Etiology	Pathophysiology
Weakness of the quadriceps	The “quads” include the vastus medialis, vastus medialis obliquus (VMO), vastus intermedius, vastus lateralis and rectus femoris. Weakness may adversely affect the patellofemoral mechanism. Quad-muscle strengthening is often recommended.3,4,7,9,10,14–17
Weakness of the medial Quadriceps, specifically VMO dysplasia	Weakness of the VMO allows the patella to track too far laterally. Although the role of the VMO is controversial,18–20 VMO strengthening is often recommended.6,7,11,15,16 However, the VMO is a difficult muscle to isolate,21 and most patients find general quadriceps strengthening easier to accomplish.
Tight iliotibial bands	A tight iliotibial band places excessive lateral force on the patella and can also externally rotate the tibia, upsetting the balance of the patellofemoral mechanism.22,23 This problem can lead to excessive lateral tracking of the patella.
Tight hamstring muscles	The hamstring muscles flex the knee. Tight hamstrings place more posterior force on the knee, causing pressure between the patella and femur to increase.7,15,16
Weakness or tightness of the hip muscles (adductors, Abductors, external rotators)	The VMO originates on the adductor magnus tendon. This is the anatomic basis for recommending adductor strengthening.11,14,16 Abductor (gluteus medius) strengthening helps to stabilize the pelvis. Dysfunction of the hip external rotators results in compensatory foot pronation; a simple stretch can improve muscular efficiency.4
Tight calf muscles	Tight calves can lead to compensatory foot pronation and, like tight hamstrings, can increase the posterior force on the knee.11,15,16