This is a standard exercise I use for knee stability. Effectively targets the Vastus Medialis Oblique (VMO), the inner thigh muscle. This muscle is very important for knee stability. A strong VMO helps to hinder the knee from valgus collapse during squats. This muscle is often underdeveloped. Maybe due to a soft and easy life in front of the TV.. or something..
I learned this exercise from reading Charles Poliquin, and I’ve used it for several years now with great success.
Stand with one foot on a step with heal lifted (on your toes), and the other on the floor beside it. Stand so you have the “floor-foot” in front of the “step-foot”. Keep the hips in linear position. Make sure to point the “step-knee” out. Lift your “floor-foot” toes slightly so you won’t cheat by pushing yourself of the floor. Shift your weight to the “step-foot” and push yourself of the floor till your leg is fully extended. Lower yourself with control and touch the floor with the heel of the “floor-foot”. Repeat a few times till you get the hang of it. You should be able to do sets of 8-10 controlled reps before adding weights. There’s no reason for doing more reps, it’s not an endurance exercise and you will easily loose focus after 10 reps.
Watch this fantastic demonstration by Alexandra and Stephan at KILO Strength Society, both former students of Charles Poliquin.