Cycling in the heat: knowing the temperature slows you down



- Alex Hutchinson (@sweatscience)


You run or bike slower in hot conditions because your body overheats… right? Actually, it’s a bit more complicated than that, according to a neat study from the University of Bedfordshire and the University of Brighton posted online at the European Journal of Applied Physiology a couple of days ago.

Here’s what they did: seven cyclists performed three 30-minute time trials under different conditions:

  1. the control trial, at 21.8 C
  2. the hot trial, at 31.4 C
  3. the “deception” trial, at 31.6 C — but thermometers were “fixed” so that the room temperature was displayed as 26.0 C, and the rectal temperatures of the cyclists were displayed as 0.3 C lower than they really were.

As expected, the cyclists covered ~4% less distance in the hot trial than the control trial. But in the deception trial, the heat didn’t hurt them at all — they covered just as much distance (slightly more, actually) than in the control trial!

This study joins a long-running dispute about exercise in the heat, sparked by work from Ross Tucker and others in Tim Noakes’s group in Cape Town. What Tucker argues is that we don’t slow down because we’re dangerously hot; we slow down to avoid getting dangerously hot: the “central governor” in our brains forces us to slow down before we reach any critical temperature. That’s why, if you do a 30-minute cycling time trial in hot conditions, you’ll already be behind your “normal temperature” pace within the first five minutes, and your brain will be recruiting fewer muscle fibres — even though, at that early stage in the trial, your core temperature is still relatively low.

Tucker argued that this is all unconscious, and your brain monitors the rate of heat storage in your body. Others like Samuele Marcora at Bangor argue that it’s a conscious process dictated by your feelings of discomfort. The authors of the new study view their results as supporting the central governor model, but there are a few interesting wrinkles. In particular, the measured skin temperature of the participants was ~0.5 C lower in the deception trial than in the heat trial for the first 15 minutes. This is a real physical effect, not just “in the head,” which the authors explain as follows:

We speculatively propose that the perceived need for heat dissipation was less during the early stage of the TT in DEC, which caused an involuntary autonomic reflex to reduce peripheral blood flow, resulting in lower [skin temperature]…It appears that a response based on incorrect conscious information may be sufficient to produce a subconscious physiological effect that resulted in an improved performance.

Strange stuff. So any practical benefits? It’s hard to imagine convincing your coach or spouse to give you fake weather reports on race days (or at least, it’s hard to imagine falling for it more than once). On the other hand, this suggests that your beliefs about how weather affects you can actually become self-fulfilling prophecies. If you’re convinced that heat will slow you down, it will. On the other hand, maybe someone like Sammy Wanjiru (RIP) simply wasn’t burdened by the belief that heat would slow him down — which could help explain his otherwordly performance in the heat of the Beijing Olympic marathon.