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A small amount of free ATP is available in the muscle for immediate use.
Phosphocreatine provides phosphates to ADP molecules, producing high-energy ATP molecules. It is present in low levels in the muscle.
Glycolysis converts glucose to pyruvate, water and NADH, producing two molecules of ATP. Excess pyruvate is converted to lactic acid which causes muscle fatigue.
Cellular respiration produces further molecules of ATP from pyruvate in the mitochondria. It is also required to resynthesize glycogen from lactic acid and restore stores of phosphocreatine and ATP in the muscle.
Muscle contractions are fueled by adenosine triphosphate (ATP), an energy-storing molecule. Four potential sources of ATP power muscle contractions.
Low levels of ATP exist within the muscle fibers and can immediately provide energy for contraction. However, the pool is very small and after a few muscle twitches will be exhausted.
Phosphocreatine, also known as creatine phosphate, can rapidly donate a phosphate group to ADP to form ATP and creatine under anaerobic conditions. Enough phosphocreatine is present in the muscle to provide ATP for up to 15 seconds of contraction.
The reaction of phosphocreatine + ADP to ATP + creatine is reversible. During periods of rest, the store of phosphocreatine is regenerated from ATP.
Glycolysis is the metabolic reaction which produces two molecules of ATP through the conversion of glucose into pyruvate, water, and NADH in the absence of oxygen.
The glucose for glycolysis can be provided by the blood supply, but is more often converted from glycogen in the muscle fibers. If glycogen stores in the muscle fibers are expended, glucose can be created from fats and proteins. However, this conversion is not as efficient.
Pyruvate is continually processed into lactic acid. With pyruvate accumulation, the amount of lactic acid produced is also increased. This lactic acid accumulation in the muscle tissue reduces the pH, making it more acidic and producing the stinging feeling in muscles when exercising. This inhibits further anaerobic respiration, inducing fatigue.
Glycolysis alone can provide energy to the muscle for approximately 30 seconds, although this interval can be increased with muscle conditioning.
While the pyruvate generated through glycolysis can accumulate to form lactic acid, it can also be used to generate further molecules of ATP. Mitochondria in the muscle fibers can convert pyruvate into ATP in the presence of oxygen via the Krebs Cycle, generating an additional 30 molecules of ATP.
Cellular respiration is not as rapid as the above mechanisms; however, it is required for exercise periods longer than 30 seconds. Cellular respiration is limited by oxygen availability, so lactic acid can still build up if pyruvate in the Krebs Cycle is insufficient.
Cellular respiration plays a key role in returning the muscles to normal after exercise, converting the excess pyruvate into ATP and regenerating the stores of ATP, phosphocreatine, and glycogen in the muscle that are required for more rapid contractions.