the first couple weeks of riding fixed i was sore alot in my legs, specifically the muscles right behind my knee on the back of my leg. Skipping, skidding and locking your legs and all that junk highly uses some muscles in your leg that before were rarely used in regular bike riding. I also had a little trouble with my ankles for like a week or more, i guess from all the strain that's placed on them from tugging and pushing with your feet.

I got over it all pretty quickly though, just takes a little time for your body to adjust