ATP is the immediate source of energy for muscle contraction. However, the ATP stores in the muscle can sustain muscle contraction for up to 3 seconds. In about 3 sec, all the ATP is depleted from the muscle cell. Thereafter, ATP is regenerated using the energy released by the dephosphorylation of creatine phosphate reserves of the muscle fiber.
Creatine phosphate + ADP-------------------- > creatine + ATP
The creatine phosphate reserves (20 mM/L) of the muscle fiber can sustain contraction for about 5 more seconds, i.e., up to 8 seconds. After depletion of creatine phosphate reserves, further supply of energy for regeneration of ATP comes from glycol sis, which can sustain muscle contraction up to 1 minute. The end product of glycol sis is pyruvate and ATP molecules. When O, is available, the acetyl-CoA is metabolized completely through the Kreb's cycle. In absence of oxygen however, the acetyl CoA molecules condense to form lactic acid.
Pyruvate + NADH <------------------> Lactate + NAD+
The lactic acid is released into blood from where it is taken up by the liver and kidney, reconverted into glucose and released back into circulation. This recycling of lactic acid is called Cori cycle.
During anaerobic glycol sis, conversion of 1 molecule of glucose- 6-phosphate to 2 molecules of pyruvate releases 3 molecules of ATP. If the muscle utilizes glucose taken up from blood, the net ATP generation drops to 2 molecules per molecule of glucose. This is because 1 ATP molecule is used for phosphorylating glucose to glucose-6-phosphate. Glycogenolysis of muscle glycogen however yields glucose-6-phophate without requiring ATP consumption and therefore, the net ATP yield remains 3 molecules of ATP per molecule of glucose.
Glycol sis alone can sustain contraction for about 1 VI minutes only since lactic acid accumulation makes prolonged contraction difficult. In presence of oxygen, the terminal product of glycol sis viz., pyruvate is converted into acetyl CoA and fed into the Kreb's cycle in which 38 molecules of ATP are generated. Kreb's cycle can sustain muscle contraction for several hours.