Or, the myth of “working up a good sweat”

On occasion my “private” workouts get interrupted by someone with the audacity to want to use the exercise room at the same time I do. That’s no biggie. But what bugs me is when the offendor will turn on the heat, or turn off the AC or close the window. I prefer to not be sweating just by walking in the room. It shortens my workout, and here’s why.

Your body is about a 100-watt heat source. That is, you you are shedding heat at level of about 100 watts, give or take, just by sitting around. There’s an overview here. Most of that is radiation, and takes into account that we would normally be shedding more heat if not clothed. Long sleeves and pants insulate more, and reduce the heat loss, or compensate for larger heat loss when it’s cooler. Imagine that.

Radiation heat transfer is covered by the Stefan-Boltzmann law, which tells us that heat transfer depends on the difference of the fourth power of temperature of our body vs. the ambient room. Your body is a heat engine, and once you start exercising that engine needs to reject more heat. So you hop on the treadmill or bike or stairmaster and start your workout. The little display tells you that the output is about 100 Watts, but your body isn’t 100% efficient — no engine can be. You’re actually burning carbs at about 400 Watts, and that extra 300 Watts needs to leave your body, so your temperature goes up. If radiation were to take the whole load, your body would need to rise about 30 degrees C, and obviously it can’t do that. So, you start sweating.

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