Changes in knee biomechanics after a hip-abductor strengthening protocol for runners with patellofemoral pain syndrome.
Over the years we have gained a better appreciation for the influence of the hip on knee injuries and symptoms. While observing patients with anterior knee pain or other knee injuries you may have noticed that during squats (or similar movements) their hip adducts causing dynamic knee valgus (probably related to weak hip abductors). While some research has supported these findings there is a need for more research assessing all of these variables together and evaluating how they change in response to interventions. The purpose of this study was to explore the relationship between hip abductor strength and frontal plane knee movement by evaluating a 3-week hip abduction strengthening protocol among runners with PFPS (15 active runners; they also assessed pain). The authors also included 10 control runners who did not have PFPS (they did not do the strength training). Among the biomechanical variables assessed was stride-to-stride knee-joint variability; which is based on the concept that our movements vary even when we plan to perform a similar motion over and over (think about walking; each step is slightly different than the others). Previous research indicated that reduced variability is associated with running-related injuries (including PFPS) and that there is an optimal amount of variability needed to allow our gait to respond to unexpected changes. In this study, biomechanics was assessed during treadmill running before and after the 3-week strengthening program. Hip abduction strength was assessed isometrically. The daily strengthening program included two exercises performed bilaterally (standing hip abduction with elastic resistance, standing hip extension with elastic resistance). At baseline, hip abduction strength was ~29% lower among runners with PFPS than control runners (pain-free). Runners with PFPS had lower stride-to-stride knee-joint variability than controls. After the 3-week daily strengthening program, hip abduction strength increased ~33% as did the stride-to-stride knee-joint variability; which made their strength and variability similar to the control group. The runners with PFPS also had less knee pain (~43% less) after the strengthening program.
This study is a nice validation of biomechanical and neuromuscular alterations many of us have seen and attempted to address in the clinical setting. The three-week strengthening program increased strength, decreased pain, and improved stride-to-stride knee-joint variability. The study did not see any differences in peak dynamic knee valgus (which is related to the hip adduction) between runners with and without PFPS. It would be interesting to see this study replicated with other functional tasks or as a randomized controlled trial with a larger sample size. Regardless, this seems to be nice evidence supporting the inclusion of hip abduction strengthening in our rehabilitation programs when we have a patient with anterior knee pain and hip abduction weakness. A part of me also wonders how the addition of hip external rotation strengthening, neuromuscular training (teaching an individual proper squatting mechanics), and balance training to their program would help. Do they add additional benefits or is a simple two exercise program sufficient for these patients? Have you started to include hip strengthening to your rehabilitation plans for patients with anterior knee pain?
Written by: Jeffrey B. Driban
Reviewed by: Stephen Thomas