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Phys Ed Gretchen Reynolds on the science of fitness.

How we hold our arms affects how we run, but probably not in ways that most of us would expect, according to a new study of upper body biomechanics. The ideal arm swing may be the one that you’re already using, the study concludes.

Distance running is, of course, physiologically costly, meaning that it requires large outlays of energy. Almost every aspect of the activity adds to that energy expenditure, like holding your body upright and metronomically swinging first one leg and then the other forward and toward the ground.

But scientists and some running-form coaches have speculated that pumping your arms, although requiring energy, reduces the overall metabolic cost of running by helping to balance the moving body, increase forward propulsion or, perhaps, provide a bit of bounce, helping to lift us off the ground with each stride. In this theory, swinging the arms makes it easier to run.

That idea, however logical it might sound, had not been proved. So for the new study, published last week in The Journal of Experimental Biology, researchers at the University of Colorado at Boulder invited 13 experienced adult runners to pull on their favorite running shoes and visit the university’s locomotion lab.

During their first session, the runners were fitted with masks to track how much oxygen they took in and carbon dioxide they puffed out. Those measures establish energy usage. The runners stood quietly for seven minutes as the scientists determined their baseline numbers.

Then they ran on treadmills at a comfortable pace while holding their arms normally or in one of three increasingly unorthodox positions. In one instance, they held their arms loosely behind their back; in another, their arms were crossed at the chest, like a mummy’s; and in the last, they held their hands, fingers entwined, at the back of their skulls. In each case, the volunteers ran for seven minutes, with a rest period between each run. Their respiration was monitored throughout.

On a separate lab visit, the runners wore reflective markers on their shoulders, trunk and legs and repeated the four variations on arm positioning, as the researchers filmed them with three-dimensional motion-capture cameras.

The results showed, as the scientists had expected, that the volunteers used the least energy and were most efficient when they ran normally, their arms swinging at their sides. With each change in arm position, their efficiency dropped. Holding their arms behind their backs required 3 percent more energy than running normally; draping them across their chests used 9 percent more; and parking them on their heads demanded 13 percent more energy.

The motion-capture recordings established why the oddball arm positions were so inefficient. When the runners did not swing their arms, the biomechanical measurements showed, they could not readily counterbalance the pendulum action of their legs. Their upper bodies began to oscillate. Like Weebles, they wobbled, increasing their bodily movements and energy expenditure. The runners’ upward momentum did not change when they did not use their arms, undercutting the idea that arm swing provides bounce.

Essentially, the scientists found that arms were a nice accessory for runners to have.

“Normal arm swing is energetically a much cheaper way to counteract the motion of the legs than using the muscles in the torso,” said Christopher Arellano, an National Institutes of Health postdoctoral fellow at Brown University and lead author of the study.

That conclusion, although foreseeable, had needed to be tested, said Rodger Kram, a professor of integrative physiology at the University of Colorado and the study’s senior author. “Obviously, it’s not likely that anyone would run with their hands on their head,” he said, “but we wanted to see what would happen if they did.” The answer is that every stride became a bit more grueling.

At the same time, the study’s results offer surprising encouragement to those whose arm swing might be idiosyncratic.

“There was tremendous variation in the normal arm swings” of the volunteers, Dr. Arellano said. All bent their elbows, but apart from that, some were stiff and robotic, others noodly. Most but not all crossed their arms slightly in front of their chest with each swing. Efficiency was largely unaffected by these differences, the researchers concluded.

“This is good news,” Dr. Kram said. “There’s been a vogue for telling runners that they have to hold their arms this way or that way and not cross them in front of the chest.”

But the study’s findings emphasize that there is no single, ideal way to swing the arms, he said, as long as you swing them at all. “Most people,” he said, “will settle into the arm swing that is the most efficient for them.”