Becs said that a lot of people think that runners don't need to do upper-body strength training because you aren't using that part of your body during runs. "They think it's just the legs that work," she said, but that's not true. "You'll see a lot of good runners spend a lot of time in the gym working on cables. They work on their posterior chain, on their back, because you need a very strong upper back in order to hold your shoulders in that great open position and drive your arms for extended periods of time." She added that when your legs get tired, having a good arm swing will help you keep moving forward. Check out this total-body strength workout for runners.