Exp Brain Res DOI 10.1007/s00221-014-4016-8

Research Article

Anodal‑tDCS applied during unilateral strength training increases strength and corticospinal excitability in the untrained homologous muscle Ashlee M. Hendy · Dawson J. Kidgell 

Received: 15 January 2014 / Accepted: 10 June 2014 © Springer-Verlag Berlin Heidelberg 2014

Abstract  Evidence suggests that the cross-transfer of strength following unilateral training may be modulated by increased corticospinal excitability of the ipsilateral primary motor cortex, due to cross-activation. Anodal-tDCS (a-tDCS) has been shown to acutely increase corticospinal excitability and motor performance, which may enhance this process. Therefore, we sought to examine changes in neural activation and strength of the untrained limb following the application of a-tDCS during a single unilateral strength training session. Ten participants underwent three conditions in a randomized, double-blinded crossover design: (1) strength training + a-tDCS, (2) strength training + sham-tDCS and (3) a-tDCS alone. a-tDCS was applied for 20 min at 2 mA over the right motor cortex. Unilateral strength training of the right wrist involved 4 × 6 wrist extensions at 70 % of maximum. Outcome measures included maximal voluntary strength, corticospinal excitability, short-interval intracortical inhibition, and crossactivation. We observed a significant increase in strength of the untrained wrist (5.27 %), a decrease in short-interval intracortical inhibition (−13.49 %), and an increase in cross-activation (15.71 %) when strength training was performed with a-tDCS, but not following strength training with sham-tDCS, or tDCS alone. Corticospinal excitability of the untrained wrist increased significantly following both strength training with a-tDCS (17.29 %), and a-tDCS alone (15.15 %), but not following strength training with sham-tDCS. These findings suggest that a single session of a-tDCS combined with unilateral strength training of the

A. M. Hendy · D. J. Kidgell (*)  Centre for Physical Activity and Nutrition Research, School of Exercise and Nutrition Sciences, Deakin University, Melbourne, Australia e-mail: [email protected]

right limb increases maximal strength and cross-activation to the contralateral untrained limb. Keywords  tDCS · Cross-activation · Strength · Cross-transfer · Corticospinal excitability

Introduction Practicing motor tasks with one limb can enhance performance of the same task within the untrained contralateral homologous limb (Munn et al. 2004). This phenomenon has been termed cross-education, or cross-transfer and has been shown to occur following both motor skill training (Lee et al. 2010; Carroll et al. 2008), and strength training (Carroll et al. 2006; Munn et al. 2004; Scripture et al. 1894). While the neural adaptations that underpin the cross-transfer effect remain somewhat unresolved, the potential to utilize the cross-transfer of motor function to enhance rehabilitation following single-limb impairment (such as musculoskeletal injury or stroke) holds significant clinical relevance, warranting further examination (Ruddy and Carson 2013). It has been speculated that cross-transfer may be induced by either ‘cross-activation’ (a spill-over of neural drive from the active to the inactive hemisphere), or ‘bilateral access’ (development of motor engrams that can be accessed by either hemisphere) (Ruddy and Carson 2013; Lee et al. 2010). Both theoretical models, which may not be mutually exclusive, suggest that the ‘untrained’ motor cortex (M1), ipsilateral to the trained limb, plays a critical role in mediating the cross-transfer effect (Ruddy and Carson 2013). Evidence of cross-activation of the untrained, ipsilateral M1 during unilateral motor tasks (previously referred to as ‘motor irradiation’ or ‘motor overflow’) is well documented

