Strength-trained adults demonstrate greater corticoreticular activation versus untrained controls.

The rapid increase in strength following strength-training involves neural adaptations, however, their specific localisation remains elusive. Prior focus on corticospinal responses prompts this study to explore the understudied cortical/subcortical adaptations, particularly cortico-reticulospinal tract responses, comparing healthy strength-trained adults to untrained peers. Fifteen chronically strength-trained individuals (≥2 years of training, mean age: 24 ± 7 years) were compared with 11 age-matched untrained participants (mean age: 26 ± 8 years). Assessments included maximal voluntary force (MVF), corticospinal excitability using transcranial magnetic stimulation (TMS), spinal excitability (cervicomedullary stimulation), voluntary activation (VA) and reticulospinal tract (RST) excitability, utilizing StartReact responses and ipsilateral motor-evoked potentials (iMEPs) for the flexor carpi radialis muscle. Trained participants had higher normalized MVF (6.4 ± 1.1 N/kg) than the untrained participants (4.8 ± 1.3 N/kg) (p = .003). Intracortical facilitation was higher in the strength-trained group (156 ± 49%) (p = .02), along with greater VA (98 ± 3.2%) (p = .002). The strength-trained group displayed reduced short-interval-intracortical inhibition (88 ± 8.0%) compared with the untrained group (69 ± 17.5%) (p < .001). Strength-trained individuals exhibited a greater normalized rate of force development (38.8 ± 10.1 N·s-1/kg) (p < .009), greater reticulospinal gain (2.5 ± 1.4) (p = .02) and higher ipsilateral-to-contralateral MEP ratios compared with the untrained group (p = .03). Strength-trained individuals displayed greater excitability within the intrinsic connections of the primary motor cortex and the RST. These results suggest greater synaptic input from the descending cortico-reticulospinal tract to α-motoneurons in strength-trained individuals, thereby contributing to the observed increase in VA and MVF.

Changes in motor unit behaviour across repeated bouts of eccentric exercise.

Unaccustomed eccentric exercise (EE) is protective against muscle damage following a subsequent bout of similar exercise. One hypothesis suggests the existence of an alteration in motor unit (MU) behaviour during the second bout, which might contribute to the adaptive response. Accordingly, the present study investigated MU changes during repeated bouts of EE. During two bouts of exercise where maximal lengthening dorsiflexion (10 repetitions × 10 sets) was performed 3 weeks apart, maximal voluntary isometric torque (MVIC) and MU behaviour (quantified using high-density electromyography; HDsEMG) were measured at baseline, during (after set 5), and post-EE. The HDsEMG signals were decomposed into individual MU discharge timings, and a subset were tracked across each time point. MVIC was reduced similarly in both bouts post-EE (Δ27 vs. 23%, P = 0.144), with a comparable amount of total work performed (∼1,300 J; P = 0.905). In total, 1,754 MUs were identified and the decline in MVIC was accompanied by a stepwise increase in discharge rate (∼13%; P < 0.001). A decrease in relative recruitment was found immediately after EE in Bout 1 versus baseline (∼16%; P < 0.01), along with reductions in derecruitment thresholds immediately after EE in Bout 2. The coefficient of variation of inter-spike intervals was lower in Bout 2 (∼15%; P < 0.001). Our data provide new information regarding a change in MU behaviour during the performance of a repeated bout of EE. Importantly, such changes in MU behaviour might contribute, at least in part, to the repeated bout phenomenon.

Determining intracortical, corticospinal and alpha motoneurone excitability in athletes with patellar tendinopathy compared to asymptomatic controls.

