Immediate increases in quadriceps corticomotor excitability during an electromyography biofeedback intervention

Abstract

The purpose of the study was to determine the effects of EMG-BF on vastus lateralis corticomotor excitability, measured via motor evoked potential (MEP) amplitudes elicited using Transcranial Magnetic Stimulation (TMS) during a maximal voluntary isometric contraction (MVIC). We also determined the effect of EMG-BF on isometric knee extensor strength. Fifteen healthy participants volunteered for this crossover study with two sessions held one-week apart. Participants were randomly assigned to condition order, during which five intervention MVICs were performed with or without EMG-BF. MEP amplitudes were collected with TMS during five knee extension contractions (5% of MVIC) at baseline and again during intervention MVICs within each session. During the control condition, participants were instructed to perform the same number of MVICs without any EMG-BF. Percent change scores were used to calculate the change in peak-to-peak MEP amplitudes that occurred during EMG-BF and Control MVICs compared to the baseline MEPs. Peak knee extension torque was recorded during MVICs prior to TMS for each condition. EMG-BF produced significantly increased MEP change scores and significantly greater torque than the control condition. The results of the current study suggest that EMG-BF may be a viable clinical method for targeting corticomotor excitability.

Introduction

Biofeedback is a common clinical modality used to provide real-time information regarding a physiological event or series of events that would typically not be perceived by the user (Prinsloo et al., 2014, Mirelman et al., 2011, Wood and Kipp, 2014, Crowell et al., 2010). Surface electromyographic biofeedback (EMG-BF) externally focuses the attention of the user away from the exclusive attempt to contract the intended muscle, and toward an outward cue that represents the underlying muscle activation (Giggins et al., 2013, McNevin et al., 2003). Shifting an individual’s focus away from the muscle contraction itself, toward an external cue, is hypothesized to improve acquisition and retention of motor skills (McNevin et al., 2003, Pascua et al., 2015). One possible explanation for improved muscle function following the use of EMG-BF is the increase in the excitability from descending neural pathways associated with brain centers involved with voluntary muscle activation.

There is not overwhelming evidence that augmenting resistance training with EMG-BF for the purpose of improving muscle activation will benefit strength development in healthy participants (Lepley et al., 2012). The strongest effects for strength development following the augmentation of EMG-BF with therapeutic exercise has been reported in patients with knee osteoarthritis, anterior knee pain, patellofemoral pain and healthy participants (Lepley et al., 2012). EMG-BF may be beneficial for improving strength in patients with knee pathology, as EMG-BF may specifically target the neuromuscular activation deficits that perpetuate muscle weakness in patients with muscle inhibition (Hart et al., 2010). Muscle strength development is predicted by an increase in voluntary quadriceps activation in patients with knee osteoarthritis (Pietrosimone and Saliba, 2012). Previous hypotheses have attributed changes in voluntary quadriceps activation following knee injury to alterations in spinal reflex excitability,(Hopkins and Ingersoll, 2000) yet changes in corticomotor quadriceps excitability have recently been demonstrated following knee joint injury (Pietrosimone et al., 2012a, Pietrosimone et al., in press, Heroux and Tremblay, 2006, On et al., 2004). The majority of interventions developed to increase voluntary activation following knee injury have focused on targeting spinal reflexive pathways and little research has been conducted to target activation of descending corticomotor pathways with disinhibitory interventions (Harkey et al., 2014). While EMG-BF has been found to improve strength in patients with anterior knee pain and osteoarthritis, there is a gap in our understanding of the neuromuscular mechanisms that play a role in how EMG-BF improves strength in specific patient populations. Knowledge of the effect that EMG-BF has on corticomotor excitability pathways would be important for the future development and optimization of treatments that target corticomotor excitability. Furthermore, elucidating a mechanism by which EMG-BF elicits strength gains may help predict specific patients that respond to EMG-BF.

Therefore the purpose of the current study was to determine the effects of EMG-BF on vastus lateralis corticomotor excitability, which was measured via motor evoked potential (MEP) amplitudes elicited with TMS during a maximal voluntary isometric contraction (MVIC) in healthy individuals. Understanding if corticomotor excitability pathways are immediately affected by EMG-BF in healthy participants will provide important evidence needed to target specific patient populations that would benefit most from an EMG-BF intervention. We also determined the effect of EMG-BF on isometric knee extensor strength in healthy individuals. We hypothesized that EMG-BF would increase vastus lateralis corticomotor excitability and knee extensor strength measured during a MVIC compared to a condition during which no EMG-BF was provided and MVICs were performed.