Why, after going on a long run, do I only start to sweat profusely immediately after I have finished. Other runners I have spoken to also experience this.
When sweat evaporates it creates a layer of saturated air close to the skin, which inhibits further evaporation. During a run, the movement of air relative to your body replaces this with fresh air, allowing evaporation to occur. Every gram of sweat that evaporates takes 2260 joules of body heat with it. When you stop running, the layer of saturated air builds up and the sweat does not evaporate. That creates the perception that you have suddenly started sweating profusely, and also raises skin temperature - which does, in fact, make you sweat slightly more.
Proof comes from the fact that the phenomenon will not occur when you stop running on a windy day.
Shane Maloney, School of Biomedical and Chemical Science, University of Western Australia, Perth
You actually begin to sweat not long after you have started to run, once your muscles have settled into a working routine and you are using more energy than you would if you were resting. What you are experiencing is the wind-chill effect of your own movement: as a result, the movement of air over your skin and through your clothes wicks the moisture away from your body before it can build up. Try wearing a small backpack or taping a section of something like plastic cooking film over an area of your chest. Sweat will build up here and not elsewhere, even after quite a short period of exercise.
When you stop running your muscles still have reserve heat to expel and this, combined with less air movement over your body, means that sweat will build up on the skin. If you warm down after a run, rather than stopping suddenly, the build-up of sweat may well not occur.
Relative humidity and ambient temperature also have a huge effect on how much sweat is produced. Higher temperatures plus high humidity leaves you more sweaty than cooler and less humid conditions. Try running through a forest on a sunny day after rain compared with open country in a breeze on a cooler day.
Dave Banks, Wellington, New Zealand
There is a second, minor effect beyond evaporative cooling at work here. When you stop exercising, your muscles stop working and generating heat, but your body's "thermostat" is still set to a higher temperature. As it gradually resets to a resting state, you continue to produce sweat to cool yourself down.
David Gibson, Leeds, West Yorkshire, UK
