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Does heat slow you down if you don’t know it’s hot?

January 29th, 2012

Why do you slow down in the heat? This may seem like a painfully obvious question, but it’s a topic of heated (oops) debate among physiologists. There are two basic camps:

  1. You slow down because the increasing temperature in your body begins to cause some sort of physical problem — maybe it’s in your muscles, or your heart, or your nervous system; there are several theories;
  2. You slow down because your brain detects that your body is getting hot, so it forcibly applies the brakes to avoid letting you reach any dangerous system failure (in your muscles, heart, brain, or whatever).

To put it another way, do you slow down in response to problems, or in anticipation of problems?

The problem with many of the experiments on both sides of this debate is that they can’t separate out the conscious psychological factors that also regulate self-paced performance. (I say “self-paced” because that’s what we’re really interested in understanding. Putting someone on a treadmill at a fixed pace and forcing them to run until they fall off is an interesting way of studying our ultimate failure mechanisms, but it offers basically no insight into what happens during a real-life race, where your decision to slow down comes long before you’re at risk of collapsing.)

Anyway, a new study from Stephen Cheung’s group at Brock University, in the journal Physiology & Behavior, takes a clever look at this problem. They told a group of cyclists that they were studying how much power output changes when you try to maintain a constant perceived exertion. To do that, they asked the cyclists to do two 60-minute rides (on separate days) where they maintained their RPE at 14 out of 20 (between somewhat hard and hard). But on the second ride, they secretly manipulated the room temperature as follows:

Now, let’s not kid each other: as the chamber heated up to 35 C, the cyclists knew something was changing. But at this point, they had oxygen tubes in their mouths, and couldn’t communicate with the experimenters. And the point is, they couldn’t consciously regulate their pace in advance to take the hotter temperature into account. Here’s what happened to their power output:

So what’s happening? Well, the power output did go down as they got hotter — but there was no real-time match between power output and any of the other variables that the researchers measured, including skin temperature, rectal temperature, heat storage (a measure of how much thermal energy is accumulating in the body), sweat rate, or heart rate. The verdict seems to be that the brain isn’t using any of these physical cues to anticipatorily regulate power output.

There are some potential limitations to the study — for example, the RPE of 14 might have been too low to cause severe enough thermal stress to trigger a response. But overall, the message seems to be that conscious psychological factors play a role in our response to thermal stress. And that fits with earlier studies like the one I blogged about last May, where lying to cyclists about the temperature allowed them to go just as fast at 31.6 C as they did at 21.8 C. This new study may not support the “anticipatory heat storage” idea of the central governor model, but it certainly reinforces the idea that the brain calls the shots.

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