I've had many questions this week about higher leg extensions - so I decided to list some articles discussing the topic:
Get Higher Developpes
Build Strength For Long and Lean Ballet Muscles
The Ultimate Ballet Line - Arabesque
You'll understand that the quads do lift the legs to the front and side, and the hamstrings lift the legs to the back (a gross simplification). And you'll understand how to strengthen the psoas muscle to lift the legs above ninety degrees, and gradually you'll stop straining the quads.