The variation of hand tremor with force in healthy subjects.

1. Total root-mean-square (r.m.s.) error over an effectively unrestricted band, and error power spectra from 0.256 to 12.5 c/s were measured for four subjects attempting to maintain fixed forces of 1-6 Lb. (0.45-2.72 kg) on a pressure joystick. The subject could see his errors as the deflexion of a cathode ray tube spot.2. The total r.m.s. error increases linearly with force with a considerable (positive) intercept when the line is extrapolated to zero force. Hence accuracy in the sense (r.m.s. error)/(force) increases with force for this type of control.3. The rate of increase of total r.m.s. error with force is greater in subjects with prominent tremor peaks in their power spectra at 8-10 c/s.4. At each frequency the r.m.s. error per unit bandwidth also increases linearly with force. Except at the very lowest frequencies the rate of increase is greater in subjects with prominent tremor peaks at 8-10 c/s. This is thought to account for the observation described in paragraph 3.5. The frequency of the tremor peak near 8-10 c/s is almost independent of the force exerted.