Vastus Lateralis Motor Unit Firing Rate Is Higher in Women With Patellofemoral Pain.

OBJECTIVE: To compare neural drive, determined from motor unit firing rate, in the vastus medialis and lateralis in women with and without patellofemoral pain.

DESIGN: Cross-sectional study.

SETTING: University research laboratory.

PARTICIPANTS: Women (N=56) 19 to 35 years of age, including 36 with patellofemoral pain and 20 controls.

INTERVENTIONS: Not applicable.

MAIN OUTCOME MEASURES: Participants sustained an isometric knee extension contraction at 10% of their maximal voluntary effort for 70 seconds. Motor units (N=414) were identified using high-density surface electromyography. Average firing rate was calculated between 5 and 35 seconds after recruitment for each motor unit. Initial firing rate was the inverse of the first 3 motor unit interspike intervals.

RESULTS: In control participants, vastus medialis motor units discharged at higher rates than vastus lateralis motor units (P=.001). This was not observed in women with patellofemoral pain (P=.78) because of a higher discharge rate of vastus lateralis compared with control participants (P=.002). No between-group differences were observed for vastus medialis (P=.93). Similar results were obtained for the initial motor unit firing rate.

CONCLUSIONS: These findings suggest that women 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.