Age-related weakness of proximal muscle studied with motor cortical mapping: a TMS study

You Are as Old as the Connectivity You Keep: Distinct Neurophysiological Mechanisms Underlying Age-Related Changes in Hand Dexterity and Strength

BACKGROUND

Aging can lead to a decline in motor control. While age-related motor impairments have been documented, the underlying changes in cortico-cortical interactions remain poorly understood.

METHODS

We took advantage of the high temporal resolution of dual-site transcranial magnetic stimulation (dsTMS) to investigate how communication between higher-order rostral premotor regions and the primary motor cortex (M1) influences motor control in young and elderly adults. We assessed the dynamics of connectivity from the inferior frontal gyrus (IFG) or pre-supplementary motor area (preSMA) to M1, by testing how conditioning of the IFG/preSMA affected the amplitude of motor evoked potentials (MEPs) induced by M1 stimulation at different temporal intervals. Moreover, we explored how age-related changes in premotor-M1 interactions relate to motor performance.

RESULTS

Our results show that both young and elderly adults had excitatory IFG-M1 and preSMA-M1 interactions, but the two groups' timing and strength differed. In young adults, IFG-M1 interactions were early and time-specific (8 ms), whereas in older individuals, they were delayed and more prolonged (12-16 ms). PreSMA-M1 interactions emerged early (6 ms) and peaked at 10-12 ms in young individuals but were attenuated in older individuals. Critically, a connectivity profile of the IFG-M1 circuit like that of the young cohort predicted better dexterity in older individuals, while preserved preSMA-M1 interactions predicted greater strength, suggesting that age-related motor decline is associated with specific changes in premotor-motor networks.

CONCLUSIONS

Preserving youthful motor network connectivity in older individuals is related to maintaining motor performance and providing information for interventions targeting aging effects on behavior.

Characterisation of motor cortex organisation in patients with different presentations of persistent low back pain

Persistence of low back pain is thought to be associated with different underlying pain mechanisms, including ongoing nociceptive input and central sensitisation. We hypothesised that primary motor cortex (M1) representations of back muscles (a measure of motor system adaptation) would differ between pain mechanisms, with more consistent observations in individuals presumed to have an ongoing contribution of nociceptive input consistently related to movement/posture. We tested 28 participants with low back pain sub-grouped by the presumed underlying pain mechanisms: nociceptive pain, nociplastic pain and a mixed group with features consistent with both. Transcranial magnetic stimulation was used to study M1 organisation of back muscles. M1 maps of multifidus (deep and superficial) and longissimus erector spinae were recorded with fine-wire electromyography and thoracic erector spinae with surface electromyography. The nociplastic pain group had greater variability in M1 map location (centre of gravity) than other groups (p < .01), which may suggest less consistency, and perhaps relevance, of motor cortex adaptation for that group. The mixed group had greater overlap of M1 representations between deep/superficial muscles than nociceptive pain (deep multifidus/longissimus: p = .001, deep multifidus/thoracic erector spinae: p = .008) and nociplastic pain (deep multifidus/longissimus: p = .02, deep multifidus/thoracic erector spinae: p = .02) groups. This study provides preliminary evidence of differences in M1 organisation in subgroups of low back pain classified by likely underlying pain mechanisms. Despite the sample size, differences in cortical re-organisation between subgroups were detected. Differences in M1 organisation in subgroups of low back pain supports tailoring of treatment based on pain mechanism and motor adaptation.

