The hypothesis that handedness stems from a greater ability to produce independent forces in the digits of the preferred than the non-preferred hand was investigated in 20 right-handed males who made a sustained isometric flexion of the distal phalanx of a single digit (the instructed digit). Instructed flexion forces were accompanied by non-instructed forces in all other digits. Mean non-instructed force was least when the thumb was the instructed digit, and increased progressively when the index, middle, ring, and little finger was the instructed digit. Both flexion and extension were recorded in non-instructed digits. There was no asymmetry in production of non-instructed force, and hence no evidence for greater independence of force production in the digits of the preferred than the non-preferred hand. (C) 2000 Published by Elsevier Science Ireland Ltd.