Whenever I have a partner help me with the v-stretch, such as helping me open up my legs wider, I get this strong pain on the inside of my left knee. Now, I make sure I don't overstretch. If I open up my legs as wide as I can without any help, I feel no pain. I've also tried using one of those leg stretching machines, but I still get the pain in my knee. The pain only seems to occur when I'm being assisted.What could this possibly be?