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When The Temperature Heats Up

Impact of heat and body mass on running performance.

The 2021 Western States 100-mile trail race saw both scorching temperatures as well as a record number of women placing well.  Temps topped out at 101 degrees Fahrenheit / 38 degrees Celsius.  For the first time in the race’s history, there were three women in the top 10 overall.  If you look at endurance running races over time when events experience really hot temperatures women tend to fare better than men.  But why?

When we run, our bodies produce heat.  There are two major factors that impact how much heat we produce:

  • Body Mass
  • Running Speed

The greater the body mass and the faster the speed is, the more heat you will produce.  Women generally have smaller bodies compared to men and won’t produce as much heat at any given running speed.  As you can see from the graph below, at 35 degrees Celsius, the greater the body mass the more they have to slow down to maintain heat balance (equal heat production and heat loss) (Dennis SC, 1999).

Figure 1 (Adapted from Dennis SC, 1999). Effects of body mass on the maximal running speeds to maintain estimated heat balance at an environmental temperature of 35°C and 60% relative humidity (rh).

You may be thinking that a greater body mass would also allow for more heat loss.  However, body mass has a 2-fold greater effect on heat production vs. heat loss (Dennis SC, 1999).  Thus, when it comes to running in the heat, a lower body mass is beneficial.  So, what are the applicable lessons?

  • You will have to slow down in hot temperatures. Even more so if the humidity is high and you have a greater body mass. This will allow your body to maintain a healthy heat balance and avoid heat stroke.


  • You can minimize the negative impact of heat by acclimatizing to heat in the weeks leading up to race day, employing cooling strategies on race day such as ice over your head or stuffed into a neck bandana, and adopting a conservative pacing strategy.

Table 1.  The impact of various ambient temperatures on running pace at the 10 km race distance.


Dennis SC, Noakes TD, (1999). Advantages of a smaller body mass in humans when distance-running in warm, humid conditions. Eur J Appl Physiol, 79: 280 – 284.

Jack Daniels Training Tables, 2020.

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