I had 5 total knee replacements between 1988 and 1997.For many years I could not kneel on the ground.Per Dr's orders I was not to kneel on any hard surface,not to run and not to jump,as I could possibly damage the repacement parts,my knee cap or the bones.Then I discovered the kneepads with jell inserts.Now I am able to kneel on the ground while recovering a target instead of bending over each time.
Did a web search and found these

atella T. Wedge Kneepads.This web site is only to show what they are.By doing a search,you may find them at a lower price.I paid $60 for mine.
http://www.mcguckin.com/index.php?main_page=product_info&cPath=70_268&products_id=583&zenid=g0ovgifsjultf2q3lc9agb2fq5