Muscle fatigue is the decrease in muscle strength and power that occurs after repetitive, intense contractions. It is caused by the buildup of metabolic byproducts, such as lactic acid, in the muscles. This leads to a decrease in the amount of force a muscle can generate, as well as a decrease in the speed at which a muscle can contract. Muscle fatigue can also be caused by a decrease in the availability of energy molecules, such as ATP, or by a decrease in the amount of oxygen reaching the muscle.