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.

Sweat can cool you down because evaporation of water is a really effective cooling mechanism. The most energetic molecules are the ones that evaporate, so they take more than the average amount of energy of the molecules, which is indicative of the temperature. And removing the water takes a bit of energy — the heat of vaporization is 580 cal/gram at normal body temperature. So the temperature tends to decrease. You can sweat more than a liter per hour (take care that you don’t get dehydrated!) for upwards of a kilowatt of heat loss.

So we don’t worry about overheating yet, because evaporation can take the load. But that assumes the water is evaporating. Once you build up a “good sweat,” the water isn’t evaporating anymore. The water that’s dripping off you isn’t cooling you, and that layer of water is actually insulating. If you’re in an environment that’s too hot, your body’s reaction is to sweat some more, and when you keep heating up, you sweat even more. But it’s not doing any good! All the while, your body’s temperature is rising, and eventually you reach the point where you can’t do any more workout because your body needs to shut down to avoid overheating. But by working out in a more comfortable, even cool environment, you can keep exercising before this limit is reached.

Now, there are physiological implications for working out when it’s too cold, and I’m not addressing that or advocating workouts in the cold. But have some consideration for others in the exercise room. If you’re cold, put on some sweats before you crank up the heat. Having the sweat pouring off you may seem like an indication of a good workout, but thermodynamically, it’s telling you that you’re overheating. You’ll get a better workout if it’s cooler.