Why do you feel warm when you exercise?

I’m looking for the most in-depth, technical answer I can get.

I understand that when calories are “burned” heat is a byproduct (but I don’t really know what burning a calorie actually means). I understand that your heart pumps faster, blood flows more quickly, breathing is heavier, so friction may even play a part in producing some of the heat initially.

But, on the most basic cellular/chemical level, why does exercise make me feel warm? And warm for quite a while afterwards too! Why does my body seem to feel warm for up to an hour or so after even a brief bit of exercise?

(I work on a very cold boat and I go exercise every once in a while to stay warm, it’s very effective, much more than when I bundle up with gloves and such).

The more technical the answer, the better! Thanks :slight_smile:

Metabolism runs molecules (carbohydrates and lipids, mostly) through a series of chemical processes. Each step in the process changes the molecule to another molecule. Energy is captured by the system by taking ADP (adenosine diphosphate) and turning it into ATP (adenosine triphosphate). Most of the end products of these reactions are either CO2 or H2O, which you excrete. When your muscles use energy, they’re taking ATP and turning it back into ADP. Wikipedia has some good pages that go into the specific chemistry if you look up metabolism there. That’s as technical as you can get, but there’s no point in repeating it here.

Waste heat is produced both while breaking down molecules to produce ATP and while using the ATP to power muscles. It’s about 50% efficient, so there’s a lot of waste heat (though it’s worth noting that the biological processes are quite close to the theoretical maximum efficiency, which is impressive).

It takes your body a while to heat up, and a while to get rid of all that heat.

In addition, there’s some work that’s done by the body after exercise ends. Muscles store some carbohydrates in glycogen; your body will take blood sugar and use it to replenish glycogen. As with all processes, this has waste heat of its own. You body may also be repairing microdamage to muscle fibers, building new muscle tissue, etc.

Thanks dracoi! That’s exactly the kind of info I was looking for. Are you a molecular biologist?

Calorie is just a measure of the energy produced by “burning” food. And any chemical energy the body uses is eventually going to reduce itself to waste heat.

No. As dracoi explains above, about half the caloric content used in muscles gets converted to mechanical work*, and half gets converted into heat. Other body process may indeed ultimately produce heat.

*Which might ultimately end up as heat outside the body, but could get stored, as when you carry a rock up a hill and leave it at the top.

I got most of the way through an organic chemistry degree before I realized that most of my career options were not as much fun as the classes. Ultimately, I wound up as an accountant, but I can still read the back of shampoo bottles.