The purpose of this study is to investigate the muscle activity of gluteus medius and tensor fascia lata in health control group and patients with PFPS during submaximal isometric muscle contraction.The research question :Is there any difference of muscle activity of gluteus medius and tensor fascia lata between health control group and patients with PFPS during submaximal isometric muscle contraction ？Null hypothesis:There is no difference of muscle activity of gluteus medius and tensor fascia lata between health control group and patients with PFPS during submaximal isometric muscle contraction.

Basic data

- Name, age, sex, BW, BH, pain VAS scale（VAS-activity）, affected side, dominate leg, symptom duration, activity level

Measurement

• Flexibility of iliotibial band：Ober test
• EMG data collection during 8s submaximal muscle contraction
• Muscle strength measurement of hip abductors

Inclusion Criteria:

• 20-50 years old
• Insidious onset of symptoms
• Pain exist at least 3 month, VAS activity>4
• Two of the following activities induce pain at anterior or retropatellar site prolonged sitting, stair-climbing, squatting, running, kneeling, and hopping/jumping
• Diagnosed by an orthopedic surgeon

Exclusion Criteria:

• History of lower extremity fracture or operation
• Neurological disease involved
• Observed knee deformity
• Other well-defined musculoskeletal problems in lower extremity