Anodal Transcranial Direct Current Stimulation Alters Elbow Flexor Muscle Recruitment Strategies

Abstract

Background

Transcranial direct current stimulation (tDCS) is known to reliably alter motor cortical excitability in a polarity dependent fashion such that anodal stimulation increases cortical excitability and cathodal stimulation inhibits cortical excitability. However, the effect of tDCS on agonist and antagonist volitional muscle activation is currently not known.

Objective

This study investigated the effect of motor cortical anodal tDCS on EMG/force relationships of biceps brachii (agonist) and triceps brachii (antagonist) using surface electromyography (EMG).

Methods

Eighteen neurologically intact adults (9 tDCS and 9 controls) participated in this study. EMG/force relationships were established by having subjects perform submaximal isometric contractions at several force levels (12.5%, 25%, 37.5%, and 50% of maximum).

Results

Results showed that anodal tDCS significantly affected the EMG/force relationship of the biceps brachii muscle. Specifically, anodal tDCS increased the magnitude of biceps brachii activation at 37.5% and 50% of maximum. Anodal tDCS also resulted in an increase in the peak force and EMG values during maximal contractions as compared to the control condition. EMG analyses of other elbow muscles indicated that the increase in biceps brachii activation after anodal tDCS was not related to alterations in synergistic or antagonistic muscle activity.

Conclusions

Our results indicate that anodal tDCS significantly affects the voluntary EMG/force relationship of the agonist muscles without altering the coactivation of the antagonistic muscles. The most likely explanation for the observed greater EMG per unit force after anodal tDCS appears to be related to alterations in motor unit recruitment strategies.

Introduction

Noninvasive brain stimulation is one of the most promising and emerging techniques in the field of neurorehabilitation [1], [2], [3]. The rationale for using noninvasive brain stimulation in rehabilitation is based on its ability to safely modulate brain excitability and functional plasticity, as well as its ability to facilitate motor learning when combined with a motor task [4]. Transcranial direct current stimulation (tDCS) is one form of noninvasive brain stimulation technique that is gaining rapid popularity among neuroscientists due to its low cost, versatility, and portability [5]. In tDCS, weak electric currents are delivered to the brain through a small battery powered device. The electric current flow induces neuromodulation through its ability to alter the neuron's resting membrane potential [6]. The direction of neuromodulation depends on the polarity of the electrical stimulation such that anodal stimulation increases cortical excitability and cathodal stimulation decreases excitability [7].

One of the proposed reasons for the neuromodulatory effects of tDCS is the long term potentiation (LTP) and long term depression (LTD)-like mechanisms of synaptic plasticity [8], [9]. Because memory and learning is considered to be mediated by LTP and LTD, there has been an upsurge of interest in studying the effects of tDCS on motor learning and memory consolidation [8]. The outcomes of these studies suggest that anodal tDCS when combined with motor training can enhance motor learning and movement control [10], [11], [12], [13], [14]. There is also some evidence to suggest that anodal tDCS facilitates memory consolidation and retrieval [15]. These observations have strong clinical implications as motor recovery after a neurological insult, such as stroke or traumatic brain injury, largely involves relearning some of the lost skills through motor practice/training [16], [17]. As a result, significant efforts have been placed in the past few years to evaluate the potential of tDCS as a therapeutic adjuvant for a wide range of neuromotor disorders. Emerging evidence from these studies suggests that tDCS may act as a powerful adjuvant for motor learning and recovery of function in individuals with neurological disorders [18], [19], [20], [21].

The effect of tDCS on the force generating capacity of the muscles has also been recently investigated [22], [23], [24], [25], [26], [27]. The findings from these studies indicate that a single bout of anodal tDCS transiently increases the force generating capacity of the hand and leg muscles. This suggests that anodal tDCS alters muscle recruitment strategies (e.g., increased neural drive) as the influences from other factors (e.g., morphological or architectural changes) that have the potential to alter a muscle's force generating capacity are highly unlikely to change in such short time intervals. While a number of studies have evaluated cortical electrophysiological changes after tDCS, evidence related to the effect of tDCS on volitional agonist and antagonist muscle recruitment strategies is lacking.

Therefore, the purpose of this study was to evaluate the effect of anodal tDCS on the EMG/force relationship in the muscles of the upper arm. We measured biceps (agonist) and triceps brachii (antagonist) muscle activation patterns across a range of force levels (12.5%–50% of maximum). We hypothesized that if anodal tDCS altered muscle recruitment strategies, we should observe a significant change in the EMG/force relationship of both the agonist and antagonist muscles.