Most of us swing our arms when we run, but why? Scientists know there is a mechanical benefit to the motion: Swinging arms counterbalance the momentum of a person’s legs, providing stability to the runner. The jury was out, however, on whether the activity saves energy. In a new study, researchers compared the energy cost of running in four different positions. The experiment looked at 13 subjects’ oxygen consumption and carbon dioxide production while running. Reporting this week in The Journal of Experimental Biology, the team concludes that swinging your arms uses 3% less energy than keeping your hands behind your back, 9% less energy than folding your arms over your chest, and 13% less energy than running with your hands above your head. The study notes that the muscular power used while holding the arms in unusual positions may contribute to some of the extra energy cost.