Interjoint coordination during handwriting-like movements

The present study investigates intrinsic preferences and tendencies in coordination of the wrist and finger movements during handwriting-like tasks. Movement of the inkless pen tip in nine right-handed subjects was registered with a digitizer. One circle-drawing task and four line-drawing tasks were included in the experiment. The line-drawing task included: (1) Drawing with the wrist only, (2) drawing with the fingers only, (3) an equivalent pattern consisting of the simultaneous flexion/extension of the wrist and fingers, and (4) a nonequivalent pattern in which wrist flexion was accompanied by finger extension and wrist extension was accompanied by finger flexion. Both the line and circle drawing were performed repetitively at four speed levels, ranging from slow to 'as fast as possible' movements. The analysis of the line drawing revealed differential variability and temporal characteristics across the four movement patterns. While the equivalent pattern had characteristics of performance similar to those observed in the wrist-only and fingers-only pattern, the nonequivalent pattern was more variable and was executed slower when as fast as possible movement was required, compared to the other three patterns. The circle-drawing task also revealed intrinsic tendencies in coordination of the wrist and fingers. These tendencies were manifested by a spontaneous transition of the circular path of the pen tip to a tilted oval with increases in movement speed. The transition to the oval shape was accompanied by decreases in relative phase between the wrist and finger movements, whereas amplitudes of these movements were not affected by movement speed manipulations. The results suggest that subjects did not display a tendency to decrease the number of joints involved when executing the patterns that required simultaneous wrist and finger movements. Instead, there were preferences during these patterns to integrate wrist and finger movements with low relative phase. The findings are interpreted in terms of biomechanical constraints imposed on the wrist-finger linkage. This interpretation was further examined by testing two left-handed subjects. The data obtained showed symmetrical preferences in joint coordination. Collectively, the findings support a supposition that the shape of cursive letters may have been adjusted to the biomechanical structure of the hand to facilitate the motor act of handwriting.