hz 3 Hz 15 Hz Figure. Actual versus predicted ratings of perceived intensity. References Borg, G. (19). A category scale with ratio properties for intermodal and intraindividual comparisons. In H. G. Geissler and P. Petzold (Eds.), Psychophysical judgment and the process of perception, (pp. 5-3). Berlin: Deutscher Verlag der Wissenschaften. Bovenzi, M. (). Epidemiologic aspects of the exposure-response relationship in the handarm vibration syndrome. In R. Lundström & A. Lindmark (Eds.) Eighth International Conference on Hand-Arm Vibration (pp. 5-15). Arbetslivsrappport : Bylund, S.H. (199). Skador och besvär av vibrationer en jämförelse mellan kvinnor och män [Vibration-induced disorders and symptoms a comparison between women and men]. Arbete & Hälsa [Work & Health], No.. Gemne, G., Lundström, R., & Hansson, J.E. (1993). Disorders induced by work with handheld vibrating tools. Arbete & Hälsa [Work & Health], No.. Griffin, M.J. (). Standards for the evaluation of hand-transmitted vibration and the prevention of adverse effects. In R. Lundström & A. Lindmark (Eds.) Eighth International Conference on Hand-Arm Vibration (pp. 3-59). Arbetslivsrappport : Griffin, M.J. (19). Vibration injuries of the hand and arm: Their occurrence and the evolution of standards and limits. Health and Safety Executive, Research Paper 9. ISO 539 (19). Mechanical vibration Guidelines for the measurement and the assessment of human exposure to hand-transmitted vibration. International Organization for Standardization. Miwa, T. (19). Evaluation methods for vibration effect: Part. Measurements of unpleasant and tolerance limit levels for sinusoidal vibrations. Industrial Health,, 1-7. Miwa (197). Evaluation methods for vibration effect: Part 3. Measurements of threshold and equal sensation contours on hand for vertical and horizontal sinusoidal vibrations. Industrial Health, 5, 13-.