Trainers and athletes already know that sprinters and marathon runners have very different muscles. Sprinters tend to have a preponderance of fast-twitch muscle fibers, which generate short bursts of strength, while endurance athletes have more slow-twitch fibers, which take longer to fatigue.

New research shows that a genetic variation found more often in long-distance athletes seems to make fast-twitch muscle fibers behave more like slow-twitch ones. The variation, known as R577X, completely knocks out a muscle protein expressed only in fast-twitch fibers. Mice genetically engineered to carry this variation had more efficient muscle metabolism and could run 33 percent farther before reaching exhaustion than their normal counterparts, according to findings published Sunday in Nature Genetics.

This variation also seems to confer an evolutionary advantage in humans. By analyzing the DNA sequence surrounding it, scientists found that is has been positively selected for in recent human history.