A Burning Question About Lactic Acid

Q: What is lactic acid?  Why does it cause that “burn”?

A: The simplest answer is that lactic acid is a byproduct of anaerobic ATP production.  But let’s backtrack a little and review some exercise physiology to put that definition into context…

A cell can produce energy in one of two ways: with or without the presence of oxygen.  If there is an adequate supply of oxygen to your cells (regardless of your activity – sleeping, typing, walking, running, etc), then your cells are involved in aerobic energy production.  Your cells are actively utilizing fat stores, sugar stores, and oxygen to produce ATP (short for the molecule adenosine triphosphate).  Think of ATP as your cells’ battery power.

Example: You are attempting to lift a dumbbell into a bicep curl.  First, your body needs fat, sugar, and oxygen in order to produce ATP molecules.  The new ATP is used to drive the muscle contraction in the bicep muscle, allowing you to curl the weight.  Realistically, this occurs on a much grander scale, but you get the idea…

In aerobic energy systems, water and carbon dioxide are the byproducts of the chemical reaction that creates ATP.  Both compounds are easy for the body to utilize or eliminate, thus aerobic energy production does not lead to muscle fatigue.

On the other hand, sometimes your body is unable to extract enough oxygen from the blood to continue the high demand for ATP production.  Now you begin to exercise anaerobically, or without oxygen.  This is often the case during vigorous workouts, sprints, heavy weight lifting – basically whenever the intensity at which you are exercising exceeds the available oxygen.  This upper limit of sustainable aerobic exercise usually corresponds with a heart rate elevated above 50-85% of maximum heart rate, and is referred to as the anaerobic threshold (sometimes called the lactate threshold).

At the anaerobic threshold, the body must continue to create ATP using alternate pathways, primarily using sugar as its fuel source (rather than a combination of fat and sugar).  In these reactions, the byproducts are lactic acid, heat, and hydrogen ions.  The lactic acid in particular is very difficult to eliminate from the muscle cells, in contrast to the water and carbon dioxide produced in aerobic energy production.  As more ATP is produced anaerobically, more lactic acid is generated and builds up within the muscle.  Lactic acid is blamed for the “burn” you feel when you exhaust a muscle (when you do enough lunges, cycle uphill, perform lots of push-ups, etc), and it causes rapid muscle fatigue.

There is nothing wrong with lactic acid build-up in the cells; it is just a natural alternative process for creating ATP when your body is short on oxygen.  The “burning” is simply due to the overproduction of acid in the muscle cell, which causes local acidosis.  The muscle then begins to fatigue quickly – usually soon after you start to feel that burn.  Once you stop the movement and allow your breathing to return to normal, your cells will be able to eliminate most of the lactic acid within a minute or two.  Continuing to move and walk around will aid in its removal and allow you to progress to the next exercise a little quicker.

References:

Bryant, Cedric X., and Daniel J. Green, eds.  ACE Personal Trainer Manual, 3rd ed.  San Diego: American Council on Exercise, 2003.

Powers, Scott K., and Edward T. Howley.  Exercise Physiology, 3rd ed. Guilford, CT: Brown & Benchmark Publishers, 1997.