Does the motor unit synchronization induced by vibration enhance maximal voluntary isometric contraction force? A randomized controlled double-blind trial.

Objectives: Motor unit synchronization has been proposed as a potential mechanism underlying muscle strength gains for vibration training, but it has yet to be definitely demonstrated. Aim of this study was to determine whether motor unit synchronization induced by vibration has an effect on isometric muscle strength. Methods: Thirty-six healthy volunteers were randomized into two groups: the vibration and the control (sham vibration) groups. Two sets of test measurements and vibration resistance training between the two sets were applied to the right wrist flexors. The maximal voluntary isometric contraction force. and flexor carpi radialis EMG activity were recorded in the first (without vibratory stimulation) and the second (with vibratory stimulation) set. Results: There was no difference in the normalized peak force between the first and the second set in the vibration group (p=0.554). Motor units fired with maximal voluntary isometric contraction synchronized at the vibration frequency (25 Hz) during vibration in all participants of the vibration group. Conclusion: The present study indicates that vibration-induced motor unit synchronization does not have a significant effect on the maximal voluntary isometric contraction force.