Corticomotor plasticity and learning of a ballistic thumb training task are diminished in older adults

Neuromuscular Training Effects on Athletes' Jumping and Lower Limb Injury: A Systematic Review

This study assessed the impact of integrated neuromuscular training (INT) on athletes' jumping performance and lower limb injury prevention. A thorough search across multiple databases, including PubMed, Web of Science, Science Direct, and the Cochrane Library, identified a sample of 19,805 athletes aged between 8.5 and 27.7 years. Results showed that INT led to a significant improvement in jumping ability (SMD = 0.45, 95% CI 0.30-0.60, P = 0.000) and a reduction in lower limb injuries (SMD = 0.68, 95% CI 0.62-0.74, P = 0.000). The most effective interventions lasted at least 25 minutes, conducted 2-5 times per week, for a minimum of 9 weeks. This approach was particularly beneficial for children and adolescents, as it enhanced their countermovement jump (CMJ) ability and helped in the prevention of injuries.

Differential modulation of corticomotor excitability in older compared to young adults following a single bout of strength -exercise

Evidence shows corticomotor plasticity diminishes with age. Nevertheless, whether strength-training, a proven intervention that induces corticomotor plasticity in younger adults, also takes effect in older adults, remains untested. This study examined the effect of a single-session of strength-exercise on corticomotor plasticity in older and younger adults. Thirteen older adults (72.3 ± 6.5 years) and eleven younger adults (29.9 ± 6.9 years), novice to strength-exercise, participated. Strength-exercise involved four sets of 6-8 repetitions of a dumbbell biceps curl at 70-75% of their one-repetition maximum (1-RM). Muscle strength, cortical, corticomotor and spinal excitability, before and up to 60-minutes after the strength-exercise session were assessed. We observed significant changes over time (p < 0.05) and an interaction between time and age group (p < 0.05) indicating a decrease in corticomotor excitability (18% p < 0.05) for older adults at 30- and 60-minutes post strength-exercise and an increase (26% and 40%, all p < 0.05) in younger adults at the same time points. Voluntary activation (VA) declined in older adults immediately post and 60-minutes post strength-exercise (36% and 25%, all p < 0.05). Exercise had no effect on the cortical silent period (cSP) in older adults however, in young adults cSP durations were shorter at both 30- and 60- minute time points (17% 30-minute post and 9% 60-minute post, p < 0.05). There were no differences in short-interval cortical inhibition (SICI) or intracortical facilitation (ICF) between groups. Although the corticomotor responses to strength-exercise were different within groups, overall, the neural responses seem to be independent of age.

Hebbian priming of human motor learning

Motor learning relies on experience-dependent plasticity in relevant neural circuits. In four experiments, we provide initial evidence and a double-blinded, sham-controlled replication (Experiment I-II) demonstrating that motor learning involving ballistic index finger movements is improved by preceding paired corticospinal-motoneuronal stimulation (PCMS), a human model for exogenous induction of spike-timing-dependent plasticity. Behavioral effects of PCMS targeting corticomotoneuronal (CM) synapses are order- and timing-specific and partially bidirectional (Experiment III). PCMS with a 2 ms inter-arrival interval at CM-synapses enhances learning and increases corticospinal excitability compared to control protocols. Unpaired stimulations did not increase corticospinal excitability (Experiment IV). Our findings demonstrate that non-invasively induced plasticity interacts positively with experience-dependent plasticity to promote motor learning. The effects of PCMS on motor learning approximate Hebbian learning rules, while the effects on corticospinal excitability demonstrate timing-specificity but not bidirectionality. These findings offer a mechanistic rationale to enhance motor practice effects by priming sensorimotor training with individualized PCMS.

Differences in motor inhibition in young and older musicians and non-musicians at rest

Introduction

Older adults experience a decline in motor inhibition. These declines have been implicated in instrumental activities of daily living. However, studies have revealed that older musicians have behavioral and neurophysiological enhancements in various motor domains compared to non-musicians. This suggests that music training may delay the decline in motor inhibition with aging. Nevertheless, motor inhibition has not been studied in young or older musicians and non-musicians. Thus, the present study aimed to investigate the neurophysiological differences in motor inhibition in aging musicians and non-musicians.

Methods

A total of 19 healthy young adult musicians, 16 healthy young non-musicians, 13 healthy older adult musicians, and 16 healthy older adult non-musicians were recruited for the study. Transcranial magnetic stimulation single-pulse (SP) and short interval cortical inhibition (SICI) were performed at rest and then converted into inhibition percentage.

Results

We did not observe significant differences between young and older musicians and non-musicians in resting SP MEP. Older adults had lower resting SICI MEP than young adults. Older adults (36%) had a greater percentage of inhibition than young adults (16%). However, when controlling for background EMG activity, musicians had a lower inhibition percentage than non-musicians.

Discussion

The results revealed that, despite the greater use of spinal mechanisms, decreased SICI, and increased inhibition percentage in older adults, motor inhibitory circuitry remains intact and functional in both young and older musicians and non-musicians. Future studies will reveal whether there are differences in motor inhibition during movement in musicians across a person's lifespan.

Effect of Motor Imagery on Corticomotor Excitability and Pain Status in Rheumatoid Arthritis Patients

OBJECTIVES

Rheumatoid arthritis (RA) has been defined by the American College of Rheumatology in 1987 as a chronic inflammatory disease characterised by joint swelling, joint tenderness, and destruction of synovial joints leading to severe disability and premature mortality. There is a paucity of literature assessing corticomotor excitability in RA patients. This study aimed to assess the effect of motor imagery on corticomotor excitability and pain status in RA patients. The specific objectives were to study the effect of motor imagery on corticomotor excitability and pain status in RA patients. We also wanted to compare the corticomotor excitability between RA patients with healthy controls. The correlation between the measures of corticomotor excitability and pain status in RA patients has also been done.

METHODS

The study was designed as a pilot clinical trial with a case-control design. Forty participants were recruited for the study. Twenty RA patients were recruited from the Department of Rheumatology and Department of Physical Medicine and Rehabilitation (PMR), AIIMS, New Delhi, and 20 healthy controls. Testing was performed at the Pain Research & rTMS Lab, Department of Physiology, AIIMS, New Delhi. The study was approved by the Institute Ethics Committee, AIIMS New Delhi, and registered in the Clinical Trials Registry-India (CTRI). For the subjective assessment of pain, the visual analogue scale (VAS), Short-Form McGill Pain Questionnaire, WHO-Quality of Life Brief questionnaire (WHO-QOL-BREF), and Rheumatoid Arthritis Pain Scale were used. For the objective assessment of pain, hot and cold pain thresholds were assessed using thermo-tactile quantitative sensory testing (QST) using the method of limits and corticomotor excitability using a transcranial magnetic stimulation device. All participants were also asked to perform motor imagery tasks which consisted of a metronome-paced thumb opposition paradigm.  Results: The resting motor threshold (RMT) decreased significantly after motor imagery when compared to the mental calculation group. The amplitude of motor evoked potential (MEP) and QST parameter value was comparable in both the groups before and after motor imagery and mental calculation. RMT was found to be significantly higher whereas MEP values were found to be significantly lower in RA compared to controls.

