RPE vs absolute power vs generating body heat

Hi all,

I’ve been wondering if there’s any good data / information around about how much heat you generate, and how this depends upon perceived effort vs actual watts of output. For example - if 2 riders of same height / weight, similar clothing go out and do the same ride together, one might end up super sweaty & hot, whilst the other might be freezing cold. In this case both will be generating similar amounts of power, but it would seem that they are generating different amounts of heat.

If this is actually the case, then it implies that one rider is more efficient - as heat is a waste byproduct. And… does this relate to your FTP (and what % of FTP you’re riding at). You see this with many other energy systems - as utilisation increases towards capacity the overall efficiency of a system declines. So does the same thing happen with human bodies?

My interest in this was sparked whilst riding with my family and some friends. We’re not the same height / weight etc, but in my case I was riding at an RPE of 1-2, whilst they were riding 7-8. I ended up taking all of their outer layers / additional jumpers / gloves / etc just trying to keep warm, whilst they were shedding clothes as fast as they could. But we were all doing roughly the same power output.

I do significantly more training than my co-riders, and obviously have a higher FTP. But what I’m wondering is - does this impact the bodies overall thermal efficiency at the same power output?

And secondly… does your body actually get more efficient thermally overall? By this I mean - does the % of power that ends up as heat vs “productive power” change in response to physiological training adaptation? I’m not thinking about losing weight - which I expect should impact this (as you’ll shed heat faster), but actual changes in either the ATP or aerobic energy systems.

Or is this a fixed % that depends on your specific genetic makeup / body type / etc?

Cheers! and happy Friday!

I think the short answer is basically yes, although there’s some nuance. At a basic level what happens inside us is chemistry and physics and one way to measure gross efficiency of any system is how much power is “lost” as, you guessed it, heat. Whether it corresponds to RPE I expect is less straightforward.

I recommend you check out the empirical cycling podcast, in particular the series about Vo2max. I caution you it is a very deep dive, but if you’re asking these questions, it probably means you’ll be into it :smiley:

1 Like

Thanks, this one? Category: Vo2max - Empirical Cycling

happy to geek out tonight whilst significant other is busy doing her own thing :slight_smile:


Yes! That’s the series

awesome, ty!

1 Like

Just listened to episode 18, which looks like the first one where they delve into vo2max… And… Brilliant! I didn’t get any answers to the questions I asked here, but they’re super interesting guys, great communicators. Will definitely keep listening. One really interesting thing they talked about today was how much your Ac contributes to results in aerobic tests. I’ve never considered myself a sprinter, but interestingly my half Monty results are very different than full Monty - and from what they were saying, I’m probably using Ac to get there (my hm numbers blow my ff out of the water). But… The hm numbers feel “right” for most workouts. By this I mean - really hard, but achievable with will power. But avdp absolutely slaughtered me with fresh hm numbers. I wound it back to 95% the next day (I did it back to back to see what happened) and limped through, but that suggests I’m relying on Ac for hm.

Anyway, haven’t got to the original answer yet but loving the journey!

1 Like

I’m just girl-guessing but I think the power/wastage as heat ratio would stay basically the same - what improves is your body’s ability to deal with the waste energy/heat. Some people just sweat more than others ( hubby sweats copiously compared to me but we weigh similar and have similar Watt outputs) and I believe that I have become “sweatier” as I’ve got fitter - but not much. I do know that heat adaptation occurs quite quickly - days to weeks - and can be trained.
Not sure that’s answered your question - but maybe helped a bit!

I don’t think sweat was a factor in this particular morning, I was literally so cold I was losing feeling in my fingers and toes. But - it’s definitely a factor when you’re warmer… (And I sweat a lot more than my wife)…

% body fat (insulating?) and overall mass would also be v important. But if you control for that…?