Asymmetric control of bilateral isometric finger forces.

We examined the ability to match the voluntary isometric finger flexion forces of the dominant and nondominant hand in humans, as well as the influence of unilateral visual feedback during this task. Right- and left-handed subjects were trained to produce a "low" force level (50 +/- 25 g) and a "high" force level (200 +/- 50 g) with the right and left index finger, separately. Following the training session, subjects were instructed to match the isometric forces of both fingers simultaneously within the required range (either low or high) so that they were perceived to be identical. The results showed an asymmetry, whereby greater forces were exerted with the index finger of the dominant hand. The asymmetry was independent of the subjects' maximum finger flexion strength. When unilateral visual feedback represented the force output of the dominant hand, the asymmetry was no longer present. In contrast, when it represented the force output of the nondominant hand, the asymmetry was not compensated. We hypothesize that these findings are the result of anatomical or physiological asymmetries inherent in the motor system controlling the production of force.