I haven't run for the last week, not by choice. The muscles behind my knee let me know I'd overextended them. Maybe I have an inherent weakness. Maybe I didn't properly prepare. Maybe I took a wrong step. The thing is, I didn't realize I'd pulled those muscles while I was running. It wasn't until later that evening, when it hurt to walk or come down the stairs that I knew. This isn't the first time, either. Those muscles have sidelined me even during the winter when I'm not running.