Neuromechanical linkage between the head and forearm during running

The main objective was to test the hypothesis of a neuromechanical link in humans between the head and forearm during running mediated by the biceps brachii and superior trapezius muscles. We hypothesized that this linkage helps stabilize the head and combats rapid forward pitching during running which may interfere with gaze stability.

Thirteen human participants walked and ran on a treadmill while motion capture recorded body segment kinematics and electromyographic sensors recorded muscle activation. To test perturbations to the linkage system we compared participants running normally as well as with added mass to the face and the hand.

The results confirm the presence of a neuromechanical linkage between the head and forearm mediated by the biceps and superior trapezius during running but not during walking. In running, the biceps and superior trapezius activations were temporally linked during the stride cycle, and adding mass to either the head or hand increased activation in both muscles, consistent with our hypothesis. During walking the forces acting on the body segments and muscle activation levels were much smaller than during running, indicating no need for a linkage to keep the head and gaze stable.

The results suggest that the evolution of long distance running in early Homo may have favored selection for reduced rotational inertia of both the head and forearm through synergistic muscle activation, contributing to the transition from australopith head and forelimb morphology to the more human-like form of Homo erectus. Selective pressures from the evolution of bipedal walking were likely much smaller, but may explain in part the intermediate form of the australopith scapula between that of extant apes and humans.