What Is Cardiac Decoupling in Cycling and What Is a Good Number

James. Welcome in to the WattPlan Podcast. I'm James. Family first, then the day job, building WattPlan, the odd podium, and a stack of physiology papers by the bed that my family has stopped commenting on. This week, a number hiding in every steady ride. Your power holds, and your heart rate creeps up anyway. It's called cardiac decoupling. So what is it telling you, and what's a good number? Lyra's our AI research voice, and every study is linked in the show notes.

Lyra. It's telling you how well your aerobic system holds its efficiency as a steady ride goes on. You compare watts per heartbeat in the first half with the second half. On a steady endurance ride, under 5 percent is the usual sign of a solid aerobic base. Over 10, your heart is working noticeably harder for the same power by the end.

James. And in between?

Lyra. Five to ten is where most trained riders sit, especially at moderate intensities or when it's warm. Nothing wrong with it. It's a rough coaching guide rather than a research norm, so treat the bands as a starting point.

James. Why does heart rate climb at all, though? Same watts, same legs.

Lyra. It's called cardiovascular drift, and it's been argued about for decades. We looked at two studies for this one. The first is a 2001 review in Exercise and Sport Sciences Reviews by Edward Coyle and a co-author. They described drift as a slow fall in stroke volume, the blood your heart pushes out each beat, starting after ten to twenty minutes of exercise. [1]

James. Because you're getting hot, and the blood goes to the skin to cool you down. That's the one I always heard.

Lyra. That's the explanation they argued against, actually. Their case was that the fall in stroke volume is mainly down to the rising heart rate itself, rather than blood steadily shifting out to the skin as you warm up. Either way, the result on your head unit is the same. More beats for the same watts as the ride goes on.

James. Okay, so a fitter rider just drifts less?

Lyra. That's the idea behind using it. As your aerobic system improves, your decoupling at a given intensity tends to come down. But heat, dehydration, caffeine, altitude and plain fatigue all push it up too, which is why one ride on its own tells you far less than the trend across similar rides.

James. And the second study?

Lyra. A 2021 review in Sports Medicine by Ed Maunder and colleagues, Stephen Seiler among them. Their point was that the numbers we measure on a fresh rider in a lab don't stay put. They shift as long exercise goes on. They called the ability to resist that durability, meaning how soon and how much your profile deteriorates over the hours, and argued it should be measured. [2]

James. Which is basically what decoupling is doing on a long ride.

Lyra. It's one simple window onto it, yes. The review doesn't name decoupling as the answer, but heart rate drifting against power over a long steady ride is exactly the kind of shift they're describing.

James. Right, but here's my question. I do intervals most evenings. Why can't it just measure those? Surely that's loads of data.

Lyra. Because on an interval session it measures the workout, not you. The hard blocks sit in the first half, the second half is easier, and heart rate takes minutes to come down after power does. So WattPlan only scores steady rides of an hour or more with power and heart rate. It sets aside a ten minute warm up and five minute cool down, and the two halves have to be close in average power.

James. So my Tuesday over-unders are just ignored.

Lyra. Ignored for decoupling, and rightly. If they weren't, you'd get a frightening number every Tuesday that had nothing to do with your fitness.

James. I've had rides where it came out negative. Does that mean I got worse, or better?

Lyra. Neither, really. Below zero means your heart rate fell in the second half for the same power. It's unusual on a steady ride, and quite often it's a quirk of the data rather than anything your body did, so don't read much into it. One negative ride is a curiosity. A run of them would be worth a closer look at the data.

James. And a hot summer ride, then a cold winter one on the same route? I'd guess the summer one drifts more.

Lyra. Almost certainly. Warm conditions push drift up whatever your fitness, so a July ride and a December ride on the same loop aren't a fair comparison. That's why the indoor trainer, with the same fan and the same room, makes such a good test bench. Compare rides done in similar conditions, at a similar effort, and the trend starts meaning something.

James. What's it actually useful for, day to day?

Lyra. Two things. First, checking your easy rides are easy. If your endurance rides keep coming in above about 10 percent, you may be riding too hard for easy days, and that's a signal to back off. Second, watching your base build. The example in the article has a rider's steady ride going from about 12 percent towards 5 over two or three months of base training.

James. If you're new to this and you haven't got a power meter, is any of this for you?

Lyra. The number itself needs both power and heart rate. But if you're new to structured training with only a heart rate strap, you can still feel the idea. On a steady hour at the same effort, notice whether your heart rate keeps creeping up. On an easy day, that creep is usually a hint to ease off.

James. And for someone with power and heart rate both?

Lyra. Make one ride a week your test. An hour or more, steady, ideally flat or on the turbo, in similar conditions each time, and ninety minutes tells you more than sixty. Chest strap rather than wrist if you can. And one catch. If your power on Strava is estimated from speed rather than measured by a power meter, WattPlan won't score it, because there's no real power to compare.

James. And anyone racing?

Lyra. If you race, look at decoupling at your target race power. Low drift there is a useful sign you can hold it for the distance, though it isn't a prediction. High drift is a warning. Either the target is too ambitious, or there's more aerobic work to do first.

James. Does WattPlan do anything with it, or is it just a chart?

Lyra. It watches the trend in the background. When a type of ride that used to drift more than 7 percent starts coming in under 5, across at least four rides, it sends you a note saying your aerobic fitness is improving, and the Coach can talk you through it.

James. What can't the research tell us here?

Lyra. Where the lines should be for you. The 5 and 10 percent marks are coaching rules of thumb, not cut-offs from either review. Nobody has shown how fast a typical amateur's number should fall in a base block, and neither review tested decoupling as a training target. It's a useful window, not a lab test, and it works best as a trend you watch over months.

James. So what should someone actually go and do?

Lyra. Ride one steady hour or more this week with power and heart rate both recording, and keep it genuinely steady, no surges. Then open the Progress page, pick Decoupling in the Metric explorer, and find that ride. Repeat a similar ride every week or two and watch the bars, not any single one.

James. So, steady rides only, an hour or more, compare like with like, and if my easy rides keep drifting high, I'm riding them too hard.

Lyra. The drift is your aerobic base talking. Ride steady enough, often enough, and it gets quieter.

Lyra. The full article and every study we mentioned are on the WattPlan blog, linked in the show notes. And if you've enjoyed this, follow the WattPlan Podcast wherever you're listening, so you don't miss the next one.

James. And if you want a plan built around the time you've actually got, get WattPlan on the App Store or Google Play. Thanks for listening.

Voices produced with ElevenLabs. James's voice is a clone of his own, Lyra is an AI voice, and James reviews every episode.