What Does Cap Stand For In Volleyball I Can't Kneel Anymore?

I can't kneel anymore? - what does cap stand for in volleyball

Ok it happened two months ago. There are about two months, played volleyball and Touche, and I landed my feet, another boy caught the ball and knocked on his knees before he landed. Since then, my knee was bothering me. This is the back of the knee, behind my knee. At this point I can not kneel on their knees without much difficulty. As if I have to kneel stand, as in softball or a church, I can not all the way straight ahead without pain. Even if I find I can not bend the knee last hit 90-100 degrees. I wonder what you think it could be, anyway. What can I do. The problem is that I am not at all, because I can relax every day softball. So basically this is to allow a stand or something else? How do I