Hearing threshold and heart rate in men after repeated exposure to dynamic muscle work, sinusoidal vs stochastic whole body vibration and stable broadband noise

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Abstract
Changes in the temporary hearing threshold (TTS2) and heart rate (HR) were examined in subjects exposed to stable noise, whole body vibration and dynamic muscular work at a dry-bulb temperature of 30°C. The exposure combinations consisted of three categories of dynamic muscular work with varying loads (2 W, 4 W, 8 W), of two categories of noise and of three categories of vibration. The noise categories were: (1) no noise, and (2) stable, broadband (bandwidth 0.2-16.0 kHz) A-weighted noise with an intensity of 90 dB. The vibration categories were: (1) no vibration, (2) sinusoidal whole body vibration (Z-axis) with a frequency of 5 Hz, and (3) stochastic broadband (bandwidth 2.8–11.2 Hz) whole body vibration. A single test consisted of a control period of 30 min, three consecutive exposure periods of 16 min, each followed by a 4-min post-exposure interval and a recovery period of 15 min. The results of the variance analyses indicated that noise had the most notable effect on the TTS2 values at the hearing frequencies of both 4 and 6 kHz. Of the paired combinations, noise plus vibration and noise plus dynamic muscular work caused the most obvious combined effects. The combined effect of all three factors (noise, vibration and work) on the TTS2 values after three consecutive exposure periods was significant at the 2.5% level at the 4 kHz hearing frequency and at the 5% level at the 6 kHz hearing frequency. The added effect of vibration on enhanced TTS2 values was particularly clear when the vibration was stochastic and when the subjects had a low (2 W) working efficiency. Increasing the working efficiency, on the other hand, seemed to retard increases in the hearing threshold. Thus TTS2 values seemed to reflect the changes in HR values. It is as if the low rate of cardiovascular activity during light dynamic muscular work had enabled the manifestation of the cardiovascular effects of noise and vibration; during strenuous dynamic muscular work, however, the high rate of cardiovascular activity aimed in some way at compensating for the effects of noise and vibration on blood circulation.