Anaerobic Respiration & Exercise

When you exercise, your muscles consume energy. They derive that energy either from aerobic respiration, which requires oxygen, or from anaerobic respiration, which does not. Anaerobic respiration is faster, but less efficient, than aerobic respiration. Your muscles use anaerobic respiration when they need energy quickly, such as during high-intensity exercise.

Alactic Anaerobic System

Energy is stored in your cells in the form of a compound called adenosine triphosphate, or ATP, which can be broken down quickly to provide immediate energy. Your muscle cells store only enough ATP for a few seconds of maximal work. However, they also store another high-energy compound called creatine phosphate, which can be used to recharge ATP. Together, ATP and creatine phosphate form the phosphagen energy system, sometimes called the alactic anaerobic system.

Replenishing ATP

The alactic anaerobic system provides most of the energy for very short-duration, high-intensity forms of exercise, such as Olympic weightlifting and 100-meter sprints. A muscle cell can exhaust its ATP and creatine phosphate stores in about 10 seconds of maximal contraction, so to continue exercising longer, your muscles must replenish their ATP supply. They do this by metabolizing fuels such as carbohydrates and fats, either using aerobic or anaerobic respiration.

Anaerobic Glycolysis

When you exercise at a high intensity, such as during interval training, your circulatory system cannot provide oxygen to your muscles fast enough to maintain aerobic respiration. When that happens, your muscles switch to anaerobic respiration. Muscle cells metabolize carbohydrates without oxygen in a process called anaerobic glycolysis. The end product of anaerobic glycolysis is lactic acid, which can build up in your muscles and bloodstream. Although anaerobic glycolysis is not as fast as the alactic anaerobic system, it is still very fast. However, there is a tradeoff. A single molecule of glucose, or blood sugar, yields just two ATPs with anaerobic glycolysis. The same glucose molecule could yield up to 36 ATPs through aerobic respiration.

Lactic Acid

Although lactic acid does not directly cause muscle fatigue, high levels of lactic acid are associated with fatigue. The point where lactic acid begins to accumulate in your blood is called the anaerobic threshold. During many types of exercise, your muscles use a combination of aerobic and anaerobic respiration. For instance, during a marathon most of your energy comes from aerobic respiration. As you increase your speed, however, your muscles must rely more on anaerobic respiration. Past your anaerobic threshold, it will be difficult to maintain that high level of energy output for an extended period of time, and you will likely see your performance drop.