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(Carson 2005). Early reports suggested that the magnitude of cross-activation is greatest when the resistance (Cernacek 1961) or effort (Hopf et al. 1974) of the unilateral task is increased, with more recent findings suggesting task complexity (Mayston et al. 1999), the presence of muscular fatigue (Arányi and Rösler 2002), and mode of muscle action (Howatson et al. 2011) may also increase cross-activation. Research utilizing transcranial magnetic stimulation (TMS) has reported increased corticospinal excitability (Hortobágyi et al. 2003; Perez and Cohen 2008; Zijdewind et al. 2006), reduced short-interval intracortical inhibition (SICI) (Perez and Cohen 2008), and reduced interhemispheric inhibition (IHI) (Hortobágyi et al. 2011; Howatson et al. 2011) in the ipsilateral M1 during varying levels of unilateral activity. In support of this, functional magnetic resonance imaging (fMRI) studies have shown increased activity in the ipsilateral M1 during unilateral voluntary isometric contractions (Kobayashi and Pascual-Leone 2003; Van Duinen et al. 2008). The increase in excitability of the ipsilateral corticospinal pathway is likely to be primarily of cortical origin, as cervicomedullary-induced motor evoked potentials (cMEPs) appear to be unaffected by moderate ipsilateral contractions, and the amplitude of the H-reflex is depressed (Hortobágyi et al. 2003; Carson et al. 2004). While these results suggest that interhemispheric facilitation via transcallosal pathways may be responsible for cross-activation, other pathways cannot be ruled out, as a number of patients with callosal agenesis have displayed significant level of cross-activation (Meyer et al. 1995; Ziemann et al. 1999). Several recent studies have used TMS to investigate the function of the ipsilateral M1 following unilateral activity, specifically, ballistic training of intrinsic hand muscles (Lee et al. 2010; Carroll et al. 2008; Hinder et al. 2011, 2013). These studies have provided evidence for increased corticospinal excitability, in the form of increased amplitude of motor evoked potentials (MEPs) in both the trained and untrained (ipsilateral) M1, accompanying the cross-transfer of performance (Lee et al. 2010; Carroll et al. 2008; Hinder et al. 2011, 2013). Gains in corticospinal excitability of the ipsilateral M1 have also been reported following longer term (2–3 weeks) unilateral training of finger tapping tasks (Koeneke et al. 2006), and heavy load strength training of the lower limb, which was also accompanied by a decrease in SICI (Goodwill et al. 2012). Perhaps the strongest evidence to support the involvement of the ipsilateral hemisphere in the cross-transfer of motor performance comes from work utilizing repetitive transcranial magnetic stimulation (rTMS) to create a ‘lesion’ in the ipsilateral M1 (Lee et al. 2010). This study found that when the untrained ipsilateral M1 was disrupted, performance gains in the untrained limb were abolished (Lee et al. 2010). Collectively, these findings highlight the involvement of the M1

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ipsilateral to the trained limb in the cross-limb transfer of both motor skill and strength. Recently, the use of transcranial direct current stimulation (tDCS) has gained popularity as a safe and noninvasive technique that can be utilized to induce plasticity in the M1 (Nitsche et al. 2008). The procedure typically involves lowlevel (1–2 mA) electrical currents delivered to the M1 over the area of interest via saline-soaked electrodes (Nitsche et al. 2008). The orientation of the electrodes and direction of current flow determines the physiological effect of stimulation, with anodal stimulation (a-tDCS) increasing excitability of underlying cortical neurons, and cathodal stimulation (c-tDCS) decreasing excitability (Nitsche and Paulus 2000). The immediate effects of tDCS are believed to be due to changes in membrane polarity, which influences the likelihood of depolarization (Nitsche et al. 2003; Purpura and McMurtry 1965). In contrast, longer lasting changes in corticospinal excitability, which have been reported up to 90 min following stimulation, are believed to be attributed to changes in synaptic efficacy (Nitsche and Paulus 2001; Liebetanz et al. 2002). Evidence has shown that in addition to the modulation of corticospinal excitability reported following tDCS, stimulation also appears to produce transient effects in motor performance, corresponding with changes in excitability (Cogiamanian et al. 2007; Hummel et al. 2005). This observation has generated interest in utilizing a-tDCS to improve motor function and performance in both healthy and motor-impaired individuals. For example, one study examined the effects of a-tDCS delivered at 2 mA for 10 min in healthy individuals, reporting a 20.5 % increase maximal isometric strength following stimulation (Tanaka et al. 2009). In addition, other research has reported a 26 % increase in corticospinal excitability of healthy participants following a-tDCS, with a corresponding 15 % increase in muscular endurance (Cogiamanian et al. 2007). Functional strength and motor performance increases have also been reported in Parkinson’s disease and stroke patients following a-tDCS of the effected M1 (Tanaka et al. 2011; Fregni et al. 2006), with one study finding a correlation between performance enhancement and neural plasticity (increased corticospinal excitability and decreased SICI) following stimulation (Hummel et al. 2005). Given the evidence for a-tDCS to enhance motor performance and corticospinal excitability following a single session, and the potential for unilateral strength training to cause cross-activation in the ipsilateral M1, the purpose of this study was to investigate the effect of unilateral training combined with a-tDCS over the ipsilateral M1. The primary aim of the study was to determine whether the application of a-tDCS to the ipsilateral M1 would produce greater gains in functional strength of the untrained limb, which would provide potential benefits for enhancing

Exp Brain Res

rehabilitation outcomes following single-limb motor impairment. Additionally, we aimed to quantify any accompanying changes in corticospinal excitability, SICI, and cross-activation of the ipsilateral M1 following the intervention. We hypothesized that a-tDCS would complement the magnitude of cross-activation evoked from unilateral training, thus enhancing neural adaptations and ultimately increasing strength transfer to the untrained limb.

