Non-uniform temporal scaling of hand and finger kinematics during typing

We examined the manner in which the keystroke kinematics of the hand and the fingers varied with the mean rate of typing by trained typists. We used words and phrases in which only one letter was typed with the right hand and all of the remaining letters were typed using the left hand. We varied the typing rate over a threefold range (intervals between keypresses ranging from 150 ms to 500 ms) with the aid of a metronome. The results from four subjects, and three letters (n, u, and o) were analyzed. We did not find a simple scaling that could account for variations in the velocity profiles with typing rate. For some subjects and some letters, the velocities were independent of typing rate. In other instances, the kinematics did depend on typing rate, but to a much greater extent prior to the time of keypress than afterward. Sometimes the velocity profiles of all of the fingers and of the hand changed in a similar manner as the interval between keypresses was varied. In other instances only the focal movement of the hand and the finger used to press the key depended on the interval, whereas the motions of the other fingers did not. We suggest that the consistencies in the velocity profiles which we observed may simplify the problem of arranging a temporally ordered sequence of goal-directed movements.