BACKGROUND: Lower capacity to generate knee extension maximal voluntary force (MVF) has been observed in individuals affected with patellar tendinopathy (PT) compared to asymptomatic controls. This MVF deficit is hypothesized to emanate from alterations in corticospinal excitability (CSE). The modulation of CSE is intricately linked to the excitability levels at multiple sites, encompassing neurones within the corticospinal tract (CST), intracortical neurones within the primary motor cortex (M1), and the alpha motoneurone. The aim of this investigation was to examine the excitability of intracortical neurones, CST neurones, and the alpha motoneurone, and compare these between volleyball and basketball athletes with PT and matched asymptomatic controls. METHOD: Nineteen athletes with PT and 18 asymptomatic controls participated in this cross-sectional study. Transcranial magnetic stimulation was utilized to assess CST excitability, corticospinal inhibition (silent period, and short-interval cortical inhibition). Peripheral nerve stimulation was used to evaluate lumbar spine and alpha motoneurone excitability, including the evocation of lumbar-evoked potentials and maximal compound muscle action potential (MMAX ), and CSE with central activation ratio (CAR). Knee extension MVF was also assessed. RESULTS: Athletes with PT exhibited longer silent period duration and greater electrical stimulator output for MMAX , as well as lower MVF, compared to asymptomatic controls (p < 0.05). CONCLUSION: These findings indicate volleyball and basketball athletes with PT exhibit reduced excitability of the alpha motoneurone or the neuromuscular junction, which may be linked to lower MVF. Subtle alterations at specific sites may represent compensatory changes to excitability aiming to maintain efferent drive to the knee extensors.

Corticospinal and spinal responses following a single session of lower limb motor skill and resistance training.

Prior studies suggest resistance exercise as a potential form of motor learning due to task-specific corticospinal responses observed in single sessions of motor skill and resistance training. While existing literature primarily focuses on upper limb muscles, revealing a task-dependent nature in eliciting corticospinal responses, our aim was to investigate such responses after a single session of lower limb motor skill and resistance training. Twelve participants engaged in a visuomotor force tracking task, self-paced knee extensions, and a control task. Corticospinal, spinal, and neuromuscular responses were measured using transcranial magnetic stimulation (TMS) and peripheral nerve stimulation (PNS). Assessments occurred at baseline, immediately post, and at 30-min intervals over two hours. Force steadiness significantly improved in the visuomotor task (P < 0.001). Significant fixed-effects emerged between conditions for corticospinal excitability, corticospinal inhibition, and spinal excitability (all P < 0.001). Lower limb motor skill training resulted in a greater corticospinal excitability compared to resistance training (mean difference [MD] = 35%, P < 0.001) and control (MD; 37%, P < 0.001). Motor skill training resulted in a lower corticospinal inhibition compared to control (MD; - 10%, P < 0.001) and resistance training (MD; - 9%, P < 0.001). Spinal excitability was lower following motor skill training compared to control (MD; - 28%, P < 0.001). No significant fixed effect of Time or Time*Condition interactions were observed. Our findings highlight task-dependent corticospinal responses in lower limb motor skill training, offering insights for neurorehabilitation program design.

Cortical and spinal responses to short-term strength training and detraining in young and older adults in rectus femoris muscle.

INTRODUCTION: Strength training mitigates the age-related decline in strength and muscle activation but limited evidence exists on specific motor pathway adaptations. METHODS: Eleven young (22-34 years) and ten older (66-80 years) adults underwent five testing sessions where lumbar-evoked potentials (LEPs) and motor-evoked potentials (MEPs) were measured during 20 and 60% of maximum voluntary contraction (MVC). Ten stimulations, randomly delivered, targeted 25% of maximum compound action potential for LEPs and 120, 140, and 160% of active motor threshold (aMT) for MEPs. The 7-week whole-body resistance training intervention included five exercises, e.g., knee extension (5 sets) and leg press (3 sets), performed twice weekly and was followed by 4 weeks of detraining. RESULTS: Young had higher MVC (~ 63 N·m, p = 0.006), 1-RM (~ 50 kg, p = 0.002), and lower aMT (~ 9%, p = 0.030) than older adults at baseline. Young increased 1-RM (+ 18 kg, p < 0.001), skeletal muscle mass (SMM) (+ 0.9 kg, p = 0.009), and LEP amplitude (+ 0.174, p < 0.001) during 20% MVC. Older adults increased MVC (+ 13 N·m, p = 0.014), however, they experienced decreased LEP amplitude (- 0.241, p < 0.001) during 20% MVC and MEP amplitude reductions at 120% (- 0.157, p = 0.034), 140% (- 0.196, p = 0.026), and 160% (- 0.210, p = 0.006) aMT during 60% MVC trials. After detraining, young and older adults decreased 1-RM, while young adults decreased SMM. CONCLUSION: Higher aMT and MEP amplitude in older adults were concomitant with lower baseline strength. Training increased strength in both groups, but divergent modifications in cortico-spinal activity occurred. Results suggest that the primary locus of adaptation occurs at the spinal level.

Reliability of corticospinal excitability and intracortical inhibition in biceps femoris during different contraction modes.

