Contralateral effect of short-duration unilateral neuromuscular electrical stimulation and focal vibration in healthy subjects

BACKGROUND: The "contralateral effect" phenomenon refers to the strength gain in the opposite, untrained homonymous muscle following unilateral training. Previous studies showed that neuromuscular electrical stimulation (NMES) of the right quadriceps facilitated maximal voluntary strength and efferent neural drive of the left knee extensors, while no previous study investigated the contralateral effect elicited by focal muscle vibration. AIM: To investigate whether quadriceps NMES and focal vibration, when applied unilaterally, have the same potential to enhance the contralateral muscle strength and the associated neural drive. DESIGN: Randomised controlled experimental study. SETTING: University laboratory. POPULATION: Healthy subjects. METHODS: Subjects completed several maximal voluntary contractions (MVCs) of the left quadriceps (tested muscle) while the right quadriceps (treated muscle) received no conditioning stimulation (control condition), NMES or focal vibration. Paired supramaximal stimuli were delivered to the left quadriceps during and immediately after the MVCs to assess voluntary activation. The EMG activity of vastus lateralis, vastus medialis, and rectus femoris muscles of the left quadriceps was also concomitantly recorded. RESULTS: MVC torque and voluntary activation of the left quadriceps increased during contralateral NMES and vibration. A remarkable inter-individual variability was observed in the contralateral effect of NMES and vibration. In fact, MVC and voluntary activation increases were particularly evident in subjects "responders" to both treatments (who showed NMES-elicited increases in MVC and voluntary activation of 22.5% and 15.8%, respectively, and vibration-elicited increases of 13.1% and 10.7%, respectively). Moreover, we found that the increases in voluntary activation and EMG activity elicited by NMES were higher than those elicited by focal vibration. We also found that voluntary activation increases were higher in subjects presenting lower baseline levels of voluntary activation. CONCLUSIONS: The short-duration unilateral application of quadriceps NMES and focal vibration increased MVC torque and efferent neural drive of the contralateral homologous muscle in healthy subjects. CLINICAL REHABILITATION IMPACT: As the two physical therapy modalities can be useful to maximize motor unit recruitment contralaterally to the side of application, they could be incorporated in rehabilitation protocols when unilateral voluntary contractions are uncomfortable, painful or not feasible.