"Pie in the Sky" Yogini: Knee flexors!
The knee flexor counterbalances the extension force of the quads. The knee flexor and quad extensors oppose each other to stabilize the knee. It is so important to avoid hyper-extension or "locking" out of the knee during standing poses in your yoga practice. This can overstretch the hamstrings and create unhealthy stress on the knee cartilage. Contract the knee flexor instead as this will avoid hyper-extending the knee. An easy way to do this is to press down on the ball of the foot. This will contract the calf muscle and stabilize your knee.
Mary Jane - www.fengshuiyoganj.com