CONCLUSION

We conclude that patients suffering from RA have decreased corticomotor excitability compared to controls. Motor imagery was effective in improving corticomotor excitability in these patients and can be used as rehabilitation in RA to relieve their pain.

Optimizing the Effect of tDCS on Motor Sequence Learning in the Elderly

One of the most visible effects of aging, even in healthy, normal aging, is a decline in motor performance. The range of strategies applicable to counteract this deterioration has increased. Transcranial direct current stimulation (tDCS), a non-invasive brain stimulation technique that can promote neuroplasticity, has recently gained attention. However, knowledge about optimized tDCS parameters in the elderly is limited. Therefore, in this study, we investigated the effect of different anodal tDCS intensities on motor sequence learning in the elderly. Over the course of four sessions, 25 healthy older adults (over 65 years old) completed the Serial Reaction Time Task (SRTT) while receiving 1, 2, or 3 mA of anodal or sham stimulation over the primary motor cortex (M1). Additionally, 24 h after stimulation, motor memory consolidation was assessed. The results confirmed that motor sequence learning in all tDCS conditions was maintained the following day. While increased anodal stimulation intensity over M1 showed longer lasting excitability enhancement in the elderly in a prior study, the combination of higher intensity stimulation with an implicit motor learning task showed no significant effect. Future research should focus on the reason behind this lack of effect and probe alternative stimulation protocols.

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.

From thinking fast to moving fast: motor control of fast limb movements in healthy individuals

The ability to produce high movement speeds is a crucial factor in human motor performance, from the skilled athlete to someone avoiding a fall. Despite this relevance, there remains a lack of both an integrative brain-to-behavior analysis of these movements and applied studies linking the known dependence on open-loop, central control mechanisms of these movements to their real-world implications, whether in the sports, performance arts, or occupational setting. In this review, we cover factors associated with the planning and performance of fast limb movements, from the generation of the motor command in the brain to the observed motor output. At each level (supraspinal, peripheral, and motor output), the influencing factors are presented and the changes brought by training and fatigue are discussed. The existing evidence of more applied studies relevant to practical aspects of human performance is also discussed. Inconsistencies in the existing literature both in the definitions and findings are highlighted, along with suggestions for further studies on the topic of fast limb movement control. The current heterogeneity in what is considered a fast movement and in experimental protocols makes it difficult to compare findings in the existing literature. We identified the role of the cerebellum in movement prediction and of surround inhibition in motor slowing, as well as the effects of fatigue and training on central motor control, as possible avenues for further research, especially in performance-driven populations.

Determining the Corticospinal Responses and Cross-Transfer of Ballistic Motor Performance in Young and Older Adults: A Systematic Review and Meta-Analysis

Ballistic motor training induces plasticity changes and imparts a cross-transfer effect. However, whether there are age-related differences in these changes remain unclear. Thus, the purpose of this study was to perform a meta-analysis to determine the corticospinal responses and cross-transfer of motor performance following ballistic motor training in young and older adults. Meta-analysis was performed using a random-effects model. A best evidence synthesis was performed for variables that had insufficient data for meta-analysis. There was strong evidence to suggest that young participants exhibited greater cross-transfer of ballistic motor performance than their older counterparts. This meta-analysis showed no significant age-related differences in motor-evoked potentials (MEPs), short-interval intracortical inhibition (SICI) and surface electromyography (sEMG) for both hands following ballistic motor training.

Strengthening the GABAergic system through neurofeedback training suppresses implicit motor learning

Gamma-aminobutyric acid (GABA) activity within the primary motor cortex (M1) is essential for motor learning in cortical plasticity, and a recent study has suggested that real-time neurofeedback training (NFT) can self-regulate GABA activity. Therefore, this study aimed to investigate the effect of GABA activity strengthening via NFT on subsequent motor learning. Thirty-six healthy participants were randomly assigned to either an NFT group or control group, which received sham feedback. GABA activity was assessed for short intracortical inhibition (SICI) within the right M1 using paired-pulse transcranial magnetic stimulation. During the NFT intervention period, the participants tried to modulate the size of a circle, which was altered according to the degree of SICI in the NFT group. However, the size was altered independently of the degree of SICI in the control group. We measured the reaction time before, after (online learning), and 24 h after (offline learning) the finger-tapping task. Results showed the strengthening of GABA activity induced by the NFT intervention, and the suppression of the online but not the offline learning. These findings suggest that prior GABA activity modulation may affect online motor learning.

Intracortical facilitation and inhibition in human primary motor cortex during motor skill acquisition

The primary motor cortex (M1) is critical for movement execution, but its role in motor skill acquisition remains elusive. Here, we examine the role of M1 intracortical circuits during skill acquisition. Paired-pulse transcranial magnetic stimulation (TMS) paradigms of short-interval intracortical facilitation (SICF) and inhibition (SICI) were used to assess excitatory and inhibitory circuits, respectively. We hypothesised that intracortical facilitation and inhibition circuits in M1 would be modulated to support acquisition of a novel visuomotor skill. Twenty-two young, neurologically healthy adults trained with their nondominant hand on a skilled and non-skilled sequential visuomotor isometric finger abduction task. Electromyographic recordings were obtained from the nondominant first dorsal interosseous (FDI) muscle. Corticomotor excitability, SICF, and SICI were examined before, at the midway point, and after the 10-block motor training. SICI was assessed using adaptive threshold-hunting procedures. Task performance improved after the skilled, but not non-skilled, task training, which likely reflected the increase in movement speed during training. The amplitudes of late SICF peaks were modulated with skilled task training. There was no modulation of the early SICF peak, SICI, and corticomotor excitability with either task training. There was also no association between skill acquisition and SICF or SICI. The findings indicate that excitatory circuitries responsible for the generation of late SICF peaks, but not the early SICF peak, are modulated in motor skill acquisition for a sequential visuomotor isometric finger abduction task.

Increasing human motor skill acquisition by driving theta-gamma coupling

Skill learning is a fundamental adaptive process, but the mechanisms remain poorly understood. Some learning paradigms, particularly in the memory domain, are closely associated with gamma activity that is amplitude modulated by the phase of underlying theta activity, but whether such nested activity patterns also underpin skill learning is unknown. Here, we addressed this question by using transcranial alternating current stimulation (tACS) over sensorimotor cortex to modulate theta–gamma activity during motor skill acquisition, as an exemplar of a non-hippocampal-dependent task. We demonstrated, and then replicated, a significant improvement in skill acquisition with theta–gamma tACS, which outlasted the stimulation by an hour. Our results suggest that theta–gamma activity may be a common mechanism for learning across the brain and provides a putative novel intervention for optimizing functional improvements in response to training or therapy.

