Wall Sit
The wall sit is used to develop strength in the legs while placing less stress on the joints than movements requiring reps, like squats and lunges. It is effective for building endurance in the knee extensors as well.
Cues:
Position yourself so your knees and hips are each bent to 90 degree angles
Keep your heels on the floor and low back against the wall
Scale by raising your body higher in order to complete the prescribed time and sets
Mistakes:
Excessive scaling to make the exercise too easy
Arching the low back away from the wall
Heels coming up and placing too much weight in the balls of the feet