Kinematics and kinetics of multijoint reaching in nonhuman primates

The present study identifies the mechanics of planar reaching movements performed by monkeys (Macaca mulatta) wearing a robotic exoskeleton. This device maintained the limb in the horizontal plane such that hand motion was generated only by flexor and extensor motions at the shoulder and elbow. The study describes the kinematic and kinetic features of the shoulder, elbow, and hand during reaching movements from a central target to peripheral targets located on the circumference of a circle: the center-out task. While subjects made reaching movements with relatively straight smooth hand paths and little variation in peak hand velocity, there were large variations in joint motion, torque, and power for movements in different spatial directions. Unlike single-joint movements, joint kinematics and kinetics were not tightly coupled for these multijoint movements. For most movements, power generation was predominantly generated at only one of the two joints. The present analysis illustrates the complexities inherent in multijoint movements and forms the basis for understanding strategies used by the motor system to control reaching movements and for interpreting the response of neurons in different brain regions during this task.