Systems Physiology – Neuromuscular and Skeletal INFLUENCE OF ELECTRODE ORIENTATION ON ELECTROMYOGRAPHIC FATIGUE INDICES OF THE VASTUS LATERALIS

Influence of Electrode Orientation on Electromyographic Fatigue Indices of the Vastus Lateralis. JOSEPH P. WEIR, KELLY P. MAHONEY, KATHERINE G. HAAN and AMANDA DAVIS. JEPonline 1999, 2(3):15-20. Monitoring of muscle fatigue in the leg extensors can provide valuable information regarding exercise and rehabilitation. Surface electromyography (EMG) can provide objective measures for monitoring fatigue. However, factors such as electrode orientation relative to muscle fiber direction can potentially affect EMG fatigue indices. That is, bipolar electrodes should be aligned parallel to the muscle fibers, but in pennated muscles alignment of electrodes parallel to the muscle fibers is problematic. The purpose of this study was to examine the effect of alterations in electrode orientation on EMG changes during fatiguing isometric contractions. Electrodes were placed parallel and perpendicular to the long axis of the vastus lateralis of thirty subjects who performed a quadriceps contraction for 60 seconds at 50% of their maximal isometric contraction. The data were analyzed for changes in median power frequency (MPF) and integrated EMG (iEMG) over time. Slopes from individual regression analyses, normalized to the subjects’ maximal values, were the dependent variables (units = %max.sec). Paired t-tests indicated that the fatigue slopes for MPF (parallel = -0.13 ± 0.13; perpendicular = -0.21 ± 0.20 ; t = 1.75, p=.09) and iEMG (parallel = 0.22 ± 0.17; perpendicular = 0.25 ± 0.19; t = 1.52, p=.14) were not significantly different between orientations. However, the correlation analysis for MPF slopes showed no significant relationship between parallel and perpendicular measures (r= 0.05, SEE = 0.29). In contrast, the correlation analysis for iEMG slopes indicated a significant relationship between orientations (r=.77, SEE = 0.13, p<.05). These findings suggest that electrode orientation can significantly affect frequency domain indices of fatigue while amplitude measures are less sensitive to alterations in electrode orientation. In pennated muscles such as the vastus lateralis, use of amplitude measures of fatigue may be preferable.