top of page

Running in the heat – why do we slow down?

Why do we slowdown in the heat? Gone off too fast? Not enough training? These are obviously just a couple of reasons why the human body just doesn’t work to plan on a specific day and time and especially as it has been recently very hot!

But what if all the training has gone to plan and for some reason your race just doesn’t work out? The answer to this can be as complicated as you would like to make it and the scientific physiological reasons for the onset of fatigue are I am sure well known to all readers.

What I want to focus on here is the current thinking surrounding the overriding power of the one organ in the body that controls everything else in our bodies which is of course is the brain.

The brain has a first and foremost duty and that is of self-preservation. So by firstly understanding this thought process we can begin to understand why we or our athletes may slow down or conversely are able to speed up in training and racing.

Hot day

Firstly let us take an easy and very topical example – running on a hot day – We all know that fatigue more quickly onsets on a hot day than on cooler days. Middle and longer distance times inevitably suffer and the incidence of collapse is heightened. So what are the scientific and logical reasons for this? Most would point to the fact that it’s due to insufficient hydration or the “inability” to run in the heat.

For this example we must assume that an athlete competing on a hot day has had the same amount of training as they would have done if it had been a cooler day? Also as it’s a hot day most will have ensured that their hydration levels were correct before toeing the start line. It is interesting therefore that some runners, whether used to the heat or not will fatigue more quickly on a hot day even though they have sufficient hydration and training. So why might this be?

Our brain performs thousands of updates and analysis of our essential systems per second and this of course continues for every second of every day. Indeed the process is accentuated still further as we put bodies through exercise or competition. One of the major areas that is monitored is core body temperature and it is this that we will take as our prime example.

Our core temperature is, under normal conditions, 37-38C . Of course this rises as we exercise and is exaggerated on a hot day, and will be more so if an athlete is undertrained. If the core heats up to 39-40C, the brain tells the muscles to slow down and fatigue sets in. At 40-41C, heat exhaustion is likely - and above 41C, the body starts to shut down and chemical processes start to be affected, the cells inside the body deteriorate and there is a risk of multiple organ failure. The body cannot even sweat at this point because blood flow to the skin stops, making it feel cold and clammy. When the core temperature begins to rise, the body reacts by increasing blood flow to the skin's surface, taking the heat from within the body to the surface in the form of sweat. As the sweat evaporates, the body cools down. Also as we live in the UK, our skin temperature is warmer than the external temperature so we are also able to lose heat to the environment, termed 'dry heat loss'.

An unduly high core temperature can and will cause organ failure which the brain, without doubt, will not allow to happen, in such case it will look for ways to prevent this from continuing. Of course you or your coached athlete will be wanting to hit a certain target and will plough on regardless using the “non-essential maintenance systems” part of the brain and its first port of call is to find ways of slowing an athlete down by withdrawing muscle recruitment which in turn slows the athlete down. The brain will continue to pursue this route until body temperature is reduced sufficiently and will allow further muscle recruitment, this is why we often hear that runners “got going again after a walk”.

Speed limiting power

Taking this one step further the brain will limit the speed that an athlete can run knowing that it has a longer way to go that the body can cope with at that speed, a good example of this would be an athlete who when running a 1500m will unconsciously slow down mid race for about a lap and then apparently, again unconsciously, begin to speed up and then deliver a sprint finish.

So breaking this down specifically lap 1 goes off quickly with the field, into lap 2 the brain makes the necessary adjustments as it believes that the body is not trained to last at that pace for the duration of the race and restricts recruitment of muscle fibre to slow the athlete down. During the latter stages of the race the brain recalculates that there is a shorter distance to run so allows the body to go faster and in the last 100m or so it allows itself to “take a risk” as there is only a short way to go and allows a sprint.

So how do we adapt the training of our athletes to override this? The obvious port of call must be to influence training and tactical decisions, so to take our 1500m runner for instance, it would be for example prudent to practice up to, say, 1200m to get the body used to running that distance at the required pace and then hence allowing the brain to “take a risk” on letting the body run the extra 300m at the usual faster pace.

With the example of running on a hot day, obviously practising running on a hot day can be usually quite difficult bearing in mind the British weather, however it would be sensible in this case to have a prior plan to amend tactics accordingly to go off slower and then maintain that pace or gradually build up as the race continues which would bring benefits in both situations, limiting core temperature increase, and allowing the brain to release sufficient muscle fibre recruitment to keep going and possibly even a sprint at the end!


UKA Level 3 Performance Endurance Coach

239 views0 comments

Recent Posts

See All

How to waste a mile

“It takes me a mile to warm up and get into the race” How many times have you heard this or even said it yourself? More than once I suspect! The first question you should ask is why waste the first mi


bottom of page