Flexibility and repeatability of finger movements during typing: analysis of multiple degrees of freedom.

The kinematics of the hand and fingers were studied during various keystrokes in typing. These movements were defined by 17 degrees of freedom of motion, and methods were developed to identify simplifying strategies in the execution of the task. Most of the analysis was restricted to the 11 degrees of freedom of the fingers, neglecting thumb and wrist motion. Temporal characteristics of the motion were defined by computing principal components, and it was found that only a few (two to four) principal components were needed to characterize motion of each of the degrees of freedom. Hierarchical relationships among patterns within and between different degrees of freedom were identified using cluster analysis. There was a considerable amount of consistency each time a given keystroke was executed by a subject, and this repeatability may imply a reduction in the number of degrees of freedom independently controlled by the nervous system. However, there also appears to be considerable flexibility in the coordination of the many joints of the hand when examined across different keys and across different subjects.