During the first few weeks of isometric resistance training there is an increase in maximal muscle force output that cannot be accounted for by muscle hypertrophy. Early on, researchers postulated the existence of neural adaptations to training primarily through the use of surface electromyographic recordings. More recent evidence also suggests that increased excitation may occur at the cortical levels following short-term resistance training. Alterations in synergistic activation and reductions in antagonist activation are neural factors that have been identified as changing during the early stages of resistance training which could contribute to maximal force generation. Neural adaptations that occur during the ramp-up phase of isometric contraction include decreases in motor unit recruitment thresholds, increased motor unit discharge rates, and increases in double discharges. An increase in the maximal rate of force development also occurs during the early stages of resistance training, but whether the neural mechanisms associated with the increase in the rate of rise are also associated with the increase in maximal force has not been elucidated. More work is needed to examine the integration of changes in cortical and spinal excitability with single motor unit firing patterns during this simple form of exercise before we can extend our understanding to different types of training.

Anodal transcranial direct current stimulation (tDCS) is known to increase the force-generating capacity of the skeletal muscles. However, when tDCS is concurrently combined with a motor task, interference may occur that hinders tDCS effects. Here, we tested the interaction and time course of tDCS effects on force production when paired with a low-level force-matching task. Twenty-two subjects were randomized into two groups: tDCS-Matching and tDCS-Resting. Each group received tDCS and a sham stimulation, separated by one week. Maximal knee extensor and flexor torques were measured before and up to twenty-five minutes following the stimulation. The tDCS-Matching group produced greater knee extension torques relative to sham when compared with the tDCS-Resting group. There was no significant effect for knee flexion. This suggests that interference does not occur for force production tasks when tDCS is combined with a motor task. Rather, the task appears to aid and isolate the effects to the muscle groups involved in the task.Copyright © 2016 IBRO. Published by Elsevier Ltd. All rights reserved.

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 cross-activation. 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 right limb increases maximal strength and cross-activation to the contralateral untrained limb.

Cross-education of strength has been proposed to be greater when completed by the dominant limb in right handed humans. We investigated whether the direction of cross-education of strength and corticospinal plasticity are different following right or left limb strength training in right-handed participants.Changes in strength, muscle thickness and indices of corticospinal plasticity were analyzed in 23 adults who were exposed to 3-weeks of either right-hand strength training (RHT) or left-hand strength training (LHT).Maximum voluntary wrist extensor strength in both the trained and untrained limb increased, irrespective of which limb was trained, with TMS revealing reduced corticospinal inhibition.Cross-education of strength was not limited by which limb was trained and reduced corticospinal inhibition was not just confined to the trained limb. Critically, from a behavioral perspective, the magnitude of cross-education was not limited by which limb was trained.

Experimental studies showed improvement in physical performance following acute application of transcranial direct current stimulation (tDCS). This study examined the neuromuscular and neural response to a single training session (Part 1) and after a 3-week resistance training (Part 2) performed with the knee extensors, preceded by tDCS over the primary motor cortex. Twenty-four participants (age, 30 ± 7 years; stature, 172 ± 8 cm; mass, 72 ± 15 kg) were randomly allocated to perform either resistance training with anodal tDCS (a-tDCS), or by a placebo tDCS (Sham). Resistance training consisted of 3 × 10 isometric contractions of 3 s at 75% MVC. Measures of neuromuscular function (MVC, voluntary activation and potentiated twitch force), corticospinal excitability, and short and long cortical inhibition were assessed. Acute tDCS did not affect neuromuscular and neural responses to a single training session (all s ≥ 0.10). Conversely, after the 3-week training program, MVC increased in both groups (< 0.01), with a greater increase observed for a-tDCS vs. Sham (⁓6%, = 0.04). Additionally, increased voluntary activation (⁓2%, = 0.04) and corticospinal excitability (⁓22%, = 0.04), accompanied with shorter silent period (-13%, = 0.04) was found following a-tDCS vs. Sham. The potentiated twitch force and measures of short and long cortical inhibition did not change after training programme (all s ≥ 0.29). Pre-training administration of tDCS only resulted in greater neuromuscular adaptations following 3-weeks of resistance training. These results provide new evidence that tDCS facilitates adaptations to resistance training in healthy individuals.