Corticospinal excitability and motor representation after long-term resistance training

It is poorly understood how the central nervous system adapts to resistance training, especially after years of exposure. We compared corticospinal excitability and motor representation assessed with transcranial magnetic stimulation (TMS) between long-term resistance trained (LRT, ≥3 years) versus untrained (UNT) males (n = 15/group). Motor-evoked potentials (MEPs) were obtained from the biceps brachii during isometric elbow flexion. Stimulus-response curves were created at the hotspot during 10% maximum voluntary torque (MVT) contractions. Maximum peak-to-peak MEP amplitude (MEPmax) was acquired with 100% stimulator output intensity, whilst 25%-100% MVT was produced. Maps were created during 10% MVT contractions, with an individualised TMS intensity eliciting 20% MEPmax at the hotspot. LRT had a 48% lower stimulus-response curve slope than UNT (p < .05). LRT also had a 66% larger absolute map size, although TMS intensity used for mapping was greater in LRT versus UNT (48% vs. 26% above active motor threshold) to achieve a target 20% MEPmax at the hotspot, due to the lower slope of LRT. Map size was strongly correlated with the TMS intensity used for mapping (r = 0.776, p < .001). Once map size was normalised to TMS intensity, there was no difference between the groups (p = .683). We conclude that LRT had a lower stimulus-response curve slope/excitability, suggesting higher neural efficiency. TMS map size was overwhelmingly determined by TMS intensity, even when the MEP response at the hotspot was matched among individuals, likely due to larger current spread with higher intensities. Motor representation appears similar between LRT and UNT given no difference in the normalised map size.

Age Differences in Motor Recruitment Patterns of the Shoulder in Dynamic and Isometric Contractions. A Cross-Sectional Study

Aging processes in the musculoskeletal system lead to functional impairments that restrict participation. Purpose: To assess differences in the force and motor recruitment patterns of shoulder muscles between age groups to understand functional disorders. A cross-sectional study comparing 30 adults (20–64) and 30 older adults (>65). Surface electromyography (sEMG) of the middle deltoid, upper and lower trapezius, infraspinatus, and serratus anterior muscles was recorded. Maximum isometric voluntary contraction (MIVC) was determined at 45° glenohumeral abduction. For the sEMG signal registration, concentric and eccentric contraction with and without 1 kg and isometric contraction were requested. Participants abducted the arm from 0° up to an abduction angle of 135° for concentric and eccentric contraction, and from 0° to 45°, and remained there at 80% of the MIVC level while isometrically pushing against a handheld dynamometer. Differences in sEMG amplitudes (root mean square, RMS) of all contractions, but also onset latencies during concentric contraction of each muscle between age groups, were analyzed. Statistical differences in strength (Adults > Older adults; 0.05) existed between groups. No significant differences in RMS values of dynamic contractions were detected, except for the serratus anterior, but there were for isometric contractions of all muscles analyzed (Adults > Older adults; 0.05). The recruitment order varied between age groups, showing a general tendency towards delayed onset times in older adults, except for the upper trapezius muscle. Age differences in muscle recruitment patterns were found, which underscores the importance of developing musculoskeletal data to prevent and guide geriatric shoulder pathologies.

Hand Focused Upper Extremity Rehabilitation in the Subacute Phase Post-stroke Using Interactive Virtual Environments

Introduction: Innovative motor therapies have attempted to reduce upper extremity impairment after stroke but have not made substantial improvement as over 50% of people post-stroke continue to have sensorimotor deficits affecting their self-care and participation in daily activities. Intervention studies have focused on the role of increased dosing, however recent studies have indicated that timing of rehabilitation interventions may be as important as dosing and importantly, that dosing and timing interact in mediating effectiveness. This study is designed to empirically test dosing and timing. Methods and Analysis: In this single-blinded, interventional study, subjects will be stratified on two dimensions, impairment level (Fugl-Meyer Upper Extremity Assessment (FM) and presence or absence of Motor Evoked Potentials (MEPs) as follows; (1) Severe, FM score 10-19, MEP+, (2) Severe, FM score 10-19, MEP-, (3) Moderate, FM score 20-49, MEP+, (4) Moderate, FM score 20-49, MEP-. Subjects not eligible for TMS will be assigned to either group 2 (if severe) or group 3 (if moderate). Stratified block randomization will then be used to achieve a balanced assignment. Early Robotic/VR Therapy (EVR) experimental group will receive in-patient usual care therapy plus an extra 10 h of intensive upper extremity therapy focusing on the hand using robotically facilitated rehabilitation interventions presented in virtual environments and initiated 5-30 days post-stroke. Delayed Robotic/VR Therapy (DVR) experimental group will receive the same intervention but initiated 30-60 days post-stroke. Dose-matched usual care group (DMUC) will receive an extra 10 h of usual care initiated 5-30 days post-stroke. Usual Care Group (UC) will receive the usual amount of physical/occupational therapy. Outcomes: There are clinical, neurophysiological, and kinematic/kinetic measures, plus measures of daily arm use and quality of life. Primary outcome is the Action Research Arm Test (ARAT) measured at 4 months post-stroke. Discussion: Outcome measures will be assessed to determine whether there is an early time period in which rehabilitation will be most effective, and whether there is a difference in the recapture of premorbid patterns of movement vs. the development of an efficient, but compensatory movement strategy. Ethical Considerations: The IRBs of New Jersey Institute of Technology, Rutgers University, Northeastern University, and Kessler Foundation reviewed and approved all study protocols. Study was registered in https://ClinicalTrials.gov (NCT03569059) prior to recruitment. Dissemination will include submission to peer-reviewed journals and professional presentations.

