What Causes a Muscle Imbalance?
Your quadriceps (thigh) muscles attach to the patella and through it to the patella tendon, which attaches to the top of your shin.
If there is a muscle imbalance between the quadriceps muscles: vastus lateralis (VL), which pulls your patella up and outwards, and the vastus medialis oblique (VMO), which is the only quadriceps muscle that pulls your kneecap up and slightly in, then your patella will track laterally in the groove.
Common reasons for a weak vastus medialis oblique (VMO) include knee injury, post-surgery, swelling or disuse.
Longstanding tightness of your lateral knee structures (lateral retinaculum, VL, and ITB) will encourage your kneecap to drift sideways over time. Especially, if your VMO is also weak.Patellofemoral pain syndrome is more common during adolescence, because the long bones are growing faster than the muscles, tendons and ligaments, putting abnormal stresses on the joints. Active children who do not stretch the appropriate muscles are predisposed to patellar malalignment.