Difference in changes of surface EMG during low-level static contraction between monopolar and bipolar lead.

The changes in surface electromyogram(EMG) during sustained contractions were compared between monopolar and bipolar lead. Ten subjects performed static elbow flexion at contraction levels of 10 and 40% of maximum voluntary contraction(MVC). Seven subjects performed static knee extension at contraction levels of 3, 8, and 15%MVC. Surface EMGs were recorded from the biceps brachii, vastus lateralis, rectus femoris and vastus medialis in monopolar and bipolar lead. Mean amplitude of EMG(AEMG) and frequency spectra of EMG were calculated. AEMG was larger in monopolar EMG than in bipolar EMG except 3 cases. AEMG changed more abruptly in bipolar EMG than in monopolar EMG. Relative power in low frequencies increased(slowing) during contractions in all cases of monopolar EMG. But bipolar EMG sometimes did not show slowing. Decrease of relative power in low frequencies(quickening) was sometimes seen during contractions. This quickening was not marked in monopolar EMG because of its large slowing. This study shows that obscure EMG slowing during a low-level fatiguing contraction is related to the lead methods. Led amplitude was compared between the lead methods using a simple model. The sphere that EMG reflects(pickup sphere) is larger in monopolar EMG than in bipolar EMG. This difference in the pickup sphere is thought not to be the main reason for the difference in the slowing.