Case studies in neuroscience: deep brain stimulation changes upper limb cortical motor maps in dystonia

Deep brain stimulation of the globus pallidus pars interna (GPi-DBS) is an effective treatment for primary dystonia; however, its therapeutic mechanism is poorly understood. Because improvement is gradual, GPi-DBS treatment likely involves short- and long-term mechanisms. Abnormal plasticity resulting in somatotopic reorganization is involved in the development of dystonia and has been proposed as a possible mechanism for this gradual improvement, yet it has not been directly investigated. We hypothesized that GPi-DBS will lead to progressive changes in the cortical representations (motor maps) of upper limb muscles. Neuronavigated robotic transcranial magnetic stimulation was used to map the cortical representation of five upper limb muscles in six healthy controls and a 45-yr-old female cervical dystonia patient before (Pre) and at four time points (Post5 to Post314), 5 to 314 days after GPi-DBS. Motor map area and volume decreased in all muscles following GPi-DBS, while changes in overlap and center of gravity distance between muscles were variable. Despite these motor map changes, only dystonic tremor improved after a year of DBS; neck position worsened slightly. These preliminary findings suggest that GPi-DBS may reduce the cortical representation and excitability of upper limb muscles in dystonia and that these changes can occur without clinical improvement.NEW & NOTEWORTHY Neuronavigated robotic transcranial magnetic stimulation was used to investigate changes in upper limb muscle representation in a cervical dystonia patient before and at four time points up to 314 days after globus pallidus pars interna deep brain stimulation (GPi-DBS). GPi-DBS altered excitability and motor cortical representation of upper limb muscles; however, these changes were not associated with clinical improvement.

Possible Contributions of Ipsilateral Pathways From the Contralesional Motor Cortex to the Voluntary Contraction of the Spastic Elbow Flexors in Stroke Survivors: A TMS Study

OBJECTIVE

The contribution of the contralesional motor cortex to the impaired limbs is still controversial. The aim of this study was to investigate the role of descending projections from the contralesional hemisphere during voluntary elbow flexion on the paretic side.

DESIGN

Eleven healthy and 10 stroke subjects performed unilateral isometric elbow flexion tasks at various submaximal levels. Transcranial magnetic stimulation was delivered to the hotspot of biceps muscles ipsilateral to the target side (paretic side in stroke subjects or right side in controls) at rest and during elbow flexion tasks. Motor-evoked potential amplitudes of the contralateral resting biceps muscles, transcranial magnetic stimulation-induced ipsilateral force increment, and reflex torque and weakness of spastic elbow flexors were quantified.

RESULTS

The normalized motor-evoked potential amplitude increased with force level in both healthy and stroke subjects. However, stroke subjects exhibited significantly higher force increment compared with healthy subjects only at low level of elbow flexion but similar at moderate to high levels. The greater force increment significantly correlated with reflex torque of the spastic elbow flexors, but not weakness.