This study aimed to determine the test-retest reliability of a range of transcranial magnetic stimulation (TMS) outcomes in the biceps femoris during isometric, eccentric and concentric contractions. Corticospinal excitability (active motor threshold 120% [AMT120%] and area under recruitment curve [AURC]), short- and long-interval intracortical inhibition (SICI and LICI) and intracortical facilitation (ICF) were assessed from the biceps femoris in 10 participants (age 26.3 ± 6.0 years; height 180.2 ± 6.6 cm, body mass 77.2 ± 8.0 kg) in three sessions. Single- and paired-pulse stimuli were delivered under low-level muscle activity (5% ± 2% of maximal isometric root mean squared surface electromyography [rmsEMG]) during isometric, concentric and eccentric contractions. Participants were provided visual feedback on their levels of rmsEMG during all contractions. Single-pulse outcomes measured during isometric contractions (AURC, AMT110%, AMT120%, AMT130%, AMT150%, AMT170%) demonstrated fair to excellent reliability (ICC range, .51 to .92; CV%, 21% to 37%), whereas SICI, LICI and ICF demonstrated good to excellent reliability (ICC range, .62 to .80; CV%, 19 to 42%). Single-pulse outcomes measured during concentric contractions demonstrated excellent reliability (ICC range, .75 to .96; CV%, 15% to 34%), whereas SICI, LICI and ICF demonstrated good to excellent reliability (ICC range, .65 to .76; CV%, 16% to 71%). Single-pulse outcomes during eccentric contractions demonstrated fair to excellent reliability (ICC range, .56 to .96; CV%, 16% to 41%), whereas SICI, LICI and ICF demonstrated good to excellent (ICC range, .67 to .86; CV%, 20% to 42%). This study found that both single- and paired-pulse TMS outcomes can be measured from the biceps femoris muscle across all contraction modes with fair to excellent reliability. However, coefficient of variation values were typically greater than the smallest worthwhile change which may make tracking physiological changes in these variables difficult without moderate to large effect sizes.

Corticospinal and spinal adaptations following lower limb motor skill training: a meta-analysis with best evidence synthesis.

Motor skill training alters the human nervous system; however, lower limb motor tasks have been less researched compared to upper limb tasks. This meta-analysis with best evidence synthesis aimed to determine the cortical and subcortical responses that occur following lower limb motor skill training, and whether these responses are accompanied by improvements in motor performance. Following a literature search that adhered to the PRISMA guidelines, data were extracted and analysed from six studies (n = 172) for the meta-analysis, and 11 studies (n = 257) were assessed for the best evidence synthesis. Pooled data indicated that lower limb motor skill training increased motor performance, with a standardised mean difference (SMD) of 1.09 being observed. However, lower limb motor skill training had no effect on corticospinal excitability (CSE), Hoffmann's reflex (H-reflex) or muscle compound action potential (MMAX) amplitude. The best evidence synthesis found strong evidence for improved motor performance and reduced short-interval cortical inhibition (SICI) following lower limb motor skill training, with conflicting evidence towards the modulation of CSE. Taken together, this review highlights the need for further investigation on how motor skill training performed with the lower limb musculature can modulate corticospinal responses. This will also help us to better understand whether these neuronal measures are underpinning mechanisms that support an improvement in motor performance.

Modulations of corticospinal excitability following rapid ankle dorsiflexion in skill- and endurance-trained athletes.

PURPOSE: Long-term sports training, such as skill and endurance training, leads to specific neuroplasticity. However, it remains unclear if muscle stretch-induced proprioceptive feedback influences corticospinal facilitation/inhibition differently between skill- and endurance-trained athletes. This study investigated modulation of corticospinal excitability following rapid ankle dorsiflexion between well-trained skill and endurance athletes. METHODS: Ten skill- and ten endurance-trained athletes participated in the study. Corticospinal excitability was tested by single- and paired-pulse transcranial magnetic stimulations (TMS) at three different latencies following passive rapid ankle dorsiflexion. Motor evoked potential (MEP), short-latency intracortical inhibition (SICI), intracortical facilitation (ICF), and long-latency intracortical inhibition (LICI) were recorded by surface electromyography from the soleus muscle. RESULTS: Compared to immediately before ankle dorsiflexion (Onset), TMS induced significantly greater MEPs during the supraspinal reaction period (~ 120 ms after short-latency reflex, SLR) in the skill group only (from 1.7 ± 1.0 to 2.7 ± 1.8%M-max, P = 0.005) despite both conditions being passive. ICF was significantly greater over all latencies in skill than endurance athletes (F (3, 45) = 4.64, P = 0.007), although no between-group differences for stimulations at specific latencies (e.g., at SLR) were observed. CONCLUSION: The skill group showed higher corticospinal excitability during the supraspinal reaction phase, which may indicate a "priming" of corticospinal excitability following rapid ankle dorsiflexion for a supraspinal reaction post-stretch, which appears absent in endurance-trained athletes.

