Central nervous adaptations following 1 wk of wrist and hand immobilization

Changes in brain functional connectivity and muscle strength independent of elbow flexor atrophy following upper limb immobilization in young females

Muscle disuse induces a decline in muscle strength that exceeds the rate and magnitude of muscle atrophy, suggesting that factors beyond the muscle contribute to strength loss. The purpose of this study was to characterize changes in the brain and neuromuscular system in addition to muscle size following upper limb immobilization in young females. Using a within-participant, unilateral design, 12 females (age: 20.6 ± 2.1 years) underwent 14 days of upper arm immobilization using an elbow brace and sling. Bilateral measures of muscle strength (isometric and isokinetic dynamometry), muscle size (magnetic resonance imaging), voluntary muscle activation capacity, corticospinal excitability, cortical thickness and resting-state functional connectivity were collected before and after immobilization. Immobilization induced a significant decline in isometric elbow flexion (-21.3 ± 19.2%, interaction: P = 0.0440) and extension (-19.9 ± 15.7%, interaction: P = 0.0317) strength in the immobilized arm only. There was no significant effect of immobilization on elbow flexor cross-sectional area (CSA) (-1.2 ± 2.4%, interaction: P = 0.466), whereas elbow extensor CSA decreased (-2.9 ± 2.9%, interaction: P = 0.0177) in the immobilized arm. Immobilization did not differentially alter voluntary activation capacity, corticospinal excitability, or cortical thickness (P > 0.05); however, there were significant changes in the functional connectivity of brain regions related to movement planning and error detection (P < 0.05). This study reveals that elbow flexor strength loss can occur in the absence of significant elbow flexor muscle atrophy, and that the brain represents a site of functional adaptation in response to upper limb immobilization in young females.

Exploring the peripheral mechanisms of lower limb immobilisation on muscle function using novel electrophysiological methods

OBJECTIVE

To explore the effects of short-term immobilisation and subsequent retraining on peripheral nervous system (PNS) measures using two novel electrophysiological methods, muscle velocity recovery cycles (MVRC) and MScanFit motor unit number estimation (MUNE) alongside lower limb muscle strength, muscle imaging and walking capacity.

METHODS

Twelve healthy participants underwent 1-week of ankle immobilisation and 2-weeks of retraining. Assessments before and after immobilisation, and after retraining, included MVRC [muscle membrane properties; muscle relative refractory period (MRRP), early and late supernormality], MScanFit, MRI-scans [muscle contractile cross-sectional area (cCSA)], isokinetic dynamometry [dorsal and plantar flexor muscle strength], and 2-minute maximal walk test [physical function].

RESULTS

After immobilisation, compound muscle action potential (CMAP) amplitude reduced (-1.35[-2.00;-0.69]mV); mean change [95%CI]) alongside reductions in plantar (but not dorsal) flexor muscle cCSA (-124[-246;3]mm2), dorsal flexor muscle strength (isometric -0.06[-0.10;-0.02]Nm/kg, dynamicslow -0.08[-0.11;-0.04]Nm/kg, dynamicfast no changes), plantar flexor muscle strength (isometric -0.20[-0.30;-0.10]Nm/kg, dynamicslow -0.19[-0.28;-0.09]Nm/kg, dynamicfast -0.12[-0.19;-0.05]Nm/kg) and walking capacity (-31[-39;-23]m). After retraining, all immobilisation-affected parameters returned to baseline levels. In contrast, neither MScanFit nor MVRC were affected apart from slightly prolonged MRRP in gastrocnemius.

CONCLUSIONS

PNS do not contribute to the changes observed in muscle strength and walking capacity.

SIGNIFICANCE

Further studies should include both corticospinal and peripheral mechanisms.

Combined effect of contraction type and intensity on corticomuscular coherence during isokinetic plantar flexions

During isometric contractions, corticomuscular coherence (CMC) may be modulated along with the contraction intensity. Furthermore, CMC may also vary between contraction types due to the contribution of spinal inhibitory mechanisms. However, the interaction between the effect of the contraction intensity and of the contraction type on CMC remains hitherto unknown. Therefore, CMC and spinal excitability modulations were compared during submaximal isometric, shortening and lengthening contractions of plantar flexor muscles at 25, 50, and 70% of the maximal soleus (SOL) EMG activity. CMC was computed in the time-frequency domain between the Cz EEG electrode signal and the SOL or medial gastrocnemius (MG) EMG signals. The results indicated that beta-band CMC was decreased in the SOL only between 25 and 50-70% contractions for both isometric and anisometric contractions, but remained similar for all contraction intensities in the MG. Spinal excitability was similar for all contraction intensities in both muscles. Meanwhile a divergence of the EEG and the EMG signals mean frequency was observed only in the SOL and only between 25 and 50-70% contractions, independently from the contraction type. Collectively, these findings confirm an effect of the contraction intensity on beta-band CMC, although it was only measured in the SOL, between low-level and high-level contraction intensities. Furthermore, the current findings provide new evidence that the observed modulations of beta-band CMC with the contraction intensity does not depend on the contraction type or on spinal excitability variations.