CONCLUSIONS

These results provide novel evidence that ipsilateral projections are not likely to contribute to strength but are correlated to spasticity of spastic-paretic elbow flexors after stroke.

TMS brain mapping of the pharyngeal cortical representation in healthy subjects

BACKGROUND

Brain mapping is fundamental to understanding brain organization and function. However, a major drawback to the traditional Brodmann parcellation technique is the reliance on the use of postmortem specimens. It has therefore historically been difficult to make any comparison regarding functional data from different regions or hemispheres within the same individual. Moreover, this method has been significant limited by subjective boundaries and classification criteria and therefore suffer from reproducibility issues. The development of transcranial magnetic stimulation (TMS) offers an alternative approach to brain mapping, specifically the motor cortical regions by eliciting quantifiable functional reactions.

OBJECTIVE

To precisely describe the motor cortical topographic representation of pharyngeal constrictor musculature using TMS and to further map the brain for use as a tool to study brain plasticity.

METHODS

51 healthy subjects (20 male/31 female, 19-26 years old) were tested using single-pulse TMS combined with intraluminal catheter-guided high-resolution manometry and a standardized grid cap. We investigated various parameters of the motor-evoked potential (MEP) that include the motor map area, amplitude, latency, center of gravity (CoG) and asymmetry index.

RESULTS

Cortically evoked response latencies were similar for the left and right hemispheres at 6.79 ± 0.22 and 7.24 ± 0.27 ms, respectively. The average scalp positions (relative to the vertex) of the pharyngeal motor cortical representation were 10.40 ± 0.19 (SE) cm medio-lateral and 3.20 ± 0.20 (SE) cm antero-posterior in the left hemisphere and 9.65 ± 0.24 (SE) cm medio-lateral and 3.18 ± 0.23 (SE) cm antero-posterior in the right hemisphere. The mean motor map area of the pharynx in the left and right hemispheres were 9.22 ± 0.85(SE) cm²and 10.12 ± 1.24(SE) cm², respectively. The amplitudes of the MEPs were 35.94 ± 1.81(SE)uV in the left hemisphere and 34.49 ± 1.95(SE)uV in the right hemisphere. By comparison, subtle but consistent differences in the degree of the bilateral hemispheric representation were also apparent both between and within individuals.

CONCLUSION

The swallowing musculature has a bilateral motor cortical representation across individuals, but is largely asymmetric within single subjects. These results suggest that TMS mapping using a guided intra-pharyngeal EMG catheter combined with a standardized gridded cap might be a useful tool to localize brain function/dysfunction by linking brain activation to the corresponding physical reaction.

Functional and structural asymmetry in primary motor cortex in Asperger syndrome: a navigated TMS and imaging study

Motor functions are frequently impaired in Asperger syndrome (AS). In this study, we examined the motor cortex structure and function using navigated transcranial magnetic stimulation (nTMS) and voxel-based morphometry (VBM) and correlated the results with the box and block test (BBT) of manual dexterity and physical activity in eight boys with AS, aged 8–11 years, and their matched controls. With nTMS, we found less focused cortical representation areas of distinct hand muscles in AS. There was hemispheric asymmetry in the motor maps, silent period duration and active MEP latency in the AS group, but not in controls. Exploratory VBM analysis revealed less gray matter in the left postcentral gyrus, especially in the face area, and less white matter in the precentral area in AS as compared to controls. On the contrary, in the right leg area, subjects with AS displayed an increased density of gray matter. The structural findings of the left hemisphere correlated negatively with BBT score in controls, whereas the structure of the right hemisphere in the AS group correlated positively with motor function as assessed by BBT. These preliminary functional (neurophysiological and behavioral) findings are indicative of asymmetry, and co-existing structural alterations may reflect the motor impairments causing the deteriorations in manual dexterity and other motor functions commonly encountered in children with AS.

