The Engine

The currency

ATP and the three energy systems

Everything your muscles do runs on a molecule called ATP, and here is the catch that shapes all of endurance: you only ever store a few seconds' worth of it. So the real question is never how much ATP you have, it is how fast you can rebuild it, and that is the entire job of what people call the energy systems. There are three of them, they burn different fuels, and THEY ALL RUN AT ONCE, with the mix set by how quickly you are demanding energy.

The phosphagen system rebuilds ATP almost instantly by borrowing a phosphate from creatine phosphate. Enormous power, almost no capacity, spent in around ten seconds. This is the system behind a standing sprint or the first moments of any hard effort. Glycolysis breaks carbohydrate down for a fast but modest amount of ATP. High power, moderate capacity, and the system that produces lactate. And oxidative phosphorylation rebuilds ATP in the mitochondria using oxygen, burning both carbohydrate and fat. Low power but effectively unlimited capacity. This is your endurance engine, and everything else in this guide, your thresholds, your fat burning, the Norwegian method, is really about how well this one system works.

The important idea is that they are not a relay, handing off one to the next. They overlap constantly. Sprint off the line and the phosphagen system leads while the others spin up. Settle into an Ironman pace and the oxidative system carries almost all of it, with the others idling quietly in the background.

Where lactate is born

Glycolysis, and where lactate really comes from

Glycolysis is where lactate is made, and lactate is the single most misunderstood thing in endurance. You don't need to try and remember this but here is the detail.Glycolysis takes one glucose, from your blood or your muscle glycogen, and splits it through a chain of steps into two molecules of pyruvate, releasing a small amount of ATP directly, two units from blood glucose or three from glycogen. Along the way it strips off some electrons, and that is the moment that decides lactate.

Pyruvate then has two roads. If your mitochondria can keep up, it goes in, gets fully oxidised, and yields roughly thirty units of ATP, around fifteen times what glycolysis managed on its own. That gap is why the oxidative system is the endurance engine. But if glycolysis is running faster than the mitochondria can take the pyruvate up, that spare hydrogen has to go somewhere, so pyruvate accepts it and becomes lactate, which frees glycolysis to keep firing.

Lactate as fuel

The lactate shuttle

So if lactate is not waste, what is it? It is FUEL. The framework here is George Brooks' lactate shuttle, once treated as heresy and now textbook. Lactate made in one place is carried, both within the same muscle cell and from cell to cell, to tissues that burn it for energy. Your slow-twitch fibres, your heart and your brain all take up lactate and oxidise it. It crosses cell membranes on dedicated transporters, and training increases the number of them, which is a large part of why a fitter athlete clears lactate so much better at the same pace. Some of it also travels to the liver, which rebuilds it back into glucose and sends it out again.

Put simply, the lactate in your blood at any moment is a running balance between how fast you are making it and how fast you are burning it back off. Hold that idea, because it is exactly what your thresholds measure.

The most useful picture in the sport

LT1, LT2, and why the best use these

If blood lactate is production minus clearance, then plotting it against intensity draws the single most useful picture in endurance physiology.

Two turning points sit on that curve. The first, LT1, the aerobic threshold, is where blood lactate first lifts meaningfully above its resting level, conventionally around 2 mmol per litre. Below it you clear lactate as fast as you make it and can keep going almost indefinitely. The second, LT2, the anaerobic threshold or maximal lactate steady state, is the highest intensity at which production and clearance still balance so blood lactate holds steady, conventionally around 4 mmol. Push past LT2 and lactate climbs without stopping, and the clock on how long you can hold that effort starts ticking fast. Those two points carve your training into three zones: easy below LT1, moderate between the two, and hard above LT2. KISS!

Thresholds, applied

The Norwegian method

The clearest example of coaching built directly on those two thresholds is the Norwegian method, and its signature move, the double-threshold day. Its origins are specific. It was worked out by Marius Bakken, a Norwegian 5000m runner who was also a physician, through more than five and a half thousand of his own lactate tests, and then applied systematically by Gjert Ingebrigtsen to his sons.

