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


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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.

6 Replies to “Does heat slow you down if you don’t know it’s hot?”

  1. Interesting. On initially reading of the study I thought of the Hawthorne effect in partially explaining the results, as the subjects certainly knew they were part of an experiment of *some* type. Nevertheless, as you noted that the subjects’ RPE was between “somewhat hard” and “hard” there was evidently sufficient physiological ‘room’ for them to maintain a consistent power output with – amazing to me – no increase in sweat, heart rate, or internal temperature despite the significantly increased surrounding temperature. I wonder – perhaps too cynically – that the suspects were possibly inadvertently tipped off by those leading the experiment to “take it easy initially” (perhaps worried about their liability otherwise!) In any case, I agree with your premise that the brain is most certainly calling the shots, as it is doing so based upon a very complex and poorly understood – most especially by the athlete themselves – set of constraints, predictive algorithms operating largely subconsciously, and complex motivations.

  2. @Mark: I should clarify that sweat, heart rate and internal temperature did change throughout the trial, drifting up and down. But the stats (using a fancy method I’d not encountered before called Auto-Regressive Integrated Moving Average) suggested that these changes weren’t synchronized in real time to the changes in power output — so the subjects apparently weren’t making immediate (and anticipatory) changes based on their thermal cues. As you say, these factors (and many others) likely still feed into some highly complex algorithm, but we’re a long way from understanding what that is.

  3. If sweat, heart rate and internal temperature did not change in response to power output, there may have been some compensatory factor – such as wind speed/flux – in the rooms. Note that control of the temperature was much less after the peak than before. Was the A/C taken into account?

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