Corticospinal and intracortical excitability is modulated in the knee extensors after acute strength training.

The corticospinal responses to high-intensity and low-intensity strength-training of the upper limb are modulated in an intensity-dependent manner. Whether an intensity-dependent threshold occurs following acute strength training of the knee extensors (KE) remains unclear. We assessed the corticospinal responses following high-intensity (85% of maximal strength) or low-intensity (30% of maximal strength) KE strength-training with measures taken during an isometric KE task at baseline, post-5, 30 and 60-min. Twenty-eight volunteers (23 ± 3 years) were randomized to high-intensity (n = 11), low-intensity (n = 10) or to a control group (n = 7). Corticospinal responses were evoked with transcranial magnetic stimulation at intracortical and corticospinal levels. High- or low-intensity KE strength-training had no effect on maximum voluntary contraction force post-exercise (P > 0.05). High-intensity training increased corticospinal excitability (range 130-180%) from 5 to 60 min post-exercise compared to low-intensity training (17-30% increase). Large effect sizes (ES) showed that short-interval cortical inhibition (SICI) was reduced only for the high-intensity training group from 5-60 min post-exercise (24-44% decrease) compared to low-intensity (ES ranges 1-1.3). These findings show a training-intensity threshold is required to adjust CSE and SICI following strength training in the lower limb.

Pain Induced Changes in Brain Oxyhemoglobin: A Systematic Review and Meta-Analysis of Functional NIRS Studies.

BACKGROUND: Neuroimaging studies show that nociceptive stimuli elicit responses in an extensive cortical network. Functional near-infrared spectroscopy (fNIRS) allows for functional assessment of changes in oxyhemoglobin (HbO), an indirect index for cortical activity. Unlike functional magnetic resonance imaging (fMRI), fNIRS is portable, relatively inexpensive, and allows subjects greater function. No systematic review or meta-analysis has drawn together the data from existing literature of fNIRS studies on the effects of experimental pain on oxyhemoglobin changes in the superficial areas of the brain. OBJECTIVES: To investigate the effects of experimental pain on brain fNIRS measures in the prefrontal-cortex and the sensory-motor-area; to determine whether there is a difference in oxyhemodynamics between the prefrontal-cortex and sensory-motor-area during pain processing; to determine if there are differences in HbO between patients with centralized persistent pain and healthy controls. METHODS: Studies that used fNIRS to record changes in oxyhemodynamics in prefrontal-cortex or sensory-motor-cortex in noxious and innoxious conditions were included. In total, 13 studies were included in the meta-analysis. RESULTS: Pain has a significantly greater effect on pre-frontal-cortex and sensory-motor areas than nonpainful stimulation on oxyhemodynamics. The effect of pain on sensory-motor areas was greater than the effect of pain on the prefrontal-cortex. There was an effect of centralized pain in the CPP group on oxyhemodynamics from a noxious stimulus compared to control's response to pain. CONCLUSIONS: Pain affects the prefrontal and sensory-motor cortices of the brain and can be measured using fNIRS. Implications of this study may lead to a simple and readily accessible objective measure of pain.

Corticospinal and spinal adaptations to motor skill and resistance training: Potential mechanisms and implications for motor rehabilitation and athletic development.

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.

High Responders to Hypertrophic Strength Training Also Tend to Lose More Muscle Mass and Strength During Detraining Than Low Responders.