The core idea is about dose, not intensity. The threshold sessions are deliberately kept just below LT2, in roughly the 2 to 4 mmol band, with the athlete taking finger-prick lactate readings mid-session and adjusting pace in real time to stay in that narrow window. Because each session stays below LT2, the fatigue it costs is low enough to do two of them in a single day, morning and evening, and then repeat the whole thing two or three days later. That is the trick. On the days in between, the running is genuinely easy, below LT1, and it makes up the bulk of the week's volume. Try to do that quality work at LT2 rather than below it, and you could not double it up or come back to it so soon. The sub-threshold ceiling is what buys the frequency.

Distribution, and what you call it

Polarised, pyramidal and 80/20

That last point sits inside a real but often-overblown debate about how to distribute training intensity, and it is worth being precise about the terms, because they get used interchangeably when they should not be. Start with the one everyone quotes. The 80/20 rule, popularised by Matt Fitzgerald, simply says keep around 80 percent of your training genuinely easy, below LT1, and the other 20 percent hard. That is a proportion, and on its own it says nothing about how that harder 20 percent is arranged.

Both approaches are, at heart, mostly easy plus a smaller dose of quality. Where the quality sits is orchestrated to the individual, the race and where you are in your annual plan.

The other axis

Fat, carbohydrate, and the crossover

So far we have talked about how hard you are working. Now the other axis: which fuel the engine is burning, because that changes with intensity too, and it is the hinge the whole of Part 2 turns on. Your body always burns a blend of fat and carbohydrate, and the mix shifts with effort. Easy, and fat dominates. As you push harder, the balance crosses over to carbohydrate.

Fat oxidation actually peaks at a fairly gentle intensity, the point called FatMax, usually somewhere between 45 and 65 percent of your maximum, and then falls away, dropping close to zero at the very high intensities where carbohydrate becomes almost the only fuel. The intensity where carbohydrate overtakes fat is the crossover, and it sits around 60 percent of VO2max for most people. Carbohydrate takes over for solid reasons: fat is slow to release from storage, it cannot be broken down fast enough to meet a high demand, and, crucially, it needs more oxygen to yield the same energy. Which brings us to the most underrated point in the whole engine.

The premium fuel

Oxygen cost, and why carbohydrate wins when it's hard

Per gram, fat holds more than twice the energy of carbohydrate, and per molecule it yields far more ATP. By those measures fat looks like the superior fuel, and for long, easy efforts it genuinely is the workhorse. But there is another measure that flips the ranking, and it is the one that decides hard racing: energy per litre of oxygen. Burning carbohydrate yields about 5.0 kilocalories for every litre of oxygen you breathe. Burning fat yields only about 4.7. Carbohydrate gives you more energy, and more ATP, for each litre of oxygen consumed.

That is why, when you go hard and oxygen becomes the thing in short supply, your body shifts toward carbohydrate. It is simply the more oxygen-efficient fuel exactly when oxygen is scarce. It is also, at the deepest level, why fuelling carbohydrate protects your performance, and there is direct evidence for the link: taking on 120 grams of carbohydrate an hour has been shown to lower the actual oxygen cost of running in elite marathoners. The engine runs more economically on its premium fuel.

The catch

Durability, and why the easy zone turns hard

One last piece, and it is the one that undoes any plan built on your fresh numbers. Everything above, your thresholds, your FatMax, your economy, is usually measured when you are rested or over short testing intervals. This doesn't hold over a long race. As the hours add up, your heart rate drifts upward at the same pace, your glycogen falls, your core temperature climbs, and your thresholds themselves deteriorate. The name for how well you resist that drift is durability, and it is increasingly treated as a fourth key quality alongside VO2max, threshold and efficiency.

The practical consequence is the one every long-course athlete feels in the legs. The pace that sat comfortably in your fat-burning zone in hour one represents a progressively higher relative intensity as you tire, edging you up toward and past the crossover and onto carbohydrate, at exactly the point your glycogen is most run down. The effort that felt easy and fat-fuelled early becomes a hard, carbohydrate-hungry one late.