Andrew K. Yegian, Yanish Tucker, Stephen Gillinov, and Daniel E. Lieberman
Stereotypically, walking and running gaits in humans exhibit different arm swing behavior: during walking, the arm is kept mostly straight, while during running, the arm is bent at the elbow. The mechanism for this behavioral difference has not been explored before. We hypothesized that a mechanical tradeoff exists between the shoulder joint and the elbow joint. Bending the elbow reduces the radius of gyration of the arm and reduces shoulder muscle torque, but at the price of increasing elbow torque. We predicted that the mechanical tradeoff would result in energetics that favored straight arms during walking and bent arms during running. The hypothesis was tested experimentally by having eight subjects walk and run with both straight arms and bent arms while recording arm swing mechanics, and oxygen consumption in a subset of six subjects. The mechanical tradeoff hypothesis was confirmed, with bent arms reducing normalized shoulder muscle torque in both gaits (walking: –33%, running: –32%) and increasing normalized elbow muscle torque in both gaits (walking: +110%, running: +30%). Bent arms increased oxygen consumption by 11% when walking, supporting our prediction that energetics favor straight arms during walking. However, oxygen consumption was equivalent for the straight and bent arm running conditions, and did not support our running prediction. We conclude that straight arms are stereotyped in walking as a result of optimal energetics, while the mechanism leading to bent arms during running remains unknown.