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Wednesday, May 21, 2008

Fatigue Series Part 4: Exercise in the Heat

Exercise in the heat: Predicting the physiological future - African runners outperform white runners in the heat

We're back with Part 4 (or is it 5 or 6? I've lost count!) of our Series on Fatigue during exercise. In our last post, we looked at exercise in the heat, and found that:

  • Laboratory research shows that human beings will stop exercise when their body temperature rises to a certain level. That level is of course dependent on the athlete's motivational levels, though interestingly, not necessarily on their training status or performance.
  • We also saw that the science has shown that when the body temperature rises to reach about 40 degrees, the brain actually activates less muscle than a "cooler" brain, and that there is evidence for reduced arousal and motivation.

So, the hypothesis, based on these constant workload studies, is that the heat affects performance because:

A high body temperature DIRECTLY inhibits the ability of the brain to activate muscle.
Therefore, exercise stops (because in these studies, remember, slowing down is not an option)

What happens when the athlete CAN slow down? Self-paced exercise

Today we turn our attention to the case where athletes can slow down - this is arguably more representative of what you will see in Beijing later this year, since any athlete can, at any stage, choose to drop off the pace. Of course, they lose their medal chance this way, but it's a much more applicable form of testing.


And to understand this, we look a few studies. We'll do it in a couple of posts, because otherwise the length would become enormous. So today, we consider one study, with more to come in the next few days.

In 2000, a study by Tatterson (J Sci Med Sport) found that cyclists slowed down soon after they started a 30-minute performance trial in hot, but not cold conditions. What was significant is that their body temperatures were not higher in the hot than in the cold when they slowed down. Obvious, yes, but quite contrary to the theory that your brain stops activating muscle AFTER your body temperature hits the "threshold". They didn't measure any index of muscle activation, however, but it was a crucial observation that something else (and not direct body temperature) was playing a role in the heat.

African runners in the heat - anticipatory regulation thanks to their smaller size?

Then, a study done by Frank Marino while he visited Cape Town a few years back, was one of the first to use the words "anticipatory", because his finding (discussed below) found differences in the pacing strategy of African runners compared to white runners in hot conditions. So the conclusion is that something is happening BEFORE the body temperature rises, slowing the runner down so that they don't overheat.


And this is obvious. Think for a moment about when you go and train on a very hot day. You do not simply go out and run or cycle at your normal pace until suddenly, overcome with a sensation of hyperthermia, you slow down! Rather, your entire approach to the session is changed and you slow down LONG BEFORE you ever get hot in the first place! Within the first few strides, you're probably already going slower. So this is one of those examples we spoke about a long time ago - intuitively, we know what happens.


The question is HOW? And also, we have to consider the prevailing expert opinion of the time. In this case, remember, the "textbook" knowledge says that exercise is impaired because the HOT BRAIN directly inhibits muscle activation after body temperatures are raised by exercise.

So, let's look at the study by Frank Marino. I'm sure he'll forgive my very rudimentary depiction of his methods below:





So he had 6 African and 6 white runners, quite well trained, doing a performance trial after a 30 minute steady run in either HOT (35 degree) or COOL (15 degree) conditions.


The starting hypotheses for this study, had you read the theories about exericse in the heat, would be:


  • Performance would be impaired in the heat, so the runners would be slower during the 8km trial in the hot condition. This is fairly obvious.
  • They'd slow down in the HOT trial because they'd be much hotter than in the cool trial - the high body temperature (and HOT brain) is failing to activate muscle, as we're told by other research.
This is what was found:


Graph of running speed (km/hr) against time for the 12 runners during 8km time-trial performances preceded by 30minutes run in hot and cold conditions

I've highlighted with a red circle one of the more significant findings - the white runners started the 8km trial much slower than the black runners did, from the first minute. Of course, both groups eventually slowed down in the heat compared to the cold (the black symbols on the graph), but it's this difference between black and white runners that should be of interest. So, why then, do the white runners start so much more slowly?

Option 1 is that they are already hot. They might be finishing the 30 minute steady run with higher body temperatures. That would agree with the theory that the hotter you are, the slower you go...

However, look at the graph below:


Graph of rectal temperatures during the course of the trials in hot and cold conditions


Again, I've highlighted the key point there - the black and white runners had THE SAME rectal temperature when they started the 8km run. And not only this, but the temperature was "only" 38.2 degrees, so they were way cooler than the supposed "limiting temperature".

