Race day heat affects on the body?

I did a 90 mile local gravel race this weekend and it was the first hot and humid day of the year. I was doing fine and on my pace but at mile 50 I cracked. By mile 65 I was having to granny gear the slightest incline and walk some of the climbs to keep from cramping and my legs were dead. By mile 75 I was head to toe demolished. I crawled across the finish line 45 minutes after everyone else.

I’m usually a mid pack finisher. I looked at my data last night and my power numbers were a little low, due to my crawling the last 40 miles but my HR averaged was in the 170’s. I spent almost 6 hours in my threshold HR zone or above. Which can’t be good. My nutrition seemed good and I was drinking plenty with electrolytes. Does this sound like being unacclimated to the heat or did I just have a random disaster of a day?

You went out too hard, that all it is .

Your threshold HR isn’t what you think it is, and/or your HRM is unreliable.

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Maybe you just went too hard and popped, but heat acclimation is a real thing and takes a few weeks. Nothing you can do about it besides continue to train in the heat for a few weeks, then you’ll be back to usual

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Based on what? I thought I was as well until I had a PH sweat test and found out I need 1200mg of sodium an hour. Not to mention preloading the night and morning before with 1600mg. I followed that up with sweat rate tests and at 90 degrees drinking 1L of water per hour and I was still at an ever increasing deficit. So riding/racing for continuous hours I can easily get dehydrated and my body/stomach can’t process the water I need and am losing.

On a side note, as you get heat adapted you sweat sooner and more profusely.


Some of this is certainly lack of adaptation if you haven’t trained much in the heat yet this year. Your body has a safety “limp mode” just like an overheating car, it sounds like you tripped it during your race and never cooled down enough for it to reset.

It could have been some dehydration/electrolytes. Did you weigh yourself before and after to see how much weight you lost? It’s a good habit to get into when training/racing to see how well you are keeping up with hydration. In the summer, it’s easy for me to be consuming over 2 liters per hour and still be in a big deficit.

Even after being adapted to heat, that doesn’t mean any of us can put out the same effort as we can on a cooler day. I have no problem pushing solid tempo for 4+ hours on a cool day, but there is no way I can do that on a 100f sunny day for an extended period. At some point, the body only has so much cooling capacity and that becomes the limiter. When I’m heat adapted, high endurance will have me right on the edge of my cooling limitations. In a race, that often means constantly pushing beyond and then backing off to keep the balance.

I raced in Kansas this past weekend and it was 95F and wind blowing 20+. I don’t have much heat adaptation yet this year, but I’m from TX and did much better than most of the locals. Over half the field DNF’d, it was a tough day since it was the first hot day of the year for most of them. For gravel racing on hot days, my strategy of choice is to go hard early while it’s cool and then just managed the power drop as the day heats up. Being in a fast group up the road early shortens the race. Everyone suffers and slows down when it gets really hot, so try to minimize that time in the heat. For all-day races, just don’t go out too hard, but still maximize the power before it heats up.


Acclimating to the heat is the ideal, as is testing your target numbers and nutrition/pacing plans In similar conditions.
If you don’t have that opportunity, I’d absolutely make adjustments and err on the side of conservative- past a certain point you can’t just ‘get used to it,’ your threshold has effectively decreased, and that will have a bigger impact if your body hasn’t acclimated to some extent. That will depend on a number of factors and is pretty individual so experience is your best friend, but I’d almost always rather go out a bit easier and have some extra gas for when everyone else is blowing up than the reverse. (Particularly for longer events.)

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HR in the heat at endurance power can be higher than HR at threshold w/ cooler temps. There is a reason HR isn’t a great measuring stick depending on conditions, caffeine intake, etc. High HR on a hot day is just your body trying to circulate more coolant volume. A high HR is worth noting, but is not typically indicative of the physiological load on a hot day. My HR on a really hot day can average “threshold HR” for 5+ hours and I’m only pushing high endurance. Obviously, I can’t do a real threshold effort for multiple hours in any conditions.

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If I’m not used to the heat, going out too fast will cause problems down the road. My body will not cool down to a balance point where I can let my endurance take over. All the cold electrolyte drinks will not put me back in my comfort zone.
I too am from Texas and do my indoor training in the not very cool garage. It does help.

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Could be. I’m just guessing here, but start at 7a…2.5 hours to get 40 miles in…getting to be 10am. Starting to heat up. Temperature up to 85F by then? Biology adjusted your FTP down but psychology refused to accept it. Over the next 40 minutes things really fall apart. Show me your strava ride profile with temp on it…let’s see if the temp ramps up about the time your ride falls apart.

That’s one possibility. There are many others.


I believe you will also lose less sodium in your sweat, at least less concentrated, as you become acclimated/acclimatized.

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I’ve had one of these for a couple of years

It’s been useful to train with and figure out what my redline is, what it feels like to start getting there, and figure out ways to not get there.

In my own case, I know if I get too hot, it can take a long time to be able to ride normally again.

Even at rest, going from an elevated core temp like 100.5 degrees f, it might take 45 minutes to get back to resting core temp. Believe me I have timed it many times sitting on my front porch after a ride.

If a person is tired of yet another wearable, know that increased core temp takes a very long time to get back down, barring ingestion of slushies. Complete inactivity is worse than moving slowly with little effort to get air moving over you.

As far as results on your day for sure it could be heat. Like someone else said, limp mode. I have a ride where I could barely do 130w sustained and probably should have gotten a ride home. There’s nothing more important to your brain than keeping you in the safe temperature range.

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HR can be a decent indicator of this too- I know from experience that if it goes past a certain point on a hot day it can take a lot longer to come down- too far and I never really get back to the same ‘baseline.’
Has it’s own caveats of course, but can be a good ‘reality check’ and occasional reference point.

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I just attempted my first intensity workout since my complete destruction last Saturday and had to cut it way short. Legs were still dead. Guess I’ll keeps it easy for a few more days. Thanks for all the input everyone.