Bow and knee joint

For the first time since months I managed to get up at 5 and do yoga before going to work. I wanted something relaxed and did Erich Schiffmann's Beginning Yoga. I was really stiff but had a nice practice nonetheless. Didn't turn on the lights but had only the screen of my laptop on so it was really peaceful.

I thought I must be somehow not "normal" because I have this strange feeling in the knees while doing bow or bridge. But I checked Anatomy of Hatha Yoga and it seems it's quite normal, because the knee joint is simply not made to be used in positions like this. So, again, patience is needed until the connective tissue capsules of the knee joints get tough enough so that one can get higher in the pose.