Yet, for some reason, despite the fact that the black and white runners have the same temperature and are not in any danger, the white runners "chose" to START an 8km time-trial slower than the black runners. We can therefore dismiss Option 1 from above, and say that it's clearly not a case of a hot athlete slowing down! If it was this simple, with some "direct effect" on the athlete, then the slowing down would happen equally in the two groups. This is an amazing finding given the prevailing view that the heat impairs performance directly, I hope it strikes you that way too!

So what, then, is the reason? Well, that's of course difficult, if not impossible to PROVE (as we've seen recently courtesy the CAS, "proof" in science is not as easy to do as people think), but here's a theory from the Marino paper:
  • The African runners were much smaller than the White runners - 59 kg compared to 77kg, to be exact. The white runners were taller, however, and had a larger body surface area.
  • We know from previous research that a smaller runner produces less heat while running at the same speed as a larger one. That is, the total heat PRODUCTION is dependent on body mass, and smaller people produce less heat.
  • Smaller runners also lose less heat, however, because they have a smaller body surface area to lose heat to environment.
  • But the key is: These two factors don't exactly cancel one another out. The result is that even though they lose less heat, smaller runners are still able to lose more heat RELATIVE to heat production than larger runners. This has to do with the ratio their mass to body surface area - they may lose on surface area, but their lighter weight more than makes up for it.
  • The net result of all this, is that smaller athletes have a reduced RATE OF HEAT STORAGE than bigger runners.
  • Now, given this fact, if two runners are going along at the same speed, the smaller one will be storing less heat, and therefore his/her body temperature will be climbing slower than that of the big runner.
  • Put differently, it means that if both athletes are concerned about how hot they are getting, then the bigger runner will have to slow down in order to prevent his heat storage from rising, which would ultimately increase his heat production.

Now, with all those facts on the table, the results start to offer an interesting theory:

The rate of heat storage is responsible for Anticipatory Regulation of exercise and pacing strategy in the heat

The theory is that the white runners, by virtue of their bigger size, have an increased rate of heat storage. (Note that this effect (the different pacing strategies, that is) is likely due to size - had the groups been matched for mass and height, the result might have been different - see the comments section to this post!)

The brain is "clever" enough to know that if the athletes starts their 8km time-trial at a fast pace, then their very high rate of heat storage is going to see their body temperature RISE very rapidly. They are in danger of reaching a core temperature of 40 degrees BEFORE the end of the time-trial (which they know is 8km long). Remember, at this temperature, the brain says "Enough" and exhaustion usually occurs (or soon after).

Therefore, the brain says "Whoa, back off a little!", long before the athlete overheats, and with the intention of making sure that they do not reach this limiting temperature before they are able to finish the trial - it would be a complete failure to do this, and reach the 6km mark by the time their brain says "enough". So instead, it REGULATES their performance IN ANTICIPATION of ever reaching that limit. That Anticipatory Regulation is achieved or mediated by the rate of heat storage, which is different from the very early stages of exercise.

On the other hand, the African runners, who are smaller, have no such problems. They thus maintain a higher speed, and a similar rate of heat storage, leading ultimately to an improved performance. Note, very importantly, that in the cold, this difference between black and white runners does not exist. Therefore, it's not a case that the white runners are just inferior to the black runners - it applies only in the heat, when the environmental temperatures bring this heat storage aspect into play.

Looking ahead

What this study does not do is measure anything related to brain function. Now, that's very difficult to do during dynamic exercise, and is often criticized, but we'll discuss a study tomorrow that looked at EMG activity (a measure of how much muscle is being activated by the brain) during trials in the hot and cold. This was the first study to find evidence for it. It was also a study I did for part of my PhD, though I'm not claiming anything here - it was be default, more than anything else!

So that's coming up in our next post - evidence of Anticipatory Regulation of Exercise Performance, along with a few more concepts to build on the ideas put forward here.

Join us then!

Ross

16 Comments:

vikram said...

Great post. A couple of questions and one request

First the request - please could you post clickable images which lead to a larger size image ? The images in the post are quite small.

If the studies had looked at runners of the same size, would the Caucasians have slowed down at the same rate as the African runners ?
Also, the studies you've mentioned talk about white skin and dark skin. What about brown skin people ? Are there any studies done on individuals whose skin colour lies between the two extremes ? Would it be safe to say that their rate of heat accumulation would also be independent of skin color but dependent on size ? Are there any such studies ?

Regards.

