The high temperatures and humidity were deemed so bad that the organizers made the unprecedented step of actually cancelling the race midway through! Three hours and thirty minutes in, at about 11:30am Chicago time, organizers sent all athletes who had not yet made haflway back to the start line, while those beyond the halfway point were advised to drop out or board cooling buses.
Statistically, it is clear that the heat affected the race. Of 45,000 entrants, about 10,000 did not even attempt the race, and of the remaining 35,000, only 24,000 actually finished.
The primary reason behind the decision to shut down the course was that organizers were concerned that the medical personnel wouldn’t be able to keep up with heat-related injuries as the day continued to get hotter and hotter. As it was, they were over-run with cases of athletes needing medical attention. In addition to the tragic death of 35-year old police officer Chad Shieber, who collapsed during the race, there were reported to be 300 cases, 49 of whom were taken to hospital, all with are being described as “heat-related ailments”. As Jonathan mentioned in an post earlier today, the autopsy was due for later Monday, and as soon as there is any announcement, we’ll definitely give it a full discussion and physiological interpretation.
The physiological view
But from a scientific, physiological perspective, this story is an interesting one, because true heat-stroke is incredibly rare. Therefore, with respect to the race competitors, one must be careful about knee-jerk reactions to the events in Chicago. Yes, it was hot and I have no doubt the heat affected many athletes, but this is not proof that running should not take place in the heat. We also need to be careful about the definitions of heat “ailments”, since this is where the problems often develop. A distinction has to be made between heat stroke, which is a very serious condition in which the body temperature has risen to in excess of 41 degrees celsius (about 105F), and heat exhaustion, where the body temperature is not as high, but the athlete feels hot and exhausted.
True heat stroke is incredibly rare, and may even require some pathology in order to occur. The reason is that the brain has been found to ‘shut down’ the activation of muscles when the body temperature reaches about 40 degrees. That is, the athlete is usually unable to continue exercise for very long after hitting what seems to be a critically high core temperature. In fact, all the evidence suggests that the athlete will slow down well before they reach this temperature – it is a protective mechanism.
Predicting heat stroke
How fast can we run in the heat?
Now, that is not to say that athletes could not develop the condition, but when one performs all kinds of mathematical models, it becomes clear that in order to develop heat-stroke, one would have to run at some remarkable speeds. For example, we recently developed a model to calculate how fast a runner would need to run in order to raise the body temperature to 41 degrees and the results demonstrate just how difficult it is.
An 80kg man, running in temperatures of 28 degrees with 60% humidity, for example, would be able to run a marathon in 2:40 without coming near to levels that would cause heatstroke. Now, there are very few 80kg men capable of running this sort of time – most are 70kg. And because smaller runners are favoured in the heat, a 70kg man can still run a marathon in 2:20 without over-heating.
The biggest predictor for heat exhaustion is physical fitness
If you look at the people who are reported to have developed heat ailments in Chicago (those 300 runners who required treatment), then you see that it happens most in the slower runners. That doesn’t seem quite right, at first glance, because in order to overheat, you need to produce a great deal of heat. And the faster you run, the more heat you produce, and so therefore, one might think that the runners in the most danger are the fastest runners. According to the equations, the runner most in danger is a fairly heavy runner who is running FAST – sub 3:00 marathon pace. And the faster you go, the more chance of overheating. Yet the race winner finished in 2:11:00 – a slow time, for sure, but not that bad compared to normal (2:07 might be ‘normal’ for this kind of runner). However, it’s the runners who finish in 4 or more hours who are most affected.
Yet, when applying mathematical models (which I admit can be debated) to these people, it’s quite clear that they are not in danger of actually hitting 41 degrees body temperature. A 100 kg man (as big as they come in marathons) running at 5:00/km (which is 3:30 marathon pace) would only reach a body temperature of 41 degrees after 25 hours of running!! This is because running these slower speeds, the heat production is much lower and hence the risk of storing heat reduced.
The paradox of the heat
Was it life-threatening or just oppressive?
So then, we have a paradox. Why do so many of the slower runners end up requiring medical treatment? And when news reports say that 300 runners had “heat illness”, what do they mean? And, most of all, was it life-threatening? I was reading reports from the race and everyone talks about ‘saving lives’ and near-death experiences.
Firstly, it’s highly unlikely that these people are genuinely over-heated. More likely, the reason they need treatment is simply that they feel dreadful because of the heat (note that this is still a valid reason not to finish – they are physiologically affected, it’s not a question of being weak). These runners feel this way because they are unaccustomed to the heat. The fact that the Chicago marathon takes place in October, when temperatures are typically in the teens means that a day of high 20 degree temperatures is very hot indeed.
I would be willing to bet that of those 300 people who required treatment, only a tiny percentage (1 or 2) actually over-heated. They FELT HOT, yes, but they were not hyperthermic. And that is a critical difference. So was it life-threatening? In some cases, it may have been. But I cannot stress enough that given the conditions, heat stroke cannot develop unless that person already has some disease or is ingesting something that contributes to heat stroke. Or, they have to be an elite runner.
A more likely problem – blood pressure?
For most of the runners, almost all of them in fact, more likely the problem was that they felt terrible, and perhaps they couldn’t regulate blood pressure. I read many reports of people fainting, becoming nauseus, dizzy, and losing consciousness – that is just as likely to be caused by a failure in blood pressure regulation than over-heating. Remember, exercise in the heat places an additional demand on the circulatory system, and this could explain what was observed just as well as hyperthermia and over-heating. So unless the athlete’s body temperature is actually measured, one is guessing as to the cause of the problem. For most people, as explained above, heat stroke is a physical impossibility, and so that leaves blood pressure as the problem. Again, this is not life-threatening, all that is required is some rest, lying down and time to normalize BP.
There is a famous story about this from World War II, where soldiers from Western Europe and the USA were sent to SE Asia for the first time. On the first day, they arrived, they simply couldn’t cope with the oppressive heat and the humidity. They were collapsing, losing consciousness. The diagnosis? Hypotension, which is where the blood pressure falls, due to the heat. But by the third day, there was no longer a problem, all the soldiers were fine, and it’s because their circulatory systems had adapted to the environment. It’s not heat, it’s blood pressure, in this case, and there’s a really good chance that is what happened in Chicago.
The explanation for the high rates of attrition is that these atheltes were not accustomed to the heat, taken by surprise by the unseasonal weather. Of course, that’s not their fault, one would not think of preparing for the heat for a marathon like Chicago (remember last year, it was so cold many of the elite didn’t even finish!). But the key to running in the heat is to prepare for the heat, and failure to do so produces the unfortunate events of yesterday.
Humans – adapted to run in the heat
I cannot make the point strongly enough that running is possible in those conditions, but requires the right level of adaptation. And failure to adapt WILL NOT result in heatstroke – it will result in an athlete who feels absolutely dreadful and is unable to continue, but this is not the same as overheating and developing a fatal condition. So while Chicago yesterday proved a tough challenge for many, it should not become a symbol for the impossible nature of exercise in the heat.
This post is part of the thread: Marathon Analysis – an ongoing story on this site. View the thread timeline for more context on this post.