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.