How I actually measure training: Power, heart rate, lactate and RPE

I got asked to talk to GCN this week about heart rate versus power. It's a question I get a lot, and I think the way it's usually framed is wrong. It's not one or the other. They measure different things, and once you understand what each one is actually telling you, the question answers itself.

Here's how I think about it.

There's one thing you produce, and a few different ways of measuring what it costs you.

Power is the output. It's what you actually produced. It's external and it's objective.

Heart rate, lactate and RPE are different. They're not what you produced, they're what it cost you to produce it. Heart rate tells you what the heart is doing to deliver the effort. Lactate tells you what's happening locally at the muscle. And RPE - rate of perceived exertion, tells you how hard it actually felt.

So power is the only external number. The other three are windows into the internal cost.

The reason this matters is that the internal numbers move around for reasons that have nothing to do with fitness.

Take heart rate. Your HR at a given power might be lower because you've adapted and you're getting fitter. Or it might be lower because you're fatigued and your body can't lift it. Same number, completely opposite meaning. It can be higher because you're unfit, or higher because you're fresh and flying. Heat, dehydration, caffeine, nerves, bad sleep, altitude, coming down with something can all affect your heart rate. So a single heart rate reading on its own is ambiguous. You genuinely can't tell what it means without more information.

Lactate isn't fixed either. It moves with fuel — ride low on glycogen and it reads lower, take on carbs and it reads higher. It climbs in the heat, with caffeine, and it's shifted by whatever hard efforts you did in the days before. 

This is where RPE comes in, and for me it's the missing piece most people ignore. Including many pros. 

RPE is what makes the other numbers make sense. Low heart rate and low RPE at a given power? Can point towards adaptation,  you're fresh and you're fit. But low heart rate and high RPE? That's a red flag. Your body can't lift the heart rate but the effort feels harder than it should. That's usually fatigue or overreaching.. A low heart rate can mean you're flying or it can mean you're fatigued. RPE can help tell you which.

We've got so much data these days. Everyone wants the next thing, the next marginal gain, the next 1%. But before you go chasing that, ask yourself a simpler question: do you even record your RPE after a session?

Data is brilliant when it helps you make better decisions. But more of it isn't always better. Sometimes it just adds noise and confusion. And the one number that ties all the others together, the one that tells you whether the data even makes sense, is free, and most people never write it down.