<P> Substrates within the muscle generally serve to power muscular contractions . They include molecules such as adenosine triphosphate (ATP), glycogen and creatine phosphate . ATP binds to the myosin head and causes the' ratchetting' that results in contraction according to the sliding filament model . Creatine phosphate stores energy so ATP can be rapidly regenerated within the muscle cells from adenosine diphosphate (ADP) and inorganic phosphate ions, allowing for sustained powerful contractions that last between 5--7 seconds . Glycogen is the intramuscular storage form of glucose, used to generate energy quickly once intramuscular creatine stores are exhausted, producing lactic acid as a metabolic byproduct . </P> <P> Substrate shortage is one of the causes of metabolic fatigue . Substrates are depleted during exercise, resulting in a lack of intracellular energy sources to fuel contractions . In essence, the muscle stops contracting because it lacks the energy to do so . </P> <P> Metabolites are the substances (generally waste products) produced as a result of muscular contraction . They include chloride, potassium, lactic acid, ADP, magnesium (Mg), reactive oxygen species, and inorganic phosphate . Accumulation of metabolites can directly or indirectly produce metabolic fatigue within muscle fibers through interference with the release of calcium (Ca) from the sarcoplasmic reticulum or reduction of the sensitivity of contractile molecules actin and myosin to calcium . </P> <P> Intracellular chloride partially inhibits the contraction of muscles . Namely, it prevents muscles from contracting due to "false alarms", small stimuli which may cause them to contract (akin to myoclonus). This natural brake helps muscles respond solely to the conscious control or spinal reflexes but also has the effect of reducing the force of conscious contractions . </P>

Muscle fatigue occurs due to a buildup of