Vastus lateralis motor unit firing rate is higher in females with patellofemoral pain

Publication date: Available online 10 February 2018
Source:Archives of Physical Medicine and Rehabilitation
Author(s): Alessio Gallina, Michael A. Hunt, Paul Hodges, S. Jayne Garland
ObjectiveTo compare neural drive, determined from motor unit firing rate, to the vastus medialis and lateralis in females with and without patellofemoral pain.DesignCross-sectional study.SettingUniversity research laboratory.ParticipantsFemales (N=56) 19-35 years old, 36 with patellofemoral pain and 20 controls. Interventions: Not applicable.Main Outcome Measure(s)Participants sustained an isometric knee extension contraction at 10% of their maximal voluntary effort for 70s. Motor units (N=414) were identified using high-density surface electromyography. Average firing rate was calculated between 5 and 35s after recruitment for each motor unit. Initial firing rate was the inverse of the first three motor unit inter-spike intervals.ResultsIn control participants, vastus medialis motor units discharged at higher rates than vastus lateralis (p=0.001). This was not observed in females with patellofemoral pain (p=0.78) due to a higher discharge rate of vastus lateralis compared to control participants (p=0.002). No between-group differences were observed for vastus medialis (p=0.93). Similar results were obtained for the initial motor unit firing rate.ConclusionsThese findings suggest that females with patellofemoral pain have a higher neural drive to vastus lateralis but not vastus medialis, which may be a contributor of the altered patellar kinematics observed in some studies. The different neural drive may be an adaptation to patellofemoral pain, possibly to compensate for decreased quadriceps force production, or a precursor of patellofemoral pain.



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