Anaerobic Metabolism and Lactic Acid in Your Body

Fueling muscles without oxygen produces lactic acid.

Male runners jumping hurdles in race
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Anaerobic metabolism is the creation of energy through the combustion of carbohydrates in the absence of oxygen. This occurs when your lungs cannot put enough oxygen into the bloodstream to keep up with the demands from your muscles for energy. It generally is used only for short bursts of activity, such as when you go for a sprint when running or cycling or when you are lifting heavy weights.

When there isn't enough oxygen in the bloodstream, glucose and glycogen cannot be fully broken down to carbon dioxide and water.

Instead, lactic acid is produced, which can build up in the muscles and degrade muscle function. Learn more about anaerobic metabolism and what it means for how you exercise.

How Anaerobic Metabolism and Lactic Acid Affect You During Exercise

Lactic acid is a by-product of anaerobic glycolysis and anaerobic metabolism, both of which occur during strenuous exercise. Although lactic acid is used as a fuel by the heart, excessive lactic acid in your skeletal muscles slows down contractions, preventing you maintaining peak performance.

When your muscles use anaerobic metabolism, lactic acid is produced in your muscle cells. With moderate intensity exercise, it is able to diffuse out of the cells, but with vigorous muscle contractions it builds up.  As you build up more and more lactic acid, your muscles burn and are fatigued.

Often this is felt in activities like weight lifting, but you can reach it when running or even race walking at high speed, such as sprinting.

You are forced to back off and slow down so your muscles can recover and allow lactic acid to diffuse out of the cells. Lactic acid is further processed by the liver into glucose to use for fuel, completing the cycle.

Here are a few useful facts about how anaerobic metabolism affects your body during exercise:

  • Weight lifting and other strength exercises use anaerobic metabolism when providing strenuous effort in a short period of time.
  • When running or race walking very fast your body turns to anaerobic metabolism to get enough energy.
  • Sprinters' muscles use anaerobic metabolism to fuel their muscles during their short burst of speed.
  • Anaerobic metabolism produces lactic acid, which can build up in the muscles to the point where you "feel the burn." This burning sensation is a normal side effect of anaerobic metabolism.
  • Fast twitch muscle fibers rely more on anaerobic metabolism for quick contractions, but they fatigue more quickly as well.
  • With high-intensity intervals, you can turn a normally aerobic exercise like endurance running into an anaerobic exercise as anaerobic metabolism is needed once you exceed 90 percent of maximum heart rate.

How to Slow the Buildup of Lactic Acid

You can improve the point at which lactic acid builds up with specific training programs. Athletes often use these to improve their performance. They include a regimen of interval or steady state training that will bring them to their lactate threshold. It is also important to have the right diet so your muscles are well-supplied with glycogen for fuel.

The lactate threshold is usually reached between 50 percent to 80 percent of an athlete's VO2 max (maximal oxygen uptake). In elite athletes it can be raised even further, allowing them to put more effort into their activities.

Anaerobic Metabolism vs. Aerobic Metabolism

Anaerobic metabolism is not as efficient as aerobic metabolism. A molecule of glucose can only produce three ATP molecules under anaerobic metabolism, while it produces 39 with aerobic metabolism. ATP is what fuels the muscles.

Anaerobic metabolism can only use glucose and glycogen, while aerobic metabolism can also break down fats and protein.

Intense bouts of exercise in the anaerobic zone and in the red-line zone with a heart rate over 80 percent of your maximum heart rate will result in using anaerobic metabolism to fuel the muscles.

While your body will naturally use the energy pathways that will best get the job done, you have a choice in how strenuously you exercise. Training programs for different sports and activities are designed to make the best use of aerobic and anaerobic metabolism.

Source:

Milioni F, Zagatto A, Barbieri R, et al. Energy Systems Contribution in the Running-based Anaerobic Sprint Test. International Journal of Sports Medicine. 2017;38(03):226-232. doi:10.1055/s-0042-117722. 

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