Single limb weakness and lower extremity asymmetry is a common issue among clients I see. Often, there is poor hip control and quadriceps weakness that contributes to anterior knee pain or altered movement patterns that may contribute to overuse injuries. This squatting exercise using a stability ball is an effective way to increase single leg strength and stability.

Execution: Position a stability ball in the small of the back against a wall. Lift the right leg up, while slowly squatting down on the left leg through the desired range of motion. The right leg will be flexed to about 90 degrees throughout and move beneath the body and toward the wall. Pause for 1-2 seconds and return to the start position. Repeat for time or reps and switch sides. Perform 2-3 sets on each side.

Focus on maintaining an erect posture and not allowing the knee to cave in (dynamic valgus). I generally advise stopping just short of full extension on the ascent to maintain some tension on the quadriceps.


1. Increase the hold time at the bottom of the squats

2. Fold the arms across the chest

3. Add external resistance

Regression: If clients have discomfort in a certain part of the range of motion or struggle to maintain stability, consider using an isometric version. In this case, simply squat down and hold for time (5-30 seconds). The client can also perform holds at different degrees of flexion to increase the difficulty or make it easier.

Application: This exercise is a simple, yet effective way to increase lower extremity strength and improve hip/knee stability. In clients who struggle with patellofemoral pain, quadriceps weakness or cannot perform a free standing single leg squat with good form, the isotonic and/or isometric version of this exercise offers a good alternative. Using the ball allows for better posture, increased stability and very small incremental changes in range of motion based on the client’s form and comfort level.