Meeting Abstract
Humans stereotypically bend their arms at the elbow while running, as opposed to maintaining relatively straight arms during walking. During both walking and running, muscles at the shoulder aid in arm swing while muscles at the elbow resist rotation at the elbow. Using a two-segment biomechanical model of arm swing we hypothesize that there is a tradeoff between the muscle moments at the shoulder and at the elbow: bending the arm reduces the shoulder moment magnitude by reducing rotational inertia of the arm, but increases the elbow moment magnitude by orienting the forearm more perpendicular to destabilizing vertical forces. Experimental data from eleven subjects asked to walk and run with straight and bent arms support the tradeoff hypothesis, and imply that the tradeoff favors straight arms during walking and bent arms during running. In addition, we added mass to the lower arm during both normal walking and normal running and observed a proportionally larger increase in the elbow moment magnitude during running. From an evolutionary perspective, these results suggest that the low Brachial Index (radius to humerus ratio) observed in the genus Homo starting with Homo erectus can be explained by the evolution of endurance running and the hunter-gatherer lifestyle. Earlier hominins, which are thought to have mostly relied on walking, invariably show higher Brachial Indices than Homo erectus. Increased reliance on running by Homo erectus likely amplified selection for shorter lower arms, which modern humans still retain.