Methods Participants Ten participants (5 males, 5 females, age: 25.9 ± 1.37 years) were selected on a voluntary basis. All participants provided written informed consent prior to participation in the study, which was approved by the Deakin University Human Research Ethics Committee. All experiments were conducted according to the standards established by the Declaration of Helsinki. Sufficient sample size was predetermined with A priori power analysis based on previous data investigating acute strength gains following tDCS in healthy individuals (Tanaka et al. 2009). All participants were right hand dominant, had not participated in strength training for a minimum of 12 months, and were free from any peripheral or neurological impairment. All participants completed the adult safety screening questionnaire to determine their suitability for TMS and tDCS application (Keel et al. 2001). Experimental approach Once recruited, participants attended a familiarization session to introduce testing procedures and minimize the effect of learning. Using a randomized, double-blinded crossover design, each participant completed three conditions with a 1 week washout period between sessions: (1) unilateral strength training of the right extensor carpi radialis (ECR) with ipsilateral anodal-tDCS (ST + a-tDCS); (2) unilateral strength training of the right ECR with ipsilateral sham-tDCS (ST + sham-tDCS) and (3) anodal-tDCS of the right M1 alone (a-tDCS alone). Maximal compound waves (M-waves) were obtained prior to and following each intervention. Single- and paired-pulse TMS was used to assess the after-effects of each condition on function of the left (untrained) ECR, by examining MEPs evoked from the right (ipsilateral) M1, with the order of TMS stimuli (single or paired-pulse) randomized throughout the trials. Dynamic muscle strength of the left (untrained) ECR for each participant was measured prior to and following each condition by completing a one-repetition maximum (1-RM) strength test. A 1-RM test was also performed on the right ECR to determine the training load.

Voluntary strength testing Maximal voluntary dynamic strength of the wrist extensors was determined by a standard unilateral 1-RM test with an adjustable weighted dumbbell. Participants were seated upright in a neutral posture, shoulders relaxed and elbow flexed at 90°, with the forearm pronated and fastened to a weight bench. The wrist was positioned such that the styloid process sat just beyond the edge of the bench, and the relaxed hand hung free. The researcher placed the dumbbell in the participants’ hand, and instructed them to grasp the dumbbell and completely extend the wrist by moving the hand upward. A trial was considered successful when the participant was able to lift the weight from a rested position hanging below the bench, to at least 15° beyond horizontal, measured by an electromagnetic goniometer (3DM-GX2®, Williston, Vermont, USA). The starting weight of the dumbbell was estimated by the researcher, and the weight of the dumbbell was increased in increments of 0.25 or 0.5 kg as appropriate, until the participant could no longer produce a successful trial. Verbal encouragement was provided during all attempts, and each trial was separated by a three min recovery period. Strength training protocol During the ST + a-tDCS and ST + sham-tDCS conditions, participants were required to perform a single session of strength training of the right wrist extensors with a weighted dumbbell. Participants completed 4 sets of 6 wrist extensions with a resistance of 70 % 1-RM, with a 3 min recovery period between sets. The forearm was pronated and rested on a horizontal bench throughout training. Repetition timing was 3 s for concentric phase and 4 s for eccentric phase, guided by an electronic metronome (Kidgell and Pearce 2010; Ackerley et al. 2011). Verbal encouragement was provided throughout the training sessions to encourage maximal efforts. The a-tDCS alone condition required participants to sit quietly without moving the wrists for the 20 min session. Transcranial direct current stimulation of primary motor cortex The ST + a-tDCS and a-tDCS alone conditions received 20 min of a-tDCS delivered with a DC stimulator (NeuroConn, Ilmenau, Germany) at an intensity of 2 mA. Two 25 cm2 (5 × 5 cm) electrodes were soaked in saline solution (0.9 % NaCl) and secured in place over the scalp. Bioelectrical impedance levels were measured by the stimulator throughout the protocol and did not exceed 55 kΩ. The stimulating electrode (anode) was located over the right M1 in the area corresponding with the participants’ left ECR

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a

b

C

EMG A

tDCS

Fig.  1  a Electrode montage for a-tDCS application (during training) ‘A’ represents the anode and ‘C’ represents the cathode. b Diagram of TMS during contralateral maximal voluntary contraction (MVC), used to quantify cross-activation

