You’re halfway through a tough set of squats, holding a plank, or pushing through the last few reps when suddenly your muscles feel like they’re on fire.
For years, that burning sensation was commonly blamed on lactic acid buildup. But exercise science tells us there’s more to the story.
What Causes the Burning Sensation?
At rest, your muscles maintain a relatively balanced chemical environment. Once you begin exercising, your muscle fibers repeatedly contract and require increasing amounts of ATP, the primary energy source used for muscle contraction.
As exercise becomes more intense, the rate of ATP breakdown increases and the chemical environment inside the working muscle begins to change. Hydrogen ions (H⁺), inorganic phosphate (Pi), lactate, and other metabolites increase during intense muscular activity.
These metabolic changes can contribute to a decrease in muscle pH and stimulate or sensitize sensory nerve endings within the muscle. Your nervous system interprets these signals as the familiar burning sensation you experience during a difficult set.
Is Lactic Acid Causing the Burn?
Not exactly.
What we commonly call “lactic acid” exists primarily as lactate under normal physiological conditions. Lactate is produced during intense exercise, but it shouldn't simply be viewed as a waste product responsible for making your muscles burn.
In fact, your body can transport lactate to other tissues where it can be recycled and used as an energy source.
So while lactate levels often rise at the same time you begin feeling the burn, lactate itself isn't simply the cause of that burning sensation.
Why Does the Burn Go Away?
Once you decrease the intensity or stop exercising, your muscles' energy demands fall. Blood flow helps transport and process accumulated metabolites, and the chemical environment within the muscle begins returning toward its resting state.
That’s why the burning sensation usually decreases relatively quickly after you stop the exercise.
The Bottom Line
That muscle burn isn't simply lactic acid building up.
During intense exercise, rapid ATP use and the accumulation of hydrogen ions, inorganic phosphate, and other metabolites change the chemical environment inside your working muscles. These changes stimulate and sensitize sensory nerve endings, contributing to the burning sensation you feel.
So the next time your quads are burning during squats or your shoulders are on fire during the last few reps, you’ll know there’s a lot more happening inside your muscles than just “lactic acid buildup.”
New Path Chiropractic – Wellness & Sports Rehab
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Dr. Steve Muscari
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