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Muscle contraction occurs via metabolism of ATP derived primarily from the simple sugar glucose.
Explain the process involved in muscle metabolism during aerobic exercise
ATP is required for muscle contraction, and four sources are available to muscle fibers: free ATP, phosphocreatine, glycolysis and cellular respiration.
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). ATP is an energy storing molecule and there are four potential sources available to 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 rapdily 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 and during periods of rest the store of phosphocreatine is regenerated from ATP.
Glycolysis the 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 supplied as glucose through the bloody 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 although 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) which inhibits further anaerobic respiration inducing fatigue.
As such glycolysis alone can provide energy to the muscle for a relatively short period of approximately 30 seconds. Although with training and conditioning of the muscle this can be increased.
Whilst 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 extended periods of exercise upwards of 30 seconds. Cellular respiration is limited by oxygen availability and as such lactic acid can still build up if insufficient pyruvate can enter the Krebs Cycle.
Cellular respiration plays a key role in returning the muscles to normal after exercise, converting the excess pyruvate into ATP and also in regenerating the stores of ATP, phosphocreatine and glycogen in the muscle required for more rapid contractions.