Abstract
PurposeNeuromuscular electrical stimulation (NMES) superimposed on voluntary muscle contraction has been recently shown as an innovative training modality within sport and rehabilitation, but its effects on the neuromuscular system are still unclear. The aim of this study was to investigate acute responses in spinal excitability, as measured by the Hoffmann (H) reflex, and in maximal voluntary contraction (MVIC) following NMES superimposed to voluntary isometric contractions (NMES + ISO) compared to passive NMES only and to voluntary isometric contractions only (ISO).MethodFifteen young adults were required to maintain an ankle plantar-flexor torque of 20% MVC for 20 repetitions during each experimental condition (NMES + ISO, NMES and ISO). Surface electromyography was used to record peak-to-peak H-reflex and motor waves following percutaneous stimulation of the posterior tibial nerve in the dominant limb. An isokinetic dynamometer was used to assess maximal voluntary contraction output of the ankle plantar flexor muscles.ResultsH-reflex amplitude was increased by 4.5% after the NMES + ISO condition (p < 0.05), while passive NMES and ISO conditions showed a decrease by 7.8% (p < 0.05) and no change in reflex responses, respectively. There was no change in amplitude of maximal motor wave and in MVIC torque during each experimental condition.ConclusionThe reported facilitation of spinal excitability following NMES + ISO could be due to a combination of greater motor neuronal and corticospinal excitability, thus suggesting that NMES superimposed onto isometric voluntary contractions may provide a more effective neuromuscular stimulus and, hence, training modality compared to NMES alone.
Highlights
Neuromuscular electrical stimulation (NMES) consists of intermittent electrical stimuli applied to one or more superficial skeletal muscles (Botter et al 2011) to preserve muscle mass and contractile function in either healthy or injured individuals (Gibson et al 1988; Caggiano et al 1994; Bax et al 2005; Sheffler and Chae 2007; Vanderthommen and Duchateau 2007)
Further analysis indicated a significant effect of time on the normalized H-reflex amplitude for the NMES (F = 4.879, ηp2 = 0.258, p < 0.05) and NMES + isometric contractions only (ISO) (F = 6.526, ηp2 = 0.318, p < 0.05) conditions
On the contrary, H-reflex amplitude significantly increased on average by 4.5% in the NMES + ISO condition compared to baseline
Summary
Neuromuscular electrical stimulation (NMES) consists of intermittent electrical stimuli applied to one or more superficial skeletal muscles (Botter et al 2011) to preserve muscle mass and contractile function in either healthy or injured individuals (Gibson et al 1988; Caggiano et al 1994; Bax et al 2005; Sheffler and Chae 2007; Vanderthommen and Duchateau 2007). The neurophysiological mechanisms underpinning motor unit recruitment induced by NMES are still unclear, with some authors suggesting that NMES leads to spatially fixed (Vanderthommen et al 2000), non-selective (Adams et al 1993) and largely incomplete motor unit recruitment (Vanderthommen et al 2003; Gregory and Bickel 2005). It has been shown that NMES induced motor unit recruitment at low intensity involves initially large high-threshold motor units, which are constituted of type II fast-twitch fibres generally located in the superficial regions of the skeletal muscles (Bickel et al 2011). Results from previous neurophysiological studies suggest that a plausible reason underpinning the effectiveness of NMES+ superimposed to voluntary movement might involve an increase in spinal excitability due to plastic changes in Ia reflex pathways, which could allow for a more comprehensive motor unit recruitment and force-generating capabilities compared to NMES alone and/or voluntary movement alone
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