‘optimal site’ as determined with TMS. The reference electrode (cathode) was placed over the left supraorbital area. Figure 1a provides a pictorial representation of the electrode montage. The ST + sham-tDCS condition followed the same protocol; however, stimulation ceased after 15 s. This procedure has been used previously, with the initial itching sensation providing an accurate pseudo-stimulation effect without inducing significant experimental effects (Hummel et al. 2005). The participants and researchers were blinded to the type of stimulation being applied (anodal or sham) and asked to rate their level of perceived sensation following tDCS on a visual analog scale (VAS) scale marked 1–10. Transcranial magnetic stimulation and electromyography TMS was applied over the right M1 using a BiStim unit attached to two Magstim 2002 stimulators (Magstim Co, Dyfed, UK) to produce MEPs recorded from the left ECR. A figure-eight coil, with an external loop diameter of 9 cm, was held over the right M1 at the optimum scalp position to elicit MEPs in the left ECR. The induced current flowed in a posterior-to-anterior direction. Sites near the estimated center of the ECR were explored to determine the ‘optimal site’ at which the largest MEP amplitude was obtained. To ensure all stimuli were delivered consistently, participants wore a fitted cap marked with a latitude–longitude matrix, positioned with reference to the nasion-inion and interaural lines. All stimuli were delivered during low-level isometric contraction of the wrist extensors, which were performed by maintaining a straight (180°) wrist and fingers. This equated to 5 ± 2 % of maximal root mean squared (rms) electromyography (EMG), with consistent muscle activation confirmed by recording pre-stimulus rmsEMG throughout testing (Hendy and Kidgell 2013). Active motor threshold (AMT) was determined as the minimum stimulus intensity that produced a small MEP (200 μV in 5 out

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of 10 consecutive trials) during isometric contraction of the ECR at 5 ± 2 % of maximal rmsEMG activity. Corticospinal excitability was determined by calculating the average peak-to-peak amplitude of 10 MEPs delivered at a stimulus intensity of 130 % AMT. To determine cross-activation, 10 stimuli were delivered to the right M1 at 130 % AMT during maximal voluntary contraction (MVC) of the right ECR (see Fig. 1b). To quantify SICI, 15 paired-pulse stimuli were delivered with a conditioning stimulus of 80 % AMT, and a test stimulus producing MEP amplitudes equal to 10 % of Mmax, with an inter-stimulus interval of 3 ms. These MEPs, along with 15 single-pulse MEPs obtained by delivering matching test stimuli, were used to calculate the SICI ratio (see “Data analyses” for more detail). Surface EMG activity was recorded from the left ECR muscle using bipolar Ag–AgCl electrodes. These electrodes were placed on the ECR muscle, with an inter-electrode distance of 2 cm. A grounding strap placed around the wrist was used as a common reference for all electrodes. All cables were fastened with tape to prevent movement artifact. The area of electrode placement was shaven to remove fine hair, rubbed with an abrasive skin rasp to remove dead skin, and then cleaned with 70 % isopropyl alcohol. The exact sites were marked with a permanent marker by tracing around the electrode, and this was maintained for the entire 3 week period by both the researcher and participant to ensure consistency of electrode placement relative to the innervation zone. An impedance meter was used to ensure impedance did not exceed 10 kΩ prior to testing. EMG signals were amplified (×1,000), bandpass filtered (high pass at 13 Hz, low pass at 1,000 Hz), digitized online at 2 kHz for 500 ms, recorded and analyzed using PowerLab 4/35 (ADInstruments, Bella Vista, Australia). Maximal compound muscle action potential Direct muscle responses were obtained from the left ECR muscle by supramaximal electrical stimulation (pulse width 200 μs) of the radial nerve under resting conditions (DS7A, Digitimer, UK). An increase in current strength was applied to the radial nerve until there was no further increase observed in the amplitude of the EMG response (Mmax). To ensure maximal responses, the current was increased an additional 20 % and the average Mmax was obtained from five stimuli, with a period of 6–9 s separating each stimulus. Mmax was recorded at baseline and following the removal of tDCS for each condition, to ensure that there were no changes in peripheral muscle excitability that could influence MEP amplitude. Data analyses Pre-stimulus rmsEMG activity was determined in the ECR 100 ms prior to each TMS stimulus during each condition.

