Your Zone 2 Is Probably Somebody Else's Zone 3

Your Zone 2 isn’t a universal temperature. It’s a percentage of something, and the question is a percentage of what. Most apps draw it as a slice of your maximum heart rate, which means a 32-year-old crit racer with a resting pulse of 42 and a 58-year-old guy coming back after a decade off both get handed the same Zone 2 ceiling if their max happens to match. One of them is soft pedaling. The other is working. Same number on the screen, two completely different afternoons. When I draw zones off your heart rate reserve instead, I’m using something nobody asked you for when you signed up: how low your heart goes when you’re doing nothing at all.

Why are my heart rate zones different in Strava?

Because we’re not starting from the same place. Percent of max is a perfectly sane default when you know nothing about a rider except their age and a number from a lab test or a hill they once blew up on. It’s cheap, it’s fast, and it’s wrong in a predictable direction rather than a random one. I don’t think less of anyone for using it.

But I know more about you than that. I know what your heart does at 4am on a Tuesday. I know what it sits at while you’re reading on the couch. That resting number is the floor of your engine, and the distance between the floor and the ceiling is your actual working range. Two riders with a max of 185 and resting rates of 44 and 62 have reserves that differ by 18 beats. Spread that difference across five zones and the boundaries drift apart fast.

Here’s what it looks like at the sharp end. The floor of Zone 4, the place where you stop riding tempo and start paying rent, lands around 78 percent of max under a reserve-based model, around 80 percent under the generic one, and around 84 percent if I’m drawing from your threshold heart rate. Same label on the screen. On a rider with a max of 185, those three floors sit about eleven beats apart, and eleven beats at threshold is the difference between a solid endurance ride and a session that costs you Thursday.

What happens when the zones move

I went back through every ride I have on file and relabeled them under each model. The old percent-of-max method put the wrong effort label on roughly one ride in seven. One in seven. A threshold-based model got it wrong about sixteen times less often on the same rides.

Think about what one in seven does to you over a season. You do a four-hour endurance ride that the app calls Zone 2, except your real aerobic ceiling was 11 beats lower, so you spent three of those hours in a dead spot that’s too hard to recover from and too easy to build anything. Then you show up Saturday wondering why your legs feel like wet sand. Nothing in the file looks wrong. That’s the part that gets riders. The data is accurate. The labels are what lied.

I had a rider push back on me about exactly this in a text thread. His argument was fair: Strava’s zones are built from heart rate, and so are mine, so what’s the big deal. The big deal is which heart rate. Theirs uses one number. Mine uses two, and sometimes three, and I pick based on what I actually hold about that specific person. I went after Strava’s fitness and form scores a while back for a related reason, and zones are the same shape of problem wearing a different jersey. Honest measurement, generic interpretation.

Karvonen method vs percentage of max heart rate

The reserve model, the one most people know as Karvonen, works like this. Take your resting heart rate. Subtract it from your max. That difference is your reserve. Every zone boundary is your resting rate plus a percentage of that reserve. So a rider with a huge reserve gets wide zones and a rider with a narrow one gets tight zones, which is exactly right, because that’s what their physiology is doing. Garmin uses this model too, for what it’s worth.

The third option is drawing from threshold heart rate instead of max. Threshold is a number you can actually test, repeatedly, without needing to nearly pass out on a climb to find your ceiling. The method is well established and it’s been checked against the standard tools. On the rides I went back through, it was the model that got the labels right most often, so it’s the one I’d rather be using when what I hold about you supports it.

Why I don’t give every rider the same model

Because I can’t. Karvonen needs both a max and a resting heart rate, and some riders simply don’t have a reliable resting number coming through from any device. No resting rate, no reserve, no Karvonen. Those riders stay on percent of max until I have something better, and I’d rather tell you that plainly than pretend everybody gets the premium treatment. There’s also a floor on how narrow a reserve I’ll accept, because when the gap between resting and max gets too small, the math stops describing a human being and starts describing a glitch.

What I don’t do is run one formula for everybody and call it personalization. I work from the best numbers I’ve actually got on you, and which model that lands on depends on which of those numbers I trust most on the day.

On the bike, none of this is the main event anyway. If you’ve got a power meter, power is the better read on effort and heart rate is my second opinion, the thing I check when power and feel disagree with each other. Heart rate drifting up on steady watts at hour three tells me something the watts can’t, and I’ve already said my piece about what the watts never record.

Where heart rate zones still fall short

Running is where I feel the limits most. Running power meters exist, but almost nobody I ride with is using one, so heart rate carries nearly the whole load on a run. It lags. It spikes in heat. It flatters you on a downhill and punishes you on a hot start. I’ve wanted a cleaner read on running effort for a long time.

Swimming is worse. A wrist heart rate underwater is closer to a rumor than a measurement, and I’ve got opinions about stroke efficiency numbers that I’d like to be able to back with more than opinions.

One last thing, and it’s the uncomfortable part. If your zones moved, the numbers you were already told about yourself changed meaning. That ride you logged in March as Zone 3 might read differently now. I’m not going to pretend that isn’t disorienting. But I’d rather hand you a number that describes your actual body than a tidy one that describes an average rider who doesn’t exist. Your heart rate reserve is yours. Nobody else is riding that engine.