I had a stress fracture in the femur in college and the pain was hard to describe. It felt like the muscle was weak when I tried to plant it while running; almost like it was going to give out. They originally diagnosed it as a quad strain and had me doing leg press and leg extenstions to strengthen it. This didn't help and I got a bone scan and verified the fracture. I had to take about 3 months off with no running and even to this day, I feel it if my mileage gets above 45 or so. Hope this helps.