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As a runner, you’ve probably not given too much thought to training your arms. After all, it’s the lower body that’s doing all the work, launching you from one foot to the other, with the upper body along for the ride. Right?

Partially, yes, says exercise physiologist Janet Hamilton, C.S.C.S., head running coach at Atlanta’s Running Strong Professional Coaching. “The arms aren’t driving this bus—the butt is!” she says. “The driving forces in running come primarily from the muscles of the lower extremity and core.” However, “if a runner is weak in his or her upper body or upper core, then there is an imbalance of strength, and they won’t be as efficient.”

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The ways you hold and swing your arms can tell you a number of things about your running, and what you can do to improve it. First, you have to understand the body mechanics. When you run, your weight is shifting from one leg to the other at a quick and regular pace. Your arms, therefore, will naturally swing as a counterbalance. “If you’re standing on your left foot and your right foot is coming forward, your left arm will also travel forward to keep you upright,” explains Alison Peters, clinical exercise specialist at NYU Langone Medical Center’s Sports Performance Center.

Here, three ways you might be using your arms—and therefore not running optimally:

Your Arms Wing Out from Your Body
This is a sign that you have weaknesses in the abs or glutes, because the arms have to go farther from the body to maintain balance. What that means you need to add to your training: more planks and more hill repeats, says Andrew Kastor, head coach of the elite Asics Mammoth Track Club in California. “My elite team does lots of core stability work—not crunches or rotation exercises—because when you’re running, you want to not flex your spine. They also do lots of hills, because running is what makes you a stronger runner.”

You Clench or Hunch Your Shoulders, Tightly Bend Your Elbows, or Cross Your Hands in Front of Your Body
These are signs of tension and wasted energy. “In anything slower than a sprint, you want your arms to be relaxed,” says Peters. That means your elbows should be bent at about a 90-degree angle, with hands gently brushing by where your front jeans pockets would be. “Cup your hands so that the thumbs rest lightly against the first two fingers, as if you’re holding potato chips but don’t want to break them,” says Hamilton. Your arms will arc slightly as they come forward, but shouldn’t cross your body’s midline.

You Arms Have Different Movements or Ranges of Motions
This is a clear sign of muscle imbalance… in your glutes. “Your push-off is weaker on one side,” says Peters, who recommends focusing attention on squeezing your butt muscles as you push off each foot to propel yourself forward. She suggests having someone film you from behind while you’re running so you can then watch where your arms travel when you’re in action.

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PLUS… When Arm Swing Matters the Most
There is one time in a runner’s repertoire that a powerful arm swing may make a difference: in a sprint, typically at the end of a race. “The arms account for about two to three percent of the forward velocity in a sprint, as opposed to less than half a percent in distance running,” says Kastor. By pumping your arms on that final stretch when your legs are fatigued, you could make the difference between a good race and personal record.

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Amy Roberts is a certified personal trainer.

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