Ross Tucker and Jonathan Dugas said...

hi Vikram

First, to respond to the request - usually an image does open in a new page when it's clicked on. I'm not sure why yours are not - perhaps it's something to do with a setting on your browser that you can change?

Then, to answer the questions:

Not sure - that's one of the important points about the study - we don't know if it's a race effect (African vs. white) or a size effect. Certainly, size is a big part of the reason for the advantage, we know that from many other studies that have shown how important size is.

Whether there is something also related to African runners, I suspect there is. There are studies, which I didn't have time or space to go into here, that suggest some differences in thermal homeostasis, and so it might be that this issue is more complex than just size.

It would be highly speculative to guess what they are though...

As for studies with different skin colours, none that I am aware of. Bear in mind that this would mostly impact on the heat gain through radiation which is considered insignificant in this study, and so I expect it would make little difference by itself.

Bear in mind, however, that whenever one starts to look at different ethnic/race groups, it becomes something of a minefield, both politically and from a genetics/anthropological point of view.

So I don't want to get too in depth into an area where an anthropologist will criticize certain things (like the use of "Caucasians", for example! I know it's not 100% literally correct - one might be able to post on that alone!).

THe key point in the post is that some people pace themselves differently, and it's hypothesized that this difference is a function of their lower heat storage - quite how that is achieved, or what underlies it, is for great debates of the future!

Thanks!
Ross

Derek said...

There's something here that's not being taken into account. The good folks over at chasingKIMbia.net noted that there are cultural differences too. Their african runners choose their pace according to how they feel AT THAT VERY MOMENT, whereas caucasian runners often take many factors ,which we are seeing in your posts, into account.

The example that was given, is that an african trying to run a 3:30 marathon will go out hard everytime, and fail the first two times, but be sucessful the third.

A caucasian runner, will pace himself carefully, and run a 3:50 marathon and then a 3:40 and then a 3:30 marathon.

Some may joke that the caucasian would never run the 3:30 marathon, but point is that while the caucasian "looks to the future" to choose his pace, the african does not.

The explaination that was given, is that the african is used to living like that everyday. It's impossible to think about tommorow's meal, so they don't think about their level of fatigue 20 minutes down the road either.

It's definatly interesting to think about!

vikram said...

thanks Ross. Does any form of training influence heat storage rates ? Are there limits on how fast or slow heat accumulates in the body ?

runningbloke said...

I read with interest your heat storage piece and the fact it has been noted that the brain already knows at the start of exercise that essentially you will overheat and therefore you start off the competition exercise slower. I used to run in normal shorts and sleeveless T-shirt like the majority of runners. For the last 6 months I have been using the BSc KompressorZ compression clothing (www.bsckompressorz.fr) and the fact when you put them on you have an instant COOL feel about you and while running it feels cooler and fresher,due to the compression qualities, my performance has instantly improved and I put a lot of it down to feeling cooler whilst running but I'm not sure if the UV rating attributes to the cool feel. Maybe you can shed some light on that. Certainly minutes after running the compression clothing seems to help my core temperature come down quicker.

Anonymous said...

Sorry to see that the study confounded the effects of race and body mass. A well-designed study would have used body-size matched individuals - as it stands these factors can't be separated in this study. For example, many Africans have proportionately lighter calf muscles, which would positively affect running economy as well.

Anonymous said...

What would be the take-away from this article and series for an average runner like myself?

Anticipatory regulation of pace as a theory seems supported. So what would most benefit my race in the heat?
1. get smaller, i.e., lose weight,
2. acclimate to racing in the heat,
3. increase will power,
4. try to have small, fast, heat- acclimated parents next time around.
5. some other factor.
Thanks. Mike McGrath

Ross Tucker and Jonathan Dugas said...

Hi everyone

just to respond to two comments:

First, to anonymous:

The study by Marino is actually very well-designed. You can't, unfortunately, go around controlling for everything, because your research would never get off the ground! If one tried to control for calf size, you might never find white runners! The same goes for mass - to find white runners with the same mass as black runners means that you lose the ability to match them for performance. for Marino's study, the key was performance - the groups had to be matched for 8km time. And if you look at the 8km times in the cool, they did it.

But the heat is the interesting thing. So to me, while I take your point and admit that in the ideal world, they'd have controlled for it, I think the study was about as good as it could be.

Next, to Derek:

Fair point - no one has ever studied pacing and perceptual differences between the Kenyans and white runners. I know of the perception that you mention, that they base things on feel, and I can certainly believe that it would affect it. However, in this particular study, I suspect it was not a factor, given the short length of the performance trial. Also note that these are not Kenyans, and I guess one has to be careful about generalizing in these kinds of studies!

