The recent warm spell in Europe has led me down a rabbit hole of heat stress reading.
If you are of smaller stature than your competition then you have an advantage in hot and humid conditions! Research shows that smaller, lighter runners accumulate less heat during hot weather comparative to taller, heavier runners AND (importantly) this allows them to run at higher speeds during races. Let’s look at why this is the case?
Firstly, we need to understand 2 things; working muscles produce significant heat as a by-product of their contractions and there are 3 processes of cooling/regulation of body temperature when we are exercising. When the air is much cooler than the temperature of the body (skin surface), heat is basically lost from the skin to the air via convection and radiation. But when there is little difference between skin and ambient air temperature, then little no heat can be shed in this way. We then rely heavily on sweating, which cools us via the process of evaporation. However, in warm, humid conditions the surrounding air can accept only little or no additional water vapour and so heat struggles to leave the body in this way also. The net result is rising core body temperature, discomfort and the need to slow down.
So, heat production (when running) is the product of an athletes mass and speed. The bigger a runner is and the faster he/she runs the more heat will be produced per minute. Dissipating that heat comes down to the surface area of the skin and the speed of the air travelling over it. Larger surface area means more sweat glands able to produce sweat for evaporation and more heat loss via both convection and radiation. The higher speed of the air means these processes are accelerated further.
At first glance it may seem that ‘bigness’ could be both good and bad; larger body mass (producing more heat) but also a larger skin surface area (to cool). Unfortunately though these factors are disproportionate in reality. That is to say that in warmer climates body heat production rises more steeply than the ability to dissipate it in the bigger runners. As an example, one study shows 2 elite marathoners of 45kg and 50kg respectively. The lighter has a skin surface area of 1.45m2 and the ‘heavier’ 1.53m2. That’s a difference of 11% body mass but only 5.5% in surface area.
One final example compares the maximum speed that athletes could run at while maintaining a stable core body temperature in very hot and humid conditions (95 degrees, 60% humidity):
45kg athlete = 5:03 per mile
55kg athlete = 6:07 per mile
65kg athlete = 7:06 per mile
75kg athlete = 7:54 per mile
Comparison of the lightest and heaviest runners here shows a difference of almost 3 mins per mile and if the heavier runner tried to close the gap he/she would soon begin to run into heat problems. Calculations show the heavier runner’s core body temp would rise 1 degree every 13mins and that by 40mins he/she would likely hit 40 degrees and suffer from heat exhaustion or possibly worse!
Ok, so what’s the take home message? If you’re heavier than 45kg (LOL – and who isn’t?) then you’d be wise to ditch plans to run a PB on the hottest of days. When the temperature is over 70 then adapting your pacing plan is likely necessary to ensure a successful race outcome. Heart rate will be useful as will RPE to help guide your efforts, however holding to a strict pace goal may lead to trouble. And finally, if your key race this year is going to be a hot one, this may be a great reason to (safely) shed another few unwanted pounds of body-fat.