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Posts Tagged ‘heat’

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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Can you “train” your fingers and toes to withstand cold?

November 28th, 2011

Winter’s coming, so here’s a topical study just published in the European Journal of Applied Physiology. Do your fingers and toes gradually adapt to being exposed to cold temperatures? There are three questions we can ask:

  1. Are the digits able to maintain a higher temperature when they’re exposed to cold?
  2. Are the digits quicker to experience “cold-induced vasodilation” (COVD)? (When you get cold, your blood vessels contract; but after a certain point, the vessel walls get so cold that they can’t stay contracted, so you get a sudden rush of blood that helps to warm up your fingers and toes — which turns out to be a very useful response to avoid frostbite.)
  3. Do your digits hurt less?

Over the years, many researchers have tested whether our digits adapt to cold, and the results are all over the map — some see a positive effect, some see a negative effect, some see no effect. Into the breach come researchers from the Netherlands and from Brock University in Canada. They took 16 suckers volunteers and had them dip their right hand and foot in 8 C water for 30 minutes at a time for 15 consecutive days. At the beginning and end of the experiment, the “trained” hand/foot was compared to the “untrained” hand/foot.

Here’s how skin temperature changed over the 15 days:

The data kind of meanders around, but there’s not much of a clear trend. Unfortunately, there was a clear trend for CIVD: during the pre-training test, 52% of subjects experienced CIVD; during the post-training test, only 24% experienced it.

And finally, the pain score:

On the surface, this might seem like good news: it hurts less as you gradually become accustomed to the unpleasant sensation of being cold. In fact, though, this is bad news. As your body gets used to the cold, you notice it less, but you also are less likely to get the warming effects of CIVD. Combine these two factors, and you become increasingly likely to get frostbite without realizing it.

So what does this mean? Well, it probably ends my dreams of being a polar explorer. I have extremely poor circulation in my fingers, and this suggests that this is unlikely to improve no matter how often I freeze my fingers off. So, despite the odd looks I get, I’m going to continue to run in my big puffy mittens whenever it gets close to freezing, because I won’t get any long-term “training” benefit from suffering.

Getting fitter doesn’t make you sweat more after all

August 1st, 2011

The fitter you get, the more you sweat during exercise in order to dissipate heat more quickly. That’s the conventional wisdom among scientists, and I’ve certainly repeated it many times here and elsewhere. So I was surprised to see a new study posted online in the American Journal of Physiology, from Ollie Jay and his colleagues at the University of Ottawa’s Thermal Ergogenics Laboratory, that contradicts this conventional wisdom. His results suggest that your sweat rate simply depends on how much physical work you’re doing, and how much skin surface area you have. Previous studies have been confused because fitter people are able to do more physical work (thus generating more heat and responding with more sweat) at the same effort level.

Let’s say I’m running at a given intensity (say 60% of VO2max) that corresponds to 6:00/km. In order to move my legs, my body is burning a combination of carbs and fat, producing heat as a metabolic byproduct. In order to dissipate that metabolic heat, I’ll sweat a certain amount.

Now let’s say I accelerate to 5:00/km (so I’m at 70% of VO2max). I’m moving my legs faster, so I generate more metabolic heat, and in response, I sweat more than at the slower pace.

The question is: what happens if I go away and train for a year, and improve my fitness so that I can run at 5:00/km (the faster speed) and have it correspond to 60% of VO2max (the lower intensity). How much will I sweat compared to my untrained state? Will it depend on my intensity, or my speed? The current conventional wisdom says it’ll depend on intensity: so running at 60% VO2max will produce the same amount of sweat whether I’m running at 6:00/km (unfit) or 5:00/km (fit). But Jay’s new study found the opposite: I’d sweat the same at 5:00/km regardless of whether my intensity is at 70% VO2max (unfit) or 60% VO2max (fit).

