Research Review by Dr. Shawn Thistle©


Aug. 2007

Study Title:

Analysis of hip strength in females seeking physical therapy treatment for unilateral patellofemoral pain syndrome


Robinson RL & Nee RJ

Publication Information:

Journal of Orthopaedic & Sports Physical Therapy 2007; 37(5): 232-238.


Patellofemoral pain syndrome (PFPS) is a common orthopedic condition which generally occurs more frequently in females. The clinical diagnosis of PFPS encompasses retro/peripatellar pain that is aggravated by prolonged sitting (“movie-goers sign”) or activities that load the patellofemoral joint such as ascending or descending stairs, squatting, kneeling, and jumping.

It is hypothesized that PFPS results from abnormal patellar tracking that causes subsequent wear on the articular surface of the patella. It is further hypothesized that hip muscle weakness can be a contributing factor to the development or proliferation of this syndrome. Poor hip control may lead to abnormal lower extremity motion, particularly femoral adduction and tibial internal rotation associated with eccentric weight-bearing activities.

This cross-sectional study investigated whether females consulting physical therapists with unilateral PFPS exhibited hip strength deficiency compared to control patients. Ten subjects with unilateral PFPS were included in the study, and met the following inclusion criteria:
  • they were between the ages of 12-35 (to exclude the possibility of degenerative changes)
  • their symptoms were insidious in onset
  • their pain was peri/retropatellar with either sports participation, ascending/descending stairs, or long-sitting
Female control subjects between the ages of 12-35 with no history of knee pain were recruited from the local community. No attempt was made to match control subjects to study subjects for age, height, or activity level. Participants in both groups were excluded if:
  • they had a history of patellar dislocation, knee surgery or trauma, or confirmed meniscus, ligamentous, or muscular pathology in either knee
  • they had a neurological condition
  • they were taking anti-inflammatory medication
  • they had received physical therapy for their knee condition in the previous 30 days
The study design was simple, and appropriate for the sample size used. Each participant completed the Anterior Knee Pain Scale – a 13-item questionnaire that has been demonstrated adequate validity and reliability in previous studies. Each subject then underwent isometric hip muscle strength testing using a hand-held-dynamometer (HHD) for the following movements: hip extension, abduction, and external rotation. These movements were measured in the same order for all subjects (abduction, extension, and external rotation), and the average of three measurements was used for analysis.

The main outcome measure used was the Limb Symmetry Index (LSI) – a percentage value of the strength in the affected limb compared to the unaffected limb. This mirrors the common clinical practice of comparing strength between limbs of the same patient.

Pertinent Results:

  • normalized hip strength values for all movements were significantly less in the PFPS group (p < 0.007)
  • differences in LSI were greatest for extension, followed by abduction and external rotation in the PFPS group
  • strength values in the affected limbs of the PFPS group were only 71-79% of the values for the unaffected limb
  • PFPS subjects exhibited lower strength in the affected limb overall compared to the weakest (non-dominant) legs of the control subjects
  • PFPS subjects has been symptomatic for an average of 35 months

Conclusions & Practical Application:

This small, simple study suggests that females with PFPS exhibit greater asymmetry in hip strength than subjects without PFPS, as well as showing lower overall strength in the three planes tested. Strength deficit values noted in this study are in accordance with previous research in this topic (only consisting of two other small studies), and identified hip extension deficits, which previous studies did not. A practical extrapolation of these results could be to focus on hip strength and stability when rehabilitating (or preventing?) PFPS in a comprehensive manner. Despite being logical, this application should be considered in the context of some limitations of this study:
  1. small sample size
  2. unmatched control group
  3. the cross-sectional nature did not allow determination of cause or effect for hip muscle weakness and PFPS – it could merely suggest an association
  4. PFPS subjects had been symptomatic for a long time, making it conceivable that they had limited their activity due to pain, which may have contributed to the reduced strength values demonstrated (no mention was made of activity level of subjects)
It still needs to be determined in quality studies whether changes in hip strength can positively affect the clinical outcome in PFPS. Further studies should also investigate the importance of hip muscle endurance as a potential factor in PFPS.