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Any pre-stimulus rmsEMG that exceeded 5 ± 2 % maximal rmsEMG were discarded and the trial repeated. The peak-to-peak amplitude of MEPs evoked as a result of stimulation was measured in the ECR muscle contralateral to the cortex being stimulated in the period 10–50 ms after stimulation. MEP amplitudes were analyzed using LabChart 8 software (ADInstruments, Bella Vista, NSW, Australia) after each stimulus was automatically flagged with a cursor, providing peak-to-peak values in μV, which were then normalized to Mmax. Average MEP amplitudes were obtained for each trial for single, paired-pulse and test TMS for each stimulation block separately. SICI was calculated as a ratio by applying the following equation:

SP − PP × 100 SICI = Mmax where: • SP represents the average MEP amplitude from the single-pulse stimuli. • PP represents the average MEP amplitude from the paired-pulse stimuli. Statistical analysis All data were screened with the Shapiro–Wilk test and found to be normally distributed (all P > 0.05), and thus, parametric analyses were performed. A split-plot in timerepeated measures ANOVA was used to calculate the effect of each condition (ST + a-tDCS, ST + sham-tDCS and a-tDCS alone) on voluntary strength and the indices of cortical plasticity (excitability, cross-activation and SICI). Univariate post hoc analysis for each dependent measure followed where significant main effects were found. For all tests, the Huynh–Feldt correction was applied if the assumption of sphericity was violated. Alpha was set at P  0.05). Average rmsEMG as a percentage of maximal voluntary EMG was calculated 100 ms prior to TMS stimulus trigger during all testing sessions (Table 1). There was no significant difference in pre-stimulus rmsEMG between conditions at baseline, and no main effects for time or time by condition interactions were detected (all P > 0.05). Likewise, there was no significant difference between conditions for perception of tDCS as reported on a VAS 1–10 scale (P = 0.72, ST + a-tDCS; 4.11 ± 1.62, ST + shamtDCS; 4.01 ± 0.71, a-tDCS alone; 4.15 ± 1.27). Corticospinal excitability

Results Dynamic strength (1‑RM) All dynamic strength measures refer to changes in wrist extension 1-RM of the left (untrained) ECR and are displayed in Fig. 2. There was no significant difference in 1-RM strength at baseline (P  = 0.47). Following the intervention, there was a main effect for time (P  = 0.02); however, no time by condition interaction was detected (P = 0.32). Mean 1-RM strength increased following an acute bout of unilateral strength training combined with a-tDCS (5.72 %, P = 0.01), but not following strength training with sham-tDCS (−0.41 %, P = 0.75) or a-tDCS alone (−0.52 %, P = 0.86).

Corticospinal excitability was determined by calculating the peak-to-peak amplitude of MEPs elicited at 130 % AMT, and then normalized to Mmax. Mean changes in corticospinal excitability following each condition are displayed in Fig. 3a. There was no significant difference in MEP amplitude at 130 % AMT between conditions at baseline (P  = 0.49). Following the intervention, there was a main effect for time (P = 0.01); however, there was no time by condition interaction (P = 0.33). Corticospinal excitability increased following unilateral strength training combined with a-tDCS (17.2 %, P = 0.01), and following the application of a-tDCS alone (15.18 %, P  = 0.04), but was not changed by strength training combined with sham-tDCS (2.02 %, P = 0.75).

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Table 1  Electrophysiological data (mean ± SD)

Mmax (mV)

a

PRE

POST

14.94 ± 2.66

14.66 ± 2.82

5.01 ± 0.11

5.26 ± 0.24

MEP 10 % Mmax (% SO)

26.80 ± 6.66 32.05 ± 7.91

26.70 ± 6.69 31.95 ± 7.87

Conditioning stimulus (% SO)

21.70 ± 4.71

21.80 ± 4.64

Pre-stimulus rmsEMG (% EMGmax) AMT (% SO)

Mmax maximum compound wave, rmsEMG root means squared electromyography, EMGmax maximal electromyography value, AMT active motor threshold, SO stimulator output, MEP 10 % Mmax motor evoked potential with an amplitude equal to 10 % of the maximum compound wave

b

Short‑interval intracortical inhibition Changes in SICI following each condition are displayed in Fig. 3b. There was no significant difference in SICI between conditions at baseline (P  = 0.16). Again, a main effect for time (P  = 0.02) was detected following the intervention; however, there was no significant time by condition interaction (P = 0.30). A reduction in SICI was observed following unilateral strength training with a-tDCS (−13.94 %, P = 0.01); however, no significant reduction in SICI was observed following strength training with shamtDCS (−1.62 %, P  = 0.57) or a-tDCS alone (−8.56 %, P = 0.63).

c

Cross‑activation MEPs were elicited during maximal contraction of the right (trained) ECR, to determine the effect of activity in the left M1 on corticospinal excitability of the right M1. Mean changes in cross-activation following each condition are displayed in Fig. 3c. There were no significant differences in MEP amplitude for the left ECR during contralateral MVC between conditions at baseline (P = 0.42). Following the intervention, there was a main effect for time (P  = 0.03); however, no time by condition interaction was detected (P  = 0.05). An increase in cross-activation was observed following strength training with a-tDCS (15.71 %, P  = 0.03), but not following strength training with sham-tDCS (−2.60 %, P  = 0.53) or a-tDCS alone (8.26 %, P = 0.15).

