Muscles require adenosine triphosphate (ATP) to function. Slow-twitch (red) muscle fibers have high levels of mitochondria and thus use oxidative phosphorylation to make ATP. However, this means that high concentrations of oxygen are required to generate the large amounts of ATP muscle cells need. There are two supplemental energy reserves in muscle: creatine phosphateis and anaerobic glycolysis.
Creatine phosphateis created by transferring a phosphate group from ATP to creatine during times of rest. This reaction can then be reversed during muscle use to quickly generate ATP from ADP:
creatine + ATP ⇌ creatine phosphate + ADP
Muscle also contains myoglobin, which binds oxygen with high affinity. As exercising muscles run out of oxygen, they use myoglobin reserves to keep aerobic metabolism going. Fast-twitch (white) muscle fibers have fewer mitochondria and must rely on glycolysis and fermentation to make ATP under most circumstances. When a person exercises, heart rate and respiratory rate increase in order to move more oxygen to actively respiring muscles. The oxyhemoglobin dissociation curve undergoes a right shift in the presence of increased carbon dioxide concentration, increased hydrogen ion concentration (decreased pH), and increased temperature. However, even with these adaptations, muscle use can quickly overwhelm the ability of the body to deliver oxygen. Then, even red muscle fibers must switch to anaerobic metabolism and produce lactic acid, at which point the muscle begins to fatigue. The difference between the amount of oxygen needed by the muscles and the actual amount present is called the oxygen debt. After the cessation of strenuous exercise, the body must metabolize all of the lactic acid it has produced. Most lactic acid is converted back into pyruvate, which can enter the citric acid cycle. This process requires oxygen, and the amount of oxygen required to recover from strenuous exercise is equal to the oxygen debt.