The science of heat management over long, hot races:
How hot is too hot for you?

Alan Couzens, M.S. (Sports Science)

Jun 2nd, 2015

"It ain't the heat, it's the humility" - Yogi Berra

I've had a lot of chats with athletes of late focused around one topic - heat!

Ironman Texas was a couple of weeks ago and, unsurprisingly, it was hot! Beyond hot, it was hot and humid. Very challenging conditions for an event that is, by virtue of its duration, already challenging!

I was nervous well before May. As the emails came in.."Coach, I signed up for Texas as my A-Race for this year", I found myself asking one question -- why? Yeah, there were a lot of slots up for grabs given that it was a championship race this year but this 'advantage' had well and truly disappeared pretty early in the piece when we saw the caliber of athlete that was going to be toeing the line this year. Honestly, I think the metaphorically 'hot' race of the year comes down more to chance. You get a few folks signing up and then they convince their buddies to sign up and before you know it, every man and his dog is signed up for that one popular race of the year - often without a lot of thought as to whether the race suits their body type or the climate/topography of their training home. But the merits of conscious race selection is the topic for another blog(s). In this one, I want to focus on that one topic of conversation that keeps coming up from the athletes with the wounds of Texas still fresh in their legs and their soul - HEAT!

The winner of the race was Matt Hanson, a guy who is 150lbs wringing wet (i.e. light) but at 5'10 in height, someone who still has a good size body surface area. A guy who, due to his light weight and good position, could move along at competitive speeds on a pretty modest (by pro standards) output of 275NP (via his slowtwitch post).

These 3 things - mass, surface area & required output to move at a competitive speed are the key variables when it comes to racing well in hot conditions.

Put another way, if you have a large frame &/or a lot of muscle mass relative to your surface area &/or, if it takes a higher power output for you to move along at competitive race speeds (i.e. you're as aero as a Mack truck :-), hot races will be especially challenging &, under extreme conditions (like Texas), your output may actually be limited, not by your fitness, but by the heat!

Let's play around with some numbers to illustrate...

First the bike: Typical efficiency numbers for an elite are in the range of 23% (average age groupers are closer to 21%). This means that of the total metabolic energy generated, ~23% of it gets applied to the pedals. The guessed lost to heat.

Power output (bike)

If you hit 'calculate' above, you can see that the estimated heat generated is in the range of W - as much as a good sized space heater!

You can also see the impact of efficiency/economy in hot races. Athletes with very good economy numbers have a big added advantage when the mercury soars!

Whichever way you look at it, that W is a lot of energy that we need to dissipate. Fortunately, as humans, we come equipped with a few pretty effective ways to deal with heat...

1. Convection

As anyone who has done a 'no fan' turbo session can attest, you get a lot hotter when you're not moving (through air). Assuming that the air temp is less than body temp, we get rid of a lot of heat on the bike purely by virtue of the fact that we're moving - the reason that climbing (at lower relative air speed) is always hotter than moving at the same power on the flat & the reason that hot races are even tougher when it comes to the run - something we'll revisit a little later...

Total heat loss here is a factor of air temp (if warmer than body temp, the opposite occurs, i.e. you're essentially exercising in a fan forced oven!), skin temp, body surface area and speed

You can plug those into the calculator below to get an idea of just how much heat we can ditch through this method

Let's start by getting an estimate of body surface area


Your estimated BSA is: m^2. Elite men who consistently race well in hot races, e.g. Kona, tend to have BSA's north of 1.8. If you are short for your weight, you will tend to not handle heat very well.

Now, for your body type, how much heat can we expect to lose through convection?..

Air TempC
Skin TempC

W of the generated is dissipated via convection. Still to go!

2. Radiation

Next up to bat, we have heat loss due to plain old radiation

Heat is lost as it 'radiates' from the source (in this case, the athlete). This is generally less effective than convection (think conventional vs 'fan-forced' oven -- only in reverse). It also is subject to the old Nieztsche quote... "he who gazes into an abyss...", i.e. just as we are radiating our heat into the environment, certain elements of the environment, namely the road, is also radiating its heat back at us. For this reason, the true amount of heat transfer is a bit hard to pinpoint as it depends on things like cloud cover, road surface type & how far we are from the road surface (if you've ridden a recumbent on a hot day, you'll know what I'm talking about!) but we can at least get an approximation for the heat that we can potentially lose...

Click calculate to get an estimate for amount of potential heat loss via radiation (derived from the BSA & temp data you gave above)...

Amount of heat loss via radiation =W. Still (at least) to go!

