Electromechanical Delay Of The Vastus Medialis During Isometric Strength Training

Abstract

PURPOSE The present investigation examined electromechanical delay (EMD) in the vastus medialis (VM) following eight weeks of isometric leg extension strength training. METHODS Thirteen adult male volunteers (age19-29) were randomly assigned to a training (TRN) group (n = 6) or a control (CTL) group (n = 7). Electromyographic (EMG) and mechanomyographic (MMG) signals were recorded at the leg extension training angle of 60° of leg flexion. Measurements were collected three times during the study (weeks 0, 4, and 8). The time delay between the onset of EMG activity and MMG activity (EMG-MMG) was recorded along with the time delay between the onset of MMG activity and torque production (MMG-torque). The TRN subjects performed unilateral isometric muscle actions with the non-dominant limb at 60° of leg flexion. The first two weeks, subjects performed two sets of muscle actions at 80% of their maximal isometric PT at 60°. For the remaining six weeks, subjects performed three sets of muscle actions at 80% of their maximal isometric peak torque (PT). Maximal PT was reassessed at the end of the fourth week of training to adjust the training torque values. Each set consisted of ten repetitions lasting six seconds each. The subjects trained three times a week. RESULTS The results indicated significant (p < 0.05) increases in PT for the trained limb of the TRN group between week 0 and week 4 and between week 0 and week 8 of 9.8% and 14.8% respectively. There were no significant changes (p > 0.05) in EMG-MMG delay or MMG-torque delay between the TRN and CTL group and no significant changes (p > 0.05) in EMG-MMG delay or MMG-torque delay found within the TRN group across the training period. CONCLUSION In the present study, delay times between EMG-MMG and MMG-torque in the VM were not significantly altered by eight weeks of isometric training. There was a significant increase in PT following eight weeks of isometric training without significant changes in EMD. It can be speculated that torque production was increased without significant changes in: 1) propagation of action potentials on muscle membrane, 2) excitation/contraction coupling, and/or 3) stretching of the series elastic components by the contractile component since these factors have been previously reported to reflect changes in EMD.