Glucose, a simple sugar, is the body's primary source of energy. Cells in the body can quickly and efficiently convert glucose to adenosine triphosphate, or ATP -- the energy currency that enables all bodily functions to occur. Glucose breaks down to ATP by 2 pathways: aerobic metabolism, which requires oxygen, and anaerobic metabolism, which occurs without oxygen. Exercise can help increase the efficiency of anaerobic metabolism of glucose.
Anaerobic Metabolism of Glucose
Anaerobic metabolism of glucose is a step-wise biochemical process called glycolysis or fermentation and can be performed by most cells in humans, animals and plants. Glycolysis occurs in the cytosol -- the liquid portion of the cell -- and produces energy quickly but not that efficiently. The anaerobic breakdown of 1 glucose molecule results in the production of 6 ATP molecules, 2 pyruvic acid molecules and 4 hydrogen ions, according to "Guyton and Hall Textbook of Medical Physiology." By comparison, aerobic metabolism of 1 glucose molecule results in 38 ATP molecules.
Intensity versus Duration
Muscle cells use anaerobic glycolysis to produce ATP very quickly, about 2.5 times faster than aerobic metabolism, enabling powerful bursts of activity. Historically this could fuel "fight or flight" survival activities, such as sprinting from predators or lifting heavy objects. Unfortunately, the intensity, or high power output, of these activities can only be maintained for short periods because of the quick use of the fuel source -- glucose -- and the production of acidic byproducts. On average, glycolysis can only produce ATP for 30 seconds to 1 minute, although with training this can be increased, according to a 2009 study published in "Sports Medicine." Activities that are sustained for more than 45 minutes increasingly use aerobic metabolism to produce ATP.
The pyruvic acid and hydrogen ions that are produced by anaerobic glycolysis must be managed. Pyruvic acid can undergo 2 different processes: It can enter the mitochondria, where it is further metabolized aerobically, or it can be converted to lactic acid. Conveniently, lactic acid can bind to hydrogen, creating lactate, which can exit the cell and eliminate buildup inside the muscle. Unfortunately, this process does not occur fast enough to balance the production of hydrogen ions by glycolysis, and the muscle cell becomes acidic, creating the burning sensation felt during intense exercise.
Training the Glycolytic System
Anaerobic training is an effort to increase the ability to produce ATP using the glycolytic pathway and to manage its byproducts. According to the American College of Sports Medicine, power training should consist of 30 to 45 seconds of intense exercise, such as weightlifting or sprinting, with 3 to 5 minutes of rest between sets. Individual physical capacity, training status and target goals should be incorporated into any training regime. Speak to a doctor or personal trainer if you have any concerns about a new exercise regime.