Motor cortex representation of deep and superficial neck flexor muscles in individuals with and without neck pain

Sensorimotor control of neck muscles differs between individuals with and without pain. Differences in the primary motor cortex (M1) maps of these muscles may be involved. This study compared M1 representations of deep (DNF) and superficial (SNF) neck flexor muscles between 10 individuals with neck pain (NP) and 10 painfree controls. M1 organisation was studied using transcranial magnetic stimulation (TMS) applied to a grid over the skull and surface electromyography of DNF (pharyngeal electrode) and SNF. Three-dimensional maps of M1 representation of each muscle were generated. Peaks in the SNF map that represented the sternocleidomastoid (SCM) and platysma muscles were identified. Unique centre of gravity (CoG)/map peaks were identified for the three muscles. In comparison to painfree controls, NP participants had more medial location of the CoG/peak of DNF, SCM, and platysma, greater mediolateral variation in DNF CoG (p = 0.02), fewer SNF and DNF map peaks (p = 0.01). These data show that neck flexor muscle M1 maps relate to trunk, neck, and face areas of the motor homunculus. Differences in M1 representation in NP have some similarities and some differences with observations for other musculoskeletal pain conditions. Despite the small sample size, our data did reveal differences and is comparable to other similar studies. The results of this study should be interpreted with consideration of methodological issues.

Anticipatory postural adjustments and spatial organization of motor cortex: evidence of adaptive compensations in healthy older adults

During anticipated postural perturbations induced by limb movement, the central nervous system generates anticipatory postural adjustments (APAs) in the trunk and hip musculature to minimize disturbances to equilibrium. Age-related changes in functional organization of the nervous system may contribute to changes in APAs in healthy older adults. Here we examined if altered APAs of trunk/hip musculature in older adults are accompanied by changes in the representation of these muscles in motor cortex. Twelve healthy older adults, 5 with a history of falls and 7 nonfallers, were compared with 13 young adults. APAs were assessed during a mediolateral arm raise task in standing. Temporal organization of postural adjustments was quantified as latency of APAs in the contralateral external oblique, lumbar paraspinals, and gluteus medius relative to activation of the deltoid. Spatial organization was quantified as extent of synergistic coactivation between muscles. Volume and location of the muscle representations in motor cortex were mapped using transcranial magnetic stimulation. We found that older adults demonstrated significantly delayed APAs in the gluteus medius muscle. Spatial organization of the three muscles in motor cortex differed between groups, with the older adults demonstrating more lateral external oblique representation than the other two muscles. Separate comparisons of the faller and nonfaller subgroups with young adults indicated that nonfallers had the greatest delay in gluteus medius APAs and a reduced distance between the representational areas of the lumbar paraspinals and gluteus medius. This study indicates that altered spatial organization of motor cortex accompanies altered temporal organization of APA synergies in older adults.

NEW & NOTEWORTHY Anticipatory postural adjustments are a critical component of postural control. Here we demonstrate that, in healthy older adults with and without a history of falls, delayed anticipatory postural adjustments in the hip musculature during mediolateral perturbations are accompanied by altered organization of trunk/hip muscle representation in motor cortex. The largest adaptations are evident in older adults with no history of falls.

Dynamic cortical participation during bilateral, cyclical ankle movements: effects of aging

The precise role of the human primary motor cortex in walking is unknown. Our previous study showed that the primary motor cortex may contribute to specific requirements of walking (i.e., maintaining a constant movement frequency and bilaterally coordinating the feet). Because aging can impair (i) the ability to fulfill the aforementioned requirements and (ii) corticomuscular communication, we hypothesized that aging would impair the motoneuronal recruitment by the primary motor cortex during bilateral cyclical movements. Here, we used corticomuscular coherence (i.e., coherence between the primary motor cortex and the active muscles) to examine whether corticomuscular communication is affected in older individuals during cyclical movements that shared some functional requirements with walking. Fifteen young men and 9 older men performed cyclical, anti-phasic dorsiflexion and plantarflexion of the feet while seated. Coherence between the midline primary motor cortex and contracting leg muscles cyclically increased in both age groups. However, the coherence of older participants was characterized by (i) lower magnitude and (ii) mediolaterally broader and more rostrally centered cortical distributions. These characteristics suggest that aging changes how the primary motor cortex participates in the cyclical movements, and such change may extend to walking.