Discussion The primary objective of this research was to examine the effects of a single session of unilateral strength training combined with a-tDCS applied to the ipsilateral (untrained) M1 on the strength of the untrained limb. In addition, we sought to quantify changes in corticospinal excitability,

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Fig.  3  a MEP amplitude (130 % AMT) of the left ECR (mean ± SD) expressed as percentage of Mmax. A significant increase in MEP amplitude was observed following the ST + a-tDCS condition (17.29 %, from 14.06 ± 5.70 to 16.50 ± 5.36 % Mmax) and the a-tDCS alone condition (15.15 %, from 15.87 ± 6.12 to 18.28 ± 8.05 % Mmax), but not the ST + sham-tDCS condition (2.02 %, from 16.86 ± 9.14 to 17.20 ± 7.52 % Mmax). *Significant time effect. b Short-interval intracortical inhibition of the right M1 (mean ± SD) expressed as a ratio of unconditioned MEP. A significant decrease in SICI was observed for the ST + a-tDCS condition (−13.94 %, from 7.14 ± 1.53 to 6.15 ± 1.81 % Mmax), but not for the ST + sham-tDCS condition (−1.62 % from 7.43  ± 0.74 to 7.31 ± 0.99 % Mmax) or the a-tDCS alone condition (−8.56 % from 6.89 ± 1.04 to 6.30 ± 0.93 % Mmax). *Significant time effect. c MEP amplitude during contralateral MVC of the right ECR (mean  ± SD), representing cross-activation, expressed as percentage of Mmax. A significant increase in MEP amplitude at 130 % AMT during contralateral MVC was observed for the ST + a-tDCS condition (15.71 %, from 22.69 ± 10.99 to 26.25 ± 11.90 % Mmax) but not for the ST + sham-tDCS condition (−2.6 %, from 24.51 ± 11.68 to 23.88  ± 9.86 % Mmax) or the a-tDCS alone condition (8.26 %, from 2,032 ± 8.46 to 22.00 ± 8.32 % Mmax). *Denotes significant time effect

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intracortical inhibition, and cross-activation of the ipsilateral ‘untrained’ M1 following the intervention, to investigate the role of these parameters in the cross-transfer of strength. Our major finding was that strength of the untrained, left ECR increased following training of the right ECR with a-tDCS of the right M1, but not following training of the right ECR with sham-tDCS or a-tDCS alone. This increase in strength was accompanied by neural modulation in the ipsilateral M1, including an increase in corticospinal excitability, a decrease in SICI, and an increase in cross-activation during maximal contractions of the right ECR. Although no between-condition interactions were demonstrated, the within-time effects warrant some discussion as to the potential role of a-tDCS applied over the ipsilateral M1 in maximizing the cross-transfer of strength. These findings may have implications for rehabilitation following single-limb impairment such as musculoskeletal injury or stroke. Voluntary strength Consistent with our hypothesis, a significant gain in strength (5.27 %) was recorded when strength training and a-tDCS were applied together. Strength training with sham-tDCS was not expected to increase strength of the untrained ECR, as typical cross-education programs require at least 2–3 weeks of training, with no evidence to suggest functional outcomes would occur following a single session. However, it was surprising that a-tDCS alone did not produce an increase in strength of the left ECR. Previous studies have shown significant strength gains following single sessions of a-tDCS (Tanaka et al. 2009, 2011; Hummel 2006). There are several factors that may be responsible for the differences in these results. Firstly, some previous studies reported improved muscular strength following a-tDCS in individuals with impaired function, not healthy individuals (Hummel 2006; Tanaka et al. 2011). Diseased populations are likely to present a greater potential for improvement due to low preexisting performance levels, whereas healthy subjects may be more likely to be limited by a ‘ceiling effect’. Despite this, strength increases have been reported in healthy individuals following a-tDCS (Tanaka et al. 2009). Changes in corticospinal excitability reported following a-tDCS appear similar regardless of the presence of motor impairment, suggesting that performance outcomes should also correspond, and the investigation of the non-dominant limb may reduce the chances of a ceiling effect on performance (Hendy and Kidgell 2013; Tanaka et al. 2009). In addition, it is likely that the use of the 1-RM dynamic strength test is less sensitive than isometric strength measured on devices such as isokinetic dynamometers, which were used in previous research (Tanaka et al. 2009, 2011; Hummel 2006). Furthermore, the

