Running in the heat and how it might affect you.

It’s set to get hot out there. Here are some practical ideas about how to cope with running in the sun and how heat affects our performance… Encouraged by doom-laden health warnings every time the mercury climbs, many of us may avoid it as best we can, by heading to the air-con gym or running…

It’s set to get hot out there. Here are some practical ideas about how to cope with running in the sun and how heat affects our performance…

Encouraged by doom-laden health warnings every time the mercury climbs, many of us may avoid it as best we can, by heading to the air-con gym or running at the crack of dawn. But it’s perfectly possible to run well even when temperatures do hot up.

In research published in Comprehensive Physiology, Daniel Lieberman 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.

Running in temperatures between 10-15°C

Most of us certainly don’t think of this as hot, but the longer the run, the more that even milder temperatures can affect us. Research back in 2007 published in Medicine & Science in Sports & Exercise, looked at decades of elite and sub-elite marathon performances at varying temperatures. They found that 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 runners, the slowdown was four to eight minutes. However, this is contradicted by other research that has found 7-15°C is in fact the ideal temperature for most runners. And in good news for women, research shows that they fare better in warmer temperatures than male runners.

Physiologically, running in warmer temperatures produces a cascade of reactions that begin with the fact that muscles aren’t actually all that efficient. 80% 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. 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. In that battle, the muscles always lose. So even when you’re only lightly sweating, your muscles are getting less oxygen and therefore are less efficient.

Running in temperatures up to 20°C

These are temperatures at which many of us start to view conditions as less than perfect, especially at the top end. The Run SMART Project calculator by Jack Daniels, calculates that at 20.5°C, a 45-min 10K runner will be 41 seconds slower.

However, your body adapts quickly when you start training in the heat. Within a week your blood plasma volume starts expanding. That may actually increase your weight by a pound or two, but more importantly 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. 

Even your perception of hot weather running effort changes. One or two hot days may feel unusually difficult, but within a week to 10 days of a more sustained spell, runners may adapt surprisingly well.

Running in temperatures between 20-26°C

At these temperatures, studies show that elite marathoners slow down by three minutes, with the sub-elites losing up to 20 minutes. However, not all runners are affected equally: again women fare better in this heat than men. A possible 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.

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, 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. 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. 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 – in a heat chamber or with multiple extra clothing layers if necessary.

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.

Running in temperatures 32°C+

Not a problem we very often have in the UK, but you may face these conditions while training on holiday or running a foreign race. And it’s no surprise that the hotter it is, the harder it is.

The key to running in extreme conditions 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. 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 – over-hydration is dangerous.

The keys to not only surviving a hot race, but also doing well, boil down to hydration, practicing the conditions, and getting there early to acclimatise as best as possible and time permits.

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