Lactic Acid Is Not What Makes Your Legs Burn

When I was a teenager, my ski coach told us that the burning in our thighs on a long climb was lactic acid "poisoning the muscles." He said it would stay in our legs for days, which is why we were sore. He was a wonderful coach. He was also wrong on almost every point, and so were most textbooks of the time.

Physiology has spent the last few decades rewriting this story. Here is what we now understand, as simply as I can put it.

Lactate is a fuel, not a waste product

When your muscles break down glucose for energy, the process called glycolysis produces a molecule called pyruvate. If there is enough capacity, pyruvate goes into the mitochondria and is burned completely with oxygen. When you work hard and fast, glycolysis runs faster than the mitochondria can keep up, and pyruvate is converted into lactate.

That conversion is useful. It regenerates a molecule the cell needs to keep glycolysis running, so you can keep producing energy quickly. The lactate is then shuttled out of the muscle and used as fuel by the heart, by other muscles and by the liver, which can turn it back into glucose. In other words, lactate is part of how your body moves energy around, not just leftover garbage.

So why does it burn?

The burning sensation during hard effort is linked to several things happening at once. Hydrogen ions accumulate and the muscle becomes more acidic. Phosphate builds up from the breakdown of energy molecules. Potassium shifts across cell membranes. Nerve endings in the muscle detect these changes and send signals that your brain interprets as burning and the urge to stop.

Interestingly, the acidity is not produced by lactate itself in the way the old story claimed. The hydrogen ions come mainly from other steps of energy metabolism. Lactate tends to rise at the same time, which is why it got blamed. It is a useful marker of how hard you are working, but it is not the villain.

And the soreness two days later?

That is a different phenomenon entirely, called delayed onset muscle soreness. Lactate levels return to normal within an hour or so after exercise. The soreness you feel 24 to 72 hours later comes mostly from small disruptions in muscle fibres and the inflammation and repair that follow, especially after unfamiliar movements or lots of downhill running.

What this means for training

• Lactate tests are still valuable, because the point where lactate starts to rise steeply tells you a lot about your endurance fitness.
• Training at moderate intensity improves your ability to clear and use lactate, which is one reason long easy sessions work.
• Stretching or "flushing out" lactate after a session is not necessary. It disappears by itself quickly.

I met my old coach last year at a race. I explained all this to him over coffee. He listened politely, then said, "Fine, but it still burns." He had a point.