Field Notes

Why we don't chase your heart rate in Zone 2

Cardiac drift, the hidden tax on heart-rate training, and the one number that actually matters

2026-06-20 · 9 min read · Brevet · AI-assisted

Drafted with AI assistance and reviewed by a human. The science is sourced and real; the article isn't medical advice.

The short version: When you ride easy for a long time, your heart rate slowly climbs even though your effort hasn't changed. That's cardiovascular drift, and it's completely normal. But if you're training to a heart-rate number, that drift quietly steals your workout — you end up doing less and less work to hold the same bpm. Here's why that happens, why heart rate is the wrong thing to chase, and what to anchor to instead.

The ride that gets easier as it goes

You've felt this even if you've never named it.

You set out for a long, easy aerobic ride. The plan says "Zone 2 — keep your heart rate around 140." The first 30 minutes feel almost too easy; you're soft-pedalling to keep the number down. An hour in, something's shifted: the same easy spin now reads 145, then 148. To get back to 140, you ease off. By the end you're barely turning the pedals — and your heart rate is still drifting up.

Nothing went wrong. Your body did exactly what a healthy body does. And yet, if you were training to that heart-rate number, your workout slowly fell apart.

This is cardiovascular drift, and almost every endurance athlete runs into it without understanding what it's doing to their training.

What's actually happening

As you ride, your body warms up. To shed that heat, it sends more blood toward your skin, and you lose fluid through sweat. With a little less blood volume in central circulation, each heartbeat moves slightly less blood — so your heart beats a bit faster to keep everything supplied. Same effort, same power at the pedals, gradually higher heart rate.

The numbers are well-documented: 5 to 15 beats per minute over an hour at a moderate, steady effort. It's larger in the heat — and larger indoors, where there's no headwind to cool you. It is a normal physiological response, not a warning sign. Every fit rider drifts.

The problem isn't the drift. The problem is what you do with it.

The trap of training to a heart-rate number

Here's the uncomfortable math. If your target is "hold 140 bpm" and your heart rate drifts up by 10 beats over two hours, then to keep hitting 140 you have to keep lowering your power. Hour one you might hold 200 watts at 140 bpm. Hour two, that same 140 bpm only buys you 175 watts. You didn't get weaker — your heart rate just moved the goalposts.

The entire point of a Zone 2 ride is the work you do down there: the slow, patient adaptations that build your aerobic engine — more capillaries feeding your muscles, bigger and more efficient mitochondria, a heart that moves more blood per beat. Those adaptations are driven by time spent at the right intensity. When drift drags your power down, you're spending that time at a lower intensity than prescribed. You're doing a real workout's worth of sitting on the bike for a fraction of the stimulus.

Chase the heart rate, and drift wins.

The reframe: heart rate was never the target

Here's the idea that fixes everything, and it's worth saying slowly.

Your heart rate and your power are both just proxies — estimates — for the one thing that actually matters: how hard you're going relative to your aerobic threshold.

That threshold (sport scientists call it LT1, the first lactate threshold, or the aerobic threshold) is the ceiling of true Zone 2 — the highest intensity you can hold while staying mostly fat-fuelled, relaxed, and able to recover quickly. That is what a Zone 2 ride is trying to dose. Power and heart rate are just two different windows onto it.

And those two windows are not equal:

So if you want to control the dose, you control the better dial: power, anchored to your threshold. Heart rate isn't useless — far from it. But its job is different.

Two ways to get it wrong

Most training falls into one of two failure modes. Both are wrong, in opposite directions.

Failure mode 1 — chase the heart rate. Hold a constant bpm, let power fall. This is what a naïve "heart-rate workout" does, and it's what we just took apart: it can't tell the difference between harmless drift and a real problem, so it eases your power for both. You under-dose the session and wonder why your aerobic base isn't building.

Failure mode 2 — ignore the heart rate entirely. Hold constant power, never look up. This is better for most days, but it has its own blind spot. On a rough day — a hot room, a tired body, a power target set too high to begin with — your "easy" watts might actually be sitting above your threshold. Hold them stubbornly and you turn a recovery ride into a hard one, paying a fatigue cost the day was never meant to charge you.

