I was a bit worried a few weeks ago, after running, about the amount of visible veins on my body. I looked like this...



... only it was just two veins, in my foot. 

I was convinced that I'd done something terrible! Was it a blood clot that had made my veins swell? Had I broken my foot in some way?

No, it just turned out I hadn't noticed before, or hadn't exercised enough before, but visible veins after exercising turns out to be perfectly normal.

This article explains it in more detail, but essentially what's happening is...

1) The increase in blood pressure when exercising forces plasma fluid of the arteries into the muscles.
2) The muscles swell and harden noticeably during exercise.
3) The swelling muscles push on the veins, pushing them closer to the skin.#

That's why you can see the veins, more so if you don't have much fat (that might be why I haven't noticed in more places).

So now we know!