Background: Delayed onset of vastus medialis obliquus activity has been described in patellofemoral pain patients. No prospective study investigating the development of patellofemoral pain has tested the onset timing of electromyographic activity of the vastus medialis obliquus and vastus lateralis muscles during a functional task.
Hypothesis: Before the development of patellofemoral pain, subjects demonstrate an altered firing order of the vastus medialis obliquus and vastus lateralis muscles compared with healthy subjects during a functional task.
Study design: Cohort study; Level of evidence, 2.
Methods: The onset of vastus medialis obliquus and vastus lateralis activity was measured with surface electromyography during a functional task (rocking back on the heels) in 79 healthy subjects subsequently submitted to a 6-week strenuous basic military training. Afterward, these subjects were reassessed.
Results: Thirty-two percent of the recruits developed patellofemoral pain during training. Patellofemoral pain subjects demonstrated a significant delay of onset of vastus medialis obliquus electromyographic activity compared with the healthy controls (P = .023), even before basic military training. In healthy subjects, no significant differences in electromyographic activity onset of the vastus medialis obliquus compared with the vastus lateralis could be identified before and after basic military training. A significant (P < .001) delay could be demonstrated in the patellofemoral pain group after basic military training. A binary logistic regression could be constructed wherein the onset of the electromyographic activity of the vastus medialis obliquus and vastus lateralis was withheld in the model. The most optimal cutoff value, which is based on the receiver operating characteristic curve, is a timing difference of -0.67 milliseconds (vastus medialis obliquus - vastus lateralis). The area under the receiver operating characteristic curve is considered as fair (0.68).
Conclusion: Delayed onset of electromyographic activity of the vastus medialis obliquus-vastus lateralis is one of the contributing risk factors to the development of patellofemoral pain.