It's certainly interesting that the Africans, easily the world's best runners, do very little "by the book". There's no laboratory testing, lactate threshold, VO2max, or preoccupation with science in their running. The eternal question is whether the introduction of science to these athletes would make them better runners or not!

An interesting debate...

I'll get to other questions and comments later!

Thanks again!
Ross

Andrew said...

Another fascinating article!

It's hard to say for sure in your plots (I, too, couldn't blow them up) but it looks like the Caucasian runners did reach a higher temp than the African runners at the end of their steady run, but managed to cool down to the same temp before starting the time trial. This seems to support heating up faster during a run due to mass, but cooling down faster when not running due to surface area.

On the heat issue for bigger runners, could it be as simple as basic geometry? Mass is proportional to volume, which increases to the 3rd power of a linear dimension, while surface area increases to the 2nd power. Heat generation is proportional to mass and heat loss is proportional to surface area, right?

Colenso said...

G'day again Ross and Jonathan - as someone who for the last fourteen years has run and competed in the high temperature, humidity and solar gain of tropical North Queensland, this is a topic in which I have enormous interest.

Writing as an ex-physics teacher, I want to comment briefly on the terminology. As Whelan and Hodgson pointed out in their classic physics text book from the early 1980s, the word "heat" is best used only as a verb. Thus, I may "heat" my flask of water on my Bunsen burner. As a noun, Whelan and Hodgson argued persuasively that it's better to use the phrase "thermal energy" rather than "heat". "Hot" as an adjective is trickier; again, perhaps it's better to refer always to the temperature, using the units of degrees Kelvin or Celsius, as indeed you seem to have done throughout these articles.

Why is this necessary (I hear the sceptics murmur), surely it's obvious here what is meant? Well, the reason why it's necessary is because it's critical to differentiate between the intrinsic parameter of core body temperature measured in degrees Celsius, and the extrinsic factors that help lead to that increase in core body temperature. Here in the coastal tropics, it is not simply the extrinsic factor of the "heat" or the ambient temperature per se that helps lead runners to overheat: it is a combination of, first, local ambient temperature (measured by convention in the shade); second, relative air humidity; third, solar gain on the race course; fourth, temperature of the tarmac surface; fifth, wind speed and direction, sixth, wind humidity; seventh, the area, mass and thermal properties of the runners' clothing and foot wear (many were the arguments I had over the years with other club members about the thermal pros and cons of wearing a running vest when competing!); eighth, course length; ninth, course steepness or gradient; tenth, mean average and maximal speeds during race, particularly during sustained efforts to surge during race or sprint at race finish.

As my local running club's president back in 2000, I felt I had to try to improve the quality of the advice we gave competitors lining up before our road races after a promising youngster collapsed with heat exhaustion at the end of a 5k and had to be carted off to hospital in an ambulance. When I looked into the matter further, I discovered that the only Olympic competitors who were really taking the thermal conditions at the Sydney Olympics seriously were the equestrians, who alone refused to compete on their massive and valuable horses when the solar gain exceeded a pre-set value. I think they had a portable electronic kit for this as used widely by the US military. I looked briefly at getting one but it was pretty pricey. One can measure solar gain more cheaply, if less conveniently, and still quite usefully by painting with blackboard paint a copper or aluminium sphere of 10 cm diameter. In the blackened sphere, one then drills a suitable hole into which one inserts a rubber bung, into which one inserts a standard thermometer (up to 110 degrees Celsius). Suspend the sphere two metres above the ground out in the open on the tarmac two hours before the race start, and record the temperature inside the sphere every ten minutes. The recorded temperature is a useful indicator of solar gain. The question of the cut-off temperature inside the sphere above which a race should not proceed, or should be cancelled once the race has started, then becomes the really interesting question. Much more work needs to be done in this area. Unfortunately, despite my best efforts to persuade them to change their ways, here in Australia the Bureau of Meteorology refuses to try to measure solar gain so that local weather forecasts are effectively useless for predicting racing hazard from overheating.

Colenso said...

Reading my comment above, I should have made it clear that:

1. By the shorthand of "solar gain", I am referring to the gain in core body temperature brought about by solar radiation load on the competing or training runner.

