Training zones are a language, not a physiology

Numbered intensity bands are a useful shorthand between coach and athlete, and they describe boundaries that do not exist in the body.

Somebody decided there were five zones, or seven, or three, and the number varies by whichever system a coach learned first. Then a watch manufacturer picked one, printed it on a screen, and now several million people believe there is a line inside their body at seventy-eight per cent of maximum heart rate on one side of which they are burning fat and on the other side of which they are not.

There is no line. Energy supply in a working muscle is a continuous blend, and the proportions shift smoothly as intensity rises. Fat oxidation does not switch off, it becomes a smaller share. The anaerobic contribution does not switch on, it grows. The thresholds coaches talk about are real inflection points in that curve, but they are gradual, they move day to day with sleep and heat and how much carbohydrate you ate, and they sit at different heart rates in the same athlete in March and in August.

So why keep the zones? Because a shared vocabulary between a coach and an athlete is worth a lot, and because the alternative, which is telling someone to run at the intensity that produces a particular internal state, is unteachable to a beginner. Zones are a map. Maps are useful and no map has ever been the terrain.

The damage starts when the map gets enforced. An athlete who slows down mid-session because a wrist reading drifted two beats above a band is not training, he is obeying. Heart rate lags effort by up to a minute, rises in heat, rises when dehydrated, and is unreliable in the first ten minutes of anything. Chasing it in real time makes an easy run harder and a hard session softer, which is precisely backwards.

What I would keep is the crude version, and honestly the crude version is where most of the benefit lives. Easy means you can talk. Hard means you cannot. There is a middle intensity that feels productive and comfortable and is the most overused pace in the sport, and if you spent a season avoiding it you would improve more than any zone model would have delivered.

The watch is not measuring your physiology. It is measuring your heart and guessing.