Confused yet? In actual fact, the study took a slightly different approach, comparing two groups matched for body mass and surface area but with dramatically different aerobic fitness (VO2max): one group averaged 40.3 mL/kg/min, the other 60.1 mL/kg/min. He had them perform cycling tests, fixing either the relative intensity (i.e. 60% of VO2max) or the metabolic heat production, and found that sweat rates depended on heat production, not aerobic fitness.

There is one important caveat, though: the study was conducted in relatively comfortable temperatures of 26 C (79 F) and 26% relative humidity:

Maximal sweating capacity and subjective tolerance to the heat are no doubt improved by aerobic fitness, and therefore individuals with a high VO2peak would certainly have a distinct advantage during exercise at a fixed heat production in a physiologically uncompensable (i.e. hot and humid) environment.

So under “normal” conditions, the amount you sweat depends only on how much physical work you’re doing (and how big you are). But if the conditions are so hot that it’s impossible for you to dissipate all your metabolic heat through sweating, less fit people will hit their maximum sweat rate earlier than fit people.

Extreme heat, dehydration and sodium balance

July 28th, 2011

Another interesting hydration study [UPDATED WITH LINK TO STUDY] from Tim Noakes and his collaborators, studying South African Special Forces soldiers marching in hot conditions — following up the one I blogged about last year. The basics: 18 soldiers did a competitive 25 km march (taking about four hours), carrying 26 kg packs and wearing full battle dress, in temperatures averaging 40.2 C and reaching a high of 44.3 C (112 F). They were allowed to drink only water. The main point: they did it, despite

environmental conditions that approached those considered to be unsafe for practice and competition by the American College of Sports Medicine. Furthermore, all soldiers completed the study successfully and none presented with either the signs or symptoms of ‘‘heat illness’’.

But it’s the details that are most interesting. They were allowed to drink as much as they wanted, and the amount they chose to drink led them to lose 3.8% of their body mass on average — too much, according to conventional thinking. But they showed no sign of trouble, and there was no link between the amount of weight each soldier lost and his finishing time. But (as their previous study showed), weight loss didn’t correspond exactly to water loss: for every 1 kg of mass lost, their total body water stores only declined by 200 g (for details of how this is possible, read the earlier blog entry).

More importantly, the sodium concentration in their blood didn’t change significantly (and neither did their overall plasma osmolality), even though weren’t taking in anything but water. They lost some salt to sweat, but they also lost some fluid, so the concentration stayed relatively constant.

At this sweat sodium concentration, average total sweat sodium losses during the march could have been >240 mmol. Yet despite such large losses that were not replaced during exercise, participants maintained their serum sodium concentration. This confirms the now well-established finding that serum sodium concentration can be maintained during exercise without the need for acute sodium replacement during exercise.

I’m sure plenty of people will disagree with that last sentence! Noakes’s argument is that, if you allow people to drink as much as they want and choose their own pace, they’ll automatically self-regulate in order to preserve homeostasis — and the crucial parameter that your body monitors is not weight or water content, it’s serum osmolality. So it’s no coincidence that the soldiers allowed themselves to get dehydrated to precisely the degree that matched the salt they lost in their sweat — that’s just the way the body works.

P.S. Random aside on dehydration: the introduction of this paper cites another study claiming that Haile Gebrselassie lost 10% of his body mass while setting the current marathon world record. Now that’s impressive!

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Pre-drinking to hyperhydrate, and other heat-related research

July 25th, 2011

There’s a great (and timely) article called “Myths About Running in Heat” in the current issue of Running Times (linked to from Amby Burfoot’s latest blog entry), in which Phil Latter takes a look at six common myths relating to topics like thirst and acclimatization. It’s all good stuff, and worth a read.

