What causes the “burn” sensation during intense exercise?

While the buildup of lactic acid during intense exercise is often associated with the "burn" sensation, the actual cause of that feeling is the accumulation of hydrogen ions, which lower the pH levels in the muscles, creating an acidic environment.

Here's a more detailed explanation:

Lactic acid and exercise

During strenuous physical activity, your body's energy production shifts from relying on oxygen to a process called anaerobic glycolysis, which produces lactate (often mistakenly called lactic acid).

The “burn” sensation

The burning sensation you feel during intense exercise is primarily due to the buildup of hydrogen ions, which are released as a byproduct of anaerobic metabolism.

Hydrogen ions and pH

These hydrogen ions lower the pH levels in the muscles, making them more acidic.

Lactate's role

Lactate itself is not the primary cause of muscle soreness or the burning sensation, though it can be converted back into glucose or used as fuel by the muscles and heart.

Muscle soreness

Delayed onset muscle soreness (DOMS), the pain you feel the day after a workout, is likely due to micro-tears in the muscle tissue, not lactic acid.

Lactate clearance

Your body efficiently clears lactate from the muscles and bloodstream, so it doesn't linger for long after exercise.

Lactate threshold

As your fitness improves, your body becomes more efficient at utilizing lactate, and it takes longer to reach the point where lactate accumulates and causes the burning sensation.