Pump your arms and your legs will follow. It's a refrain we've all said to ourselves during the last stretch of a long run, our legs losing the will to live, let alone run.

It's opposition, and although we learned to do it back in elementary school gym class, most of us have never given it much thought since then.

If you're a runner, you might want to. Opposition keeps you from falling over to one side. (Thanks, arms!) When running, your arms act as a counterbalance against your legs, says Alison Peters, M.S., senior clinical exercise physiologist at the Center for Musculoskeletal Care at NYU Langone Medical Center. And vice versa: When you're running along and speed up your arm swings, your legs will naturally pick up the pace so that, again, you don't fall over.

And while having less-than-strong arms probably won't land you on the pavement, strengthening your arm muscles is vital to keeping you upright, in proper form, and running efficiently, Peters says.

Plus, when you hit it hard during the home stretch of an endurance event, your arms have to kick into overdrive. If they are worn out and fatigued by the time you get to mile 12 in a 13.1, they may very well be why you don't hit a new PR.

That said, bulging biceps will only get you so far on the race course. When running, your triceps and deltoids are doing most of the work, Peters says. A lot of the forward swing, which you would think is thanks to your biceps, is actually momentum.

(Of course, it's still important not to ignore your biceps, because 1. they look good and 2. you don't want any muscular imbalances.) To train your triceps, try the diamond push-up. And for your delts, you can't go wrong with the dumbbell clean and press.

Don't forget your back: Strengthening your lats as well as the muscles around your shoulder blades can help keep your back straight, transfer power to your arms, and stabilize your shoulders, Peters says. Every runner should add this 15-minute back workout to his routine.