Intervention group
patellofemoral joint stress, knee flexion
Participants in the intervention group completed one-on-one sessions with a physiotherapist to learn the technique of each exercise and 18 exercise sessions over 6 weeks, with 3 sessions per week (Table 
Exercise program performed by the intervention group+The load was adjusted every 2 weeks to maintain an effort of perception between 6 and 7 on the OMNI scaleSince knee strength is associated with PFP, most exercises in this program are strength-based exercises that included strength, stretching, and core stability exercises. Before the exercise session, the participants performed a warm-up by cycling or walking for 10 min to prevent injuries before the formal exercise protocol. To minimize patellofemoral joint stress while performing quadriceps exercises, a single leg squat against the wall was performed to between 0° and 60° of knee flexion, and leg extension was performed to between 90° and 45° of knee flexion [Physiotherapists remotely reviewed the participants progress on intervention, adjusted the load of exercises if necessary and provided health education via 3 biweekly health education sessions. Health education session was delivered by WeChat APP (Tencent Computer System Co. Ltd., Shenzhen, China) and booklets that covered the information of clinical manifestations, risk factors, treatment of PFP, and benefits of exercise on PFP.