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.

Morphometry of Macaca mulatta forelimb. III. Moment arm of shoulder and elbow muscles.

The conversion of muscle activity into smooth, purposeful movement of the limb depends complexly on the morphometry of muscles and their mechanical action on the skeleton. Although nonhuman primates are common subjects in motor control experiments (Scott [2000] Can J Physiol Pharmacol 78:923-933), little information is available on the morphometric properties of their upper limbs. One key variable is muscle moment arm, or mechanical advantage, which defines how linear motion or force of a muscle is translated into angular motion or torque at a joint. This study reports moment arm values with respect to joint angle (flexion/extension) of 14 muscles spanning the shoulder and elbow in Macaca mulatta. The magnitude of moment arm values ranged widely across muscles. In some muscles mechanical advantage remained constant with joint angle, whereas the moment arm of others varied strongly. The angle (Theta(f)(o)) at which optimal fascicle length (L(f)(o)) occurred showed strong trends, where the elbow-spanning muscles had Theta(f)(o) values clustered at mid-flexion and the shoulder musculature Theta(f)(o) values tended to be grouped around the neutral joint angle of 0 degrees. Estimates of peak muscle torque for flexor and extensor muscle groups at each joint were surprisingly similar in both magnitude and dependency on joint angle. The present study, along with the previous two in this series (Cheng and Scott [2000] J Morphol 245:206-224; Singh et al. [2002] J Morphol 251:323-332), provides a comprehensive description of the morphology of the proximal portion of the limb suitable for the development of a musculoskeletal model of the M. mulatta upper limb.