The relation between pen force and pen-point kinematics in handwriting

This study investigates the spectral coherence and time-domain correlation between pen pressure (axial pen force, APF) and several kinematic variables in drawing simple patterns and in writing cursive script. Two types of theories are prevalent: “biomechanical” and “central” explanations for the force variations during writing. Findings show that overall coherence is low (<0.5) and decreases with pattern complexity, attaining its lowest value in cursive script. Looking at subjects separately, it is found that only in a small minority of writers “biomechanical coupling” between force and displacement takes place in cursive handwriting, as indicated by moderate to high negative overall correlations. The majority of subjects display low coherence and correlation between kinematics and APF. However, APF patterns in cursive script reveal a moderate to high replicatability, giving support to the notion of a “centrally” controlled pen pressure. The sign of the weak residual average correlation between APF and finger displacement, and between APF and wrist displacement is negative. This indictes that small biomechanical effects may be present, a relatively higher APF corresponding to finger flexion and wrist radial abduction. On the whole, however, variance in APF cannot be explained by kinematic variables. A motor task demanding mechanical impedance control, such as handwriting, apparently introduces a complexity that is not easily explained in terms of a passive mass-spring model of skeleto-muscular movement.