Sometimes after long runs (18+ miles), directly behind my knee gets tight. It doesn't feel like the hamstring is tight, because it is the area right behind my knee. What muscle is that, and how can I stretch it?

There are several structures in the back of the knee that might be involved, but the most likely is the popliteus muscle. In order to stretch it, try straightening out your knee when it is stiff, and note the way you must apply force to make it feel a little uncomfortable. That is the force application you should apply to stretch, when it is not so stiff, but after some exertion to get the muscle supple.

If your knee is getting worse, giving way, locking or having increasing problems, get in to see your provider.

                     ~The Doc