Heat Paralysis in Endurance Athletes


When I raced Cairns 70.3 in 2015, something weird happened. It was my first 70.3 so I took it really easy on the bike and then came out of T2 with a full tank and ready to attack the run. I ran well until about 14k, where I started to notice some strange sensations. My fingers started to tingle, and my lips also felt a bit numb. I remember thinking “Surely I haven’t been running long enough to get hyponatraemia?!” I thought maybe I wasn’t taking on enough fluids/fuel so I started drinking flat coke and hoping for the best. At the 18k mark I began feeling dizzy, the same kind of feeling as if you’re drunk and the world starts to sway around you. My legs also felt insanely heavy and my head was pounding. When I hit the 20k mark I was bordering on delirious,then I actually WAS delirious. I was struggling to go in a straight line and the only thoughts I had were “keep going” and “god this is embarassing” because I couldn’t figure out what was happening to me, much less what I should do about it. When I rounded the last corner before the chute I was using all my energy just to stay upright, I had all my muscles clenched in an effort to prevent the blood from draining out of my brain because I was so convinced I was about to pass out. It was not pretty. I remember trying to run and it was like a scene out of a nightmare where you try to run but you cant. I heard my mum screaming somewhere in the distance and was vaguely aware that people were running around me in the finish chute because I was staggering from left to right with zero control. When I collapsed over the line I was dragged out of the way and put into a wheelchair, Mum had jumped the fence by then and was crying – sorry Mum :). They measured my temperature at nearly 40˚C and my heart rate was over 210, 5 mins after I’d finished. I had some water and a cold pack on my head in the medical tent and felt better, but I could never figure out what had happened. Surely you can’t get hyponatraemia in a 70.3, and it wasn’t a hypo because I was having gels and coke on the run, with a BSL of 5 at the end. It was a complete unknown and I just put it down to one of those things I’d probably never know, until I was watching the ITU world champs and saw exactly the same thing happen to Jonny Brownlee.

Around a fortnight after watching the Brownlee brothers incident I had a phonecall from mum asking if I had heard the latest Endurance Planet podcast. Tawnee had Paul Laursen on talking about racing in the heat. Mum told me that Paul had explained what had happened to Jonny, and that it was eerily similar to what had happened to me in Cairns so I quickly downloaded it and listened. Amongst other things heat-related, Paul spoke about the concept of ‘heat paralysis’ in endurance athletes. As I listened it became clear that he was describing exactly what I had felt, and explained how it had happened. I started looking up heat paralysis online, but there doesn’t seem to be much information available.  Whether it exists under the label of “heat stroke” or just hasn’t been a very popular/sexy research subject in recent years I am not sure. So here I have put together most of the important points that I have found in the weeks since I first heard about it, in the hope that maybe I can help stop it happening to someone else.

Essentially, without delving into too much physics, your core body temperature is a direct result of your heat production vs heat loss. In a running race or the run leg of a triathlon, the majority of this heat comes from your metabolism that is the contraction of your muscles and the reactions used to power them. In this way, heat production is directly proportional to how fast you are running i.e the internal factors are the most important. Heat loss comes mainly from convection, evaporation and radiation, which can be maximised through the use of cold sponges, ice and very cool fluids to help improve heat losing ability. Thus, heat loss is primarily a function of the surrounding environment at the time of competition. Athletes have been using these cooling strategies for years because they know it makes them go faster, but few of them fully realise (and thank goodness do not experience) what happens when the body is no longer able to clear the heat as fast as they are producing it.

Studies done in both human and animals relating to exercise and fatigue date back to the 70-80’s where the main theories were that at a particular body temperature, fatigue developed secondary to a lack of blood supply to the muscles and so they refuse to work, slowing the athlete down. However what the scientists noticed when doing these tests was that the subjects became increasingly confused, demonstrating symptoms of dizziness, confusion, lack of co-ordination and inability to control their limbs. It became apparent that, at a high body temperature, brain function was impaired and this affected the drive to exercise. This created a paradigm shift in the way researchers thought about fatigue and its causes.

Further research into this new “central fatigue theory” showed that both humans and animals began to stop running on treadmills at a particular core temperature. For humans, this was 40˚C, and this was constant despite changes in environment, hydration, pre-cooling etc. It is thought that the brain becomes so hot that the motor control centres are affected, forcing the body to slow down in order to facilitate cooling. At this stage, athletes start to develop a paralysis – which you can see happening in the footage of that Cozumel race.

Though interesting, this is not a blanket rule because we know that some highly motivated individuals are able to push through these limits and reach higher body temperatures whilst competing (e.g running for olympic gold). When athletes begin to push these barriers and refuse to slow down despite a torrential influx of signals from the body telling them to stop, we see things like Jonny Brownlee’s finish in Mexico. Certainly, in professional competition sometimes over-pacing for the conditions is the risk they take to get to the line the fastest, but in my case there was no podium finish or medal on the line- I just wanted to GET to the finish. An alternative or addition to this ‘motivation’ theory is that athletes in a case like mine initially don’t quite understand what is happening to them, and they then become progressively more and more confused and disorientated. Highly motivated athletes are usually able to hold on to one thought, which is “keep going”, yet they don’t realise that by continuing to fight against overwhelming fatigue they are digging themselves into an even bigger hole.

The development of heat paralysis is more common in very humid conditions, where the athlete is unable to effectively lose heat through evaporation of sweat. In these conditions, the heat build up from the metabolic processes of running reaches catastrophic levels and the brain starts to malfunction. Most athletes will receive this afferent heat feedback from the body early on and reduce exercise intensity before their core temperature can rise, but there are always a some who can continue into the suffering and override the urge to slow down or stop. The theory of “overriding the central governor” can indeed be applied here, but is a long topic in itself for another day. Whether I should be proud to have been one of these people or embarrassed because of the result (and the finisher photos – yikes) the jury is still out, but I am glad to finally get to the bottom of what happened and why. There is always room for growth and development to come from failures, so with this one I will live and learn- and hopefully never do it again!


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