Background and Aim: The structure and function of the hamstring and quadriceps can contribute to the development and process of patellofemoral pain syndrome. As open and closed kinetic chain exercises have different effects on the knee joint, the aim of the current study was to investigate motor control of knee joint in both kinetic chains according to voluntary response index aspect.

Materials and Methods: 12 healthy knees and 12 injured knees (6 females in each group) were examined. Surface electromyography were performed in knee muscles including vastus medialis oblique, rectus femoris, vastus lateralis and biceps femoris while the subjects were doing flexion-extension of knee joint by isokinetic dynamometer system and one leg semi-squat movement. Then voluntary response index was calculated.

Results: There were no significant difference between the patients and healthy groups (p>0.05). In the patient group, correlation between similarity index and magnitude was seen.

Conclusion: Probably in the low intensity patellofemoral syndromes, voluntary response index could not differentiate between healthy and the patient groups. In the patients with patellofemoral syndrome group, correlation between similarity index and magnitude may represent co-contraction in their muscles.

Keywords: Patellofemoral pain syndrome, Motor control, Voluntary response index, Surface electromyography