The effects of power hand tools on grip force were studied in humans. Fourteen university students, seven males, operated a simulated hand tool vibrating at frequencies of 0, 40, or 160 hertz (Hz) with accelerations of 9.8 or 49 meters per second squared (m/sec2) and loads of 1.5 or 3.0 kilograms. The vibrations were directed along the three coordinate axes specified by the International Organization for Standardization. Grip force of the subjects was measured by a strain gauge dynamometer. Additionally, hand flexor and extensor muscle responses to a handle vibrating with frequencies of 20, 40, 80, or 160Hz, acceleration 8m/sec2, and grip forces corresponding to 5, 10, or 15 percent of maximum voluntary contraction (MVC) were investigated in five subjects, three males. The muscular responses were measured by an electromyographic technique and MVCs by a hand dynamometer. In the first experiment, average grip force increased with both increasing acceleration and vibration frequency. The largest grip force increase was from 25.3 newtons (N) for no vibration to 35.8N for a frequency of 40Hz and acceleration 49m/sec2. Grip force was sensitive to vibration frequencies in the Y-direction and the Z-direction, but not the X- direction. Grip force increased slightly but significantly with load. The greatest flexor response occurred at a grip force of 15 percent of MVC at a frequency of 20Hz in the Z-direction. The greatest extensor response occurred at a grip force of 15 percent of MVC at 40Hz in the Y-direction and Z-direction. The authors conclude that occupational exposure to power hand tool vibrations causes neuromotor disturbances. Cumulative trauma disorders associated with power tools are secondary lesions, caused indirectly by vibrational effects on grip exertion.