Patellofemoral pain syndrome in young women. II. Muscle function in patients and healthy controls.

Muscle function was evaluated in 40 patients with patellofemoral pain syndrome (PFPS) and 20 healthy controls. Patients with PFPS had a significantly lower knee extensor strength in the most symptomatic knee compared to the least symptomatic knee. Further, the patients had less vertical jumping ability and were weaker in the most symptomatic knee compared to the controls, with the largest differences in eccentric knee extension. There were lower strength and EMG activity, in the patients compared to the controls, in the range closer to full extension and significant differences in muscle activity between the vastus medialis and the rectus femoris muscle. The results can be explained by inhibition selective to knee angles and to the vastus medialis muscle.