What hot temperatures can do to your running and how to cope

However many bad-conditions-will-make-you-stronger quotes we collect, there’s still one aspect of weather that most of us runners do our best to dodge: heat. Encouraged by the health warnings every time the mercury climbs, many of us do everything we can to avoid it: running at dawn or in the late evening, or seeking shelter on treadmills in air-conditioned gyms. But it’s possible to run well in the heat. In the 1996 Atlanta Olympics, Portuguese distance star Maria Fernanda Moreira Ribeiro set an Olympic 10,000m record in hot, humid conditions of 27.7°C.

In research published in Comprehensive Physiology, Daniel Lieberman, the renowned Harvard evolutionary biologist and author of The Story of The Human Body suggests that our heat tolerance probably results from the fact that our ancestors evolved to hunt and forage on the African savannah at midday, when they were relatively safe from less heat-tolerant predators. Wherever we may live now, he suggests, we still largely retain these ancient heat-tolerance genes. ‘Heat is a paradox,’ says Lieberman. ‘On the one hand, we evolved to run in heat. But on the other, if you are not well adapted, heat can be mighty dangerous.’

With the weather forecast for the London Marathon looking warm, here’s your complete guide to what to expect as the mercury rises – and how to handle it.

Related: London Marathon 2018 Weather Forecast 

Running in temperatures between 10-15°C

Most of us don’t think of this as hot, but the longer the run, the more the heat will affect you. In research published in Medicine & Science in Sports & Exercise, a team led by exercise physiologist Matthew Ely studied decades of elite and sub-elite marathon performances at varying temperatures. What they found was surprising: even at temperatures of 10-15°C there was a drop in performance of one to two minutes for a 2:10 marathoner, depending on whether the race was at the top or bottom end of the temperature range. For three-hour males, the slowdown was four to eight minutes. (Slower runners weren’t studied.)

Physiologically, running in heat produces a cascade of reactions that begin with the fact that our muscles aren’t actually all that efficient. In fact, says Yannick Molgat-Seon, who spent over two years at the Thermal Ergonomics

Laboratory at the University of Ottawa, Canada, 80 per cent of the energy generated by our muscles ends up as heat. In cold weather, that inefficiency is what keeps us warm. But during exercise, the body has to get rid of it. One way is by sweating. ‘Less obvious is promoting blood flow to the skin,’ says Molgat-Seon. That’s important because it’s what carries excess heat from your muscles to the skin, where it can be lost to the environment.

But the body only has so much blood. ‘You have a competition between blood going to the skin and blood to the active muscles,’ says Molgat-Seon. ‘In that battle, the muscles always lose.’ So even when you’re barely sweating, your muscles are getting less oxygen and therefore are less efficient.

Running in temperatures between 15-20°C

These are temperatures at which most of us start to view conditions as less than perfect. The Run SMART Project calculator by Jack Daniels, author of Daniels’ Running Formula, calculates that at 20.5°C, a 45-min 10K runner will be 41 seconds slower. For marathoners, Ely’s study found a one- to four-minute slowdown for top-level elites and three-hour men.

Your body adapts quickly when you start training in the heat. Within a week, says Armstrong, your blood plasma volume starts expanding. That may increase your weight by a pound or two, but it gives you more fluid to sweat away without leading to dehydration. It also makes it easier for your body to supply blood to the skin without overly reducing flow to the muscles.

You start to sweat earlier in your run, as your body learns to anticipate the build-up in core temperature. You also sweat more and your sweat becomes less salty, as the body conserves sodium. And your heart rate slows down slightly, allowing the heart to fill more between beats, so it has more blood to pump out. ‘That’s stroke volume,’ says Armstrong. ‘With each beat, you’re pushing more blood to exercising muscles, and to cool the body.’

Even your perception of hot weather running effort changes, says Armstrong. Brett Ely, Matthew’s wife, was a heat researcher for the US Army, looking at how to train soldiers for conditions in Iraq. She found it could be done in 10 days. ‘Start gradually,’ she says, ‘rather than overheating yourself. Ease into it.’

Running in temperatures between 20-26°C

At these temperatures, Matthew Ely’s elite marathoners slowed down by three minutes, with the sub-elites losing up to 20 minutes. However, not all runners are affected equally: Ely’s data crunching revealed conclusively that women fare better in heat than men. The most likely explanation for this is that women, being smaller than men, have a higher surface-to-mass ratio, which allows them to get rid of heat more efficiently.

It’s not just women who benefit from being petite, however. In the men’s marathon at the Atlanta Olympics (which was run at a starting temperature of 23°C, with 90 per cent relative humidity), the winner, South African Josia Thugwane, weighed just over 7st. The silver medallist, South Korean Lee Bong-ju, weighed a shade under 9st.

And the correlation between heat and body size doesn’t only affect marathoners. In laboratory experiments conducted shortly before the 2004 Athens Olympics, exercise physiologist Tim Noakes, author of Lore of Running, found that when the room temperature was ramped up to 35°C, smaller men ran an average 45 seconds per mile faster than larger men in an 8km treadmill time trial. In cool temperatures the two groups performed about equally. The principle also applies at less extreme temperatures.

Running in temperatures between 26-31°C

There comes a point when additional sweating doesn’t do you any good. ‘When you’re dripping sweat, there’s lost water as opposed to lost heat,’ says Molgat-Seon. You also begin to run into the limits of not only your body, but of physics: there are conditions in which no matter how efficiently you sweat, it won’t evaporate fast enough to keep pace with the rate at which you are generating heat. Your only alternative is to slow down.

‘When air temperature exceeds 26°C and the humidity exceeds 70 per cent, performance drops markedly,’ says Armstrong. Training your body to do its best under these conditions requires dedication. If you have a race coming up in which you anticipate hot, humid conditions, training for that environment will make a difference. But just as you taper off your training volume before important races, you should also back off any heat training in the day or two before the race, trusting that the adaptations have already taken place and what you now need to do is make sure your body is not overstressed on the start line.

Related: 6 of the best sunscreens for runners 

Running in temperatures 32°C+

Not a problem we often have in the UK, of course, but you may face these fierce conditions while training on holiday or running a foreign race. And it’s no surprise that the hotter it is, the harder it is. When American elite Kara Goucher was preparing for the 10,000m World Championships in 2007, she knew that Japanese summers are notorious for hot, humid conditions. Race day in Osaka was no exception. ‘[It] was humid and stifling,’ she says, remembering a temperature of 31°C, with high humidity.

But she’d trained carefully for the event, doing summer track workouts in tights and long sleeves, as well as easier runs in ‘sauna suits – jacket and pants that were basically like rubber’. She went to Japan two weeks before the race. ‘By race day, I knew I could handle it,’ she says. She took the bronze medal.

The key to running in extreme conditions, says Greg Pressler, a veteran of the brutal Badwater 135 – an ultra run in the searing temperatures of Death Valley, California – is thinking about everything that might affect your performance, whether it’s monitoring pace or your choice of clothing. Stints in a sauna can also help with preparation.

It’s also useful to work on hydration, even for shorter races. Use electrolyte tablets while also ensuring maximum hydration. Taking such steps will pay off with larger blood volume and greater resistance to dehydration.

You can also train yourself to drink more liquids. You won’t be able to exceed one litre per hour, but most people aren’t used to consuming even that much, says Pressler, which means that when you’re training for hot conditions, it’s quite easy to become dehydrated. However, remember that when increasing your fluid intake, it’s important to take electrolyte supplements so you don’t create dangerous imbalances.

The keys to not only surviving a hot race, but also doing well, boil down to ‘hydration, practicing the conditions, and getting there early,’ says Goucher.