Theres one you can do with a fitball that will help CFH. Sit with your back to the wall nice and straight, with a leg over a fitball hip knee and foot in line. Pushing with your Hamstrings into the Ball and your foot pushed back to shins bend the knee so the Heel is pushing into the Ball, while all the time pushing the Hamstrings into the Ball with pressure. Then after 10 secs raise the foot and straighten the leg at the top hold for 10 secs always pushing down into the Ball with the foot bent towards the shin do it 3 times each leg. you will tremble well the muscles will keep correct form at all times, Enjoy