Training zones and the energy systems behind them

Most training plans hand you zones without explaining them. You get told to spend an hour in zone 2 and thirty seconds in zone 5, and the numbers do work, but they are easier to apply well once you know what they represent. Zones are a bookkeeping system laid over something continuous: the shifting mix of fuels and pathways your body uses to keep producing energy.

Here is what is happening underneath, and what follows from it.

Three ways to make energy

Every muscle contraction runs on a chemical called ATP. You store very little of it, so it has to be regenerated continuously by three systems.

The phosphagen system rebuilds ATP from creatine phosphate, a small high-energy store your muscles keep on hand. It is immediate, needs no oxygen, and is nearly exhausted after roughly ten seconds of maximal effort. This is what powers a standing start or the last few metres of a sprint finish.

Glycolysis breaks down glucose and stored muscle glycogen without requiring oxygen. It comes up fast and dominates efforts from about ten seconds to around two minutes. It is the system that makes a 400 metre run or a hard minute up a climb feel the way it does.

Oxidative metabolism burns fat and carbohydrate with oxygen inside your mitochondria. It is slower to respond and produces far less ATP per second, but it can run essentially indefinitely as long as fuel and oxygen keep arriving. Beyond the first couple of minutes of any effort, this is the main event.

The important part: these are not switches. All three run all the time, and what changes with intensity is the proportion each contributes. Zone boundaries are lines we draw across a smooth gradient because discrete labels are easier to train with.

Fat and carbohydrate, and why the ratio moves

Within the oxidative system, the interesting question is which fuel it is oxidising.

At low intensities a large share of your energy comes from fat. As intensity rises, carbohydrate takes over an increasing share, until at high intensities you are running almost entirely on carbohydrate. Physiologists call this shift the crossover effect.

There are two reasons for it. Carbohydrate can be broken down much faster than fat, and it yields more ATP per litre of oxygen consumed. When demand is high, your body uses the more oxygen-efficient and faster fuel, because fat cannot supply energy quickly enough.

One nuance that gets lost in the popular version. The proportion of energy from fat is highest at very low intensity, but the absolute rate of fat oxidation peaks somewhere in the moderate range, usually reported between about 45 and 65 percent of VO2max. Where exactly varies a lot between individuals, and it tends to sit higher in better trained athletes. Below that point you are burning a high proportion of fat but not much energy in total.

This is why the phrase "fat burning zone" is misleading rather than wrong. Training does improve your ability to oxidise fat, which matters for endurance performance because it spares limited glycogen. But if the goal is body composition, total energy expenditure and diet do the heavy lifting, not the fuel mix during a given hour.

Lactate is not the enemy

Lactate has a bad reputation it does not deserve. It is not a waste product and it is not what makes your legs hurt. It is a usable fuel, shuttled between muscle fibres and oxidised by your heart, brain and working muscles.

Here is how that works. When you go hard, glycolysis churns out fuel faster than your mitochondria can take it in and burn it. The overflow gets turned into lactate, and rather than being a dead end, that lactate is carried off in the blood to tissues that can still use it: nearby easy-effort muscle fibres, the heart, and the brain burn it for energy. In effect, the fibres working hardest hand their spare fuel to the parts of you best placed to use it. And the burn you feel in a hard effort is not the lactate either. It comes from the muscle turning more acidic as glycolysis ramps up, so lactate is really just a marker that this is happening, not the cause of it.

What lactate is also genuinely useful for is measurement. Blood lactate concentration tracks how hard your glycolytic system is working relative to your capacity to clear the products, which makes it a good marker for two important intensities.

The first threshold, often called the aerobic threshold or LT1, is where lactate first rises consistently above resting levels. Below it you can go for a very long time. It marks the practical top of genuinely easy training.

The second threshold, variously called LT2, maximal lactate steady state, or approximated by critical power and functional threshold power, is the highest intensity where production and clearance still balance. Below it, lactate stabilises. Above it, it climbs until you stop. Most trained athletes can hold this for somewhere between thirty and seventy minutes.

Those two points, not any formula, are what zones should be built around.

Mapping it onto the zones

A common five-zone model lines up roughly like this.

Zone 1, recovery. Very easy. Almost entirely oxidative, high proportion of fat. The purpose is circulation and movement without adding meaningful fatigue.

Zone 2, endurance. Below the first threshold. Still mostly fat with a growing carbohydrate contribution. Sustainable for hours. This is where mitochondrial density and capillary networks are built, and it is the bulk of most well-constructed weeks.

Zone 3, tempo. Between the two thresholds. Carbohydrate is now the majority fuel. Comfortably hard, sustainable for a long while but with a real cost.

Zone 4, threshold. At or around the second threshold. Almost entirely carbohydrate. Glycolytic contribution is substantial. Sustainable for tens of minutes at most.

Zone 5, VO2max and above. Above the second threshold. Heavily glycolytic, lactate rising throughout, duration measured in minutes or seconds.

Some coaches prefer a three-zone model, which is arguably more honest: below the first threshold, between the two, above the second. It maps more cleanly onto the underlying physiology, and the choice between five zones and three is a matter of convenience rather than correctness.

What to do with this

Get your thresholds from testing, not from a formula. The old age-based maximum heart rate estimate carries an error of roughly ten beats per minute in either direction, which is wide enough to put your zone 2 inside someone else's zone 3. A lactate test, a field test, or a well-executed ramp test is worth far more than an arithmetic guess.

Respect the intensity of easy sessions. Most of the cost of drifting from zone 2 into zone 3 is invisible in the session and shows up two days later. Easy work accumulates adaptation cheaply. Slightly-harder-than-easy work accumulates fatigue without buying much extra.

Fuel according to the zone. High intensity work depends on carbohydrate availability, so arriving at an interval session underfuelled limits the session before you start. Long sessions need carbohydrate too, both to sustain output and to protect the quality of the days that follow.

Trust heart rate less on short efforts. Heart rate lags behind changes in output by tens of seconds, and drifts upward over long or hot sessions even at constant effort. For anything measured in seconds, use power or pace. For long steady work, heart rate is genuinely informative, drift included.

Retest occasionally. Thresholds move as fitness changes. Zones set in January are usually wrong by April, in the good direction, and training to stale numbers means progressively easier sessions than intended.

The short version

Zones are a language for describing which fuel and which pathway you are leaning on. Low intensity leans on fat and oxidative metabolism and can be sustained almost indefinitely. High intensity leans on carbohydrate and glycolysis and cannot. Almost everything sensible in endurance training follows from that trade, including why most of your week should be easy and why the hard parts should be genuinely hard.

If you would rather not maintain the bookkeeping yourself, the AI coach in Wyder sets your zones from your own data and adjusts sessions as your thresholds move. But the reasoning above is worth understanding either way, because it is what makes a plan make sense.