NOISE IN CONTINUOUS FORCE PRODUCTION 135

Abstract

The purpose of this experiment was to investigate the long standing hypothesis that the increment in force variability at higher levels of force output is due to increments of noise in the perceptual-motor system. This goal was accomplished by examining the structure of the force variability signal, in addition to, the normal indices of the amount of variability. A group (n = 10) of young adult males performed with the index finger a continuous isometric force production task of matching to a target on a computer screen 10 different levels of force (5 through 85% of maximum) for 15 s trials. There were 10 trials at each force level condition for each subject. The results showed that force variability increased exponentially with increments of force, that the coefficient of variation was a U-shaped function over the force range, and that the complexity of the signal as measured in the time domain [Approximate entropy (Ap En) and frequency domain (spectral analysis] was an inverted U-shaped function. The findings suggest that: a) the signal/noise ratio in force output is directly related to the complexity of the signal; b) that this relation in force output varies as an inverted U-shaped function over the force range; and c) that 50% of maximum force production affords the optimal information transmission. The data provide strong evidence that the concepts of noise and variability are not synonymous in motor control.