post-intervention values for strength were obtained approximately 10 min following the cessation of a-tDCS, when potential strength gains may have subsided. For example, one study reported a 20.5 % increase in strength during a-tDCS of the lower limb, and while significant gains were still present following 30 min, they appeared to be of a lesser magnitude (Tanaka et al. 2009). Other factors to be considered may include differences in the muscle groups investigated, and differences in intensity and duration of stimulation. Corticospinal excitability Our results indicate that the application of a-tDCS increases corticospinal excitability, which is consistent with previous a-tDCS studies (see Nitsche et al. 2008 for review). An increase in MEP amplitude was observed following both the strength training with a-tDCS, and a-tDCS alone conditions, but not following strength training with sham-tDCS. Although no significant between-condition interaction was present, the within-effects warrant further discussion. The observation that the application of a-tDCS to the ipsilateral M1 during unilateral strength training produced a main effect for both an increase in corticospinal excitability and strength of the contralateral untrained limb is noteworthy, and adds to the current tDCS literature. On this basis, the current findings show some potential for combining unilateral strength training and a-tDCS in short-term training programs to further exploit adaptations in the ipsilateral hemisphere, with the view to producing longer lasting neurological and functional outcomes that may be relevant for rehabilitation purposes. It was somewhat disappointing that the combination of strength training and a-tDCS produced only marginally greater increases in MEP amplitude than a-tDCS alone (17.29 and 15.15 % respectively). We hypothesized that the combined effect of a-tDCS and high intensity contractions of the ipsilateral ECR may further increase corticospinal excitability of the right M1 based upon the increase in crossactivation and decrease in SICI, which may allow disinhibition of corticospinal neurons, increasing the efficacy of descending neural transmission. While the present results indicate that combining a-tDCS and unilateral strength training does not produce a cumulative effect on MEP size following a single session, it does not rule out the potential for such an effect to occur following multiple sessions as part of a strength training program. Cross‑activation We found that cross-activation of the ipsilateral M1 was present both before and after all three interventions, i.e.; when MVCs were performed with the right ECR, MEP

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amplitudes recorded from the left ECR were greater than those recorded while the right ECR remained at rest. Crossactivation occurred in the absence of any change in prestimulus rmsEMG of the left ECR (i.e., no mirror activity was present). This is consistent with previous TMS studies, which have reported greater MEP amplitudes in the inactive muscle during high intensity contractions of the opposite limb, despite comparable or matched pre-stimulus EMG (Hortobágyi et al. 2003; Perez and Cohen 2008; Zijdewind et al. 2006). Of some interest, the magnitude of cross-activation was greater only following a single strength training session of the right ECR performed during a-tDCS of the ipsilateral (right) M1. Although this increase was only a within-effect, it should be noted that when strength training was performed with sham-tDCS, or a-tDCS was performed alone, similar pre-intervention levels of cross-activation remained present, but were not significantly enhanced post training. This was somewhat surprising, as previous research has suggested that cross-activation increases as muscles become fatigued (Arányi and Rösler 2002), which was expected to occur following the high intensity strength training protocol used in this study, regardless of a-tDCS application. However, a limitation to this interpretation is that maximum dynamic strength of the right (trained) ECR was not recorded following the training intervention, thus the effect of muscular fatigue following each condition could not be directly quantified. Despite this, our results tend to suggest that the strength training session did not produce sufficient fatigue to cause an increase in crossactivation. It can therefore be speculated that the increase in cross-activation observed following strength training with a-tDCS may be a result of a cumulative effect on the untrained M1, induced via multiple neural pathways. For example, the application of a-tDCS is reported to modulate superficial neural circuits, specific to the site of stimulation, producing ‘experimentally induced plasticity’ in the form of altered membrane polarity and increased synaptic efficacy (Nitsche et al. 2003; Nitsche and Paulus 2001; Liebetanz et al. 2002). On the other hand, strength training of the right ECR is likely to produce ‘use dependent plasticity’ in the right M1 via increased corticospinal excitability, reduced SICI, and a reduction in IHI from the ‘trained’ to the ‘untrained’ M1 (Hortobágyi et al. 2003, 2011; Howatson et al. 2011; Perez and Cohen 2008). Our observations provide some evidence to suggest that the combination of the ‘use dependent’ and ‘experimentally induced’ plasticity models may provide greater adaptive outcomes when applied in conjunction, rather than independently. Intracortical inhibition Once again, a significant reduction in SICI was only observed following strength training with a-tDCS, in the

