Training zones are a bookkeeping tool, not a biology

The lines between zones are drawn for the convenience of coaches, and athletes treat them as physical law.

Somebody decided there were five zones. Or seven, or three, depending on which coaching manual landed on your desk first, and the fact that the number keeps changing should tell you how real the boundaries are.

Energy production is continuous. At any running speed the body uses every pathway at once, in a mix that shifts gradually as intensity rises. There is no moment when the aerobic system clocks off and the anaerobic one takes over. Yet the language of zones implies a switch, and athletes act on that implication in ways that quietly damage their training. I have met runners who would rather stop a session than let a heart rate cross a line on a watch, as though the extra beat converts good work into bad.

What zones are actually for is accounting. They let a coach describe a week without a lecture, and they let two coaches compare notes. That is a genuine use and I am not proposing we bin them. I am proposing we stop pretending the numbers were discovered rather than agreed.

Consider what moves a threshold around. Sleep, heat, the previous session, the time of day, how much the athlete ate, whether they are fighting something off. A tested threshold pace from March describes one morning in March. By July the athlete is fitter and the number is stale, and if every zone was set as a percentage of a stale number then every session that month has been aimed slightly wrong.

The practical fix is unfashionable because it sounds like guesswork. Use effort. Teach an athlete what comfortably hard actually feels like in the chest and the legs, and let the watch confirm rather than command. An experienced runner can find threshold within a few seconds a kilometre by feel on any given day, which is more accurate than a spring test applied to an August body.

The strongest objection is that feel is unreliable in beginners, and that is true. Beginners need the rails. Fine, give them the rails, and then take them away. A runner who has spent five years obeying a coloured bar has learned to read a screen instead of learning to read themselves, and only one of those two skills survives a flat battery on the start line.