Transcranial direct current stimulation (tDCS) can increase cortical excitability of a targeted brain area, which may affect endurance exercise performance. However, optimal electrode placement for tDCS remains unclear. We tested the effect of two different tDCS electrode montages for improving exercise performance. Nine subjects underwent a control (CON), placebo (SHAM) and two different tDCS montage sessions in a randomized design. In one tDCS session, the anodal electrode was placed over the left motor cortex and the cathodal on contralateral forehead (HEAD), while for the other montage the anodal electrode was placed over the left motor cortex and cathodal electrode above the shoulder (SHOULDER). tDCS was delivered for 10min at 2.0mA, after which participants performed an isometric time to exhaustion (TTE) test of the right knee extensors. Peripheral and central neuromuscular parameters were assessed at baseline, after tDCS application and after TTE. Heart rate (HR), ratings of perceived exertion (RPE), and leg muscle exercise-induced muscle pain (PAIN) were monitored during the TTE. TTE was longer and RPE lower in the SHOULDER condition (P<0.05). Central and peripheral parameters, and HR and PAIN did not present any differences between conditions after tDCS stimulation (P>0.05). In all conditions maximal voluntary contraction (MVC) significantly decreased after the TTE (P<0.05) while motor-evoked potential area (MEP) increased after TTE (P<0.05). These findings demonstrate that SHOULDER montage is more effective than HEAD montage to improve endurance performance, likely through avoiding the negative effects of the cathode on excitability.Copyright © 2016 The Author(s). Published by Elsevier Ltd.. All rights reserved.

Several studies have used transcranial magnetic stimulation to probe the corticospinal-motoneuronal responses to a single session of strength training; however, the findings are inconsistent. This systematic review and meta-analysis examined whether a single bout of strength training affects the excitability and inhibition of intracortical circuits of the primary motor cortex (M1) and the corticospinal-motoneuronal pathway. A systematic review was completed, tracking studies between January 1990 and May 2018. The methodological quality of studies was determined using the Downs and Black quality index. Data were synthesised and interpreted from meta-analysis. Nine studies (n=107) investigating the acute corticospinal-motoneuronal responses to strength training met the inclusion criteria. Meta-analyses detected that after strength training compared to control, corticospinal excitability [standardised mean difference (SMD), 1.26; 95% confidence interval (CI), 0.88, 1.63; p<0.0001] and intracortical facilitation (ICF) (SMD, 1.60; 95% CI, 0.18, 3.02; p=0.003) were increased. The duration of the corticospinal silent period was reduced (SMD, -17.57; 95% CI, -21.12, -14.01; p=0.00001), but strength training had no effect on the excitability of the intracortical inhibitory circuits [short-interval intracortical inhibition (SICI) SMD, 1.01; 95% CI, -1.67, 3.69; p=0.46; long-interval intracortical inhibition (LICI) SMD, 0.50; 95% CI, -1.13, 2.13; p=0.55]. Strength training increased the excitability of corticospinal axons (SMD, 4.47; 95% CI, 3.45, 5.49; p<0.0001). This systematic review and meta-analyses revealed that the acute neural changes to strength training involve subtle changes along the entire neuroaxis from the M1 to the spinal cord. These findings suggest that strength training is a clinically useful tool to modulate intracortical circuits involved in motor control.

