Effects of geometric joint constraints on the selection of final arm posture during reaching: a simulation study

Significant debate exists regarding the neural strategies underlying the po sitioning and orienting of the hand during voluntary reaching movements of the human upper extremity. Some authors have suggested that positioning and orienting are controlled independently, while others have argued that a st rong interdependence exists. In an effort to address this uncertainty, our study employed computer simulations to examine the impact of physiological limitations of joint rotation on the proposed independence of hand position and orientation. Specifically, we analyzed the effects of geometric constr aints on final arm postures using a 7 degree-of-freedom model of the human arm. For 20 different hand configurations within the attainable workspace, we computed sets of achievable joint angles by applying inverse kinematics. From each set, we then calculated the locus of possible elbow positions fo r the particular final hand posture. When the joints were allowed 360 degre es of rotation, the loci formed complete circles; however, when joint range s were limited to physiological values, the extent of the loci decreased to an average are angle of 54.6 degrees (+/-27.9 degrees). Imposition of join t limits also led to practically linear relationships between joint angles within a solution set. These theoretical results suggest a requirement for coordinated interaction between control of the joints associated with hand position and those involved with hand orientation in order to ensure attain able joint trajectories. Furthermore, it is conceivable that some of the co rrelations observed between joint angles in the course of natural reaching movements result from geometric constraints.