Kevin had a great post about how hot it has been in North Texas. 17 of the last 18 days it has been 100+ degrees in Fort Worth (today should make it 18 of 19). Sunday was 107 degrees! I rode on Thursday and yesterday after work and it is just too damn hot – pretty depressing when you roll out and it is 105 degrees. This is the first time since I started riding that the heat has had a dramatic impact on my riding (luckily I’m pretty heat tolerant growing-up in South Louisiana).
Cyclingnews.com actually has an interesting answer to the question of how heat impacts training today in their Form & Fitness Q&A:
There are several interesting effects of training in high temperatures and high humidity. Principally at such temperatures the dissipation of heat becomes unavoidably the limiting factor to performance. That means the speeds or powers one can put out in extreme heat are less than what one can do at lower temperatures and one simply cannot train as hard or as effectively.
The reasons are that humans use sweat evaporation as the principle mode of cooling and that we cannot function with body temperatures more than a few degrees above normal. Dissipating heat by evaporation of sweat requires delivery of heat from the core and working muscles to the skin. That heat is moved from the core to the skin by blood which would be flowing through muscles at lower temperatures, so the first problem is that there is less blood available to deliver oxygen to the muscles. Then, evaporative cooling is less effective in high humidity, so one simply can’t effectively cool oneself by sweating in those conditions. The brain will shut down muscle activity to prevent overheating. Overheating causes coma and death, so not overheating is a better choice. I don’t mean that you can’t use muscles when it’s hot, but that the effort required to make a contraction force that would feel easy in cooler temperatures becomes greater in warmer temperatures.
So, to answer you question directly: The downsides of training in 40 degree Celsius (104F) and 80-90% humidity are that it is relatively ineffective and you are at risk for death by heat-stroke. The upside is that if you ever race somewhere warm, you will be well adjusted to the heat.
If you want to train effectively you need to do the same things that your competitors who live in snow country do in the winter: Find someplace safe to ride where the temperatures are more conducive to physiologically effective training: Get out the indoor trainer and ride somewhere air-conditioned.
So for the next few weeks I’ll be doing most of my riding in the morning. Hopefully we get a reprieve from the heat when TX Tough rolls around.