Zone 2 by heart rate is the intensity at which you can still speak in full sentences — roughly the effort you could hold for hours and finish feeling you could have gone on. Heart rate marks it out less sharply than power does, because the same effort produces different numbers at different points of a ride: stroke volume begins to fall after 10 to 20 minutes of steady exercise, and heart rate rises to make up the difference. That drift is why holding one fixed number for four hours means riding progressively easier, and why a heart rate eight beats higher at hour three does not by itself mean you are working harder than you were at hour one.
What Zone 2 is when you have no power meter
Zone 2 is the pace you could hold for a very long time. Not the pace that feels comfortable for twenty minutes — the one you could still be riding at in three hours, and finish thinking you had more in you.
If you train with power, that intensity has a hard edge: a band of watts, set as a percentage of a number you tested. Our guide to the training zones lays out the full table if you want it.
On heart rate, the edge is soft, and it is worth understanding why. Power is what you produce. Heart rate is how your body answered — and the answer moves with the weather, your sleep, your last coffee, how much you have drunk and how long you have already been riding. Two identical efforts, a fortnight apart, can show numbers ten beats apart with nothing wrong at all.
So on heart rate, Zone 2 is a band with fuzzy edges rather than a line you either sit on or miss. That is not a defect in your training. It is what the measurement is, and the riders who get the most out of it are the ones who stop treating the number as a verdict.
The three tests that need no equipment at all
Before the numbers, the checks that work on any bike, on any road, with nothing strapped to you.
Can you speak in full sentences? Not single gasped words between breaths — an actual sentence, delivered normally, without your voice breaking to breathe. If you cannot, you are above Zone 2, and it does not matter what any device says. This is the test to trust when the two disagree.
Can you breathe through your nose alone? Rough, and not everyone can manage it even when easy, but for most riders the moment the mouth has to open is close to the top of the easy range.
Does it feel almost annoyingly easy? This is the one riders resist. A proper Zone 2 ride should feel underwhelming while you are doing it. If you finish a session feeling like you trained, you probably went too hard.
None of these are laboratory measures, and they should not be dressed up as any. But they fail in a useful direction: they catch you going too hard, which is the error almost everybody actually makes.
Why the same effort shows you two different numbers
Two things happen on every long ride, and neither is a fault in your fitness or your strap.
At the start, your heart rate is behind. For the first ten minutes or so it is still climbing towards the effort you are already making. Ride to the number in that window and you will push harder to make it move — which is exactly backwards. Warm up on feel and let the number catch up.
Later, your heart rate drifts up on its own. This is the better-documented half. In prolonged steady exercise the amount of blood your heart moves per beat begins to decline after 10 to 20 minutes, and heart rate rises to compensate for it. The classic review of this argues the rise in heart rate is itself the main driver, rather than blood being diverted to the skin as you warm up.
The practical consequence is the part worth keeping. If you hold one fixed heart-rate number for four hours, you are riding progressively easier. And if your heart rate is eight beats higher at hour three on the same effort, that is the expected behaviour of the measurement, not evidence that you are cooking yourself.
Worth saying plainly: this research is not cycling-specific, and it dates from 1998 and 2001. It is well replicated and it describes how the system behaves. It does not describe your particular Sunday.
The mistake that quietly costs you a whole training block
There is one failure that does more damage than all the others put together, and it does not feel like a failure at any point.
It is riding slightly too hard. Every time. Not hard enough that any single ride is a problem — just above easy, week after week. The group lifts the pace and you go with it. The drag on the flat road is annoying so you press a little. There is a rise, and nobody soft-pedals a rise.
What it feels like from the inside is the deceptive part: nothing hurts, no session goes badly, every ride feels fine. What is missing is at the other end of the week. The hard session never quite lands. The intervals that should feel sharp feel heavy. You are always slightly tired and never properly rested, and the easy rides were not easy enough to let the hard ones be hard.
The fix is unglamorous and it is mostly social. Let the group ride away. Accept the slower average. Ride the flat road you find boring. Riders who commit to this usually notice it first in their hard sessions, a few weeks later — not because they trained harder, but because they finally arrived at them fresh.
The things that move your heart rate without moving your effort
Before you conclude your fitness has changed, run through the list of things that move the number on their own.
Heat is the big one, and it does not need to be a heatwave. A short night will lift it. Caffeine before a ride will lift it. A hill will lift it for reasons that have nothing to do with your training zone. Stress, an illness arriving, and simply being nervous before an event all show up here.
Dehydration deserves its own line, because its effect is larger than most riders assume. When exercise leaves you dehydrated and overheated, roughly half of the fall in blood pumped per beat is accounted for by the drop in blood volume itself. Your heart rate rises to cover the gap. That is not a training signal. That is a bottle you did not drink.
So the rule is: respond to the pattern, not to the reading. One ride sitting eight beats high tells you almost nothing. Ten days of the same effort costing more beats than it used to, on ordinary sleep and in ordinary weather, is worth paying attention to. And any single morning where the number looks strange is a question about yesterday, not a verdict on your fitness.
Setting a range without a laboratory
Most riders start from a formula, which is reasonable. It is worth knowing which formula, because the famous one is the one the evidence does not support.
220 minus age is the one to drop. In 2001 researchers pooled 351 studies covering 492 groups and 18,712 people and found the relationship was better described by 208 − 0.7 × age. They then re-ran it as a controlled laboratory study on 514 healthy people and got 209 − 0.7 × age — practically the same line. The relationship did not differ between men and women, and it was not shifted by how active people were. Six years later a different group followed 132 people through 908 exercise tests across 25 years and arrived at 207 − 0.7 × age.
Three different designs, three times essentially the same answer, and all three appreciably different from 220 − age — which, as the first group put it, underestimates maximum heart rate in older riders.
Now the caveat that matters more than the formula. That very strong correlation was between age and the average maximum heart rate of a study group. It tells you where a room full of forty-year-olds will centre. It does not tell you where you sit in that room. Two riders the same age, the same fitness and the same sex can genuinely have maximum heart rates far apart, and neither of them is broken.
So treat any range you calculate as a starting point to be corrected, not a prescription to be obeyed. Ride it for a few weeks with the talk test running in the background. If you are comfortably holding conversation at the top of your calculated band, the band is too low for you and should move up. If you are already clipping words halfway up it, it is too high. Where the formula and the talk test disagree, believe the talk test. It is measuring you; the formula is measuring everybody.
Your numbers, explained the moment you ask.
The awkward thing about riding easy by heart rate is that the number keeps raising questions the number cannot answer. Your heart rate climbed ten beats over four hours at the same effort — is that drift, the heat, or the fact that you slept badly? A session is marked easy and every instinct says you are wasting the afternoon.
You can ask the Peakfy coach why today's ride is meant to be easy and get a reason rather than an instruction, and ask what a heart rate that drifted upward on Sunday actually means for Tuesday. It will also tell you plainly when a value has not been computed, instead of showing you a figure that merely looks plausible.
Questions riders ask
Keep reading
- Cycling training zones explained: what each one is for
- Recovery and readiness in cycling: reading the morning signals
- A beginner cycling training plan that you will actually keep