The Lactate Myth: What's Actually Happening When Your Legs "Burn"

Ask most runners what lactate does, and you'll get some version of the same answer: it's the waste product that builds up in your muscles and makes them burn. Push too hard, produce too much lactate, and you slow down or stop.

That explanation is incomplete enough to be misleading, and it changes how you should actually train.

Lactate isn't the villain

Lactate isn't just a waste product your body needs to get rid of. It's a signalling molecule, it helps trigger the very adaptations (like mitochondrial growth) that make you fitter. Your heart and brain use it as fuel too. The "burn" itself is more likely caused by other byproducts of hard effort, not lactate directly, a nuance still debated in exercise science, but one that matters because it changes the story from "lactate is bad" to "lactate is part of how you get better."

Even the term most runners have heard, "anaerobic threshold," is a bit of a misleading holdover. What's actually happening at that intensity is more nuanced than a simple aerobic/anaerobic switch flipping.

The three thresholds that actually matter for training

Rather than one single cutoff point, there are effectively three intensity markers worth knowing:

• Aerobic threshold (AeT) — the pace where you shift from purely easy, all-day effort into the start of real aerobic work
• Lactate threshold (LT) — the pace you can sustain for roughly an hour, the classic "comfortably hard" tempo effort
• Functional threshold (FT) — closer to your race-specific limit, where effort becomes genuinely hard to sustain

Training too much time in the murky gap between these (not easy enough to recover from, not hard enough to build real threshold fitness) is one of the most common ways runners plateau without understanding why.

Why threshold actually rises with training

It's not just one adaptation. Several things improve together: your mitochondria and how efficiently they use fuel, how well your muscles shuttle lactate around and reuse it as fuel, how your neuromuscular system handles the stress of sustained hard effort, and how efficiently your heart and circulation deliver oxygen to working muscle. Threshold training earns its place in a plan because it's genuinely one of the highest-value sessions you can run, consistently ranked above VO2max work and well above classic hard interval sessions for the fitness return it produces.

What this means for your training

This is really the same idea behind the 80/20 principle: roughly 80% of your running should sit at a genuinely easy effort, and the other 20% should be purposeful, mostly threshold-focused work, rather than a week of everything sitting in the same uncomfortable medium gear. Know which of the three zones a given session is actually targeting, and train there on purpose instead of by accident.

Credit: some of the physiological detail above (lactate as a signalling molecule, the lactate shuttle, the fuller list of threshold mechanisms) builds on training material from Stazza's Stable.