Repetitive shock and vibration are routinely encountered with the use of powered and non-powered hand tools, and are associated with neurological and neurovascular disorders of the hand, as well as more proximal injuries. In this work-RO1 OHO4025, Temporal and Impulsive Characteristics of Hand Tools, we looked at the effects of dynamic factors, force variable over cycle time and repetition rate on temporary threshold shifts (TTS) in three populations of mechanoreceptors at the fingertips in professional and non-professional subjects. We had posed the question whether changes in dynamic forces that are intrinsic to tools, such as force variation over cycle time (impulsiveness) and temporal patterns (repetition rates) were more strongly associated with physiological changes in human mechanoreceptor response than tool transmission energy. The results were remarkably clear. A physiologic response of the FAll receptor (Pacinian Corpuscle) to the impulsive stimulus is less than that to a continuous stimulus. The physiologic responses of the other mechanoreceptors (SAI and FAI) in the fingertip were not affected by the differences between foot mean squared (RMS) and peak accelerations, and forces, demonstrating an equivalent response to oscillatory and impulsive stimuli. The cycle time or repetition rate of a tool had little bearing on physiologic response. When grip force, RMS acceleration and posture were controlled experimentally, the physiologic response was the same or less than that expected from the cumulative energy transfer. Based on stimuli produced in the laboratory, the effects of frequency on the digital mechanoreceptors were not subordinate to energy and time- related effects; it appeared to dominate the responses for most exposures. Finally, the thesis that neuromuscular response patterns, as inferred from the EMG, might be differentially affected by frequency transmission was not observed. The results suggest that response is related to the frequency-weighting function used to quantify exposure [aw(t)], and not to the procedure for summing shocks, and that the frequency weighting used to identify hazard from vibration by national and international standards is significantly in error (ISO 5349: 1, 2001). In addition, the experiments used low (125 Hz) and high (1250 Hz) sinusoidal carrier frequencies as non-shock or oscillatory stimuli, with the TTS being much larger at the lower carrier frequency than at the higher. The results lead to a simplification of future experimental modeling, since health effects can be studied using sinusoidal signals at different frequencies, which are simpler to produce than the complex signals (particularly for low frequencies) that were developed for these experiments.
Ergonomics Technology Center, Department of Medicine, UCONN Health Center, 263 Farmington Avenue, Farmington, CT 06030