I am self taught using the frog kick. I do a modified kick mostly where the legs are not 100 percent vertical and it works good for me for 90% of my diving. But it got me to thinking that I should work on a "true frog kick." Problem is that my kness start to hurt after a few minutes. It feels like I am twisting my knee when doing the kick. Is this normal, or better yet, what should I be doing to make it right?