2. I can't remember now for sure what lead me down the solar loading route. It wasn't something that I thought up; I came across it when I was researching risk factors for racing in the tropics. I think it was an ACSM paper I read that first suggested to me the importance of measuring solar load. However, the ACSM's latest position paper relevant to this issue, the 2005 American College of Sports Medicine Roundtable on Hydration and Physical Activity: Consensus Statements seems to say a lot less on this than I remember them saying back in 2000! (Perhaps it's just my ageing (sun-baked) brain!).

3. I deliberately tried to avoid commenting on intrinsic factors, physiological or psychological, but rereading my comment and others it seems to me essential not to overlook the intrinsic factors of a runner’s hydration level and of thermal energy loss from sweating.

As Andrew correctly points out, because body mass increases with the cube, while surface area only increases with the square, if proportions remain constant a 75 kg runner is one and half times the mass of the 50 kg runner, but has only 1.145 squared ( slightly less than one and a third) the surface area.

A simple model, then, that compared the body mass and surface area of different sized naked runners, and assumes that thermal energy loss is only a function of surface area might seem indeed to provide a clue to the faster running of the lighter runner. But is this only how naked Homo regulates his body temperature under thermal stress - what about the role of the sweat glands: their numeracy, their flow rates, and their efficiency? And remember, it's a long time since runners last competed naked in the Olympic Games. National pride and corporate sponsorship as much as any concerns about propriety dictate that today’s competitors probably wear too much clothing when they race. My point is that evaporative cooling only really takes place efficiently under the right ambient conditions from naked, exposed skin: in a modern race, this means from the scalp (to a small extent, provided the hair is very short); the face; the neck; the d├ęcolletage; the arms, armpits and hands; the part of the legs from beneath the shorts down to the sock tops. A thick covering of body hair may further reduce cooling from evaporation from the exposed skin.

For my height, I am particularly heavy as competitive long distance runners go and as hairy as a goat. I have never won a race in the tropics where corporate sponsorship meant that the race rules required me to wear a vest – psychologically or physiologically, a vest when racing means I feel that I am overheating by the first corner and all I want to do is rip the wretched garment off my body.

Anonymous said...

I agree with the earlier poster that failure to match body mass is a critical error or omission, for whatever reason, in this study. It can't tell us much I feel.

Surely one would expect differences in heat handling with such differences. And 77kgs is rather heavy for an endurance athlete.

maria said...

This sort of study cries out to have a repeat done in the UK or some other country where there are runners whose ancestors came from all over the world - take out cultural and acclimatisation aspects. (For example when you have a Mr. M.F. talking about how much he learned from sharing a house with some Kenyan runners and having to give up greasy burgers, not to mention the infamous bleached blonde hair of his youth, you would not leap to the conclusion that he spent the first nine years of his life in Somalia!)
Having said that, if you match on 8K times then you already have a difficulty due to the well-known issue of big folk just plumb not being as fast as little'uns (in general) and people with West African grannies being bigger than those with East African grannies. Maybe 2 studies needed there - one matching on "size" somehow, another on performance?

Erik said...

Great post as always. I am intrigued by a thought I had earlier today while contemplating this post and cycling in the (so far) hottest day of the year.

If running or cycling in heat causes less muscle activation and slower pacing, could one logically conclude that training in cooler climates will be more productive? It seems that a greater training stress could be produced by an effort in cool conditions.

Bob, NY said...

I dont know how significat trhe slower starting pace of white runners really is.

It seems to me irregardless of temperature African runners take it out much harder than runners of other races.

Just take a look at any World Cross country race.

Then take a look at the one held a few years back in Mombasa (2006?). Extremely hot conditions, and the Africans took off like bullets.

You simply cant say this only happens in hot weather races.

Ross Tucker and Jonathan Dugas said...

Hi Bob

Thanks for the comment. We're not saying this only happens in hot weather races though, I think you've misread the conclusion from the study.

If you have a look at the graph in the post above (the one showing running speed), you'll see that in fact, it was only in the hot conditions that the African runners ran faster, not in the cool.

That is, the study we discuss here showed that African runners pace themselves differently in HOT, but NOT IN COOL conditions.

I think that what you are perhaps getting confused about is a difference between African and white runners with regards to overall performance, because yes, the Africans do run faster than white runners. But in this study, the groups were actually matched for performances in NORMAL temperatures. They had the same 8km time trial ability, but in the heat, the Africans did better.

So it's two separate issues - when you watch the AFricans beating all the other runners in global marathons, you're seeing a very complex system at work, because there, it could be any number of things that allows the Africans to perform so well.

But in this study, the data disagrees with you - it was the heat that made the difference to their performance and pacing.

Ross