Hello, I have also experenced the same kinda thing with my knee that you have explaned and I also would like to no if it is something that I have to worry about or not. What happeneds to my knee is that when I kneel down my knee gives way. Its also feels like it is sliding out of place. The pain I get is very extream and sometimes it is hard to walk after it happenes.The pain could last for a couple of hour`s to days. This has been happening for the last half a year. I am only 15 and in good shape. I have always been involved in sports but I havent been training since september since my sport was of for winter. Now that it is starting back up I am worry to go back training incase I badly injure my knee. But first I have to find out what is wrong with it. If anyone knows anything about what im am experencing please let me know as I am very worried about it.