The effects of the motion mode on surface electromyograms during dynamic contractions

Abstract

Surface electromyograms (EMG) during dynamic contractions of elbow flexors were studied in reference to muscle contraction speed, the change of muscle length, contraction strength and the change of muscle contraction strength. Subjects flexed their arms at elbow joint angle of 40-150 degrees with loads 1, 3 and 6kg at the flexion-extension period of 2, 4 and 8sec. One flexion was divided into five phases by the elbow joint angle ; shortening contraction with the start of contraction (phase1), shortening contraction without the start of contraction (phase2), the period when shortening contraction changes to lengthening contraction (phase3), Iengthening contraction without the end of contraction (phase4) and lengthening contraction with the end of contraction (phase5). Surface EMG of muscle biceps brachii was integrated and the power spectrum was calculated. The value of integrated EMG increased in parallel with the speed of contraction. The degree of the increment was larger in shortening contractions than in lengthening contractions. This result by integrated EMG was explained by the energy loss caused by muscle shortening. Relative power in low frequency was more at fast contraction, Iengthening contraction, Iight strength contraction and contraction when the muscle is lengthened. These effects were discussed from the viewpoint of the synchronization of motor unit activity.