Age-related decline in cortical inhibitory tone strengthens motor memory

Ageing disrupts the finely tuned excitation/inhibition balance (E:I) across cortex via a natural decline in inhibitory tone (γ-amino butyric acid, GABA), causing functional decrements. However, in young adults, experimentally lowering GABA in sensorimotor cortex enhances a specific domain of sensorimotor function: adaptation memory. Here, we tested the hypothesis that as sensorimotor cortical GABA declines naturally with age, adaptation memory would increase, and the former would explain the latter. Results confirmed this prediction. To probe causality, we used brain stimulation to further lower sensorimotor cortical GABA during adaptation. Across individuals, how stimulation changed memory depended on sensorimotor cortical E:I. In those with low E:I, stimulation increased memory; in those with high E:I stimulation reduced memory. Thus, we identified a form of motor memory that is naturally strengthened by age, depends causally on sensorimotor cortex neurochemistry, and may be a potent target for motor skill preservation strategies in healthy ageing and neurorehabilitation.

Evaluation of Training with Elastic Bands on Strength and Fatigue Indicators in Paralympic Powerlifting

Background: Variable resistance training has recently become a component of strength and conditioning programs. Objective: This randomized counterbalanced cross-over study aimed to investigate the use of elastic bands (EB) and the traditional method (TRAD) and force indicators in a training session. Methods: 12 Paralympic athletes (age: 28.60 ± 7.60 years) participated in this three-week study. In the first week, the participants were familiarized with EB and TRAD and were tested for maximal repetition (1-RM). The research occurred in weeks 2 and 3, which included the pre-post training, during which the following measures were extracted: maximum isometric force (MIF), the peak torque (PT), rate of force development (RFD), fatigue index (FI), and time to MIF (Time). The athletes performed two tests, EB and TRAD, separated by a one-week interval. Results: Significant differences were found between the pre- and post-test for 1RM (p = 0.018, η2p = 0.412), MIF (p = 0.011, η2p = 0.415), PT (p = 0.012, η2p = 0.413), and RFD (p = 0.0002, η2p = 0.761). With the use of EB, there was a difference in RFD between TRAD before and EB after (p = 0.016, η2p = 0.761). There were significant differences in the before and after for FI between TRAD and EB (p < 0.001) and for Time (p < 0.001), indicating that training with the use of elastic bands promotes overload, characterized by increased fatigue and decreased strength. Conclusions: Training with EB did not decrease 1RM, PT, MIF or RFD, however, there was an increase in fatigue and time to reach MIF when compared to the method with fixed resistance.

Inhibition, excitation and bilateral transfer following a unilateral complex finger-tapping task in young and older adults

Neuroplasticity underpins motor learning, with abnormal neuroplasticity related to age-associated motor declines. Bilateral transfer of motor learning, through rehabilitation, may mitigate these declines; however, the magnitude of transfer may be reduced in older populations. This study investigated excitatory and inhibitory pathways in the trained and untrained hemispheres following unilateral training of a complex finger-tapping task across ageing. Fifteen young (26.2 ± 3.8 years) and 11 older adults (63.7 ± 15.4 years) received transcranial magnetic stimulation, although surface electromyography was recorded from the extensor digitorum communis (EDC) and abductor pollicis brevis (APB), before and after practicing a complex finger-tapping task with the dominant hand. Excitability, inhibition (expressed as percent change scores from pre- to post-training), motor task performance and bilateral transfer were assessed between groups. Investigation of hemispheric differences within each group was completed for measures that significantly differed between groups. There were no between-group differences in task performance or bilateral transfer, with task performance improving post-training irrespective of group for both hands (p < 0.05). Pre- to post-inhibition change scores of the untrained EDC muscle increased (p = 0.034) in older compared with younger adults, indicating reduced inhibition in older adults. Inhibition change scores significantly differed between hemispheres for the young group only (p = 0.037). Only the younger group presented with hemispheric lateralisation, providing some support for the Hemispheric Asymmetry Reduction in OLDer adults (HAROLD) hypothesis. Whether this reduction is evidence of de-differentiation or compensation will need to be confirmed with additional measures.

Stereoscopic Visual Perceptual Learning in Seniors

BACKGROUND

We showed that seniors can improve their stereoscopic ability (stereoacuity) and corresponding reaction time with repetitive training and, furthermore, that these improvements through training are still present even after a longer period of time without training.

METHODS

Eleven seniors (average age: 85.90 years) trained twice a week for six weeks with dynamic stereoscopic perception training using a vision training apparatus (c-Digital Vision Trainer®). Stereoscopic training was performed in 12 training session (n = 3072) of visual tasks. The task was to identify and select one of four figures (stereoscopic stimuli) that was of a different disparity using a controller. The tests included a dynamic training (showing rotating balls) and a static test (showing plates without movement). Before and after training, the stereoacuity and the corresponding reaction times were identified with the static stereotest in order to determine the individual training success. The changes in respect to reaction time of stereoscopic stimuli with decreasing disparity were calculated.

RESULTS

After 6 weeks of training, reaction time improved in the median from 936 arcsec to 511 arcsec. Stereoscopic vision improved from 138 arcsec to 69 arcsec, which is an improvement of two levels of difficulty. After 6 months without training, the improvement, achieved by training, remained stable.

CONCLUSIONS

In older people, visual training leads to a significant, long-lasting improvement in stereoscopic vision and the corresponding reaction time in seniors. This indicates cortical plasticity even in old age.

Physical activity, motor performance and skill learning: a focus on primary motor cortex in healthy aging

Participation in physical activity benefits brain health and function. Cognitive function generally demonstrates a noticeable effect of physical activity, but much less is known about areas responsible for controlling movement, such as primary motor cortex (M1). While more physical activity may support M1 plasticity in older adults, the neural mechanisms underlying this beneficial effect remain poorly understood. Aging is inevitably accompanied by diminished motor performance, and the extent of plasticity may also be less in older adults compared with young. Motor complications with aging may, perhaps unsurprisingly, contribute to reduced physical activity in older adults. While the development of non-invasive brain stimulation techniques have identified that human M1 is a crucial site for learning motor skills and recovery of motor function after injury, a considerable lack of knowledge remains about how physical activity impacts M1 with healthy aging. Reducing impaired neural activity in older adults may have important implications after neurological insult, such as stroke, which is more common with advancing age. Therefore, a better understanding about the effects of physical activity on M1 processes and motor learning in older adults may promote healthy aging, but also allow us to facilitate recovery of motor function after neurological injury. This article will initially provide a brief overview of the neurophysiology of M1 in the context of learning motor skills, with a focus on healthy aging in humans. This information will then be proceeded by a more detailed assessment that focuses on whether physical activity benefits motor function and human M1 processes.

