Investigating heatstroke: How fast do you actually have to run?

Recent marathon running news has been dominated by the Chicago Marathon and the heat which forced the closure of the race at around 11h30. There has been a great deal of reaction to this, with many people suggesting that even attempting to run in such conditions was "suicidal". Take this quote from one particularly scathing commenter:

"If you are foolhardy enough to run a marathon when the temperature outdoors is up to 88 degrees, then it is your fault, no one else's...when 35,000 people jump off a ledge, you can only catch so many in safety nets. The rest are going to fall. This is a professional competition that the public sometimes confuses with a company picnic."

But is it heatstroke?

Yet the other day, we featured a post which suggested that actually running yourself into true heatstroke is incredibly rare. We said that one should be cautious about making a knee-jerk reaction that just because it was hot, heatstroke and heat illness was the problem. And here it's really important to define the conditions very accurately - when we refer to heat stroke, we talk of a potentially fatal condition where the body temperature rises excessively because either:
a) The body is unable to lose the heat it produces, or;
b) The body is producing too much heat

The physics of heat stroke - what does it take to heat up?

Without going into massive detail on the physics of heat transfer, your body temperature at any time is dependent on heat production versus heat loss. Heat production, in the case of marathon running, comes from muscle contraction and is directly proportional to how fast you run. In otherwords, if you double your speed, you produce twice as much heat.

Heat loss is a little more complex, because it depends on evaporation, convection and radiation. Now, the specifics of each are complex and we won't go into them, but let's just say that the environment is the crucial factor that affects heat loss - high humidity prevents evaporation, high air temperatures prevent both evaporation and convection from cooling the body.

The point of this is that if we know the environmental conditions, and we know how fast a runner is going, then we can calculate what their body temperatures would be doing during the marathon. And this is what we have attempted to do in the table below, where we look at a few cases. The point of doing this is to illustrate just how difficult it is to get heat stroke, unless you are a big person running really fast.

The values calculated in this table, incidentally, are calculated based on a number of equations that have been published in the scientific literature. And while I acknowledge that they may have some small errors, they have generally been found to be very accurate and quite valid, so we use them to make a point.


Case 1 - a 60 kg elite runner going for a world record in the marathon

Take an elite Kenyan or Ethiopian runner. They tend to be very light (60kg is assumed in this case), and we're assuming the air temperature to be 15 degrees celsius. The runner goes off and runs at 2:55/km. Under these conditions, the heat production is equal to 6232 kj/hour, and heat loss is equal to 8346 kJ/hour. Therefore, this runner can lose more heat than he gains, and under normal physiological circumstances, he has NO THERMAL LIMIT. In other words, he can run without concern for over-heating. Note that there are occasions when heat stroke does occur in this kind of athlete, but that is due to something else, which we will address lower down in this post.

Case 2A - 60 kg elite going for a world record, but in hot and humid conditions

Now we have the same 60kg elite runner, but the air temperature is 28 degrees Celsius. Our runner sets off at 3:00/km, hoping for a fast time, but the problem is that because it's so hot and humid, his heat loss is vastly reduced compared to our previous example (Case 1). Therefore, our calculations predict that this runner will NOT BE ABLE TO FINISH THE MARATHON IN THIS PACE! He will gradually gain heat, because heat loss is LESS THAN heat production and his body temperature will hit 41 degrees after 1 hour 43 minutes. Therefore, this runner would be a candidate for heat stroke.

Case 2B - the 60kg elite does not get heat stroke. Why? Because he slows down!

So having just said that our 60kg elite could get heat stroke, we ask when was the last time you saw this? If it was going to happen, surely on Sunday in Chicago, it would have. Yet none of the elite runners overheated. Why? Because they slow down. Case 2B in the table above shows this clearly. If our 60kg runner is in the SAME conditions as before, but instead of running 3:00/km, he now goes 3:06/km (which is a 2:10:50 marathon), we then see that he can now finish the marathon, because his body would only reach 41 degrees after two hours and 18 minutes.

So it would be a close run thing, he would certainly be very hot and probably feel terrible at the end, from the exertion, but he would NOT be in danger of overheating. His marathon time would be around 2:10:50, and his body temperature in the mid-40's, which is hot, but safe. That is the difference made by slowing down by 6 seconds per kilometer!

The really interesting thing here is that these conditions are the same as in Chicago - 28 degrees, give or take a few. And the winning time was 2:11:11. And yes, I acknowledge that all kinds of other factors affect performance, but it's interesting to note that the prediction of the equations regarding temperature are borne out by performances.

Case 3A - an 80kg runner aiming to break 3 hours

Now we move onto a runner who perhaps more of you can relate to - weighing in at 80kg, his goal is to crack 3 hours (pace of 4:15/km). First, for Case 3A, we look at air temperatures of 20 degrees. Here, you can see that our runner is in no danger at all. His heat production is equal to 5293 kJ/hour and total heat loss is equal to 7207 kJ/hour. Therefore, running at 20 degrees celsius, an 80kg runner will have no risk of heat stroke or hyperthermia.

