LOS ANGELES—Many of us have experienced the familiar burning sensation in our muscles as they tire during exhaustive exercise. This is most common during periods of intense exertion such as running or cycling over a set sprinting distance. It can also be felt when weight lifting to a high intensity where a specific muscle is isolated and trained to exhaustion. It is widely assumed that this burning sensation is a buildup of lactic acid in the muscle tissue, while other theories believe differently.

When we exercise, our body finds different ways to use energy according to our physical demands. Normally we use energy aerobically, meaning that oxygen is readily available. To allow muscles to move (i.e. contract and relax) efficiently, energy is used from a storage system by a series of chemical reactions. Adenosine Triphosphate (ATP) is stored in the muscle tissue. In the production of energy, it is broken down into Adenosine Di Phosphate (ADP) and Phosphate, and in this process, the bonds between the ATP molecules are broken and energy is released.

ATP Hydrolysis. Photo by Jocelyn Holt

However, at higher levels of physical demand, the oxygen needed for aerobic energy synthesis cannot be reached, and energy demand is greater than the body’s ability to deliver oxygen. The muscles will then go on to produce energy anaerobically via the breaking down of stored glucose molecules in a process called glycolysis. Pyruvate is converted into lactate at this level of depleted oxygen to allow energy production to continue. It is important to note in this instance that lactate is not the same as lactic acid, despite common belief that they are interchangeable.

Lactic Acid Fermentation. Photo by Jocelyn Holt

At higher levels of intense exercise for longer periods, the lactate produced cannot be removed fast enough from the muscle tissue. This can result in high levels of buildup in lactate and other metabolites, and in this process of acidosis, a lowering of pH in the muscles occurs. This causes the lactate threshold to be reached as the acidity of the muscle environment is increased. As the muscle is forced to slow down, however, more oxygen then becomes available (via oxidation) and the lactate is converted back into pyruvate, the muscle pH returns to normal, and so the cycle continues. This allows the natural recovery mechanism to take place, slowing down the main energy systems necessary for muscle movement. Lactate is a necessary by-product of energy production, which ensures the muscle does not undergo permanent damage from overexertion.

The increase in acidity from excess H+ ions within the muscle tissue during acidosis results in irritation of the nerve endings. This causes the burning sensation in the muscle, forcing us to stop, and thus preventing overexertion and permanent muscle tissue damage. It is therefore not the buildup of lactic acid that causes the pain from extreme muscle contractions, but the actual onset of muscle fatigue resulting from a lack of oxygen, as well as the production of lactate forcing the body into submission.

So the next time you are training hard in the gym and you “feel the burn,” you will know exactly what is going on in those hardworking muscles. Be sure to rest up and eat well afterward to allow efficient muscle repair and recovery to take place.

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