ABSTRACT: Räntilä, A, Ahtiainen, JP, Avela, J, Restuccia, J, Kidgell, DJ, and Häkkinen, K. High responders to hypertrophic strength training also tend to lose more muscle mass and strength during detraining than low responders. J Strength Cond Res 35(6): 1500-1511, 2021-This study investigated differences in individual responses to muscle hypertrophy during strength training and detraining. Ten weeks of resistance training was followed by 6 weeks of detraining in men (n = 24). Bilateral leg press (LP) one-repetition maximum (1RM) and maximal electromyography (EMGs) of vastus lateralis (VL) and vastus medialis, maximal voluntary activation (VA), transcranial magnetic stimulation for corticospinal excitability (CE), cross-sectional area of VL (VLCSA), selected serum hormone concentrations were measured before and repeatedly during training and detraining. In the total group, VLCSA increased by 10.7% (p = 0.025) and LP 1RM by 16.3% (p < 0.0001) after training. The subjects were split into 3 groups according to increases in VLCSA: high responders (HR) > 15% (n = 10), medium responders (MR) 15-4.5% (n = 7), and low responders (LR) < 4.5% (n = 7). Vastus lateralis CSA in HR and MR increased statistically significantly from pre to posttraining but not in LR. Only HR increased LP 1RM statistically significantly from pre to post. Maximal EMG activity increased 21.3 ± 22.9% from pre- to posttraining for the total group (p = 0.009) and for MR (p < 0.001). No significant changes occurred in VA and CE or serum hormone concentrations. During detraining, HR showed a decrease of -10.5% in VLCSA, whereas MR and LR did not. None of the subgroups decreased maximal strength during the first 3 weeks of detraining, whereas HR showed a slight (by 2.5%) rebound in strength. The present results suggest that strength gains and muscle activation adaptations may take place faster in HR and decrease also faster compared with other subgroups during detraining.

Determining the Intracortical Responses After a Single Session of Aerobic Exercise in Young Healthy Individuals: A Systematic Review and Best Evidence Synthesis.

ABSTRACT: Alibazi, RJ, Pearce, AJ, Rostami, M, Frazer, AK, Brownstein, C, and Kidgell, DJ. Determining the intracortical responses after a single session of aerobic exercise in young healthy individuals: a systematic review and best evidence synthesis. J Strength Cond Res 35(2): 562-575, 2021-A single bout of aerobic exercise (AE) may induce changes in the excitability of the intracortical circuits of the primary motor cortex (M1). Similar to noninvasive brain stimulation techniques, such as transcranial direct current stimulation, AE could be used as a priming technique to facilitate motor learning. This review examined the effect of AE on modulating intracortical excitability and inhibition in human subjects. A systematic review, according to PRISMA guidelines, identified studies by database searching, hand searching, and citation tracking between inception and the last week of February 2020. Methodological quality of included studies was determined using the Downs and Black quality index and Cochrane Collaboration of risk of bias tool. Data were synthesized and analyzed using best-evidence synthesis. There was strong evidence for AE not to change corticospinal excitability and conflicting evidence for increasing intracortical facilitation and reducing silent period and long-interval cortical inhibition. Aerobic exercise did reduce short-interval cortical inhibition, which suggests AE modulates the excitability of the short-latency inhibitory circuits within the M1; however, given the small number of included studies, it remains unclear how AE affects all circuits. In light of the above, AE may have important implications during periods of rehabilitation, whereby priming AE could be used to facilitate motor learning.

Task-specific strength increases after lower-limb compound resistance training occurred in the absence of corticospinal changes in vastus lateralis.

NEW FINDINGS: What is the central question of the study? Are corticospinal responses to acute and short-term squat resistance training task-specific? What is the main finding and its importance? A single bout of resistance training increased spinal excitability, but no changes in corticospinal responses were noted following 4 weeks of squat training despite task-specific increases in strength. The present data suggest that processes along the corticospinal pathway of the knee extensors play a limited role in the task-specific increase in strength following resistance training. ABSTRACT: Neural adaptations subserving strength increases have been shown to be task-specific, but responses and adaptation to lower-limb compound exercises such as the squat are commonly assessed in a single-limb isometric task. This two-part study assessed neuromuscular responses to an acute bout (Study A) and 4 weeks (Study B) of squat resistance training at 80% of one-repetition-maximum, with measures taken during a task-specific isometric squat (IS) and non-specific isometric knee extension (KE). Eighteen healthy volunteers (25 ± 5 years) were randomised into either a training (n = 10) or a control (n = 8) group. Neural responses were evoked at the intracortical, corticospinal and spinal levels, and muscle thickness was assessed using ultrasound. The results of Study A showed that the acute bout of squat resistance training decreased maximum voluntary contraction (MVC) for up to 45 min post-exercise (-23%, P < 0.001). From 15-45 min post-exercise, spinally evoked responses were increased in both tasks (P = 0.008); however, no other evoked responses were affected (P ≥ 0.240). Study B demonstrated that following short-term resistance training, participants improved their one repetition maximum squat (+35%, P < 0.001), which was reflected by a task-specific increase in IS MVC (+49%, P = 0.001), but not KE (+1%, P = 0.882). However, no training-induced changes were observed in muscle thickness (P = 0.468) or any evoked responses (P = 0.141). Adjustments in spinal motoneuronal excitability are evident after acute resistance training. After a period of short-term training, there were no changes in the responses to central nervous system stimulation, which suggests that alterations in corticospinal properties of the vastus lateralis might not contribute to increases in strength.

