Exploitation of elasticity as a biomechanical property in the production of graphic stroke sequences

In the present study we report several findings which indicate that subjects exploit elasticity of muscles and tendons as a biomechanical property of the motor system in the execution of graphic stroke sequences. The drawing movements of 15 right-handed subjects were analyzed, who copied a geometrical pattern consisting of four line segments. Three of these segments were connected by an acute and an obtuse angle. A first analysis concerning stroke-direction preferences shows that subjects tended to produce final strokes in preferred movement directions and obeyed an end-state stability constraint. Subsequently, we analyzed the copying movements with respect to (1) pauses at acute and obtuse angles, (2) local deviations in angle size, and (3) size variations of the strokes surrounding the angles. The results reveal a higher incidence of pauses at obtuse than at acute angles. Furthermore, a local sharpening of angles was found which was most pronounced at obtuse angles. Finally, systematic size variations of the strokes surrounding the angles were found. The results are considered to reflect the functional use of elasticity during task performance. It is concluded that biomechanical properties of the motor system significantly influence higher-order preparatory processes involved in multi-trajectory control.