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Exp Brain Res

absence of any between-condition interactions. Nonetheless, the combination of a-tDCS with strength training did appear to modulate a release of SICI, which is a novel finding. However, it was perplexing that SICI remained unchanged following the a-tDCS condition, which is in contrast to previous research which has reported a reduction in SICI following a-tDCS (Hummel et al. 2005; Nitsche et al. 2005; Edwards et al. 2009). Reductions in SICI reported following a-tDCS are believed to be a result of changes in N-methyl-d-asparate (NMDA) and γ-amino-butyric (GABA) receptor efficacy (Liebetanz et al. 2002; Kujirai et al. 1993). It is difficult to draw direct comparisons between our findings and previous research due to differences in tDCS and TMS stimulation protocols, such as longer a-tDCS interventions, and varied methods for calculating conditioning and test stimulus intensities during TMS (Hummel et al. 2005; Nitsche et al. 2005; Edwards et al. 2009). All previous studies reporting a reduction in SICI used lower intensity a-tDCS (1 mA), which may be more likely to target superficial inhibitory neurons, whereas higher intensity stimulation (2 mA used in the present study) may have a greater effect on deeper pyramidal neurons, potentially masking the effect of inhibitory circuits (Purpura and McMurtry 1965). Similarly, previous findings indicate that muscle contractions reduce intracortical inhibition, most likely in order to facilitate movement (Ridding et al. 1995). Greater voluntary drive has been shown to produce greater reductions in SICI (Zoghi and Nordstrom 2007; Rantalainen et al. 2013), and a long lasting reduction in SICI has been reported following several weeks of high intensity strength training (Weier et al. 2012). Evidence suggests that these reductions in SICI occur not only in the active M1, but also in the ipsilateral, ‘inactive’ M1 during high intensity contractions (Perez and Cohen 2008), directly following ballistic motor practice (Hinder et al. 2011), and several days after a 4 weeks heavy load training program (Goodwill et al. 2012). As such, we hypothesized that the strength training with shamtDCS condition would produce a significant reduction in SICI; however, this was not the case. It is possible that the strength training protocol used in the present study, consisting of a total of 24 repetitions (compared to the ballistic practice protocol requiring up to 300 repetitions) did not produce sufficient bilateral activity to significantly effect SICI in the ipsilateral M1. The fact that SICI was significantly reduced only when the strength training and a-tDCS conditions were combined, again suggests that a cumulative effect of separate neural mechanisms (i.e., the ‘experimentally induced’ and ‘use dependent’ plasticity) may be responsible for this finding.

Exp Brain Res

Limitations There are several limitations to the present study that must not be overlooked when interpreting the results. First, the application of a-tDCS via 5 × 5 cm electrodes results in a somewhat widespread area of stimulation, which may extend beyond the intended target of the ipsilateral M1 (Nitsche et al. 2008), potentially influencing the pre-motor and supplementary motor areas. Therefore, the possibility that increased activation of brain areas outside the M1 contributes to the observed increase in strength cannot be excluded. Also, the measurement of strength using a 1-RM test, although providing a functional and training specific representation of strength, lacks the sensitivity of strength testing with isokinetic dynamometry. In addition, there is good evidence that the corticospinal responses to a-tDCS evolve over time and may remain elevated for up to 90 min (Nitsche and Paulus 2000), leaving a possibility that meaningful findings may have gone undetected due to the fact that time course measures were not obtained. These limitations, along with the relatively small sample size tested and high variability in individual responses to tDCS, may contribute to the lack of interaction effects observed in this study. Despite this, the within main effects observed following the strength training and a-tDCS condition is a new and important finding regarding the benefits of combining tDCS with motor training.

Conclusions This study found that unilateral strength training of the right ECR combined with a-tDCS of the ipsilateral M1 produced a significant increase in functional strength of the contralateral, untrained ECR muscle. The strength gain observed was accompanied by the modulation of the ipsilateral, ‘untrained’ M1, including; an increase in corticospinal excitability, a reduction in SICI, and an increase in cross-activation during ipsilateral MVC. Taken together, these results provide some preliminary evidence for the potential role of the ipsilateral M1 contributing to the cross-transfer of strength when a-tDCS is applied during unilateral strength training. Although no interactions were observed, the important within-effects of a-tDCS applied to the ipsilateral M1 suggest that a cumulative effect of experimentally induced plasticity (from a-tDCS) and use dependent plasticity (from strength training) may be responsible for our findings. Given the acute outcomes observed following this single session of unilateral strength training combined with a-tDCS, we believe that future research should seek to examine the effect of a strength training program with simultaneous a-tDCS over a period of several weeks. This may have significant

clinical implications for rehabilitation following singlelimb impairment, such as musculoskeletal injury or stroke. Acknowledgments  D.J. Kidgell is supported by an Alfred Deakin Postdoctoral Fellowship. Conflict of interest The authors declare that they have no conflict of interest.

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