The way our arms swing when we walk doesn’t seem to make much sense. We don’t need to move our arms to move our legs, so why do it? It’s a question that’s long bothered scientists, whose theories included speculation that it was a good-for-nothing practice that we haven't evolved out of. But in 2009, researchers took a closer look to figure out why exactly we flail while we walk.

University of Michigan scientists measured the energy used by 10 people who walked a number of ways—swinging their arms, holding them to their sides, and so on. They ran similar tests on mechanical models of arms, and they found that the swinging actually has a purpose: It reduces the overall amount of energy it takes to walk.

The study found that people who hold their arms still while walking use twelve percent more metabolic energy than people who swing their arms naturally.

“Normal arm swing is the most economic way of walking,” says biomedical researcher Sjoerd Bruijn. A “normal” arm swing is what researchers call moving your left arm forward as you step with your right foot, and vice versa.

Swinging your arms in sync with your legs—putting your left foot forward with your left arm out—is much harder on your body, using 26 percent more energy than a normal walk.

The researchers even went so far as to strap the test subjects' arms down to their sides to see what happened if people didn’t have to exert any energy to keep their arms from swaying. Even with the help, they used seven percent more energy than when they walked with a natural swing.

Swinging your arms conserves energy because your arm muscles don’t have to do much work when your limbs are swaying. Think of it this way—your arm is like a pendulum; when your leg moves, your body moves, and that movement forces the pendulum to sway. Your arms move passively because of this natural motion, meaning your muscles aren’t exerting energy to make it happen. They do kick in a little effort to keep the swing in control, Brujin says, but the energy savings from swinging more than offset the tiny amount of energy needed to keep the swing going. Especially since the arm-swinging helps offset some of the force when your leg hits the ground, meaning your legs are using less energy.

Researchers have learned to harness the effects of arm-swinging for use in rehab practices with patients who have conditions like Parkinson’s disease or stroke damage. Studies show that vigorous arm movement in those patients helps lengthen their stride and improves their ability to walk.

Just because going against your natural arm swing uses more energy doesn’t mean it’s a good way to burn more calories, though. If you suppress your normal movements, you can do damage to your spine. Stick with your natural swing to keep your walk efficient.