Resistance-training causes changes in the central nervous system (CNS); however, the sites of these adaptations remain unclear.To determine sites of neural adaptation to resistance-training by conducting a systematic review and meta-analysis on the cortical and subcortical responses to resistance-training.Evidence from randomized controlled trials (RCTs) that focused on neural adaptations to resistance-training was pooled to assess effect estimates for changes in strength, cortical, and subcortical adaptations.The magnitude of strength gain in 30 RCTs (n = 623) reported a standardised mean difference (SMD) of 0.67 (95% CI 0.41, 0.94; P < 0.001) that measured at least one cortical/subcortical neural adaptation which included: motor-evoked potentials (MEP; 19 studies); silent period (SP; 7 studies); short-interval intracortical inhibition (SICI; 7 studies); cervicomedullary evoked potentials (CMEP; 1 study); transcranial magnetic stimulation voluntary activation (VA; 2 studies); H-reflex (10 studies); and V-wave amplitudes (5 studies). The MEP amplitude during voluntary contraction was greater following resistance-training (SMD 0.55; 95% CI 0.27, 0.84; P < 0.001, n = 271), but remained unchanged during rest (SMD 0.49; 95% CI -0.68, 1.66; P = 0.41, n = 114). Both SP (SMD 0.65; 95% CI 0.29, 1.01; P < 0.001, n = 184) and active SICI (SMD 0.68; 95% CI 0.14, 1.23; P = 0.01, n = 102) decreased, but resting SICI remained unchanged (SMD 0.26; 95% CI - 0.29, 0.81; P = 0.35, n = 52). Resistance-training improved neural drive as measured by V-wave amplitude (SMD 0.62; 95% CI 0.14, 1.10; P = 0.01, n = 101), but H-reflex at rest (SMD 0.16; 95% CI - 0.36, 0.68; P = 0.56; n = 57), during contraction (SMD 0.15; 95% CI - 0.18, 0.48; P = 0.38, n = 142) and VA (MD 1.41; 95% CI - 4.37, 7.20; P = 0.63, n = 44) remained unchanged.There are subtle neural adaptations following resistance-training involving both cortical and subcortical adaptations that act to increase motoneurone activation and likely contribute to the training-related increase in muscle strength.

Optimal strategies for enhancing strength and improving motor skills are vital in athletic performance and clinical rehabilitation. Initial increases in strength and the acquisition of new motor skills have long been attributed to neurological adaptations. However, early increases in strength may be predominantly due to improvements in inter-muscular coordination rather than the force-generating capacity of the muscle. Despite the plethora of research investigating neurological adaptations from motor skill or resistance training in isolation, little effort has been made in consolidating this research to compare motor skill and resistance training adaptations. The findings of this review demonstrated that motor skill and resistance training adaptations show similar short-term mechanisms of adaptations, particularly at a cortical level. Increases in corticospinal excitability and a release in short-interval cortical inhibition occur as a result of the commencement of both resistance and motor skill training. Spinal changes show evidence of task-specific adaptations from the acquired motor skill, with an increase or decrease in spinal reflex excitability, dependant on the motor task. An increase in synaptic efficacy of the reticulospinal projections is likely to be a prominent mechanism for driving strength adaptations at the subcortical level, though more research is needed. Transcranial electric stimulation has been shown to increase corticospinal excitability and augment motor skill adaptations, but limited evidence exists for further enhancing strength adaptations from resistance training. Despite the logistical challenges, future work should compare the longitudinal adaptations between motor skill and resistance training to further optimise exercise programming.

