Went for a 50 mile ride today on our tandem (was supposed to be 64). Temperatures got over 100 degrees and 35 miles in we couldn't get our ride up a 4% grade without my getting really hot: some walking was involved.
Now we don't intend to repeat this experience, but I'm wondering physiologically, why heat has such a drastic effect on performance? I'm assuming this is the bodies self-protection mechanism to keep core temperature in line. Wound up taking a cab from th e 50 mile point back to the car and returning to pick up bike and stoker.
Background - returned to cycling last August after 20 years. both in our early 60's. Used to run decently fast marathons into my early 40's and didn't like the heat then either. This was a distance we've riddent 2-3 times and I'm confident we would have been fine with moderate temperatures. We did keep hydrated and I had electrolyte supplements (Nuun tablets plus gatorade/water)
There are limited ways your body can cool itself. They depend on natural laws of chemistry and thermodynamics. When the ambient air temperature is above body temperature, you can't radiate heat to the environment anymore. Even at 90 F, the radiation from you to the surroundings is slowed considerably.
When humidity is high, sweating (the body's most effective mechanism of cooling) becomes ineffective. The water has to evaporate to do much good; when it boils, it removes energy (heat) with it. When it just sits on your skin, it isn't helping much. And, just because you don't see sweat on your skin doesn't mean you aren't sweating. It is just doing its job of evaporating.
When heat and humidity are both high, it can be dangerous to do extended bouts of exercise. Your natural mechanisms simply won't work very well. I seem to remember the Chicago Marathon being delayed or cancelled a couple of years ago for this reason.
Your core temperature cannot rise too much for many reasons, and your body will try to prevent you from doing so. Specifically,biological enzymes, being proteins, have a very specific temperature range in which they function properly. Also, the oxygen/hemoglobin dissociation curve becomes less favorable (shifts to the right) with a rise in blood temperature. Basically, you transport oxygen less efficiently. This isn't drastic, but is significant. Of course, higher heat also leads to more dehydration, which is in itself a major problem.
next time (which hopefully wont happen), you'd have to hydrate better before, during, and after. thirst is the worst indicator of dehydration Also, high sweating removes electrolytes.. A good jersey and shorts are also good, as they wick away water, allowing you to cool faster. Helmets with good ventilation are also effective; i've heard that about half the heat lost from the body comes from the head.
I do intend to look for a helmet with better ventilation. We were hydrated and taking in electrolytes, but not well-adapted to high temperature as this was our first spell of heat this year save for a freakish hot spell in March. After a long stop for lunch at mile 40 we were doing ok albeit dreading one last climb, but we hit a stretch of new, blacktop, dark as the night, and the heat radiating from that stuff was just too much. Don't think it affected my stoker as much. Prior to that we had been pretty comfortable at 20 mph on a very slight downslope. Chalk this one up to experience.
The first hot ride each year, before you are acclimated, is tough. Heat acclimation takes about two weeks (not sure how many rides that is though) and does not last through the winter so you have to do it again each summer. Although I think that at least the mental part of being able to handle heat can improve over the years, with practice. Instead of avoiding riding in the heat, do it, albeit with lots of water, and at a lower pace.
Heat has such a large impact because as we ride, we are already producing waste heat that needs to be gotten rid of. Humans are about 20% effecient, (18-22% or so depending on the individual). The other 80% is waste heat. Some calculations I found show that a 70kg person running for an hour at a rate that consumes 900 Calories wastes 780 of those as heat. If they didn't cool via sweating (or convection to a colder environment), their core temp would go up by 22 degrees F and they would be dead.
That's why when the temps get close to body temp, or above, it is difficult to cool enough to keep the core temp down.