Age-related changes in motor cortex plasticity assessed with non-invasive brain stimulation: an update and new perspectives

It is commonly accepted that the brains capacity to change, known as plasticity, declines into old age. Recent studies have used a variety of non-invasive brain stimulation (NIBS) techniques to examine this age-related decline in plasticity in the primary motor cortex (M1), but the effects seem inconsistent and difficult to unravel. The purpose of this review is to provide an update on studies that have used different NIBS techniques to assess M1 plasticity with advancing age and offer some new perspective on NIBS strategies to boost plasticity in the ageing brain. We find that early studies show clear differences in M1 plasticity between young and older adults, but many recent studies with motor training show no decline in use-dependent M1 plasticity with age. For NIBS-induced plasticity in M1, some protocols show more convincing differences with advancing age than others. Therefore, our view from the NIBS literature is that it should not be automatically assumed that M1 plasticity declines with age. Instead, the effects of age are likely to depend on how M1 plasticity is measured, and the characteristics of the elderly population tested. We also suggest that NIBS performed concurrently with motor training is likely to be most effective at producing improvements in M1 plasticity and motor skill learning in older adults. Proposed NIBS techniques for future studies include combining multiple NIBS protocols in a co-stimulation approach, or NIBS strategies to modulate intracortical inhibitory mechanisms, in an effort to more effectively boost M1 plasticity and improve motor skill learning in older adults.

Effect of Integrative Neuromuscular Training for Injury Prevention and Sports Performance of Female Badminton Players

Objectives

Investigate the effects of integrated neuromuscular training (INT) on injury prevention and the performance of professional female badminton athletes by comparing their preintervention and postintervention tests. The study hypothesized that integrated neuromuscular training can improve the asymmetry and improve the sport performance of female badminton players.

Methods

According to pretest value based on functional movement screening, 38 participants were divided into a high-risk group (HG) and a low-risk group (LG) with 22 and 16 people in each group. Two groups of athletes took part in an 8-week INT program consisting of four 90-min sessions each week. The asymmetries in movement, physical fitness, and special abilities were tested before and after the intervention. Independent sample t-test was used for the statistical analysis.

Results

This research found indicated that 8 weeks of INT influenced FMS scores in both groups (HG and LG). The change of inline lunge (ES_H = 0.42, ES_L = 0.21) and trunk stability push-up (ES_H = −0.58, ES_L = −0.20) showed significant differences (P < 0.05), and the change of the FMS scores (ES_H = 0.81, ES_L = 0.65), deep squat (ES_H = 0.6, ES_L = 0.3), and rotation stability (ES_H = −0.65, ES_L = −0.72) showed very significant differences (P < 0.01). Compared to the pretest, most of the physical fitness parameters improved significantly in the HG and LG groups except strength index, and special abilities of the HG and LG group women badminton athletes showed a substantial increase.

Conclusion

Integrated neuromuscular training can effectively improve the asymmetry of female badminton athletes' limbs, prevent sports injury, and improve the athlete's performance ability. However, athletes in different risk groups have certain differences in the degree of improvement in their motor skills.

Contribution of TMS and TMS-EEG to the Understanding of Mechanisms Underlying Physiological Brain Aging

In the human brain, aging is characterized by progressive neuronal loss, leading to disruption of synapses and to a degree of failure in neurotransmission. However, there is increasing evidence to support the notion that the aged brain has a remarkable ability to reorganize itself, with the aim of preserving its physiological activity. It is important to develop objective markers able to characterize the biological processes underlying brain aging in the intact human, and to distinguish them from brain degeneration associated with many neurological diseases. Transcranial magnetic stimulation (TMS), coupled with electromyography or electroencephalography (EEG), is particularly suited to this aim, due to the functional nature of the information provided, and thanks to the ease with which it can be integrated with behavioral manipulation. In this review, we aimed to provide up to date information about the role of TMS and TMS-EEG in the investigation of brain aging. In particular, we focused on data about cortical excitability, connectivity and plasticity, obtained by using readouts such as motor evoked potentials and transcranial evoked potentials. Overall, findings in the literature support an important potential contribution of TMS to the understanding of the mechanisms underlying normal brain aging. Further studies are needed to expand the current body of information and to assess the applicability of TMS findings in the clinical setting.

Age-related abnormalities of thalamic shape and dynamic functional connectivity after three hours of sleep restriction

Background

Previous neuroimaging studies have detected abnormal activation and intrinsic functional connectivity of the thalamus after total sleep deprivation. However, very few studies have investigated age-related changes in the dynamic functional connectivity of the thalamus and the abnormalities in the thalamic shape following partial sleep deprivation.

Methods

Fifty-five participants consisting of 23 old adults (mean age: 68.8 years) and 32 young adults (mean age: 23.5 years) were included in current study. A vertex-based shape analysis and a dynamic functional connectivity analysis were used to evaluate the age-dependent structural and functional abnormalities after three hours of sleep restriction.

Results

Shape analysis revealed the significant main effect of deprivation with local atrophy in the left thalamus. In addition, we observed a significant age deprivation interaction effect with reduced variability of functional connectivity between the left thalamus and the left superior parietal cortex following sleep restriction. This reduction was found only in young adults. Moreover, a significantly negative linear correlation was observed between the insomnia severity index and the changes of variability (post-deprivation minus pre-deprivation) in the functional connectivity of the left thalamus with the left superior parietal cortex.

Conclusions

The results indicated that three hours of sleep restriction could affect both the thalamic structure and its functional dynamics. They also highlighted the role of age in studies of sleep deprivation.

Use of explicit processes during a visually guided locomotor learning task predicts 24-h retention after stroke

Implicit and explicit processes can occur within a single locomotor learning task. The combination of these learning processes may impact how individuals acquire/retain the task. Because these learning processes rely on distinct neural pathways, neurological conditions may selectively impact the processes that occur, thus, impacting learning and retention. Thus, our purpose was to examine the contribution of implicit and explicit processes during a visually guided walking task and characterize the relationship between explicit processes and performance/retention in stroke survivors and age-matched healthy adults. Twenty chronic stroke survivors and twenty healthy adults participated in a 2-day treadmill study. Day 1 included baseline, acquisition1, catch, acquisition2, and immediate retention phases, and day 2 included 24-h retention. During acquisition phases, subjects learned to take a longer step with one leg through distorted visual feedback. During catch and retention phases, visual feedback was removed and subjects were instructed to walk normally (catch) or how they walked during the acquisition phases (retention). Change in step length from baseline to catch represented implicit processes. Change in step length from catch to the end of acquisition2 represented explicit processes. A mixed ANOVA found no difference in the type of learning between groups (P = 0.74). There was a significant relationship between explicit processes and 24-h retention in stroke survivors (r = 0.47, P = 0.04) but not in healthy adults (r = 0.34, P = 0.15). These results suggest that stroke may not affect the underlying learning mechanisms used during locomotor learning, but that these mechanisms impact how well stroke survivors retain the new walking pattern.NEW & NOTEWORTHY This study found that stroke survivors used implicit and explicit processes similar to age-matched healthy adults during a visually guided locomotion learning task. The amount of explicit processes was related to how well stroke survivors retained the new walking pattern but not to how well they performed during the task. This work illustrates the importance of understanding the underlying learning mechanisms to maximize retention of a newly learned motor behavior.

