After my 3 miles of fartleks on Saturday, I felt fine through out the day. However, during the evening I started to feel a little bit of tenderness in my right leg, between my hip and my knee, and that soreness continued during Sunday. It was very minor, and I didn't notice it most of the time. At first I thought the problem might be with my quad, so I did extra quad stretches on Sunday. But then I recalled that the tenderness was on the outside edge of my leg and thus probably wasn't a quad problem. I remembered that the IT Band goes down the outside of my leg, and I decided it might be an ITB problem. I started doing my ITB exercise, and I could tell that the soreness was in the same place as the muscle being stretched. The soreness was gone this morning. I'm continuing the ITB exercise throughout this day and tomorrow and maybe on into the week.
I first heard of the ITB about six months ago when it came up in discussions on running.about.com Even though I'd never had a problem with my ITB I decided to do a stretch for it, and for the past six months, I've been doing 20 seconds of ITB stretching before and 20 seconds after my runs (per leg). I think I'll double that. During my fartleks on Saturday, I did a few sprints for about 150 feet, and that burst of speed might have caused the soreness. IT Band pain is an overuse problem. Most of my fartleks were at slower paces.
Later: I read about IT Band injuries on the Internet and learned that running too fast for ones body is one cause of the injuries. I just added a page to my running site about IT Band injuries.
Next day Tuesday February 13, 2007: I haven't had any soreness in the IT Band since Sunday evening. Stretching did the trick, and I caught it before it became an injury!