Shout out to everyone on the forum that helped me think through openers and nutrition! So I did some “socially distant racing” aka time trialing over the weekend. Did well, but missed my power goal by a few watts. Talking to some other guys, seems like everyone came out a few watts low. It was 97 on the course. I’m assuming that has some effect, but how much is normal??? Y’all have any experience with extreme heat? I’ve raced a lot in heat but not a TT where I have a very specific power goal. Give me tips!
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When I train in hot weather, I’ve reduced intensity as much as 10% for bike rides or runs. RPE and HR help guide that.
If I’m not heat adapted, I see more issues running than riding, due to cooling. I can lose a minute on an 18:00 5K run split, for example, in heat like that. The effect on the bike is less, but still there. Sorry I can’t really quantify it, but yes, totally normal even with some degree of heat acclimation.
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%10 is almost spot on what I lost. Shooting for 280 average. Did 260. I’m wondering how much of that I could get back by riding my TT position more, vs heat acclimation. I’ve been on the trainer a lot in lockdown, which means no heat and sitting upright haha.
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Would be cool to hear from several people:
Your hottest TT (or power test or outside workout), what was your power goal based off of your normal FTP and then how much (if anything) did you lose in the heat? I’d be interested to find an average range.
My personal experience is that heat massively impacts performance; my N of 1 experience is admittedly as someone who only regularly rides is pretty cool temperate weather being UK based so it could be fairly likely that if a rider is not regularly exposed to warm weather that the impact is more acute?
I rode in Yosemite last June and it was super hot (40+ degrees Celsius).
Just looking at my Strava file for a long-ish climb, I averaged only 204w but my HR was up over 165 on average. At the time my FTP was around 330 and I was approx 3.4 to 3.5 W/Kg.
Update / Edit:
I’ve just looked again at the Strava file and for the last third of the climb (same average gradient) I averaged the same HR and pretty identical cadence but my average power was 256w.
So in make that a 20% difference in power for the same HR and cadence on similar gradient climbing on smooth tarmac. The only major difference being the temperature which dropped considerably to below 25 degrees C.
I had been spot/on with my fluid and electrolyte and salt intake and was very well fuelled.
It just felt like a ‘limiter’ was on my power. Some TR podcasts have covered this talking about an internal ‘Governor’ so I suspect that may be what occurred. Apparently you can do stuff to acclimate to heat (something to do with saunas and heat shock proteins?) but that’s beyond my understanding.
It was extremely noticeable that as we climbed higher and the air cooled down, it was like a power switch had been flipped back on again and I felt great, despite the altitude, and my power returned.
Very weird to experience.
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If everyone puts their N of 1 experiences together, eventually we have a data set!
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My running is abysmally bad when is too hot. I think 10 - 15% is about right. I think Jack Daniels put me 10s per mile on a 5k tt i did back in April, the temperature was 77, humidity was 88.
Temperature will also affect cycling, but mostly will be way different. You create more air flow while riding plus you can coast. Also its easier to drink water and rest for a few seconds while coasting.
I think indoor cycling is closer to running when it comes to how temperature affects it.
point taken 
Living in Arizona, I have some pretty good experience, if not scientific as to how heat impacts.
In the Fall, Winter and Spring where the temperatures I was cycling in outside were normally between 45F (7C) to 90F (32C), when doing climbing intervals I always seemed to have 5 - 10% more watts available than I would when doing indoor workouts of the same intervals. At least the perceived exertion was less.
However, this has flipped around totally now that the daily temperature for summer are from the mid 80s F to 110F generally. Even if I get out in the morning at the weekend I am normally cycling in 90s to the early 100s Fahrenheit (not a really early morning person!), so the 30s up to 40 in Celsius. I find the same intervals much harder and I seem to have more power now when doing the same intervals inside with decent cooling. This gets progressively worse when doing climbing intervals, as the wind cooling goes away and the second / third intervals around threshold are not achievable as my body does not cool back down.
I have switched around my training to do most of my interval work indoors during the week, but I still go out at the weekend and do my z2 / 3 rides on flat terrain where I can maintain 20+ MPH and get the cooling effect. My heart rate seems to edge more towards sweetspot / threshold when doing those Z2/3 rides in the heat though.
I find about a 5% loss for zone 2 and for short zone 5 intervals (3-6 min), for longer high zone 3/zone 4 work, I see a 10% loss. Training in steamy South Carolina, I don’t do any sweet spot or threshold after the first week of June – too much heat fatigue for too few watts.
