Wall Sits 

The wall sit is a gateway to functional squatting. It is appropriate for anyone experiencing patellofemoral pain or front knee pain.
Start: Place your back against a wall with your feet out away from the wall. You will slide down the wall until you are in a sitting position. You want this to be pain-free. You should not go further than 90 degrees and make sure that your knees are aligned with your ankles.
You can perform a "Sit and Hold" and hold the wall sit for 30 seconds plus for a number of sets or you can perform multiple repetitions, 3x10.
For more information, visit: www.stoneclinic.com/rehab