The exact mechanism that results in the failure of muscles to continue contractions is unknown, but it is probably related to either chemical failure at the point where the nervous system connects with the muscles, or to a protective mechanism in the central nervous system meant to prevent muscular damage.
Exercising at high intensities may fortify the body against muscle failure by training the nervous system to call on more of its muscles for endurance activities. At higher levels of intensity, as in intervals, more fast-twitch muscles are recruited than when exercising at low intensity, as in long, slow workouts. Fast-twitch muscles contract quickly and need a long time to recover. They are best for sprinting, but by training zat moderately high effort, some of these take on the characteristics of slow-twitch, or endurance, muscles allowing the athlete to continue exercising longer.
Daryl Conant, M.Ed.