3. Evaporation

Here is where things get 'sticky' for the competitive endurance athlete.

We have the potential to lose a good amount of heat from evaporative losses of sweat from the skin's surface. However, this evaporation is dependent on somewhat dry conditions. If you've ever had the 'pleasure' of finding an evaporative (swamp) cooler as the primary cooler in a humid holiday locale, you'll know exactly what I'm talking about!. Cooling via evaporation simply doesn't work when the surrounding air is already moist/humid!

It is a challenging fact, that the 'super bowl' of our sport - the World Championships, takes place in one of the more humid places on the planet - with an average humidity during October of ~70%! It has become apparent to me, as a coach of many folks who have raced in that venue, that anyone who is to truly conquer that course needs to have a virtual PhD in the science of heat management - both the conditions of the race and their own response. Texas is one of the few places that can actually be a little worse!

If we look at the stats from Weather underground for Ironman Texas we find an average humidity for May 16th of 82%! It goes without saying that this makes getting rid of that extra heat extra challenging!

In addition to the ambient conditions, 2 related factors come into play in the athlete's ability to get rid of heat via evaporation...

  1. The athlete's sweat rate -- i.e. the athlete must have trained their ability to get rid of heat by sweating at relatively high rates

  2. The athlete's fluid absorption rate -- i.e. the athlete must have trained their ability to get high volumes of fluid across the gut by 'training' it in appropriately hot conditions with a sports drink designed for optimal absorption in the heat

There is a lot of stupidity circulating at the moment suggesting that athlete's shouldn't be worried about 'a bit of dehydration'. While this may have an element of truth over relatively short durations (up to an elite marathon), it is dangerous to apply the same principle over 10+hrs of racing. Make no mistake, the 'no plan' plan simply doesn't work over the Ironman distance in hot conditions. It takes a very conscious, focused, attentive, hydration plan to even somewhat keep up with fluid (& electrolyte) loss and arrive at that point of 'a little bit of dehydration'. In the absence of a plan, or if the athlete loses focus from the plan for even a little bit, things can get very nasty late in the race!

This 'skill' of absorbing large amounts of fluid is something that is very common (& perhaps selective) among the elite Ironman athletes that I've trained with. Flying in the face of conventional scientific wisdom, I remember sessions where Gordo would return to the van & chug a liter or more of fluid after each 40min or so loop around the Boulder Res! The man could put away some serious fluid & (by necessity) this ability is pretty common at the top of the sport. Maybe those college-day keg stands weren't for nought after all ;-)

All kidding aside, it is important to also highlight the importance of not over-doing hydration. The fear of hyponatremia is absolutely legit and is the reason that an athlete needs to be sure to replicate the heat of the race when training to tolerate high levels of fluid intake. Fortunately, this is quite easy to do if we strip out the benefits of convective cooling (i.e. hit the trainer for some 'no fan' sessions). In the interests of health, the athlete should always lose some weight/fluid over the course of the race & over the course of 'heat training'. Hyponatremia is not to be taken lightly. The key is to pace these losses so that they are manageable over the course of the very challenging Ironman duration

Let's run the math to see just what sort of numbers we're looking at here...

The upper limit of fluid transport across the gut wall is in the range of 1.5L/hr. Therefore, we don't want sweat rates to exceed this number by too much or we will run the risk of depleting the body's fluid stores (& the consequent risk of losing control of our core temperature). If we plug that rate into the calculator below, along with the 80% humidity of Texas, you can see the potential for heat loss from evaporation

Sweat/Ingest rateL/hr

Amount of heat loss via evaporation = W. Leading to a total of W of heat remaining 'in the system'

If this number is a negative, congratulations, you stand a good chance of keeping your core temp under control (at least for the bike). If you play around with the numbers a bit for a race of these extreme conditions, you'll see it doesn't take much for the environment to start to beat the athlete. If your efficiency on the bike isn't great, if you're bigger/carrying a few extra pounds, if your initial power output is a bit aggressive, if you lose focus for a bit (or shift focus to catching some guy up the road) and your fluid intake falls. All of these can amount to you stepping off the bike already dehydrated/at a fluid deficit going into the toughest leg from a heat management perspective!

That's right - staying cool on the bike is the easy bit! If you look back to your first heat management mechanism - convection, you'll remember the importance of air speed. Now that the air coming at you is moving at about a third the rate of the bike and the day is warming up, you can imagine that things are going to get extra challenging. I'll run the math on that challenge in my next post. Until then, stay cool and...

Train smart,



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