Glycolysis

 

The energy for muscle contraction comes from ATP (adenosine triphosphate). ATP is formed by aerobic (oxygen-dependent) respiration in the mitochondria of muscle cells, using energy released by glycolysis. Breaking of a high-energy bond in ATP releases ADP (adenosine diphosphate), inorganic phosphate (Pi), and one proton or hydrogen ion (H^þ). The protons are utilized by the mitochondria.

The final step of glycolysis yields two molecules of pyruvate along with protons from the splitting of ATP. Under conditions of sufficient oxygen, pyruvate is further metabolized in the mitochondria; however, under conditions of insufficient oxygen availability, as is common during intense exercise, lactate (lactic acid) builds up in muscle.

Until recently, it was thought that this buildup of lactic acid caused acidosis, characterized by muscle soreness and fatigue, but it is now recognized that acidosis or muscle burn results from the release of more protons than can be utilized by the mitochondria. Pyruvate absorbs the protons to form lactate, temporarily slowing the development of acidosis and muscle fatigue.

Lactate is metabolized within 15–30 minutes after exercise. Muscle cells also store energy in phosphate bonds in compounds such as phosphocreatine, which is known as the phosphagen system. Release of phosphate from these compounds to provide energy also produces protons that contribute to acidosis.