Primate upper limb muscles exhibit activity patterns that differ from their anatomical action during a postural task.

The present study examined muscular activity in the primate proximal forelimb during a posture task. By applying loads selectively to the shoulder, elbow, or both joints, we observed that monoarticular shoulder and elbow muscles varied their activity with loads at the unspanned joint. Shoulder monoarticulars changed activity with elbow torque and elbow monoarticulars changed activity with shoulder torque. Due to this additional modulation, the maximal activation of monoarticular muscles was deviated from their anatomical action toward either shoulder-extension/elbow-flexion or shoulder-flexion/elbow-extension. Biarticular muscles also expressed deviations in their preferred torque direction toward either shoulder-extension/elbow-flexion or shoulder-flexion/elbow-extension. The biased distribution of preferred torque directions in proximal forelimb muscles could be modeled by the minimization of a global measure of muscle activity. Moreover, arm-related neurons of primary motor cortex exhibit a similar bias in preferred torque directions consistent with the intimate relationship between the primary motor cortex and the motor periphery.