Fixed Firm Pose, or Supta Vajrasana, is a great pose for compressing and clearing your kidneys, compressing and subsequently increasing blood flow to the knees, and increasing flexibility in your quadriceps. Begin by kneeling down "Japanese style". Wiggle your thighs out so that your bottom reaches the floor. If this is already impossible, then try to get close, but keep your hands out in front to take some of the body weight off of your knees. You can also move the knees further out to the side to ease the pressure.
Then, with your hands on your feet, slowly bring your elbows down to the floor--if this is all you can do, stop there! If you are still ok, then bring your shoulders down to the floor, bring your arms over your head, and lift through the chest. Hold three full breaths. Reverse to come out.