Modeling motor-evoked potentials from neural field simulations of transcranial magnetic stimulation

OBJECTIVE

To develop a population-based biophysical model of motor-evoked potentials (MEPs) following transcranial magnetic stimulation (TMS).

METHODS

We combined an existing MEP model with population-based cortical modeling. Layer 2/3 excitatory and inhibitory neural populations, modeled with neural-field theory, are stimulated with TMS and feed layer 5 corticospinal neurons, which also couple directly but weakly to the TMS pulse. The layer 5 output controls mean motoneuron responses, which generate a series of single motor-unit action potentials that are summed to estimate a MEP.

RESULTS

A MEP waveform was generated comparable to those observed experimentally. The model captured TMS phenomena including a sigmoidal input-output curve, common paired pulse effects (short interval intracortical inhibition, intracortical facilitation, long interval intracortical inhibition) including responses to pharmacological interventions, and a cortical silent period. Changes in MEP amplitude following theta burst paradigms were observed including variability in outcome direction.

CONCLUSIONS

The model reproduces effects seen in common TMS paradigms.

SIGNIFICANCE

The model allows population-based modeling of changes in cortical dynamics due to TMS protocols to be assessed in terms of changes in MEPs, thus allowing a clear comparison between population-based modeling predictions and typical experimental outcome measures.

Long-term motor skill training with individually adjusted progressive difficulty enhances learning and promotes corticospinal plasticity

Motor skill acquisition depends on central nervous plasticity. However, behavioural determinants leading to long lasting corticospinal plasticity and motor expertise remain unexplored. Here we investigate behavioural and electrophysiological effects of individually tailored progressive practice during long-term motor skill training. Two groups of participants practiced a visuomotor task requiring precise control of the right digiti minimi for 6 weeks. One group trained with constant task difficulty, while the other group trained with progressively increasing task difficulty, i.e. continuously adjusted to their individual skill level. Compared to constant practice, progressive practice resulted in a two-fold greater performance at an advanced task level and associated increases in corticospinal excitability. Differences were maintained 8 days later, whereas both groups demonstrated equal retention 14 months later. We demonstrate that progressive practice enhances motor skill learning and promotes corticospinal plasticity. These findings underline the importance of continuously challenging patients and athletes to promote neural plasticity, skilled performance, and recovery.

Age effect on functional connectivity changes of right anterior insula after partial sleep deprivation

Neuroimaging studies revealed that emotion and cognition dysfunction after sleep deprivation is associated with the disturbance of the salience network. However, the age effect on the functional connectivity changes of the network after sleep deprivation remains unclear. The current study investigated the functional connectivity changes of the right anterior insula after partial sleep deprivation in young and old adults by using resting-state functional magnetic resonance imaging. We found a significant age × deprivation interaction effect on the functional connectivity between the right ventral anterior insula and the right ventral lateral prefrontal cortex and between the right dorsal anterior insula and the right anterior temporoparietal junction and left medial prefrontal cortex. Post-hoc analysis indicated that only young adults showed reduced functional connectivity of the right anterior insula. The changes in the functional connectivity between the right anterior insula and the right ventral lateral prefrontal cortex and anterior temporoparietal junction were negatively correlated with the insomnia severity index. Results suggested that sleep deprivation affects the salience network of young and old adults differently and highlighted the crucial role of age in sleep deprivation-related studies.

External Resistance Is Imperative for Training-Induced Efferent Neural Drive Enhancement in Older Adults

Strength training performed with heavy loads and maximal intended velocity is documented to enhance efferent neural drive to maximally contracting musculature in older adults. However, it remains unclear whether the neural plasticity following training result from motor skill learning or if external resistance is a prerequisite. To investigate this, we assessed electrically evoked potentials (H-reflex and V-waves normalized to maximal M-wave) and voluntary activation (VA) in 36 older adults (73 ± 4 years) randomized to 3 weeks of plantar flexion strength training, with (maximal strength training [MST]) or without (unloaded ballistic training [UBT]) heavy external loading (90% of one repetition maximum), or a control group. Both training groups aimed to execute the concentric phase of movement as fast and forcefully as possible. The MST group improved maximal voluntary contraction (MVC) and rate of force development (RFD) by 18% ± 13% (p = .001; Hedges g = 0.66) and 35% ± 17% (p &lt; .001; g = 0.94), respectively, and this was different (MVC: p = .013; RFD: p = .001) from the UBT group which exhibited a 7% ± 8% (p = .033; g = 0.32) increase in MVC and a tendency to increase RFD (p = .119; g = 0.22). Concomitant improvements in efferent neural drive (Vmax/Msup ratio: 0.14 ± 0.08 to 0.24 ± 0.20; p = .010) and a tendency towards increased VA (79% ± 9% to 84% ± 5%; p = .098), were only apparent after MST. No changes were observed in Hmax/Mmax ratio for the groups. In conclusion, external loading during exercise training appears to be a prerequisite for efferent neural drive enhancement in older adults. Thus, strength training with heavy loads should be recommended to counteract the typically observed age-related decline in motoneuron firing frequency and recruitment.

Central and Peripheral Neuromuscular Adaptations to Ageing

Ageing is accompanied by a severe muscle function decline presumably caused by structural and functional adaptations at the central and peripheral level. Although researchers have reported an extensive analysis of the alterations involving muscle intrinsic properties, only a limited number of studies have recognised the importance of the central nervous system, and its reorganisation, on neuromuscular decline. Neural changes, such as degeneration of the human cortex and function of spinal circuitry, as well as the remodelling of the neuromuscular junction and motor units, appear to play a fundamental role in muscle quality decay and culminate with considerable impairments in voluntary activation and motor performance. Modern diagnostic techniques have provided indisputable evidence of a structural and morphological rearrangement of the central nervous system during ageing. Nevertheless, there is no clear insight on how such structural reorganisation contributes to the age-related functional decline and whether it is a result of a neural malfunction or serves as a compensatory mechanism to preserve motor control and performance in the elderly population. Combining leading-edge techniques such as high-density surface electromyography (EMG) and improved diagnostic procedures such as functional magnetic resonance imaging (fMRI) or high-resolution electroencephalography (EEG) could be essential to address the unresolved controversies and achieve an extensive understanding of the relationship between neural adaptations and muscle decline.

