To diagnose exertional compartment syndrome, they measure the pressure of the muscle compartment (with a needle) before exercise, and then when you feel the pain. If the pressure of the compartment skyrockets, you have it. It's pretty rare, but not unheard of (when i found out about it, I asked on the Ultrarunning list, and found about 10 people who had it) and also hard to convince a doc you have it. My pain was anterior and lateral. Just to the outside of the shin bone. That big front muscle. (and the one behind it that you can't feel from the outside) You can't see it on an x-ray. There's a surgery to fix it.

Nanci