This Posture can be particularly challenging for athletes and those suffering from knee injuries. Fixed firm is of the best for your knees. In the beginning, it may be enough that you are on your knees, not yet going back all the way. In this first stage, its very important to feel OK with where you are. Know that you are increasing the flexibility in your knees and ankles so that you are able to move to the next stage, leaning back on the floor and arms over the head.
Believe it or not, I used to be so stiff around my knees that I could only stand on my knees in this pose. The constant pounding on the field from footwork drills and tackling took a toll on my joints. It took me about six months of practicing this pose before I was able to get my head on the floor in the full expression. The most important part was allowing myself to be patient with my process.