Task-dependent modulation of corticospinal excitability and inhibition following strength training

This study determined whether there are task-dependent differences in cortical excitability following different types of strength training. Transcranial magnetic stimulation (TMS) measured corticospinal excitability (CSE) and intracortical inhibition (ICI) of the biceps brachii muscle in 42 healthy subjects that were randomised to either paced-strength-training (PST, n = 11), self-paced strength-training (SPST, n = 11), isometric strength-training (IST, n = 10) or to a control group (n = 10). Single-pulse and paired-pulse TMS were applied prior to and following 4-weeks of strength-training. PST increased CSE compared to SPST, IST and the control group (all P < 0.05). ICI was only reduced (60%) following PST. Dynamic strength increased by 18 and 25% following PST and SPST, whilst isometric strength increased by 20% following IST. There were no associations between the behavioural outcome measures and the change in CSE and ICI. The corticospinal responses to strength-training are task-dependent, which is a new finding. Strength-training that is performed slowly could promote use-dependent plasticity in populations with reduced volitional drive, such as during periods of limb immobilization, musculoskeletal injury or stroke.

Age effect on gray matter volume changes after sleep restriction

Sleep deprivation disrupted functional and structural brain areas which are associated with cognition and emotion in healthy participants. However, the effect of age on the structural changes after sleep restriction remains unclear. In the current study, gray matter volume was calculated in 43 young adults and 37 old adults before and after sleep restriction. Two-way mixed analysis of variance (between-subject factor: deprivation; within-subject factor: age) was then employed to investigate differences in gray matter volume changes between young and old adults. Gaussian random field theory was applied for multiple comparison correction. Results revealed that sleep restriction decreases gray matter volume in the right thalamus, left precuneus, and postcentral gyrus. More importantly, we found a significant deprivation × age interaction effect mainly in the right dorsal/ventral anterior insula where the gray matter volume increased in young adults after sleep restriction but showed no difference in old adults. These findings highlight the crucial role of the anterior insula in the neural mechanisms underlying sleep lose, especially among young adults. The current work provided structural evidence for describing emotional dysfunction and suggests the potential effect of age on functional and structural changes after sleep restriction.

Practice-dependent motor cortex plasticity is reduced in non-disabled multiple sclerosis patients

OBJECTIVES

Skill acquisition after motor training involves synaptic long-term potentiation (LTP) in primary motor cortex (M1). In multiple sclerosis (MS), LTP failure ensuing from neuroinflammation could contribute to worsen clinical recovery. We therefore addressed whether practice-dependent plasticity is altered in MS.

METHODS

Eighteen relapsing-remitting (RR)-MS patients and eighteen healthy controls performed 600 fast abductions of index finger in 30 blocks of 20 movements. Before and after practice, transcranial magnetic stimulation (TMS) was delivered over the hot spot of the trained first dorsal interosseous muscle. Movements kinematics, measures of cortical excitability, and the input/output curves of motor evoked potentials (MEPs) were assessed.

RESULTS

Kinematic variables of movement improved with practice in patients and controls to a similar extent, although patients showed lower MEPs amplitude increase after practice. Practice did not change the difference in resting motor threshold values observed between patients and controls, nor did modulate short-interval intracortical inhibition. Clinical/radiological characteristics were not associated to practice-dependent effects.

CONCLUSIONS

Practice-induced reorganization of M1 is altered in non-disabled RR-MS patients, as shown by impaired MEPs modulation after motor learning.

SIGNIFICANCE

These findings suggest that in RR-MS physiological mechanisms of practice-dependent plasticity are altered.

The effects of mechanical tactile stimulation on corticospinal excitability and motor function depend on pin protrusion patterns

Somatosensory stimulation modulates corticospinal excitability. Mechanical tactile stimulation (MS) activates cortical activity depending on tactile stimulation patterns. In this study, we examined whether the effects of mechanical tactile stimulation on corticospinal excitability and motor function depend on different pin protrusions patterns. This single-blind study included 18 healthy subjects. Two types of MS interventions were used: repetitive global stimulus (RGS) intervention was used to stimulate the finger by using 24 pins installed on a finger pad, and sequential stepwise displacement stimulus (SSDS) intervention was used to stimulate the finger by moving a row of 6 pins between the left and right sides on the finger pad. MS interventions were applied to the right index finger for 20 min (stim on/stim off, 1 s/5 s) at a frequency of 20 Hz. After RGS intervention, motor evoked potentials (MEPs) by transcranial magnetic stimulation were observed to be significantly smaller than pre-intervention MEPs; however, motor function using the grooved pegboard task remained unchanged. After SSDS intervention, MEPs were significantly larger and motor function significantly improved compared with pre-intervention values. Our results demonstrated that MS intervention can modulate corticospinal excitability and motor function and that the effects of MS intervention depend on MS intervention patterns.

Age-Related Changes in the Plasticity of Neural Networks Assessed by Transcranial Magnetic Stimulation With Electromyography: A Systematic Review and Meta-Analysis

Objective: The excitability of cerebral cortical cells, neural pathway, and neural networks, as well as their plasticity, are key to our exploration of age-related changes in brain structure and function. The combination of transcranial magnetic stimulation (TMS) with electromyography (EMG) can be applied to the primary motor cortex; it activates the underlying neural group and passes through the corticospinal pathway, which can be quantified using EMG. This meta-analysis aimed to analyze changes in cortical excitability and plasticity in healthy elderly individuals vs. young individuals through TMS-EMG.

Methods: The Cochrane Library, Medline, and EMBASE databases were searched to identify eligible trials published from database inception to June 3, 2019. The Cochrane Risk of Bias Tool and improved Jadad scale were used to assess the methodological quality. A meta-analysis of the comparative effects was conducted using the Review Manager 5.3 software and Stata 14.0 software.

Results: The pooled results revealed that the resting motor threshold values in the elderly group were markedly higher than those reported in the young group (mean difference [MD]: −2.35; 95% confidence interval [CI]: −3.69 to −1.02]; p < (0.00001). The motor evoked potential amplitude significantly reduced in the elderly group vs. the young group (MD: 0.18; 95% CI: 0.09–0.27; p < 0.0001). Moreover, there was significantly longer motor evoked potential latency in the elderly group (MD: −1.07; 95% CI: −1.77 to −0.37]; p =(0.003). There was no significant difference observed in the active motor threshold between the elderly and young groups (MD: −1.52; 95% CI: −3.47 to −0.42]; p =(0.13). Meanwhile, only two studies reported the absence of adverse events.

Conclusion: We found that the excitability of the cerebral cortex declined in elderly individuals vs. young individuals. The findings of the present analysis should be considered with caution owing to the methodological limitations in the included trials. Additional high-quality studies are warranted to validate our findings.

Integrative Neuromuscular Training in Young Athletes, Injury Prevention, and Performance Optimization: A Systematic Review

The aim of this systematic review was to evaluate the current evidence by assessing the effectiveness of integrative neuromuscular training programs in injury prevention and sports performance in young athletes. Different data sources were analyzed up to January 2018. Eligible studies contained information on population (young athletes), intervention (neuromuscular training), comparator (control group or another exercise intervention), outcomes (injury prevention or sport performance), and study design (randomized trials or prospective studies). The trials were restricted based on the language (English) and for publication date (after January 1st, 2007). Fourteen randomized controlled trials were included: Seven included dynamic stability-related outcomes. Three assessed the coordination performing fundamental movements and sport-specific skills, while other five studies analyzed muscle strength and two assessed plyometric tests. Agility was evaluated in three studies and speed tests were also considered by four studies. Finally, fatigue resistance in three studies and injury risk in four were assessed. This review provides evidence that integrative neuromuscular training programs can enhance performance and injury prevention in young athletes, taken into account that adherence to the training program is adequate. Collectively, well-designed, randomized studies are necessary to collaborate with the present findings.

