I had minor knee soreness for about the first two or three days after converting to fixed gear riding, after that I felt perfectly fine. I read that the minor discomfort was due to the fact that when you start riding fixed, you begin to use certain muscle groups around the knee area in new ways that are different from walking or running. Once your legs adjust to the new routine (in my case two to three days) you'll be fine. I should mention that I do run a front brake. I only use my legs to slow down, and the brake to come to a complete stop or when I need to stop on a dime.