During exercise, the body needs more energy than usual. It cannot take in enough oxygen to create energy, so the muscles in the body take over. This creates lactic acid, which causes a burning feeling when it builds up. Running or an intense workout will often cause burning legs.
The common belief is that this burning sensation is caused by the lactic acid build-up in our muscles, which eventually forces us to stop exercising. But does lactic acid really cause muscle burn? The answer is a resounding “No”, it doesn’t. In fact, lactic acid build-up helps reduce the burn.
Burning during intense exercise is caused by the acidity from the accumulation of lactic acid. When your muscles cannot get all the oxygen they need to convert food to energy during intense exercise, lactic acid accumulates in muscles, makes them more acidic, and the acidity causes a burning feeling.
Have you ever felt a burning sensation in your muscles after an intense sprint, or maybe peddling up a hill on a bicycle?
The common belief is that this burning sensation is caused by a lactic acid build-up in our muscles, which eventually forces us to stop exercising.
But does lactic acid really cause muscle burn?
The answer is a resounding “No”, it doesn’t. In fact, lactic acid build-up helps reduce the burn.
How Lactic Acid Build Up Works
Your body needs energy to function and glucose is its primary fuel source during exercise. Under a process called glycolysis, your body breaks down the glucose into a substance called pyruvate.
Then, within the cell, the pyruvate can be converted into (1) water and carbon dioxide through the Krebs cycle (a series of chemical reactions that occur using pyruvate to generate energy) and (2) oxidative phosphorylation (process for the creation of ATP – adenosine triphosphate – that cells use for energy). This requires oxygen.
So, during times when there is not enough oxygen, such as when you are exercising intensely, the cell cannot keep up production of ATP to meet its energy demands, leading to increased glucose break down into pyruvate, Now the pyruvate, instead of going through the Krebs cycle, is converted to lactate.
Even when there is adequate oxygen, in some cells, lactate is still made continuously. Note that lactate is created and not lactic acid.
While lactate is actually the salt of lactic acid, there is a big difference in what they do.
Lactate, as the salt, is missing a hydrogen ion or proton. Therefore, their properties inside the cell differ. A measurement of the amount of the protons tells you how acidic something is. Lactate, not having any protons, is not acidic. Lactate, because it is not acidic, is NOT the cause of the burning sensation in your muscles.
OK, Then What Causes Muscle Burning?
If neither lactate nor lactic acid causes muscle burn, then what does?
Remember ATP, that substance used as an energy source by your cells? During exercise, your body uses large quantities of ATP to meet your muscle cells’ high energy demand. When you use this ATP, it produces a proton.
And what happens when protons are produced? The area becomes more acidic. As the protons increase in number they come into contact with nerves near the muscles, creating the sensation known as muscle burn. As you continue to work your muscles, you use more ATP, producing protons and increasing the acid in the muscles.
None of this involves lactate or lactic acid.
Lactic Acid Build Up Reduces Muscle Burning
It might be hard to believe, with the bad rep given to lactic acid & lactate over the years, but they’re actually beneficial.2 Lactate in the cell can act as a buffer by reducing the cell’s acidity.
But the most benefit comes when lactate leaves the cell. At this point, it can either enter other cells, like your heart cells, to generate energy for them. Or, it can undergo the Cori cycle, where it is taken from the muscles and brought to the liver to be turned back into glucose and recycled.
Lactic acid is not your enemy, but a perfect fuel source for your muscles. Helping to reduce the acidity that comes from exercising, it is also used as a vast energy source for muscles to work on endurance.
So, next time you exercise, you can thank lactic acid for keeping your muscles moving!
- Berg, J. M., Tymoczko, J. L., & Stryer, L. Biochemistry. New York: W.H. Freeman and Company. 2007.
- Roberts, R. A., Ghiasvand, F., & Parker, D. Biochemistry of exercise-induced metabolic acidosis. American Journal of Physiology. 2004
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