Imagine that you're standing on a step three inches above the floor. You hop off the step and land on one foot. Easy, right?
Now imagine that your fear is that when you land on that one foot, your leg won't be able to support you, and your leg will buckle underneath your weight. So, even though you want to land softly by bending your leg, you keep landing with your leg held almost straight.
That was one of the things I was doing last night in physical therapy for my knee. I got better at it, but there was always that fear that my leg would give way as it most certainly would have before my ACL reconstruction surgery. Also, I know that my knee doesn't have the strength when it's bent that I want.
Another simple thing that's difficult (though it's getting easier with practice); standing on a step with my weight on my left leg, and simply bending that leg so my right heel will dip to touch the floor. Again, it's an exercise to rebuild that flexing strength in the knee.
Despite all this, I'm coming to the end of my physical therapy. I have my full range of motion in my knee, and my quad muscles have good tone. I'm hoping this will mean returning to martial arts soon. I'm seeing my doctor in a week, we'll see what he says.