Synergistic finger movements in a skilled motor task.

When skilled typists strike one key, typically all of the fingers of one hand are in motion simultaneously. We studied the extent to which the motion of the individual fingers was similar. Subjects were asked to type lists of words, each word designed so that only one key would be pressed by a finger of one hand, the remaining letters being typed with the other hand. Changes in the lengths (flexion-extension) and orientation (abduction-adduction) of each finger were measured and the similarity of the motion of pairs of fingers was assessed by computing correlation coefficients. For each pair of fingers, the correlation coefficients were broadly distributed, but in most instances the mean was significantly greater than zero. Adjacent fingers showed a higher degree of correlation than did non-adjacent fingers. When one of the fingers was actually used to press the key, the degree of correlation decreased substantially. The results demonstrate that in this skilled motor task, the fingers usually tend to be moved together, but they can be moved independently when the task so requires.