Revision as of 16:11, 29 August 2011

In hot conditions, your maximum running pace is limited by your ability to cool off. It is possible to calculate your theoretical maximum running pace at a particular temperature and humidity given your height and weight.

1 Introduction

When you run, you produce heat that must be removed from your body. In cold conditions, this is not a problem, but as the temperature and humidity go up, it becomes harder to remove the generated heat. If you generate more heat than you lose, your core temperature will rise and you will have to slow down. For most people, the rise in core temperature causes a gradual, though unpleasant, slow down. However, extreme dedication can force some people to push hard enough to raise their core temperature enough to cause collapse, coma and even death.

2 Heat limited pace table

The table below shows an example set of maximum running paces for someone who is 174cm/70inches high and weighs 60Kg/132lbs. You can create a customized version of this table by filling in the form below

3 Heat Acclimation Training

Main article: Heat Acclimation Training

Heat Acclimation can improve your ability to keep cool by sweating more and sweating earlier, as well as making your sweat more dilute. Heat adaptation can also teach you the warning signs of an elevated core temperature so you can adjust your pace or take action to cool off before things get dangerous. However, heat adaptation does not change the limits of physics, nor does it allow you to tolerate a higher core temperature.

4 Basic Heat Calculation

The basic heat calculation is from

1. Heat produced from running. This is dependent mainly on your weight.
2. Heat lost (or gained) from convection. This is mostly dependent on your surface area (calculated from weight and height), and the air temperature.
3. Heat lost (or gained) from radiation. This is mostly dependent on your surface area and the air temperature.
4. Heat lost from evaporation of sweat. This is mostly dependent on your surface area, the air temperature and humidity.

5 Important caveats

The calculations of maximum pace make lots of assumptions. This means that the pace indicated may not be safe.

• This is the maximum pace based on the limits of heat; there are other limiting factors that mean you may not be able to achieve these paces.
• Your running efficiency may be different to the average. A higher efficiency will produce less heat at a given pace, a lower efficiency will produce more heat.
• The only air movement is assumed to be from running through still air.
  • Any wind can make a significant difference, with a headwind or crosswind cooling your off. A tailwind less than twice your running pace will reduce your cooling; it takes a tailwind of at least twice your running pace to cool you off more than still air, but of course a tailwind will reduce the energy required to run.
• The calculations assume negligible clothing. If you are wearing more than the absolute minimum clothing, this will impair your ability to cool off.
• It is possible to actively cool off with ice, which will allow you to tolerate higher temperatures.
• Any direct sun will increase the heat stress and dark clothing will make this worse.

6 Advanced notes on calculations

These notes are for those with an interest in the underlying details.

• There are multiple formulas for calculating water vapor pressure which produce slightly different results.
• The calculation of skin temperature is not straightforward. The calculator uses Neilson's simplistic approach for 0 Celsius to 15 Celsius, then Mehnert for above 15 Celsius.
• The maximum running pace is calculated using a goal seek algorithm.
• The various formulas for calculating the surface area of the skin appear to give similar results.