that it would be rather substantial.Although many theories exist to explain the cause of PFPS, improper activation of the quadriceps muscle remains a commonly proposed mechanism. It has been suggested that delayed onset timing and reduced activation magnitude of the vastus medialis oblique (VMO) with re-P atellofemoral pain syndrome (PFPS) is one of the most common conditions seen in orthopaedic and sports physical therapy. Of all individuals with knee injuries treated in a sports medicine clinic, 25% were diagnosed with PFPS.14 In addition, the incidence rate of PFPS in the general population has been reported to be between 10% and 25%.21,32 While the annual cost of treating PFPS has not been reported, such a high prevalence would suggest t Study deSign: A case control study, with single observation.t ObjectiveS: To compare the onset timing and activation of the vastus medialis oblique (VMO) and vastus lateralis (VL) between subjects with and without patellofemoral pain syndrome (PFPS) at various step heights.t backgrOund: It has been theorized that delayed or reduced VMO activity relative to the VL contributes to lateral patellar tracking and PFPS. However, conflicting evidence exists in the literature regarding this proposed mechanism. The lack of agreement among studies may be attributed to inconsistent knee flexion angles used in previous studies.t MethOdS and MeaSureS: Twenty subjects with PFPS (mean 6 SD age, 29.5 6 10 years) and 20 control subjects (mean 6 SD age, 25.4 6 3.1 years) ascended 5 different step heights, while knee kinematics and quadriceps EMG data were collected. Knee flexion angle at foot-step contact, VMO-VL onset timing, and VMO/VL activation ratios were analyzed between groups and step heights using 2-factor analyses of variance (ANOVAs) with repeated measures (a = .05).t reSultS: Individuals with PFPS demonstrated 4.7° (P = .038) more knee flexion at foot-step contact than control subjects. Despite greater knee flexion with increased step height (P,.001), no differences in onset timing or activation magnitude ratio were present between groups or across step heights. However, individuals with PFPS displayed a significantly increased activation duration ratio compared to the control group (P = .043).t cOncluSiOn: Quadriceps onset timing and activation magnitude during stair ascent was similar between individuals with and without PFPS, regardless of step height. Thus, the results of this study are in agreement with evidence indicating no difference in VMO-VL timing and VMO/VL activation magnitude ratio between individuals with and without PFPS.
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