Muscle fatigue is weakness or weariness resulting from exertion or prolonged stress and the failure to maintain an expected power output. (Amussen) The process by which your body produces energy is called glycolysis. During glycolysis, glycogen is broken to produce creatine phosphate, which releases energy. The energy released catalyzes a reaction to produce ATP. The ending product of glycolysis is lactic acid, which is created by breaking pyruvate acid down. Then lactic acid is broken down to produce lactate. For each compound of lactic acid that is formed the cell gains a lactate compound and an H+. The increase of H+ in a cell causes the pH to decrease, which causes the cell to become acidic. The acids in muscles cause the fibers' calcium-binding capacity to decrease, which then limits muscle contraction, which causes muscle fatigue. (Mullick) During muscle fatigue, feedback of nerve impulses from the fatigued muscles interrupts on a part of the reticular formation and causes a hang-up of voluntary effort.
(Amussen) It is important to understand that muscle fatigue results from metabolic reasons and not structural changes in your muscle.
Peripheral muscle fatigue involves the motor units, such as motor neurons, peripheral nerves, motor endplates, and muscle fibers. There are two different sites where repeated contractions may cause muscle fatigue. One is the transmission mechanism, which is basically the neuromuscular junction, muscle membrane, and endoplasmic reticulum. The other one is the contractile mechanism, which is the muscle filament. A muscle fiber's mechanical response declines with fatigue. Peripheral muscle fatigue is caused by changes in the internal conditions of the muscle. The changes can be biochemical, depletion of substrates, high-energy phosphate compounds in the muscle fibers, and acetylcholine in the terminal motor nerve branches, or they may be caused by the accumulation of metabolites. (Amussen)
Cnetral muscle fatigue...