NEUROMUSCULAR EFFECTS OF VIBRATING HAND TOOLS ON GRIP EXERTIONS, TACTILITY, DISCOMFORT, AND FATIGUE

Hand tool vibration influences grip force, affects forearm muscle contraction, impairs tactility, promotes discomfort, and causes muscle fatigue. A survey of selected power tools used in automobile assembly found significant vibration primarily in the 20 Hz to 160 Hz frequency range. Based on these findings, three experiments were conducted. Fourteen subjects operated a simulated hand tool vibrating at 9.8 m/s('2) and 49 m/s('2) acceleration magnitudes, 40 Hz and 160 Hz frequencies, three orthogonal directions, and with 15 N and 30 N tool weights. Average grip force increased from 25.3 N without vibration to 35.8 N (42%) with vibration at 40 Hz and 49 m/s('2). This increase was the same magnitude as doubling the weight where average grip force increased 56%. A second experiment studied hand flexor and extensor EMG using five subjects holding a handle vibrating at 8 m/s('2) ISO weighted acceleration, frequencies of 20 Hz, 40 Hz, 80 Hz, and 160 Hz, and grip forces of 5%, 10%, and 15% MVC. Muscle responses were greatest at frequencies where grip force was affected, indicating the tonic vibration reflex was likely the cause of increased muscle contraction. In a third experiment five subjects operated a simulated hand tool for 30 min vibrating at 8 m/s('2) ISO weighted acceleration, frequencies of 20 Hz, 80 Hz, and 160 Hz, in three orthogonal directions using a 30 s: 30 s work/rest duty cycle. Average falling ridge thresholds shifts, measured using a ridge aesthesiometer, increased 0.01 mm (50%) at 20 Hz and 80 Hz, and increased 0.03 mm (150%) at 160 Hz. No significant rising ridge or two point discrimination threshold shifts were observed. No EMG indication of muscle fatigue was observed using RMS amplitude and mean power frequency shifts after the 30 min exposure. Average muscle force tremor amplitude did not change after vibration exposure at 20 Hz, but increased 17% at 80 Hz, and 25% at 160 Hz. Average discomfort was greater after holding a vibrating handle than holding one without vibration, and was slightly greater for vibration parallel to the long forearm axis than other directions. It was concluded that operating vibrating hand tools can result in increased grip force, which may increase the risk of cumulative trauma disorders. Vibration diminishes hand tactility, and increases tremor and upper extremity discomfort, which could impair performance in manipulative tasks. Current hand vibration exposure guidelines do not adequately control for these effects.