Strength training results in changes in skeletal muscle; however, changes in the central nervous system also occur. Over the last 15 years, non-invasive brain stimulation techniques, such as transcranial magnetic stimulation, have been used to study the neural adaptations to strength training. This review explored the hypothesis that the neural adaptations to strength training may be due to changes in corticospinal excitability and inhibition and, such changes, contribute to the gain in strength following strength training. A systematic review, according to PRISMA guidelines, identified studies by database searching, hand-searching and citation tracking between January 1990 and the first week of February 2017. Methodological quality of included studies was determined using the Downs and Black quality index. Data were synthesised and interpreted from meta-analysis. Nineteen studies investigating the corticospinal responses following strength training were included. Meta-analysis found that strength training increased strength [standardised mean difference (SMD) 0.84, 95% CI 0.55 to 1.13], decreased short-interval intracortical inhibition (SMD -1.00, 95% CI -1.84 to -0.17) and decreased the cortical silent period (SMD -0.66, 95% CI -1.00 to -0.32). Strength training had no effect on motor threshold (SMD -0.12, 95% CI -0.49 to 0.25), but a borderline effect for increased corticospinal excitability (SMD 0.27, 95% CI 0.00 to 0.54). In untrained healthy participants, the corticospinal response to strength training is characterised by reduced intracortical inhibition and cortical silent period duration, rather than changes in corticospinal excitability. These data demonstrate that strength training targets intracortical inhibitory networks within the primary motor cortex (M1) and corticospinal pathway, characterising an important neural adaptation to strength training.© 2017 Federation of European Neuroscience Societies and John Wiley & Sons Ltd.

The present study investigated the effects of anodal transcranial direct current stimulation (tDCS) on lower extremity muscle strength training in 24 healthy participants. In this triple-blind, sham-controlled study, participants were randomly allocated to the anodal tDCS plus muscle strength training (anodal tDCS) group or sham tDCS plus muscle strength training (sham tDCS) group. Anodal tDCS (2 mA) was applied to the primary motor cortex of the lower extremity during muscle strength training of the knee extensors and flexors. Training was conducted once every 3 days for 3 weeks (7 sessions). Knee extensor and flexor peak torques were evaluated before and after the 3 weeks of training. After the 3-week intervention, peak torques of knee extension and flexion changed from 155.9 to 191.1 Nm and from 81.5 to 93.1 Nm in the anodal tDCS group. Peak torques changed from 164.1 to 194.8 Nm on extension and from 78.0 to 85.6 Nm on flexion in the sham tDCS group. In both groups, peak torques of knee extension and flexion significantly increased after the intervention, with no significant difference between the anodal tDCS and sham tDCS groups. In conclusion, although the administration of eccentric training increased knee extensor and flexor peak torques, anodal tDCS did not enhance the effects of lower extremity muscle strength training in healthy individuals. The present null results have crucial implications for selecting optimal stimulation parameters for clinical trials.

This study aimed to analyze the acute effect of anodal transcranial direct current stimulation (a-tDCS) over the primary motor cortex (M1) on the volume, perceived exertion, and neuromuscular performance measurements in trained and untrained adults. Twenty-four male adults (12 trained and 12 untrained) participated in this single-blind, randomized, and sham-controlled study. The participants performed three back squat repetitions using the 15RM load with maximal concentric velocity to assess neuromuscular performance before tDCS and 30-min after resistance exercise. Next, they were randomly assigned to a-tDCS over M1 or the sham condition. Participants performed ten sets of parallel back squat with 15RM load and repetitions sustained to momentary muscular failure. The total number of repetitions was higher (p<0.05) and perceived exertion was lower (p<0.05) after a-tDCS in both groups. Peak power, velocity, and force decreased in both groups after the RE session (p<0.05), but with a higher rate in untrained individuals (p<0.05). No significant effect was found for peak power, peak velocity, and peak force (p>0.05). This study suggests that using a-tDCS may improve the total volume of repetitions and perceived exertion in trained and untrained individuals.Thieme. All rights reserved.