I move to zone 5 work in June-August. It takes me about two weeks at the start of July to get adapted to the humidity , but then I can match my cool weather watts on 4-5min intervals. I’ll train in the morning when it’s in the high 70s/low80s and about 90% humidity. When school starts and I have to do those hill repeats in the 95-degree afternoon, the power drops about 5% on those 4-5 minute efforts, but after two weeks it comes back up.
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When the warm weather started (27-33°C) my power indoor dropped around 10% - SS@90% felt like threshold (it was 25-27°C inside). After 3 weeks of riding in this conditions everything seems back to closer to norm (a little more sweat even during Z2 when normally I didn’t sweat at all).
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An athlete I train with had a similar question recently. He felt like he absolutely died the first time it reached above 30°C this summer. So we looked at his power-duration curve for the past season (2019 through 2020). Was pretty interesting.
Clearly his PD Curve is much lower >30°C across the board. By a ridiculous 80+ W at FTP! But only around 1% of his total rides are at this high temp, so definitely too small a sample size to be realistic. But as a comparison, he also had only ~1% of his rides at <10°C, and that curve was at the high end of his performance.
The other interesting trends we noticed were: At 25-30°C his power only starts to fall off around FTP and beyond. So he’s fine over shorter durations and high intensities. But his fatigue resistance is lower. And for lower temps, at 10-15°C his anaerobic (~10sec to 2min) power was better, if anything. But again power beyond FTP starts to fall off.
Overall he seems to perform worse over longer durations at higher temps, which aligns with his perception. His sweet spot is definitely 15-20°C.
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Yup, heat — especially coupled with humidity — can have a significant impact on performance. Basically, it may limit the amount of cooling you feel, which, in turn, limits how much power your body can generate. (AFAIK for each watt in your legs, your body produces an additional 3 watts in heat.) That limits with how much power you can turn the pedals. Like the others have said, have a look at your heart rate, it should be higher than usual. Use heart rate as a governor.
The aero position puts additional strain on you, but that is something you can train.
I think 10% must be smack dab within the sigma-1. That’s pretty much how much I lose, like others stated before me.
If you’re seeking hard n=1 data, here are two recent sets for hot day vs nominal day, both taken outdoors on “fresh legs day”.
Hot
Temp: 97°F / 36°C
HRavg: 181bpm
Watts(avg): 259w
Time: 25min
Nominal:
Temp: 72°F / 22°C
HRavg: 177bpm
Watts(avg): 288w
Time: 25min
So -29w lost during heat or…
258/288 = 0.899
So chalk it up to 10% lost, nearly on the dot.
Going from a fresh, cool early morning around 70F/21C (which only happen during 2-3 weeks each winter) to the hottest summer days in Miami at 93F/34C and 90% humidity costs me about 25W to 30W, meaning 12% to 15% of my 199W FTP, for a given HR (usually 140bpm for Zone 2 rides, 75% of my 186bpm max).
Well when I ran - up to 5k no effect - I ran a 17 min 5k when it was 34C and my pb was 16:48. Longer than that though and it made a difference. Half marathon I ran 1:20 when it was 32C once against a pb of 1:17 - mind you the field self destructed and I finished 3rd out of 1000 in that race and won £200 plus beat some guys I never finished in front of again! Marathons - never did a hot one really - London was warm one year and I ran 2:52 against a pb of 2:47 - but then I ran it carefully as even though it was only 21C you can get in serious trouble in a marathon in the heat. On the bike last year I was doing 25 mile TT at about 252W and the one really hot one at over 30C I was down at 238W - only made about 40s difference time wise though.
Plus I lose about 20W on the TT bike 270W for a road bike tt for an hour - 250 ish on the TT bike…but that gap closes as the season goes on…well it would normally!
This is also exactly mine experience. Longer threshold fall off over 25C . Short power duration stays similar.
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Went out for a hard ride in UK sunshine (294 TSS, similar ride week before was 236 TSS)
So this is my Garmin HRV stress record of what happened that night/ next morning. It stopped recording when my watch ran out of power. I am usually in the 20s, max 30s on a tough day.
I’ve never had heat stroke before and it took me a while to realise what it was. Once you’ve had it, it can be triggered again in response to heat.
I currently pay more attention to HRV and resting HR in the heat. If I notice any signs prior to riding I either keep it shorter or reduce intensity.
Edit: more susceptible for a week or so afterwards. Not forever 
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That’s a super interesting observation - helpful insight
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