Post-exercise depression following submaximal and maximal isometric voluntary contraction

It is well known that corticomotor excitability is altered during the post-exercise depression following fatigue within the primary motor cortex (M1). However, it is currently unknown whether corticomotor reorganization following muscle fatigue differs between magnitudes of force and whether corticomotor reorganization occurs measured with transcranial magnetic stimulation (TMS). Fifteen young healthy adults (age 23.8±1.4, 8 females) participated in a within-subjects, repeated measures design study, where they underwent three testing sessions separated by one-week each. Subjects performed separate sessions of each: low-force isometric contraction (30% maximal voluntary contraction [MVC]), high-force isometric contraction (95% MVC) of the first dorsal interosseous (FDI) muscle until self-perceived exhaustion, as well as one session of a 30-min rest as a control. We examined changes in corticomotor map area, excitability and location of the FDI representation in and around M1 using TMS. The main finding was that following low-force, but not high-force fatigue (HFF) corticomotor map area and excitability reduced [by 3cm(2) (t(14)=-2.94, p=0.01) and 56% respectively t(14)=-4.01, p<0.001)]. Additionally, the region of corticomotor excitability shifted posteriorly (6.4±2.5mm) (t(14)=-6.33, p=.019). Corticomotor output became less excitable particularly in regions adjoining M1. Overall, post-exercise depression is present in low-force, but not for HFF. Further, low-force fatigue (LFF) results in a posterior shift in corticomotor output. These changes may be indicative of increased sensory feedback from the somatosensory cortex during the recovery phase of fatigue.

The Effects of Dietary Omega-3s on Muscle Composition and Quality in Older Adults

This review will focus on findings from the few studies performed to date in humans to examine changes in muscle protein turnover, lean or muscle mass and physical function following fish oil-derived omega-3 fatty acid treatment. Although considerable gaps in our current knowledge exist, hypertrophic responses (e.g., improvements in the rate of muscle protein synthesis and mTOR signaling during increased amino acid availability and an increase in muscle volume) have been reported in older adults following prolonged (8 to 24 weeks) of omega-3 fatty acid supplementation. There is also accumulating evidence that increased omega-3 fatty acid levels in red blood cells are positively related to strength and measures of physical function. As a result, increased omega-3 fatty acid consumption may prove to be a promising low-cost dietary approach to attenuate or prevent aging associated declines in muscle mass and function.

Successful aging: Advancing the science of physical independence in older adults

The concept of 'successful aging' has long intrigued the scientific community. Despite this long-standing interest, a consensus definition has proven to be a difficult task, due to the inherent challenge involved in defining such a complex, multi-dimensional phenomenon. The lack of a clear set of defining characteristics for the construct of successful aging has made comparison of findings across studies difficult and has limited advances in aging research. A consensus on markers of successful aging is furthest developed is the domain of physical functioning. For example, walking speed appears to be an excellent surrogate marker of overall health and predicts the maintenance of physical independence, a cornerstone of successful aging. The purpose of the present article is to provide an overview and discussion of specific health conditions, behavioral factors, and biological mechanisms that mark declining mobility and physical function and promising interventions to counter these effects. With life expectancy continuing to increase in the United States and developed countries throughout the world, there is an increasing public health focus on the maintenance of physical independence among all older adults.

Reproducibility of transcranial magnetic stimulation metrics in the study of proximal upper limb muscles

OBJECTIVE

Reproducibility of transcranial magnetic stimulation (TMS) metrics is essential in accurately tracking recovery and disease. However, majority of evidence pertains to reproducibility of metrics for distal upper limb muscles. We investigate for the first time, reliability of corticospinal physiology for a large proximal muscle - the biceps brachii and relate how varying statistical analyses can influence interpretations.