Exercise performance is influenced by many physical factors, such as muscle strength and endurance. Particularly in the physical fitness and sports performance contexts, there are many types of ergogenic aids to improve muscular strength and endurance performance, with non-athletes and even athletes using illegal drugs to reach the top. Thus, the development of innovative methods to aid in exercise performance is of great interest. One such method is transcranial direct current stimulation (tDCS). A systematic search was performed on the following databases, until January 2019; PubMed/MEDLINE, SCOPUS, and Pedro database. Studies on tDCS for muscular strength and endurance performance improvement in non-athletes and athletes adults were included. We compared the effect of anodal-tDCS (a-tDCS) to a sham/control condition on the outcomes muscular strength and endurance performance. We found 26 controlled trials. No trial mentions negative side effects of the intervention. The data show differences between the studies investigating muscle strength and the studies evaluating endurance, with regard to successful use of tDCS. Studies investigating the efficiency of tDCS on improving muscular strength demonstrate positive effects of a-tDCS in 66.7% of parameters tested. In contrast, in studies evaluating the effects of a-tDCS on improving endurance performance the a-tDCS revealed a significant improvement in only 50% of parameters assessed. The majority of the data shows consistently influence of a-tDCS on muscular strength, but not to endurance performance. The results of this systematic review suggest that a-tDCS can improve muscular strength, but not to endurance performance.

Transcranial direct current stimulation (tDCS) has been used to improve exercise performance, though the protocols used, and results found are mixed.We aimed to analyze the effect of tDCS on improving exercise performance.A systematic search was performed on the following databases, until December 2017: PubMed/MEDLINE, Embase, Web of Science, SCOPUS, and SportDiscus. Full-text articles that used tDCS for exercise performance improvement in adults were included. We compared the effect of anodal (anode near nominal target) and cathodal (cathode near nominal target) tDCS to a sham/control condition on the outcome measure (performance in isometric, isokinetic or dynamic strength exercise and whole-body exercise).22 studies (393 participants) were included in the qualitative synthesis and 11 studies (236 participants) in the meta-analysis. The primary motor cortex (M1) was the main nominal tDCS target (n = 16; 72.5%). A significant effect favoring anodal tDCS (a-tDCS) applied before exercise over M1 was found on cycling time to exhaustion (mean difference = 93.41 s; 95%CI = 27.39 s-159.43 s) but this result was strongly influenced by one study (weight = 84%), no effect was found for cathodal tDCS (c-tDCS). No significant effect was found for a-tDCS applied on M1 before or during exercise on isometric muscle strength of the upper or lower limbs. Studies regarding a-tDCS over M1 on isokinetic muscle strength presented mixed results. Individual results of studies using a-tDCS applied over the prefrontal and motor cortices either before or during dynamic muscle strength testing showed positive results, but performing meta-analysis was not possible.For the protocols tested, a-tDCS but not c-tDCS vs. sham over M1 improved exercise performance in cycling only. However, this result was driven by a single study, which when removed was no longer significant. Further well-controlled studies with larger sample sizes and broader exploration of the tDCS montages and doses are warranted.Copyright © 2018 Elsevier Inc. All rights reserved.

Neuromuscular fatigue is the exercise-dependent decrease in the ability of muscle fibres to generate force. To investigate whether manipulation of brain excitability by transcranial direct current stimulation (tDCS; 1.5 mA, 10 min, 0.026 C/cm(2)) modulates neuromuscular fatigue, we evaluated the effect of brain polarization over the right motor areas of the cerebral cortex of healthy subjects on the endurance time for a submaximal isometric contraction of left elbow flexors. In 24 healthy volunteers the study protocol comprised an assessment of the maximum voluntary contraction (MVC) for the left elbow flexors and a fatiguing isometric contraction (35% of MVC), before and immediately after brain polarization. One hour elapsed between baseline (T0) and postconditioning (T1) evaluation. After tDCS, MVC remained unchanged from baseline (mean +/- SEM; anodal tDCS: T0, 154.4 +/- 18.07; T1, 142.8 +/- 16.62 N; cathodal tDCS: T0, 156 +/- 18.75; T1, 141.86 +/- 17.53 N; controls: T0, 148.8 +/- 6.64; T1, 137.6 +/- 7.36 N; P > 0.1). Conversely, endurance time decreased significantly less after anodal than after cathodal tDCS or no stimulation (-21.1 +/- 5.5%, -35.7 +/- 3.3% and -39.3 +/- 3.3%, respectively; P < 0.05). None of the evaluated electromyographic variables changed after tDCS. Anodal tDCS could improve endurance time by directly modulating motor cortical excitability, modulating premotor areas, decreasing fatigue-related muscle pain, increasing motivation and improving synergist muscle coupling. Our findings, showing that anodal tDCS over the motor areas of the cerebral cortex improves muscle endurance, open the way to increasing muscle endurance and decreasing muscle fatigue in normal (i.e. sports medicine) and pathological conditions.

