Patellofemoral pain syndrome (PFPS) is defined as pain appearing in the anterior or posterior
knees during kneeling or squatting [1]. The etiology of PFPS is not clear yet but considered to
include pressure on or tilting of the patella and imbalance between the vastus medialis oblique
(VMO) and vastus lateralis (VL); In particular, imbalance between the muscles near the knees
resulting from the weakening f the VMO is regarded the most important cause [2]. In a study which examined VMO/VL ratios in PFPs patients and normal people during voluntary
isometric knee extension, Makhsous et al.[3] reported that the muscle activity ratios of PFPS patients were lower than those of normal people. Such imbalance between VMO and VL triggers lateral tracking and malalignment of the patella and further increases pressure on the interface of the patellofemoral joint, triggering pain [4]. Therefore, for balance between and stability of the VMO and the VL, training for selective strengthening of the VMO is very crucial [5].
The most universal physical therapy intervention for PFPS patients is quadriceps femoris
muscle (QFM) strengthening exercise [6]. Muscle strengthening exercise is divided into open
kinetic chain exercise and closed kinetic chain exercise and traditionally open kinetic chain
exercise has been used much [7]. However, it was reported that increase in the force of the
QFM by open kinetic chain exercise increased pressure on the patellofemoral joint and pain in
a lot of PFPS patients [8]. On the other hand, closed kinetic exercise like squat exercise provides high stability resulting from co-contraction of the QFM and biceps femoris muscle
(BFM) and provides minimal stress on the patellofemoral joint within functional range;
therefore, it is an effective and safe exercise method