Cross education effect on EMG observed in unilateral electromyostimulation and voluntary strength training

Abstract

This study investigated the effects of four weeks of unilateral electromyostimulation (EMS) versus voluntary isometric strength training (VIM), on knee extension strength of contralateral limb in thirty previously untrained young adults. Ten subjects were assigned into each of the EMS, VIM and Control groups. The training groups performed or evoked 40 isometric knee extensions, at the intensity of 65% of maximum voluntary contraction (MVC) force, three times per week. Pre and post the four weeks training, both the trained and contralateral limbs were tested for isometric MVC. Surface EMG was recorded from the rectus femoris, vastus lateralis and vastus medialis muscles during MVC. The results showed that the EMS and VIM training had a similar effect on strength improvement in the trained limb (21.1% in EMS, 24.5% in VIM, both P\u3c0.05. -0.4% in Control, non significant (n.s.)). Both types of training induced significant cross-education effect on isometric MVC of the contralateral limb (21.1% in EMS, 21.4% in VIM, both P\u3c0.05. 2.4% in Control, n.s.). The analysis of integrated EMG (iEMG, sum of three muscles) showed a trend of increment in both trained limb (16.6% in EMS, 29.8% in VIM, -7.0% in Control, all n.s.), and the contraleteral limb in the training groups (10.2% in EMS, 21.6% in VIM, -7.8% in Control, all n.s.). It appeared that neural adaptation, as reflected in the increased EMG, could be attributed at least partly to the improved the voluntary strength in both trained and contralateral limbs. However, the mechanism underlying the cross-education effect and EMG variation in response to the EMS training is not clear and requires further study