The test-retest reliability of the onset of concentric and eccentric vastus medialis obliquus and vastus lateralis activity in a stair stepping task

Study design: Test–retest reliability using a repeated measures design. Objectives: The aim of this study was to evaluate the test–retest reliability of measurement of onsets of electromyographic (EMG) activity of vastus medialis oblique (VMO) and vastus lateralis (VL) during a stair stepping task. Background: Differences in the timing of onset of VMO and VL have been hypothesized to contribute to patellofemoral pain syndrome. However, no studies have investigated the reliability of the EMG onset of VL and VMO using a direct measure of EMG onset timing. Methods and measures: Ten asymptomatic subjects were assessed by the same examiner on two occasions one week apart. The relative difference in the timing of onset of EMG activity of VMO and VL was quantified by subtracting VMO EMG onset from that of VL. Results: The difference in EMG onsets during the concentric task (step up) had an intraclass correlation coefficient (ICC) of 0·91 with a standard error of measurement of 6·20 ms (95% CL 12·20 ms). The difference in EMG onsets during the eccentric task (step down) had an ICC of 0·96 with a standard error of measurement of 5·90 ms (95% CL 11·56 ms). Conclusions: The results indicate that in a normal population a difference of greater than 12·20 ms in the concentric and 11·56 ms in the eccentric phase of stair stepping would be required to demonstrate a significant difference in EMG onsets of VMO and VL between groups. Further research is required to determine the degree of measurement error in a population with specific pathologies