The one that I hadn’t really thought about before was the idea of “hyperhydration” before running in the heat. In general, if you try to load up on fluid in the days or hours before a run, you’ll just pee it out. But Latter suggests two options. First:

Hyperhydrating, or drinking more fluid than is necessary to maintain fluid balance within the body, is effective right before an event because blood flow is severely reduced to the kidneys during exercise, thus limiting fluid excretion. “The trick then is to be able to absorb quickly and then tolerate the bloating feeling for a couple of minutes into the exercise period,” says [University of Sherbrooke exercise physiologist Eric] Goulet. “As the exercise progresses the intestine will slowly absorb the fluid, which will then be used for physiological regulation.”

Second is the idea of drinking “lightly salted water in the several hours preceding hot weather exercise” — a technique with a long anecdotal history that Goulet is currently testing in the lab:

The biggest trick, Goulet concedes, is making the substance palatable. For his trials, Goulet had the salt water ( just over ¼ teaspoon of table salt per cup) blended with Crystal Light and served at roughly 35 degrees, but adds, “You have to find what works best for you.”

One other interesting point is the idea that drinking fluids helps you deal with heat better — a claim that most people (including exercise physiologists) accept absolutely uncritically. Not everyone agrees, though:

After performing a thorough meta-analysis, Loyola University’s Jonathan Dugas, a well-known blogger on the Science of Sport website, explains why. “I’m not saying there’s no effect of fluid on body temperature, but you have to really qualify it,” he says. “The effect is really small. Maybe a half degree in temperature, maybe less.” [...]

His research suggests hydration levels have almost no effect on one’s likelihood of suffering from even the most extreme of all heat-related issues, heat stroke. [...]

“On a very hot day,” Dugas says, “no amount of drinking is going to change the fact that you’re going to go slower. You can drink up to 100 percent of your body mass, and it won’t keep you from running slower.

For practical purposes, of course, this doesn’t mean that water isn’t a special concern on hot days. The heat will make you sweat more, so you’ll need to drink more that match thirst. But it challenges the widespread assumption that when someone at a race collapses from heat stroke, one of the causes was that they didn’t drink enough water:

Current research suggests that some combination of genetic predisposition, infection, muscle damage, sleep deprivation and high levels of exertion may lead to heat stroke… water intake (or the lack thereof) isn’t mentioned.

 

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Jockology: exercising in the heat

July 3rd, 2011

This week’s Jockology column in the Globe and Mail is a round-up of a few recent studies on exercise in hot weather: how the brain slows you down more than the body; how acclimatization does (and doesn’t) work; and how cooling your palms can make your workout feel easier.

[...] “Slowing down in the heat could be a subconscious regulation to protect us from damage, such as heat stroke,” explains University of Bedfordshire researcher Paul Castle, the lead author of the study.

In other words, you don’t slow down because your body has reached some critical temperature. Instead, your brain slows you down to prevent you from ever reaching that critical temperature. It’s a subtle difference – but as the cyclists in the study discovered, it means that our physical “limits” are more negotiable than previously thought… [READ THE WHOLE ARTICLE]

 

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Heat acclimatization: what does it take?

June 3rd, 2011

Conventional wisdom says that we adapt to deal with heat after a week or two of high temperatures. But a study in the current European Journal of Applied Physiology suggests that it doesn’t happen automatically. These days, we spend a lot of time in air-conditioned homes, offices, cars and even gyms — so we may no longer get the stimulus we need to adjust to exercising in heat.

To test this proposition, researchers at the University of Ottawa tested a group of 8 volunteers in mid May and early September. They measured core temperature, skin temperature, skin blood flow, sweat rate, heart rate, and a few other variables during a 90-minute bike session at 60% VO2max — and found no significant differences even after a long, hot summer. The key: their subjects reported spending an average of just 18 minutes a day doing “moderate” or “intense” physical activity outdoors over the summer.

In comparison, chamber-based heat acclimation protocols known to elicit physiological adaptations require a minimum of 1 h of exercise at 50% of VO2max for ten successive days in order to elicit a physiological acclimatization.

There’s no doubt that heat acclimatization effects are real — enhanced sweat rate and greater blood flow to the skin, resulting in lower core temperatures. But you have to get out there and sweat to make it happen.