90 patients with patellofemoral pain syndrome who less than 50 years old will be included in this study, and randomly distributed to hip adduction combine knee extension exercise group, knee extension exercise group, and control group. Following 8 weeks exercise training by an experimental physical therapist, ultrasound measurement of muscle thickness and cross-sectional area of vastus medialis oblique muscle, vastus lateralis muscle, and rectus femoris muscle component of quadriceps, 10cm-VAS patellofemoral joint pain evaluation, including VAS-U, VAS-W, and VAS-activity, and Lysholm scale scores will be measured. A two-way mixed ANOVA will be used to compare the mean differences between theses three groups.