A Scientific Explanation Into Why Jonathan Brownlee Collapsed

Photo: EPA

Photo: EPA

What Happened to Jonathan Brownlee?

If you haven’t already seen the remarkable display of sportsmanship shown by Alistair Brownlee to his brother, Jonny, do yourself a favour and view it here.

Controversies aside, what caused Jonny to become suddenly so fatigued to the point of literally being unable to withstand his own body weight?

Heat Exhaustion.

Jonny became so hot that his mind stopped sending signals to his body to move. Here’s a brief overview of the factors which lead to this disaster:

Jonny is Racing, Hard.

The average human core body temperature is 37°C. Aerobic metabolism involves oxygen being breathed in, circulated to the muscles via the bloodstream, and utilised for energy production. Oxygen enters the mitochondria in the muscle (mitochondria are responsible for converting oxygen and fuel into usable energy), goes through a number of processes, and creates energy, carbon dioxide and heat. The more energy we create (higher intensity = more energy required), the more heat we create as a byproduct, which will increase our core body temperature if we cannot regulate it.

It Was Hot, But it Was Also Humid.

The commentators mentioned that temperatures were “on the decline” mid-way through the bike leg, however temperatures were still in the low 30s, which, depending on your geographic location, is considered fairly hot for most of us (particularly the Brownlees). It was also evident that the sun was out for most of the race, albeit with some cloud cover at times. This combination of high ambient temperatures and direct radiation from the sun caused Jonny’s body to heat up. To make matters worse, it was horribly humid. High relative humidity makes cooling the body through evaporative cooling very difficult. Your body sweats, but the sweat remains as a fluid, rather than evaporating into a gas and releasing heat from the body… The heat is trapped.

Speaking of Sweat…

Sweating is initiated by the body redirecting blood (and therefore oxygen) away from the working muscles and to the surface of the skin. This allows us to diffuse heat out of the body and secrete sweat. Unfortunately, however, the outside temperature was in the 30s, so much of the skin’s heat remained inside the body. Jonny is constantly sweating, but it isn’t helping to cool his body down. Sweat needs to evaporate in order to release heat from the body, but the high humidity made this impossible. This is disastrous as sweating would usually account for up to 80% of heat loss during exercise. Sweat contains water and a number of electrolytes, the main ones of which are sodium and potassium. Sodium and potassium play a crucial role in muscular contractions. Without adequate concentrations of these, athletes will cramp and their power output will decrease.

Losing Fluid Leads to Dehydration

Jonny sweat out a lot of fluid. Did you know that, on average, 60% of an adult human body is made up of water? Blood contains a lot of fluid too, in the form of blood plasma. If we lose sweat at a quicker rate than we can replenish it, we become dehydrated. Dehydration causes a decrease in blood volume (as much of the plasma has been sweat out), and subsequent thickening of the remaining blood. This reduces the heart’s stroke volume (amount of blood ejected each beat), because less total blood is being circulated to the heart. Our heart rate increases in an attempt to counteract this drop in cardiac output, but as we know, maximum heart rate can only go so high. Our body also increases carbohydrate utilisation for energy, lactate levels rise, and adrenaline is released. Our body is beginning to struggle.

Unfortunately for Jonny, he reached heat exhaustion. A combination of factors, including racing in a hot ambient temperature, high relative humidity, substantial loss of fluid from sweat and subsequent dehydration, has led to Jonny’s core body temperature rising well above the normal 37°C, and closer towards 40-41°C. The major consequence of this? A large decrease in voluntary muscle activation and central drive (see figures below). A lot of athletes can push themselves to huge extremes and put up with enormous punishment, however there comes a time when your subconscious mind literally takes over your body and stops sending signals to your muscles to move. This is done as a protective mechanism to avoid death, as a core body temperature 45°C or above can destroy proteins and enzymes, and literally kill you. Unfortunately for Jonny, his mind forced him to stop only 400m from the finish line. At this point, there is nothing that can be done.

Nybo and Nielsen (2001)

Nybo and Nielsen (2001)

Nybo and Nielsen (2001)

Nybo and Nielsen (2001)

Sometimes race day conditions will just get the best of you. There are, however, some simple but highly effective strategies to combat heat stress and dehydration, which may save you from being in Jonny’s position late in a race, particularly if you’re racing in a different climate to which you train in. Find out more here.

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Written by Luke McIlroy – Director of Sport Science at METS Performance Consulting