The term that is used to describe the conditions associates with anterior knee pain from insult to the cartilage is chondromalacia patella, which literally means sick cartilage of the kneecap. For many patients pain at the anterior compartment (front part of the knee) starts in adolescence and persists into adulthood, and can even lead to osteoarthritis.
The patella is designed to glide in its groove (trochlea) as a fulcrum for the actions of the quadriceps. Walking, running, and jumping are contingent on this extensor mechanism being sound. A disruption of the quadriceps tendon, patella, or patella tendon impairs this mechanism and makes even basic activities such as rising from a chair difficult or impossible. When there are slight changes in the alignment of the patellofemoral joint there can be pain present with any of the above activities. Sometimes that pain is related to abnormal biomechanics and poor muscle integration.
The purpose of a single leg squat is to help balance and improve knee pain. It has been said that the knee is a “slave” to the hip and ankle. In other words if the ankle and/or hip has weakness or dysfunction, the knee joint suffers. This is because the hip and ankle are relatively more mobile joints and the knee is more of a hinged mechanism. When there is torque that stresses the knee outside of that simple hinged mechanism, the knee can glide abnormally and wear away at the cartilage of the patella.
A single leg squat is the final target for rehabilitation of anterior knee pain after performing more simple exercises to strengthen the rotator cuff of the hip and ankle mobility/stabilization exercises. Balance is another important aspect that needs to be trained prior to initiating this movement. Spending at least 1-2 weeks focusing on the above can help to set the foundation for good movement in single leg training.
The focus of the single leg squat is to stand with the opposite hip flexed, pelvis level, core engaged, shoulders back, head up, and a smooth, controlled movement. The toes are forward or slightly outward and the knee should move over the great toe when flexing into the squat. The quadriceps and gluteus muscles/rotator cuff of the hip should be working together to control the the plane of movement of the knee. The foot and ankle should be stable from practicing balance, also maintaining the single plane of motion of the knee into flexion. The focus is smooth and controlled movement. The number of repetitions is less important than correct form. Practice regularly and set a goal of symmetry of motion for both knees. Practice with a mirror or spotter.