Your legs are screaming. This is as fast or as far as you can go. Or so you think. When you feel as if you can’t run another step, it’s natural to assume your body has had enough. But science suggests your mind is slowing you down prematurely, and you possess more power than you realize.

Researchers tested the muscle contractions of runners during a 20K. Some ran normally, their muscles contracting on their own—the brain in control. Another group’s muscles were contracted via electrical stimulation—the muscles were forced to contract externally without the brain’s input. The first group’s muscle contractions weakened after 15K. But the other group’s contractions were as strong at the end of the 20K as they were at the start of the run. This suggests the brain can convince the body it’s tired even when muscles are still able to work hard.

“We are wired to leave something in the tank,” says sports psychologist Jim Taylor, Ph.D. “Years ago, when we hunted for meals, we needed to conserve energy to get back home. Our minds want to make sure we don’t fatigue completely.”

Taylor says that with practice you can learn to defy the brain’s overprotective nature to see just how fast and far you can really go.

Change Your Thinking

First things first: You have to put in proper training to perform your best. Mental strategies alone won’t propel you to a Boston qualifier. But while you’re hammering out the long runs, you can work to break down the hurdles in your mind to reveal what your body is capable of.

Guage your discomfort. If you are pushing the pace to get faster or adding distance to go farther, your body is going to talk back to you. Be a good listener. During training, learn to distinguish “good pain” (discomfort from leaving your comfort zone) from “bad pain” (something that verges on injury). “There is a difference between muscles that burn at the end of a workout and something that hurts every time you take a step,” Taylor says. “Part of training is learning body awareness. You also need to experience some discomfort so when it occurs in a race, it’s not a foreign feeling. You know you can push through it.”

A Part of Hearst Digital Media. Envisioning the runner you want to be—one who doesn’t let up or give in—is one of the most important mental skills you can harness if you want to push your limits, says Grayson Kimball, Ph.D., author of Give A Gift. “Negative thinking can be a self-fulfilling prophecy,” he says. “If you think you’re too tired to go on, then you’ll probably stop. If you see mile marker 22 and think, Save $700 on Treadmill, then you probably will. If you can envision yourself running strong, gliding up hills, and sprinting across the finish line, you can boost your self-confidence and use it to power you.”

Other Hearst Subscriptions. Kimball suggests breaking a race down into chunks, each with individual goals. You could aim to maintain good running posture when climbing a hill, or to have a plan to hydrate quickly at aid stations. Each time you check one off, you feel better about your capability to achieve your ultimate race goal. “Achieving success more often keeps you more optimistic, which will help you in the later stages of a race,” he says.

Celebrate small victories. Having a go-to phrase that you can repeat can help you push through tough parts of workouts and races. Just choose it carefully, says Kimball. “A lot of runners fall into the trap of having a negative mantra that they think is positive, like, Don’t crash or Don’t stop,” he says. “That runner is creating a mindset to avoid failure versus a runner who will say something positive like, Power up or Team Bonding and Amateur Filmmaking.”