This is a Randomized Controlled Trial (RCT) regarding conservative treatment of Patellofemoral Pain Syndrome (PFPS). Patellofemoral Pain Syndrome (PFPS) is a very common cause of knee pain in young active adults with a high rate of recurrent and/or chronic occurrence. PFPS is notoriously difficult to treat and has been referred to as "one of the most vexatious clinical challenges in rehabilitative medicine". Its etiology is unclear but is commonly thought to be related to pathomechanics in the patellofemoral joint (PFJ). There are many factors that can influence PFJ mechanics. Among these, quadriceps strength and timing has been shown to be important. As such, treatment of PFPS has traditionally been based on correction of pathomechanics through influencing quadriceps strength and timing. However, a growing body of evidence is revealing the importance of strength and control of hip abduction and external rotation in PFPS. Hip strength in ab/adduction and rotation is thought to influence femoral positioning in the patellofemoral joint, thereby affecting PFJ mechanics. Several cohort and smaller RCT studies within the last 7 years have shown that additional exercises for hip strength and control give an improved effect in pain and function compared with quadriceps based training alone. A smaller RCT from 2012 compared isolated hip strengthening exercises to a control group and found surprisingly good results on pain in function in the hip strengthening group. The investigators plan a RCT in which isolated hip strengthening will be compared to traditional quadriceps training and a control group which will receive no structured training. Primary outcomes will be pain and function. This high-quality study will include 40-50 patients in each group, making it one of the largest of its kind on conservative treatment for PFPS. In contrast to the vast majority of studies of this type, this study will also include men, which will potentially help to fill a significant gap in the literature on this subject. The investigators study will therefore be an important contribution to elucidating the etiology of PFPS and improving treatment options for both men and women in the future. As well, the role of psychometric parameters will be examined and a standardized clinical test for hip abduction endurance will be developed. Follow-up at 3 months and 12 months is completed and published. A 5-year follow-up of the same patients is underway.

Inclusion Criteria:

• Age 16-40 years
• Insidious onset of symptoms not related to trauma
• Symptoms more than 3 months
• A minimum of VAS 3 on VAS-W
• Peri- or retropatellar pain during or after at least two of the following: Stair ascent or descent, hopping, running, prolonged sitting, squatting, kneeling.
• Pain on one of the following: Compression of the patella, palpation of the patellar facets

Exclusion Criteria:

• Clinical findings indicative of meniscal or other intraarticular injury
• Clinical findings indicative of injury to or increased laxity of cruciate or collateral ligaments
• Findings on MRI indicative of other intraarticular pathology.
• Clinical and/or x-ray findings (plain anterior-posterior, lateral and skyline view) indicative of osteoarthritis, Osgood-Schlatter or Sinding-Larsen-Johanssen syndrome
• Clinical findings indicative of knee joint effusion
• Significant pain from hip or lumbar spine on clinical evaluation, with potential for causing referred pain to the knee or hindering the patient's ability to perform the prescribed exercises.
• Recurrent patellar subluxation or dislocation.
• Previous surgery to the knee joint
• NSAID or cortisone use over an extended period of time
• Having suffered trauma to the knee joint judged during clinical evaluation to have a significant effect on the presenting clinical condition.
• Physiotherapy or other similar treatment for patellofemoral pain syndrome within the previous 3 months