The first mistake throws away the workout. The second one digs a hole.

The right way: power-led, threshold-anchored, heart-rate-governed

The fix is to stop treating it as "heart rate or power" and start using each for what it's good at.

  1. Power is the dose. Ride your easy days at a steady power, held honestly. Don't let normal drift pull it down.

  2. Your own threshold is the anchor. Not a generic "65% of FTP" or a textbook "180 minus your age" heart-rate zone — your measured aerobic threshold. When your easy power is genuinely set below your threshold, a few beats of drift is provably harmless, and you can ignore it with confidence.

  3. Heart rate is the governor, not the target. This is the subtle part. A small, expected drift means nothing — leave the power alone and protect the dose. But when the gap between your power and your heart rate opens up too far — when your heart rate has climbed well past where that power sat at the start — that's no longer harmless drift. That's the moment to ease off, because now the data is telling you the effort has genuinely crossed a line.

The difference between a heart-rate chaser and a heart-rate governor is everything. The chaser reacts to every beat and bleeds your workout dry. The governor sits quietly, protects the prescribed work, and only steps in when the drift actually means something — and tells you why when it does.

Drift isn't just noise — it's information

There's a bonus hiding in all of this. How much you drift on an easy ride is one of the most honest readouts of your form you can get.

A well-trained, well-rested, well-fuelled rider holds a long Zone 2 effort with barely any drift — heart rate and power stay locked together. When that gap starts opening up earlier and wider than usual, it's a sign your body is working harder than the wattage suggests: a readout that's outside your typical range.

That's not something to panic about and it's certainly not a diagnosis — it's a signal worth respecting. The smart move is to let it shape tomorrow: a little more recovery, a slightly easier next session. Measured over weeks, the trend tells you whether your aerobic durability is improving or whether you've been digging too deep. Throw away the drift by chasing it in real time, and you throw away that signal too.

Beginners and elites: same idea, different settings

This isn't only an elite concern — it just looks different at each end.

If you're new, your power numbers can't be trusted yet (an FTP test you did on a bad day can be wildly off), and the most valuable thing you can learn is what easy actually feels like. So heart rate earns a bigger role for you — not because it's the perfect dial, but because it keeps you honest and protects you from a bad power estimate. The goal is to teach your body the feeling of true Zone 2.

If you're experienced, your power and threshold are well known, so power is the dose, full stop. For you, drift isn't something to correct mid-ride — it's a durability readout, a quiet measure of how deep your aerobic engine runs. Ease your easy days for normal drift and you'd just be sandbagging the work that builds your base.

Same physiology. Same principle. The dials are simply tuned to where you are.

How we think about it at Brevet

Most platforms do one of two things with cardiac drift: ignore it, or build a naïve "ride to heart rate" mode that chases your bpm and under-doses you. We think both are wrong, and we've built toward the third path.

We measure your drift — the actual decoupling between your power and your heart rate — with a method that throws away the warm-up and locks onto a clean baseline, so the number reflects real drift and not the normal settling of your first few minutes.

We anchor your easy rides to your threshold, not a population average. A textbook "65% of FTP" band can sit above the aerobic threshold we actually measured for you — so we cap your Zone 2 where your body says it ends, not where a formula guesses.

And we treat your drift as information: it shapes how hard tomorrow should be, and over weeks it tells you which way your aerobic durability is trending.

The principle underneath all of it is simple, and it's the one thing we'd want you to take away:

Power is the dose. Your threshold is the anchor. Your heart rate is the governor — never the target.

Train smarter than the number on the strap. Your aerobic base will thank you for it.

A note on what this is

Brevet is a training and coaching tool, not a medical device. It does not diagnose, treat, or predict any medical condition. Cardiac drift, aerobic decoupling, and threshold estimates are used here only as training signals, described relative to your own history — never as health assessments. As always, you make the final call on every session; the platform recommends, you decide.

Our methods are built on peer-reviewed sport science and calibrated against thousands of real training rides. Curious about the details? Our individualized-baseline methodology is written up in plain language — we believe a sceptical sports scientist should be able to read exactly what we do and why.