METHODS

14 young right-handed healthy participants completed two sessions assessing resting motor threshold (RMT), motor evoked potentials (MEPs), motor map and intra-cortical inhibition (ICI) from the left biceps brachii. Analyses included paired t-tests, Pearson's, intra-class (ICC) and concordance correlation coefficients (CCC) and Bland-Altman plots.

RESULTS

Unlike paired t-tests, ICC, CCC and Pearson's were >0.6 indicating good reliability for RMTs, MEP intensities and locations of map; however values were <0.3 for MEP responses and ICI.

CONCLUSIONS

Corticospinal physiology, defining excitability and output in terms of intensity of the TMS device, and spatial loci are the most reliable metrics for the biceps. MEPs and variables based on MEPs are less reliable since biceps receives fewer cortico-motor-neuronal projections. Statistical tests of agreement and associations are more powerful reliability indices than inferential tests.

SIGNIFICANCE

Reliable metrics of proximal muscles when translated to a larger number of participants would serve to sensitively track and prognosticate function in neurological disorders such as stroke where proximal recovery precedes distal.

Weaker Seniors Exhibit Motor Cortex Hypoexcitability and Impairments in Voluntary Activation

BACKGROUND

Weakness predisposes seniors to a fourfold increase in functional limitations. The potential for age-related degradation in nervous system function to contribute to weakness and physical disability has garnered much interest of late. In this study, we tested the hypothesis that weaker seniors have impairments in voluntary (neural) activation and increased indices of GABAergic inhibition of the motor cortex, assessed using transcranial magnetic stimulation.

METHODS

Young adults (N = 46; 21.2±0.5 years) and seniors (N = 42; 70.7±0.9 years) had their wrist flexion strength quantified along with voluntary activation capacity (by comparing voluntary and electrically evoked forces). Single-pulse transcranial magnetic stimulation was used to measure motor-evoked potential amplitude and silent period duration during isometric contractions at 15% and 30% of maximum strength. Paired-pulse transcranial magnetic stimulation was used to measure intracortical facilitation and short-interval and long-interval intracortical inhibition. The primary analysis compared seniors to young adults. The secondary analysis compared stronger seniors (top two tertiles) to weaker seniors (bottom tertile) based on strength relative to body weight.

RESULTS

The most novel findings were that weaker seniors exhibited: (i) a 20% deficit in voluntary activation; (ii) ~20% smaller motor-evoked potentials during the 30% contraction task; and (iii) nearly twofold higher levels of long-interval intracortical inhibition under resting conditions.

CONCLUSIONS

These findings indicate that weaker seniors exhibit significant impairments in voluntary activation, and that this impairment may be mechanistically associated with increased GABAergic inhibition of the motor cortex.

Rethinking stimulation of the brain in stroke rehabilitation: why higher motor areas might be better alternatives for patients with greater impairments

Stimulating the brain to drive its adaptive plastic potential is promising to accelerate rehabilitative outcomes in stroke. The ipsilesional primary motor cortex (M1) is invariably facilitated. However, evidence supporting its efficacy is divided, indicating that we may have overgeneralized its potential. Since the M1 and its corticospinal output are frequently damaged in patients with serious lesions and impairments, ipsilesional premotor areas (PMAs) could be useful alternates instead. We base our premise on their higher probability of survival, greater descending projections, and adaptive potential, which is causal for recovery across the seriously impaired. Using a conceptual model, we describe how chronically stimulating PMAs would strongly affect key mechanisms of stroke motor recovery, such as facilitating the plasticity of alternate descending output, restoring interhemispheric balance, and establishing widespread connectivity. Although at this time it is difficult to predict whether PMAs would be "better," it is important to at least investigate whether they are reasonable substitutes for the M1. Even if the stimulation of the M1 may benefit those with maximum recovery potential, while that of PMAs may only help the more disadvantaged, it may still be reasonable to achieve some recovery across the majority rather than stimulate a single locus fated to be inconsistently effective across all.