The increase in corticospinal excitability in response to anodal transcranial direct current stimulation (a-tDCS) may contribute to decrease neuromuscular fatigability.This study investigated the effects of a-tDCS on neuromuscular fatigability in relation with changes in corticospinal excitability.Eleven adults participated in two experimental sessions consisting of two submaximal voluntary contractions (35% maximal torque) performed to failure, one hour apart with the right elbow flexor muscles. Sham stimulation (90 s) and a-tDCS (10 min) were applied in two separate sessions 10 min prior to the second fatiguing contraction. Corticospinal excitability was assessed by recording motor-evoked potential (MEP), elicited by transcranial magnetic stimulation (TMS) of the motor cortex, in biceps brachii, brachioradialis and triceps brachii during the first (C1) and second (C2) fatiguing contractions. The silent period (SP) in electromyogramme (EMG) that followed MEP was also recorded for biceps brachii and brachioradialis.Time to failure was briefer for C2 than C1 in both experimental sessions, but the decrease was less pronounced after a-tDCS (-14.4±12.7%) than sham stimulation (-23.3±11.9%; p=0.04). MEP amplitude (+9.7±4.0%) and SP duration (+22.5±12.8%) in biceps brachii and brachioradialis increased significantly (p<0.05) during C1 and C2, but to a similar extent in both sessions (p>0.05). A similar result was observed for MEP amplitude in triceps brachii. No association was found between changes in time to task failure of C2 and corticospinal excitability.The reduced neuromuscular fatigability induced by a-tDCS does not rely on changes in the excitability of the corticospinal pathway of both agonist and antagonist muscles.Copyright © 2016. Published by Elsevier Ltd.

The dorsolateral prefrontal cortex (DLPFC) is a crucial brain region for inhibitory control, an executive function essential for behavioral self-regulation. Recently, inhibitory control has been shown to be important for endurance performance. Improvement in inhibitory control was found following transcranial direct current stimulation (tDCS) applied over the left DLPFC (L-DLPFC). This study examined the effect tDCS on both an inhibitory control and endurance performance in a group of healthy individuals. Twelve participants received either real tDCS (Real-tDCS) or placebo tDCS (Sham-tDCS) in randomized order. The anodal electrode was placed over the L-DLPFC while the cathodal electrode was placed above Fp2. Stimulation lasted 30min with current intensity set at 2mA. A Stroop test was administered to assess inhibitory control. Heart rate (HR), ratings of perceived exertion (RPE), and leg muscle pain (PAIN) were monitored during the cycling time to exhaustion (TTE) test, while blood lactate accumulation (∆B[La]) was measured at exhaustion. Stroop task performance was improved after Real-tDCS as demonstrated by a lower number of errors for incongruent stimuli (p=0.012). TTE was significantly longer following Real-tDCS compared to Sham-tDCS (p=0.029, 17±8 vs 15±8min), with significantly lower HR (p=0.002) and RPE (p<0.001), while no significant difference was found for PAIN (p>0.224). ∆B[La] was significantly higher at exhaustion in Real-tDCS (p=0.040). Our findings provide preliminary evidence that tDCS with the anodal electrode over the L-DLPFC can improve both inhibitory control and endurance cycling performance in healthy individuals.Copyright © 2019 IBRO. Published by Elsevier Ltd. All rights reserved.