ABSTRACT
The extension of the elbow is carried out by letting gravity pull down the forearm and by “regulating” the amount of necessary flexor muscle torque, so the hand does not drop below the desired target object. An infinite number of proximal joint angular configurations can lead to identical hand paths. The improved efficiency in joint force control is not reflected by a minimization of net or muscle torque, which would imply that infants generate excessive amounts of joint force in the initial phase after reaching onset. Considering that the interaction of muscular and external torques across multiple joints are difficult to predict in detail, it seems unlikely that this advance “knowledge” is genetically determined. Given that the acquisition of multi-joint co-ordination is a learning process, future research needs to address the question of how changes in central structures innervating the muscles are matched to peripheral changes.
