The first source of energy that is used to reconstitute the ATP is the substance Phosphocreatine. The combined energy of both the stored ATP and the phosphocreatine in the muscle is capable of causing maximal muscle contraction for only 5 to 8 seconds. The second impoant source of energy, which is used to reconstitute both ATP and phosphocreatine, is glycolysis. The third and final source of energy is Oxidative metabolism If the energy demands of exercise cannot be met by oxidative phosphorylation, an oxygen debt is incurred. After completion of exercise, respiration remains above the resting level in order to "repay" this oxygen debt. The extra oxygen consumption during this recovery phase is used to restore metabolite levels (such as creatine phosphate and ATP) and to metabolize the lactate generated by glycolysis. Pathways for regenerating ATP during muscle activity