Large quantities of ATP are required for the movement of the myosin heads, the active transport of calcium ions into the SR and the sodium-potassium pump restoring the sarcolemma to its resting potential after an action potential.Most of the ATP comes from oxidative phosphorylation which occurs in the presence of oxygen during aerobic respiration. Aerobic respiration is therefore sufficient for long periods of moderate exercise.If oxygen is used up faster than it can be replaced, e.g. in very active muscle during intense exercise, creatine phosphate can combine with ADP to form ATP for a short burst, but this is used up quickly and ATP is then made anaerobically by glycolysis only. This results in the build-up of lactic acid and muscle fatigue.