HR is an unreliable metric to determine effort level

Hi, yesterday I noticed something during my ride that some people are probably aware of.
During some climbing parts of the ride I could average 234 watts with 136 bpm:

While other times in the flat at 200 watts I was at 138bpm:

Same ride, same day, similarly tired, same hydration level… more than 30 watts difference at same heart rate.
At FTP I am pretty sure I can do roughly the same in the flat than climbing (definitely not 30 watt difference).
I wonder about the underlying reasons of this discrepancy.
What are your thoughts?

Also later into the ride so you’ll see some increase for sure. Here’s Boarstone, watch how each interval of the same wattage gets slightly higher in HR as time goes

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If your heart rate stayed the same at a given power output indefinitely there would be no such thing as fatigue. Doesn’t mean it’s not a reliable piece of data. You just may want to further explore in what contexts you can use it.


Before the presidential elections here in the US I was watching some political stuff while in ERG mode on the trainer. Wasn’t paying attention H.R. but was locked in on a long Z2 ride. After a couple minutes I could feel myself breathing harder and thought “What the heck?” Looked over at the laptop to see my HR had climbed nearly 9% just from being frustrated watching what was on the screen. Totally surprised me because my HR is usually very consistent (+/- 2%) during these types of efforts. Turned the TV to a different program and HR went back to “normal”. Thought it was interesting.

You’re very right. Heart rate takes into account many more things than just effort.


And by the same token, heart rate can be very useful!

I donated blood on Thursday, totally forgot because it has no seeming effect, and went on an 80 mile bike ride Saturday morning. My HR was crazy high, but my watts were not. Uh, hello McFly, take it down a notch on the target power numbers.

Like all data, it can useful or not depending on the circumstances. I love monitoring or reviewing drift or decoupling on 3-4 hour Z2 rides as a “measure” of fatigue and or general fitness. But it becomes useless past 6-7 hours when heart rate starts being suppressed.


There’s a podcast episode where they go into it - it’s very irregular as it’s affected by so many things. Tiredness is a big one for me, but also strange things like a certain musical track can have it climb right up.

^^^x1000. I feel like external factors can be huge. If I have a hard ride to do my HR is elevated at the warmup cause I’m thinking of what’s coming. Same if I think about how to plan my sprints in our local crit.

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Could it be that your strap wasn’t wet enough? Or that it is about time to change the battery?

I have this sometimes where my HR is 20 lower than I’d expect. Thus far, it has been the above.

When I have exceptionally fresh legs, I’m typically 10 points max under what I’d expect, but perhaps this varies from individual to individual.

The first point I looked at, is your cadence. At least with me, my heart rate elevates as my cadence increases, and on harder efforts, where my cadence drifts down, my heart rate also decreases


It’s called fatigue and comes from lack of endurance assuming food and hydration and overheating are under control . I bet if you’d ridden another 50km or so you’d have seen your power drop further for same HR.

This is what a lot of the long Z2 rides take care of. They dramatically reduce the amount of fatigue you suffer over longer distances so you can hold the power at lower HR for longer.