Case 3B - our 80kg runner aiming to break 3 hours on a hot day

Now, consider the same runner running at 30 degrees, as it was later in the day in Chicago. Our 80kg runner is still trying to run sub-3 hour pace (4:15/km). But now, because it's hotter, it's a lot more difficult to lose the heat he produces, and our equations show that he would overheat, with a body temperature of 41 degrees reached in 1 hour 58 minutes. So that means heat stroke could occur in this runner if he ran for this long at this speed.

Case 3C - 80 kg runner who slows down slightly in the heat

However, as I have no doubt all of you know, if you run at 30 degrees, and your best performance is 3 hours, you DON'T run a 3 hour marathon in 30 degree heat. So let's see what happens if our runner slows down, as for Case 2B. Assume he slows down to a 3:20 marathon time (4:44/km). Now, we see that because his HEAT PRODUCTION is much lower, it takes much longer for him to store heat and lift his temperature. In fact, we work out that it would take 5 hours 9 minutes to get up to 41 degrees. Therefore, in a marathon, there is no risk, so he's able to run his 3:19:43.

Case 4 - 100 kg runner aiming for four hours on a hot day

Now we turn to the kind of person who is most likely to feature in the medical tent. I mean this with the greatest of respect, but from experience having worked in the medical tents of Ironman triathlons and Comrades and Two Oceans Ultra-marathons, there is no doubt that MOST of the people in the medical tent are the heavier runners who run times greater than 4 hours for the marathon.

So if you take a 100kg runner, who runs at 5:41/km pace (4 hour marathon), then you will see that even if the air temperature is 30 degrees and humidity 60%, this runner would be able to run the marathon in 3:39:43 without ever getting close to overheating. In fact, it would take this person 7 hours and 53 minutes to actually reach 41 degrees celsius!

Case 5 - A typical runner in the Chicago Marathon

Lastly, let's have a look at a typical runner, weighing say 85 kg and running the Chicago marathon in about 4 hours 30. This runner would be producing heat at a much lower rate than they could lose it. And therefore, there would be no danger. It is for this reason that I am highly sceptical that most of the runners treated on Sunday had any type of heat illness.

Who is most at risk?

From the above cases, I hope it's clear that the runner most at risk during running is the slightly heavier athlete (that is, someone weighing over 75kg) who is running at a fairly fast pace and doesn't slow down to accommodate the higher temperatures. The slower runner, who finishes in times over 4 hours, is far LESS LIKELY to be in any danger. Yet on Sunday, from the comments we've received, we've heard that the biggest problems seemed to be in the guys aiming for about 3h30 to 4h30, so that they were at between 15 and 24 miles after about 2 to 3 hours. We must emphasize that unless this runner is particularly large, he's simply not running fast enough to be in REAL danger of heatstroke. So instead, we're left to suggest that maybe Chicago wasn't heatstroke - people just felt really hot. But again, that's NOT the same as BEING hot, and that's been somewhat overlooked.


The alternative to heat illness - why such high attrition in the heat?

Instead, I would suggest that the reason it was so tough on Sunday is because the vast majority of people who ran the race were simply not accustomed to the heat. The race takes place in the early winter in Chicago and for most of its participants, temperatures higher than about 20 degrees would constitute a tough day out. So standing on a start line when it is closer to 25 degrees (80F) is a near impossible task.

We know from scientific research that heat acclimitazation makes massive differences to performance. The other day, I mentioned the story of the soldiers who were sent to South-East Asia and almost all lost consciousness, felt ill and were completely incapacitated on the first day. But within 3 days, they had recovered. What was discovered there is that on arrival, it was their blood pressures that were dropping in the heat - we know this happens and it is a possible explanation for why so many people collapsed in Chicago. This is not a life-threatening condition, it simply requires some rest and then the athlete is 100% fine again.

Finally, the heat poses a huge challenge because of how it alters the perceptions of the runner. I am not for a second suggesting that these people could have continued, because clearly, some tried and failed. But to suggest it was because they overheated is not correct. They were hot, yes, and the felt like they had over-heated. But how one feels is not always how one IS, and all it takes is some experience and training in those conditions, and most of those runners would have been able to finish the race, in slower than usual times, but without any danger.

Last word on heatstroke

So having said that it's just about impossible for a runner to get heatstroke, unless they are heavy and running really fast, we still have a question - why does heatstroke occur? This is something we will look at in a future post, because it's a really fascinating question. But let me leave you with the observation that MOST of the heat stroke cases in the last ten or so years have happened in COLD conditions. Believe it or not, it's true. One case, which comes to mind, saw a runner collapse after 19km of a race where the temperature was 4.1 degree Celsius (that is 39Fahrenheit). Go figure...but we'll look at that in more detail in the future.

Ross