Multi-session anodal transcranial direct current stimulation enhances lower extremity functional performance in healthy older adults.

The aim of this study was to examine the effects of 5 days of anodal-transcranial direct current stimulation (a-tDCS) over the primary motor cortex (M1) on lower extremity functional performance in healthy elderly people. This was a randomized, double-blinded, sham-controlled study whereby 32 healthy older individuals participated in two groups. The intervention group received 20 min of a-tDCS (1 mA) over the M1 on five consecutive days. The sham group received the same stimulation, but the tDCS device was turned off after 30 s of stimulation. Participants were asked to perform the Timed Up and Go (TUG), 30-s Chair Stand Test (30-s CST), and a Modified Figure of Eight Walk Test (MFEWT) on the first day before tDCS application, immediately, 30 min, and 1 week after the last session of stimulation. Results of the a-tDCS group showed that most of the test values had significant changes in post-test assessments compared to the pre-test (p < 0.05). When comparing the anodal and sham tDCS groups, the results showed a significant improvement in TUG and time-MFEWT immediately after (p = 0.02, p = 0.01), 30 min after (p = 0.04, p = 0.01) and 1 week after the last session of stimulation (p = 0.01, p = 0.01). Improvements in performance of the 30-s CST and the number of steps-MFEWT were not significant, except at 1 week after the last session for the steps-MFEWT (p = 0.04). The application of 20 min a-tDCS over the M1 for 5 consecutive days improves lower extremity functional performance in the healthy older participants.

Corticomotor correlates of somatosensory reaction time and variability in individuals with post concussion symptoms.

Persistent post concussion symptoms (PPCS) describe the condition when an individual experiences chronic symptoms, particularly fatigue, beyond the expected time of recovery. The aim of this study was to quantify the effect of fatigue and related ongoing symptoms on somatosensory and corticomotor pathways using reaction time (RT) testing, and single-pulse and paired-pulse transcranial magnetic stimulation (TMS). Eighty-three participants (nine female, mean age 37.9 ± 11.5 years) were divided into two groups (persistent symptoms versus asymptomatic) following self-report based upon previously published clinical symptom scores. All participants completed somatosensory and visuomotor RT testing, as well as corticomotor excitability and inhibition measurements via TMS. Participants in the persistent symptom group (n = 38) reported greater number of previous concussions (t = 2.81, p = 0.006) and significantly higher levels of fatigue and related symptoms in the asymptomatic group (n = 45; t = 11.32, p < 0.006). Somatosensory RT showed significant slowing and increased variability in the persistent symptoms group (p < 0.001), however no significant differences were observed between groups for visuomotor RTs. Transcranial magnetic stimulation revealed differences between groups for intracortical inhibition at all stimulus intensities and paired pulse measures. The results indicate that somatosensory and corticomotor systems reflect on-going fatigue. From a practical perspective, objective and simplistic measures such as somatosensory and corticomotor measures can be used in the assessment of PPCS and gauging the efficacy of post concussion rehabilitation programmes.

Tracking the corticospinal responses to strength training.