Contraction intensity-dependent variations in the responses to brain and corticospinal tract stimulation after a single session of resistance training in men

The aim of this study was to determine the effects of acute resistance training (RT) intensity on motor-evoked potentials (MEPs) generated by transcranial magnetic brain stimulation and on cervicomedullary motor-evoked potentials (CMEPs) produced by electrical stimulation of the corticospinal tract. In four experimental sessions, 14 healthy young men performed 12 sets of eight isometric contractions of the elbow flexors at 0 (Control session), 25, 50, and 75% of the maximal voluntary contraction (MVC). Before and after each session, MEPs, CMEPs, and the associated twitch forces were recorded at rest. MEPs increased by 39% (P < 0.05 versus 25% in the control condition, Effect size (ES) = 1.04 and 1.76, respectively) after the 50% session and by 70% (P < 0.05 vs. all other conditions, ES = 0.91-2.49) after the 75% session. In contrast, CMEPs increased similarly after the 25%, 50%, and 75% sessions with an overall increase of 27% (P < 0.05 vs. control condition, ES = 1.34). The amplitude of maximal compound muscle action potentials (M_max) was unchanged during the experiment. The MEP- and CMEP-associated twitch forces also increased after RT, but training intensity affected only the increases in MEP twitch forces. The data tentatively suggest that the intensity of muscle contraction used in acute bouts of RT affects cortical excitability.NEW & NOTEWORTHY Resistance training (RT) can acutely increase the efficacy of the corticospinal-motoneuronal synapse, motoneuron excitability and motor cortical excitability. We show that motor-evoked potential generated by transcranial magnetic stimulation but not cervicomedullary electrical stimulation increased in proportion to the intensity of training used during a single session of RT. The data suggest that the intensity of muscle contraction used in acute bouts of RT affects cortical excitability.

Determining the Corticospinal Responses to Single Bouts of Skill and Strength Training

Mason, J, Frazer, AK, Jaberzadeh, S, Ahtiainen, JP, Avela, J, Rantalainen, T, Leung, M, and Kidgell, DJ. Determining the corticospinal responses to single bouts of skill and strength training. J Strength Cond Res 33(9): 2299-2307, 2019-Neuroplastic changes in the primary motor cortex accompany performance improvements following motor practice. Recent evidence suggests that the corticospinal responses to strength and skill training are similar, following both a single session and repeated bouts of training, promoting discussion that strength training is a form of motor learning. However, these findings are limited by the lack of a light-load strength training group. Therefore, the aim of the current study was to determine whether a single session of heavy-load strength training, light-load strength training or skill training differentially modulates the corticospinal pathway. Transcranial magnetic stimulation was used to assess the excitatory and inhibitory circuitry of the motor cortex following a single session of skill training, and following a single session of light-load and heavy-load strength training. Following a single session of training, participants in all groups experienced comparable increases in corticospinal excitability (ranging from 38 to 46%, all p < 0.05); however, disparity was observed in the inhibitory responses. Corticospinal inhibition was reduced in all 3 single-sessions, although to a greater magnitude in the heavy-load and skill-training sessions (22 and 18% respectively, compared with 11% following light-load training, all p < 0.05). Short-interval intracortical inhibition was reduced immediately following single sessions of heavy-load strength training (40% p < 0.05) and skill training (47% p < 0.05), but remained unchanged the following light-load strength training session. It appears that the corticospinal responses to single sessions of different types of strength and skill training are task-dependent. These findings reinforce the notion that strength training, at least when heavily-loaded, can be considered a form of motor learning, potentially because of the sensory feedback involved.

Visuomotor task acquisition is reduced by priming paired associative stimulation in older adults

Transcranial magnetic stimulation may represent an effective means for improving motor function in the elderly. The aim of this study was therefore to investigate the effects of paired associative stimulation (PAS; a plasticity-inducing transcranial magnetic stimulation paradigm) on acquisition of a novel visuomotor task in young and older adults. Fourteen young (20.4 ± 0.6 years) and 13 older (69.0 ± 1.6 years) adults participated in 3 experimental sessions during which training was preceded (primed) by PAS. Within each session, the interstimulus interval used for PAS was set at either the N20 latency plus 5 ms (PAS_LTP), the N20 latency minus 10 ms (PAS_LTD), or a constant 100 ms (PAS_Control). After training, the level of motor skill was not different between PAS conditions in young subjects (all p-values > 0.2), but was reduced by both PAS_LTP (p = 0.02) and PAS_LTD (p = 0.0001) in older subjects. Consequently, priming PAS was detrimental to skill acquisition in older adults, possibly suggesting a need for interventions that are optimized for use in elderly populations.

Combining reward and M1 transcranial direct current stimulation enhances the retention of newly learnt sensorimotor mappings

Background

Reward-based feedback given during motor learning has been shown to improve the retention of the behaviour being acquired. Interestingly, applying transcranial direct current stimulation (tDCS) during learning over the primary motor cortex (M1), an area associated with motor retention, also results in enhanced retention of the newly formed motor memories. However, it remains unknown whether combining these distinct interventions result in an additive benefit of motor retention.

Methods

We investigated whether combining both interventions while participants learned to account for a visuomotor transformation results in enhanced motor retention (total n = 56; each group n = 14). To determine whether these interventions share common physiological mechanisms underpinning learning, we assessed motor cortical excitability and inhibition (i.e. SICI) on a hand muscle before and after all participants learned the visuomotor rotation using their entire arm and hand.

Results

We found that both the Reward-Stim (i.e. reward + tDCS) and Reward-Sham (i.e. reward-only) groups had increased retention at the beginning of the retention phase, indicating an immediate effect of reward on behaviour. However, each intervention on their own did not enhance retention when compared to sham, but rather, only the combination of both reward and tDCS demonstrated prolonged retention. We also found that only the Reward-Stim group had a significant reduction in SICI after exposure to the perturbation.

Conclusions

We show that combining both interventions are additive in providing stronger retention of motor adaptation. These results indicate that the reliability and validity of using tDCS within a clinical context may depend on the type of feedback individuals receive when learning a new motor pattern.

•

Concurrently administering reward and M1 tDCS during learning results in enhanced motor retention.

•

The combination of these interventions also leads to a reduction in M1 inhibitory mechanisms.

•

No benefits of motor retention were found when reward or M1 tDCS were given alone.

