Power vs Heart Rate: Which Should You Trust?

Anthony Walsh (0:00)

What if I told you that your Zone 2 power could change while you're actually riding in Zone 2?

What? Sounds confusing. Stay with me.

Your FTP number hasn't changed. Your power meter, it's calibrated and it's working correctly. And the training zones programmed into your Garmin or your bike computer are exactly where they were when you left the house this morning. So what's going on? The physiological boundary you're trying to train below may well have moved, and researchers have measured something rather extraordinary. After just two hours of cycling, the power at which riders crossed their first ventillary threshold dropped significantly. So I've got a question. If our physiology changes during a ride, why do we train as those zones are fixed? And when your power and heart rate tell you different stories, which one of those two should you actually believe? Now, before you head out the door and get your power meter and toss it into the bin, I'm not suggesting there's anything wrong with measuring watts at all. Power meters have absolutely transformed cycling. If I prescribe you 250-watt session, you know exactly what mechanical output you're producing and putting into the pedals. You don't have to worry about the wind. You don't have to worry about the gradients. You don't have to worry about sitting on. You can compare intervals, quantify work, measure performance, and build remarkably precise training sessions, at least I can, as a coach. But there's something power can't measure directly, and that is what the effort is actually costing your body. And I think that's where this discussion becomes a lot more interesting than simply asking whether heart rate is better than power. We can ask a much better question, because we're measuring different things. Power is an external measure. It's the work you're producing. Heart rate is one component of your internal physiological response, and perceived exertion, that's another. Now, you might assume those three things should generally move together, and generally they do. More watts, higher heart rate, greater rate of perceived effort. And broadly during increasing levels of exercise, that's what happens. But that relationship isn't always that way. It's not fixed. And understanding why might completely change how you interpret your training sessions. Imagine inside our not-done-yet coaching community that I'm coaching two cyclists. Both have an FTP of 300 watts. Both are doing a two-hour endurance ride at 210 watts. On paper, that's a relatively straightforward bread and water prescription. 70% of FTP, say. Same relative power, same duration. But one rider is sitting at 130 beats per minute, while the other is sitting at 155 beats per minute. Which one's working harder? Well, we don't actually know from the facts that I've given you there. We don't know their maximum heart rates. We don't know their individual thresholds. We don't know their training histories or how their cardiovascular systems respond. A heart rate of 155 might be quite comfortable for one cyclist and uncomfortable for the next rider. And here's the complication. Even if we were comparing the same cyclist on two different days, the answer still wouldn't necessarily be straightforward. I know this is confusing, but other things matter that we're not taking into account. Temperature matters, hydration matters, the accumulated work in the days and weeks leading up to that point, fatigue, that matters. The intensity of the proceeding work on that day matters also. How many kilojoules have they put out that day? The same power output can place a totally different relative demand on the body as a ride progresses. And that's not just a coaching observation that I've observed, it's something researchers have actually measured. There was a 2022 study, there was a group of researchers, they investigated 14 endurance cyclists. They wanted to know what happens to the boundary between moderate and heavy exercise, intensity domains after prolonged riding. We're talking about that first ventillary threshold. In simple terms, one of those physiological boundaries that helps us distinguish between relatively easy, aerobic and more demanding work. Before the prolonged ride, the average power at that threshold was 217 watts. After two hours of cycling, it had fallen to 196 watts.

21 watts, that's nearly a 10% difference. But the really fascinating part is what happened to heart rate. The average heart rate measured at that physiological threshold, has actually increased from 142 to 151 beats per minute. So think about what's actually happening here. The power associated with the thresholds, it's fallen. And the heart rate associated with the thresholds, it's risen. Two measurements moving in opposite directions, while the physiological boundary itself is changing. And remember, those groups were average. Individual cyclists won't all respond in exactly the same way. Some might have responded more, some might have responded less. But it raises a pretty uncomfortable question for us as coaches and for you as athletes.