PURPOSE: The motor cortex (M1) appears to be a primary site of adaptation following both a single session, and repeated strength-training sessions across multiple weeks. Given that a single session of strength-training is sufficient to induce modification at the level of the M1 and corticospinal tract, this study sought to determine how these acute changes in M1 and corticospinal tract might accumulate across the course of a 2-week heavy-load strength-training program. METHODS: Transcranial magnetic stimulation (TMS) was used to infer corticospinal excitability (CSE), intracortical facilitation (ICF), short and long-interval intracortical inhibition (SICI and LICI) and silent period duration prior to and following each training session during a 2-week heavy-load strength-training period. RESULTS: Following 2-weeks of strength-training, increases in strength (15.5%, P = 0.01) were accompanied by an increase in CSE (44%, P = 0.006) and reductions in both silent period duration (14%, P < 0.0001) and SICI (35%, P = 0.0004). Early training sessions acutely increased CSE and ICF, and acutely reduced silent period duration and SICI. However, later training sessions failed to modulate SICI and ICF, with substantial adaptations occurring offline between training sessions. No acute or retained changes in LICI were observed. Co-contraction of antagonists reduced by 36% following 2-weeks of strength-training. CONCLUSIONS: Collectively, these results indicate that corticospinal plasticity occurs within and between training sessions throughout a training period in distinct early and later stages that are modulated by separate mechanisms of plasticity. The development of strength is akin to the previously reported changes that occur following motor skill training.

Modulation of intracortical inhibition and excitation in agonist and antagonist muscles following acute strength training.

PURPOSE: Transcranial magnetic stimulation (TMS) usually investigates the corticospinal responses of the agonist muscle to strength training, despite the role of the antagonist muscle in strength development. We examined the intracortical responses from an agonist and antagonist muscle following a single session of heavy-loaded strength training (dominant-arm only) to identify the early antagonistic responses to a single session that may accompany improvements in strength. METHODS: Corticospinal and motor cortical excitability and inhibition was collected from agonist and antagonist muscles prior to and following a single session of heavy-loaded wrist flexor training in 18 individuals. Training consisted of four sets 6-8 repetitions at 80% of 1-repetition maximum (1-RM). Recruitment curves for corticospinal excitability and inhibition of the right wrist flexor and wrist extensor muscles were constructed and assessed by examining the area under the recruitment curve. Intracortical measures were obtained using paired-pulse TMS. RESULTS: Following a single training session, increases in corticospinal excitability were observed in both the agonist and antagonist muscles. This was accompanied by decreases in corticospinal inhibition in both muscles. Intracortical inhibition was reduced and intracortical facilitation was increased for the agonist muscle only. Intracortical measures in the antagonist muscle remained unchanged after training. CONCLUSIONS: These findings indicate that the corticospinal responses to a single session of strength training are similar between agonist and antagonist muscles, but the intrinsic cortico-cortical circuitry of the antagonist remains unchanged. The corticospinal responses are likely due to increased involvement/co-activation of the antagonist muscle during training as the agonist muscle fatigues.

Priming the Motor Cortex With Anodal Transcranial Direct Current Stimulation Affects the Acute Inhibitory Corticospinal Responses to Strength Training.

Frazer, AK, Howatson, G, Ahtiainen, JP, Avela, J, Rantalainen, T, and Kidgell, DJ. Priming the motor cortex with anodal transcranial direct current stimulation affects the acute inhibitory corticospinal responses to strength training. J Strength Cond Res 33(2): 307-317, 2019-Synaptic plasticity in the motor cortex (M1) is associated with strength training (ST) and can be modified by transcranial direct current stimulation (tDCS). The M1 responses to ST increase when anodal tDCS is applied during training due to gating. An additional approach to improve the M1 responses to ST, which has not been explored, is to use anodal tDCS to prime the M1 before a bout of ST. We examined the priming effects of anodal tDCS of M1 on the acute corticospinal responses to ST. In a randomized double-blinded cross-over design, changes in isometric strength, corticospinal excitability, and inhibition (assessed as area under the recruitment curve [AURC] using transcranial magnetic stimulation) were analyzed in 13 adults exposed to 20 minutes of anodal tDCS and sham tDCS followed by a ST session of the right elbow flexors. We observed a significant decrease in isometric elbow-flexor strength immediately after training (11-12%; p < 0.05), which was not different between anodal tDCS and sham tDCS. Transcranial magnetic stimulation revealed a 24% increase in AURC for corticospinal excitability after anodal tDCS and ST; this increase was not different between conditions. However, there was a 14% reduction in AURC for corticospinal inhibition when anodal tDCS was applied before ST when compared with sham tDCS and ST (all p < 0.05). Priming anodal tDCS had a limited effect in facilitating corticospinal excitability after an acute bout of ST. Interestingly, the interaction of anodal tDCS and ST seems to affect the excitability of intracortical inhibitory circuits of the M1 through nonhomeostatic mechanisms.