It has been hot but I am not complaining. I love that we are actually having a summer and I hope it continues. Having said that, I am aware that it is seriously affecting my running. I take it for granted that it will, but recently the scientist in me wants to know more. When the temperature soars to 28C what can I expect ? How much will it affect my running pace when training and perhaps more interestingly, how will it affect my race times?

We know that body temperature is a crucial factor in how we function physiologically. All the metabolic systems in our bodies that control how we operate are designed to work optimally within a very narrow range of temperatures.  When running, we generate heat by burning fuel in our muscles for energy and we lose excess heat by sweating. In lower temperatures this generally isn’t a problem, but when we add ambient heat from outside the body it becomes more of an issue. If there is any danger of the body temperature going above (or below) its optimal range then our brain does everything it can to readjust the situation – when running in the heat this means slowing us down or even stopping us running to enable our bodies to cool down.  This is achieved in a numbers of ways but the aim is to get more blood flowing closer to the skin so that evaporating sweat can cool the blood down. You will notice your heart rate increase in the heat.

But what this means is that for a slower pace, your body is getting the same work-out.

Humidity is an additional factor as high humidity (air saturated with water) slows down evaporation. A side effect of increased blood flow to the skin is there is that less blood (carrying the much needed oxygen ) travelling to the muscles in the legs, which results in our legs having less power. One of the adaptations made by training is to make all these systems more efficient so that body temperature stays down whilst running. But in extreme conditions most runners are still affected to some extent.

The question was: how much does it affect me? There seems to be some disagreement.

The London marathon this year provided a good example. We had trained in the cold winter but the indications this year were that the temperature on race day would be high; in fact it turned out to be the hottest on record at just under 24C. Before the race I found some calculators (here and here) which claimed to adjust race times in relation to temperature, the idea being if I knew how much the increase in temperature was going to affect my finish time, then I could adjust my pace from the start and hopefully have a more comfortable (and successful) race. However when I looked at these tables I was surprised to find how little they altered my predicted finish time. I had run 3:49 in Manchester a few weeks before in cool conditions (10C) and was predicted to run 3:54 or 3:55 at the forecast 24C.

I did in fact run 4:23, struggling from the, start having not adjusted my pace sufficiently.  (This was my 7th London Marathon and my second slowest – my slowest was 4:31 in 2016, in one of the first races I completed after I had broken my leg).

I was left wondering whether it was me underperforming in the heat – perhaps because of my age or some other factor. I couldn’t use Cam’s time as a comparison as she was pacing 4:45, well below her race pace.

So I looked at other London results for 2016, 2017 and 2018. As a crude indication of the effect of this on the runners, I looked at the results for championship times for men (sub 2:45 ) and women (sub 3:15) over the past three years. In 2016, when temperatures were unseasonably low (6-9C), the numbers obtaining those times were 540 men and 350 women. In 2017, in almost perfect conditions (14C), the numbers were 542 men and 286 women. However in 2018 the corresponding figures were 277 men and 154 women. So it appears that I wasn’t the only one significantly affected.

A bit more research led me to an article by Jonathan Savage. He points out that calculators are often based on elite runners’ stats, and the effect is likely to be greater on non-elite runners; so he has extended the figures to slower paces. Using his calculations, my London finish time was predicted to be around 4:10 to 4:15 – closer to what I ran. Maybe if I’d gone out at a slower pace I could have achieved that. Two weeks later in Mainz Germany  I ran 4:07 in similar temperatures (on admittedly a slightly cooler feeling course – not very scientific!).

Obviously all these calculations have their limitations, but it makes me feel better about my race times. It’s also good to know when I set out for a run in the current weather conditions that despite running slower I’m still getting a good training.

If you want to read more about temperature and race times, Ross Tucker has some good articles.