The emergence of global coordination during multijoint pulls made while standing

Abstract

This study evaluated whether global coordination improves and body rigidity decreases as subjects learn to perform a complex, multijoint action. Freely-standing subjects practiced making abrupt impulse-like horizontal pulls in the sagittal plane, against a handle held bilaterally, to different target forces. Global coordination was modeled as a second-order equation relating the dynamics associated with anterior-posterior center of mass motion (CM AP) to horizontal pulling force ( F P). The stiffness ( K E) parameter of the model provided an estimate of global body rigidity. Five subjects had three days' practice in producing peak forces equalling 25, 40, and 55% of their estimated maximum, receiving verbal feedback about force production after every other trial. CM AP coordinates and anterior-posterior pulling force ( F P) were measured on Days 1 and 3. Force errors, model fit and K E were compared between the first ten trials to each target on Day 1, and the last ten trials at each target on Day 3. Practice resulted in significantly more consistent force production, better fits of the model to the data, and lower global K E. These findings suggest that the modeled global coordinative function was learned and that global stiffness decreased as subjects became more skilled on the multijoint pulling task.