How Much Does The Heat Slow You Down?

I was caught in the rain halfway through my run 2 days ago. Since it has been so hot recently, the coolness the rain brought was very welcomed. I also noticed that I managed to run faster with lesser effort. 

So if the heat (and humidity) affects a short run so easily, how much does it affect us when we race?

When running in hot weather, we need more oxygen since some of the blood flow is redirected from working muscles to the skin to cool us down. This requires more energy usage, increases lactic acid production and a higher heart rate at a given pace compared to cooler weather.

Warmer weather can also cause us to fatigue faster by making us sweat more. This can lead to reduced stroke volume, cardic output and blood pressure.

Research has shown that the optimal temperature range for most groups of runners are between 7-15 degrees Celsius (or 44 to 59 degrees Fahrenheit). Below and above this range, marathon finishing times tend to be slower on average.

Most published research studied elite marathoners timings. At the Boston marathon (Miller-Rushing et al, 2012) more than two-thirds of the men and women's course records were set below 13.3 degrees Celsius (56 degrees Fahrenheit). More than a third were set when the temperature was under 9 degrees Celsius (48 degrees Fahrenheit).

The top 10 fastest marathon performances of all time were set when temperatures were between 10-15 degrees Celsius (50 to 59 degrees Fahrenheit). 

However, I found a comprehensive study (El Helou et al, 2012) that included data from the Paris, London, Boston, Chicago and New York Marathons. The researchers found that most non elite runners (finishing times between 3:30 and 5:00 hours) performed best at temperatures around 6.67 degrees Celsius (44 degrees Fahrenheit).

Once the temperature is higher than 15 degrees Celsius (59 degrees Fahrenheit), perceived effort rises and pace slows for most runners. Runners averaging 5:45 min per mile pace slowed approximately 1 second per mile for each 1 degree Celsius (1.8 degrees Fahrenheit) increase in temperature.

You can use this information on race day for your pace. Say you are capable of a 3:30 hrs marathon. You will need to add 2 to 2.5 seconds to your race day pace for every degree above 15 degrees Celcius (59 degrees Fahrenheit).

Those who averaged between 7:25 to 10:00 per mile slowed between 4 to 4.5 seconds per mile for 1degree Celsius (1.8 degrees Fahrenheit) higher than 15 degrees Celsius (59 degrees Fahrenheit).

Optimal running performances for women tend to be at the cooler end of the range (especially for faster female runners. Women tend to be less affected by rising temperatures than men since women (and elite runners) tend to be lighter. They will usually have lower metabolic heat production due to their lower body mass and their higher surface area to body mass ratio. Less energy is then needed to cool the body, this translates to less stress on the cardiovascular system.

Faster runners also spend less time exposed to hotter temperatures (since they finish ealier) which may be a reason they are less affected than slower runners.

The only consolation is that running in the heat does get easier (especially for those of us who live here in Singapore) with repeated exposures to running in the heat (Lorenzo et al, 2010). If you race in a cooler climate, chances of a faster time is much higher.

Organizers of the Singapore International Marathon used to offer a 1 million dollar bonus to world class runners to break the break the world record here. Rumour has it that the organizers (back in 2014) tried to convince Haile Gebrselassie to make an attempt but was turned down immediately after he learnt about our temperature. He did agree to run the 10 km event though and won it.

Now you know that it will never happen. I do not even think a sub 2:10 hrs run will be possible given our heat and humidity.

References

El Helou N, Tafflet M, Berthelot G et al (2012). Impact Of Environmental Parameters On Marathon Running Performance. PLoS One. 7(5): e37407. DOI:10.1371/journal.pone.0037407.

Ely MR, Cheuvront SN and Montain SJ (2012). Neither Cloud Cover Nor Low Solar LoadsAre Associated With Fast marathon Performance. Med Sci Sports Ex. 39(11): 2029-2035. DOI: 10.1249/mss.0b013e318149f2c3.

Lorenzo S, Halliwill J, Sawka M et al (2010). Heat Acclimation Improves Aerobic Performance. J App Physiol. 109:1140-1147. DOI: 10.1152/japplphysiol.00495.2010.

Miller-Rushing AJ, Primack RB, Phillips N et al (2012). Effects Of Warming Temperatures On Winning Times In The Boston Marathon. PLoS One. 7(9): e43579. DOI:10.1371/journal.pone.0043579.

*Note that wind, wet-bulb temperature, dew point, precipitation and cloud cover can affect running performance to some degree. But none of them have more influence than air temperature. 

Drink It or Pour It?

Picture by richseow from Flickr

You've often seen the elite athletes pour water over their heads to cool themselves while racing. So is it really better to pour the water over your head on a really hot day or is it better to drink it?

Picture by richseow from Flickr

Well, here are some numbers obtained in the paper referenced below. In the article, kilojoules and calories are a measure of the energy dissipated as heat. The heat is produced as a by product of muscular exertion, which is powered by the energy from food.

Drinking a 250 mL cup of water at just above freezing ( 1 degree Celsius or 34 degress Fahrenheit) helps you get rid of 39 kJ of heat (about 9 calories).

If you drink a slushie that is half water and half ice, it's better as the ice can melt inside your body. A 250 mL slushie will get rid of 81 kJ of heat (or 19 calories).

Pouring a 250 mL cup of water on your head will get rid of 607 kJ (145 calories) if the water can be spread around your body surface so it all evaporates rather than dripping to the ground. Yes, evaporation is a great way of dissipating heat.

But, that's a very big "if" in my opinion as it's hard to pour a bottle over your head without spilling any water. although the authors suggested that even if you spill 85 percent of the water you still get more heat loss from pouring than from drinking the slushie.

Do take note that the water you pour over yourself has to evaporate. If it's a hot and super humid day like it always is in Singapore where your sweat is dripping like there's no tomorrow you won't have the water evaporating. Pouring cold water over your head probably makes your feel cooler (and hopefully better) for a while.

It works best when the weather is dry and breezy so that your sweat can evaporate. (Local runners who race overseas take note).

My suggestion would be to drink when you need and pour some cold water over your head and neck too to cool off if you don't mind your running shoes getting a little wet.

Best to be sure it is water and not a sports drink that you pour over yourself. Yes, I've accidentally poured 100 Plus over myself before when I was racing, so beware.


Reference

Morris N and Jay O (2016). To Drink Or To Pour: How Should Athletes Use Water To Cool Themselves? Temperature. 3(2): 191-194. DOI: 10.1080/23328940.1185206.