Muscle carnosine levels, as measured by proton magnetic resonance spectroscopy, exhibit a negative sigmoidal relationship with the logarithm of an athlete’s best competitive running distance, such that carnosine concentration decreases progressively as the endurance demand of the athlete’s primary event increases from 100 meters to the marathon.

When athletes train for long-distance running, their muscles adapt by increasing the proportion of slow-twitch fibers, which naturally contain less carnosine. This shift reduces the total amount of carnosine in the muscle because slow-twitch fibers produce and hold less of this molecule than fast-twitch fibers. As the training becomes more endurance-focused, the muscle becomes dominated by these low-carnosine fibers, causing carnosine levels to drop in a predictable, stepwise pattern from sprinters to marathoners.