The Burn Explained

Anyone who's pushed through a hard sprint or a brutal set knows the feeling: that deep, searing burn in the muscles that builds until you have to back off. For decades we've had a name for the culprit, and for most of that time, we had the wrong one. The real story is metabolic acidosis, and understanding it is changing how athletes train and supplement.

What metabolic acidosis actually is

When you exercise hard, your muscles burn through energy at a furious rate. Producing that energy quickly, without enough oxygen to keep up, generates a flood of hydrogen ions (protons) inside the muscle. More protons means lower pH, which is just another way of saying the muscle environment becomes more acidic. That shift toward acidity is metabolic acidosis, and it's a big part of what you feel as "the burn."

It's not just an unpleasant sensation, either. It's your body signalling that the chemistry inside the muscle is changing fast.

The lactate myth

For years, the burn and the fatigue that follows were blamed on lactic acid. "Lactate is building up," people would say, treating it as the villain that poisons your muscles and forces you to stop.

We now know that's largely wrong. Lactate isn't the cause of the burn, and it isn't a waste product to be feared. In fact, lactate is a useful fuel that your muscles, heart, and brain can burn for energy, and its production is actually tied up with helping manage the acid load, not creating it. The acidity comes mainly from the broader cascade of rapid energy metabolism, not from lactate itself. Lactate just happened to show up at the scene of the crime, so it took the blame for a long time.

Why it impacts muscle function

Acidosis matters because muscle is a finely tuned chemical machine, and it works best within a narrow pH range. As things turn more acidic, the processes that let muscle fibres contract forcefully start to falter. Enzymes work less efficiently, calcium handling is disrupted, and the muscle's ability to generate force and power drops. That's the physiological reason your output fades during a hard effort, even when your mind is still willing.

Where bicarbonate comes in

This is also why sodium bicarbonate, also ordinary baking soda, has become a well-known performance aid. Bicarbonate acts as a buffer in the blood, helping to mop up and shuttle away excess hydrogen ions from working muscles. By blunting the rise in acidity, it can help athletes hold higher intensities for a little longer during the kind of short, brutal efforts where acidosis is the limiting factor. It's one of the more genuinely evidence-backed supplements in sport. There's a catch, though: how you take it matters just as much as whether you take it. Bicarbonate is notorious for causing stomach upset, and getting the dose, timing, and format wrong can leave you with gut distress that hurts performance more than the buffering helps. Taking it alongside a carbohydrate-rich meal, splitting the dose, timing it well ahead of your session, or using newer slow-release and hydrogel-encapsulated formats can dramatically improve digestibility. The science is solid, but the delivery is where many athletes get tripped up.

Why monitoring could change the game

Here's the catch: most of what we know about acidosis comes from blood draws in a lab, taken after the fact. Athletes are left guessing about what's happening inside their muscles in real time, and supplementation is often a blunt, one-size-fits-all approach.

Imagine being able to actually track your acid-base balance as you train. You could see exactly when acidosis starts to bite, tailor your interval intensities and rest periods to your real physiology rather than a generic formula, and time supplements like bicarbonate precisely to when and how much you actually need. Supplementation would shift from guesswork to something measured and personal.

That's the promise of continuous metabolic monitoring: turning the invisible chemistry of the burn into data you can train with. The burn has always been telling you something. Soon, you might finally be able to hear exactly what.