Motor training modulates intracortical inhibitory dynamics in motor cortex during movement preparation

BACKGROUND

The primary motor cortex (M1) has a vital role to play in the learning of novel motor skills. However, the physiological changes underpinning this learning, particularly in terms of dynamic changes during movement preparation, are incompletely understood. In particular, a substantial decrease in resting gamma-amino butyric acid (GABA) activity, i.e. a release of resting inhibition, is seen within M1 as a subject prepares to move. Although there is evidence that a decrease in resting inhibition occurs within M1 during motor learning it is not known whether the pre-movement "release" of GABAergic inhibition is modulated during skill acquisition.

OBJECTIVE

Here, we investigated changes in pre-movement GABAergic inhibitory "release" during training on a motor skill task.

METHODS

We studied GABA_A activity using paired-pulse TMS (Short-Interval Intracortical Inhibition (SICI)) during training on a ballistic thumb abduction task, both at rest and at two time-points during movement preparation.

RESULTS

Improvement in task performance was related to a later, steeper, release of inhibition during the movement preparation phase. Specifically, subjects who showed greater improvement in the task in the early stages of training showed a reduced level of GABAergic release immediately prior to movement compared with those who improved less. Later in training, subjects who performed better showed a reduction in GABAergic release early in movement preparation.

CONCLUSIONS

These findings suggest that motor training is associated with maintained inhibition in motor cortex during movement preparation.

Neuromuscular adaptations to healthy aging

Even in the absence of disease or disability, aging is associated with marked physiological adaptations within the neuromuscular system. An ability to perform activities of daily living and maintain independence with advanced age is reliant on the health of the neuromuscular system. Hence, it is critical to elucidate the age-related adaptations that occur within the central nervous system and the associated muscles to design interventions to maintain or improve neuromuscular function in the elderly. This brief review focuses on the neural alterations observed at both spinal and supraspinal levels in healthy humans in their seventh decade and beyond. The topics addressed are motor unit loss and remodelling, neural drive, and responses to transcranial magnetic stimulation of the motor cortex.

Reduced GABAergic cortical inhibition in aging and depression

The neurobiology underlying depression in older adults is less extensively evaluated than in younger adults, despite the putative influence of aging on depression neuropathology. Studies using transcranial magnetic stimulation (TMS), a neurophysiological tool capable of probing inhibitory and excitatory cortical neurotransmission, have identified dysfunctional GABAergic inhibitory activity in younger adults with depression. However, GABAergic and glutamatergic cortical neurotransmission have not yet been studied in late-life depression (LLD). Here, we used single- and paired-pulse TMS to measure cortical inhibition and excitation in 92 LLD patients and 41 age-matched healthy controls. To differentiate the influence of age and depression, we also compared these TMS indices to those of 30 younger depressed adults and 30 age- and sex-matched younger healthy adults. LLD patients, older healthy adults, and younger depressed adults demonstrated significantly lower GABA_A receptor-mediated cortical inhibition than younger healthy controls. By contrast, no significant differences in cortical inhibition were observed between older adults with and without depression. No significant differences in GABA_B receptor-mediated inhibition or cortical excitation were found between the groups. Altogether, these findings suggest that reduced cortical inhibition may be associated with both advancing age and depression, which (i) supports the model of depression as a disease of accelerated aging, and (ii) prompts future investigation into diminished GABAergic neurotransmission in late-life as a biological predisposing factor to the development of depression. Given that cortical neurophysiology was similar in depressed and healthy older adults, future prospective studies need to establish the relative influence of age and depression on cortical inhibition deficits.

Differential plasticity of extensor and flexor motor cortex representations following visuomotor adaptation

Representations within the primary motor cortex (M1) are capable of rapid functional changes following motor learning, known as use-dependent plasticity. GABAergic inhibition plays a role in use-dependent plasticity. Evidence suggests a different capacity for plasticity of distal and proximal upper limb muscle representations. However, it is unclear whether the motor cortical representations of forearm flexor and extensor muscles also have different capacities for plasticity. The current study used transcranial magnetic stimulation to investigate motor cortex excitability and inhibition of forearm flexor and extensor representations before and after performance of a visuomotor adaptation task that primarily targeted flexors and extensors separately. There was a decrease in extensor and flexor motor-evoked potential (MEP) amplitude after performing the extensor adaptation, but no change in flexor and extensor MEP amplitude after performing the flexor adaptation. There was also a decrease in motor cortical inhibition in the extensor following extensor adaptation, but no change in motor cortical inhibition in the flexor muscle following flexor adaptation or either of the non-prime mover muscles. Findings suggest that the forearm extensor motor cortical representation exhibits plastic change following adaptive motor learning, and broadly support the distinct neural control of forearm flexor and extensor muscles.

The effects of aging on cortico-spinal excitability and motor memory consolidation

We investigated whether cortico-spinal excitability (CSE), a marker of synaptic plasticity, is associated with age-related differences in the consolidation of motor memory. Young and older participants practiced a visuomotor tracking task. Skill retention was assessed 8 and 24 hours after motor practice. Transcranial magnetic stimulation applied over the primary motor cortex at rest and during an isometric muscle contraction was used to assess absolute and normalized to baseline CSE at different points after practice. When skill performance was normalized to baseline level, both groups showed similar gains in acquisition, but the young group showed better retention 24 hours after practice. The young group also showed greater absolute CSE assessed during the isometric muscle contraction. Although young participants with greater absolute CSE showed better skill retention, it was the capacity to increase CSE after motor practice, and not absolute CSE, what was associated with skill retention in older participants. Older adults who have the capacity to increase CSE during motor memory consolidation show a better capacity to retain motor skills.

Learning new gait patterns: Age-related differences in skill acquisition and interlimb transfer

Evidence from upper-extremity literature suggests that the normal ageing process affects an individual's ability to learn and retain a motor skill, but spares their ability to transfer the skill to the untrained, opposite limb. While this phenomenon has been well-studied in the upper-extremity, evidence in the lower-extremity is limited. Further, it is unclear to what extent age-related differences in motor learning and transfer are dependent on visual feedback of the motor task. Therefore, the purpose of this study was to examine the effects of ageing on motor learning, retention, and interlimb transfer during walking with and without visual feedback. Forty-four subjects (24 young; 20 older adults) were tested on a treadmill over two consecutive days. On day 1, subjects learned a new gait pattern by performing a foot-trajectory tracking task that necessitated greater hip and knee flexion during the swing phase of the gait. On day 2, subjects repeated the task with their training leg to test retention, then with their untrained leg to test interlimb transfer. Trials without visual feedback were also collected on both days. Results indicated that older adults had reduced ability to learn the task, and also exhibited lower retention and inter-limb transfer. However, these differences were dependent on visual feedback as the groups performed similarly when feedback was removed. The findings provide novel evidence indicating that ageing impairs learning, retention, and transfer of motor skills in the lower-extremity during walking, which may have implications for gait therapy after stroke and other geriatric conditions.