Loss of neuromuscular junction integrity and muscle atrophy in skeletal muscle disuse

Physical inactivity (PI) is a major risk factor of chronic diseases. A major aspect of PI is loss of muscle mass and strength. The latter phenomenon significantly impacts daily life and represent a major issue for global health. Understandably, skeletal muscle itself has been the major focus of studies aimed at understanding the mechanisms underlying loss of mass and strength. Relatively lesser attention has been given to the contribution of alterations in somatomotor control, despite the fact that these changes can start very early and can occur at multiple levels, from the cortex down to the neuromuscular junction (NMJ). It is well known that exposure to chronic inactivity or immobilization causes a disproportionate loss of force compared to muscle mass, i.e. a loss of specific or intrinsic whole muscle force. The latter phenomenon may be partially explained by the loss of specific force of individual muscle fibres, but several other players are very likely to contribute to such detrimental phenomenon. Irrespective of the length of the disuse period, the loss of force is, in fact, more than two-fold greater than that of muscle size. It is very likely that somatomotor alterations may contribute to this loss in intrinsic muscle force. Here we review evidence that alterations of one component of somatomotor control, namely the neuromuscular junction, occur in disuse. We also discuss some of the novel players in NMJ stability (e.g., homer, bassoon, pannexin) and the importance of new established and emerging molecular markers of neurodegenerative processes in humans such as agrin, neural-cell adhesion molecule and light-chain neurofilaments.

Effects of short-term arm immobilization on motor skill acquisition

Learning to sequence movements is necessary for skillful interaction with the environment. Neuroplasticity, particularly long-term potentiation (LTP), within sensorimotor networks underlies the acquisition of motor skill. Short-term immobilization of the arm, even less than 12 hours, can reduce corticospinal excitability and increase the capacity for LTP-like plasticity within the contralateral primary motor cortex. However, it is still unclear whether short-term immobilization influences motor skill acquisition. The current study aimed to evaluate the effect of short-term arm immobilization on implicit, sequence-specific motor skill acquisition using a modified Serial Reaction Time Task (SRTT). Twenty young, neurotypical adults underwent a single SRTT training session after six hours of immobilization of the non-dominant arm or an equivalent period of no immobilization. Our results demonstrated that participants improved SRTT performance overall after training, but there was no evidence of an effect of immobilization prior to task training on performance improvement. Further, improvements on the SRTT were not sequence-specific. Taken together, motor skill acquisition for sequential, individuated finger movements improved following training but the effect of six hours of immobilization was difficult to discern.

Immediate and short-term effects of continuous theta burst transcranial magnetic stimulation over contralesional premotor area on post-stroke spasticity in patients with severe hemiplegia: Study protocol for a randomized controlled trial

Background

Post-stroke spasticity is an important complication that greatly affects survivors' functional prognosis and daily activities. Increasing evidence points to aberrant contralesional neuromodulation compensation after brain injury as a possible culprit for increased spasticity in patients with severe stroke. Hyperactivity of the contralesional premotor area (cPMA) was supposed to be highly correlated with this progression. This study aims to demonstrate the immediate and short-term efficacy of continuous theta-burst stimulation (cTBS) targeting cPMA on upper limb spasticity in severe subacute stroke patients.

Methods

This trial is a single-center, prospective, three-group randomized controlled trial. Forty-five eligible patients will be recruited and randomized into three groups: the sham-cTBS group (sham cTBS targeting contralesional PMA), the cTBS-cM1 group (cTBS targeting contralesional M1), and the cTBS-cPMA group (cTBS targeting contralesional PMA). All subjects will undergo comprehensive rehabilitation and the corresponding cTBS interventions once a day, five times a week for 4 weeks. Clinical scales, neurophysiological examinations, and neuroimaging will be used as evaluation tools in this study. As the primary outcome, clinical performance on muscle spasticity of elbow/wrist flexor/extensors and upper-limb motor function will be evaluated with the modified Ashworth scale and the Fugl-Meyer Assessment of Upper Extremity Scale, respectively. These scale scores will be collected at baseline, after 4 weeks of treatment, and at follow-up. The secondary outcomes were neurophysiological examinations and Neuroimaging. In neurophysiological examinations, motor evoked potentials, startle reflex, and H reflexes will be used to assess the excitability of the subject's motor cortex, reticulospinal pathway, and spinal motor neurons, respectively. Results of them will be recorded before and after the first cTBS treatment, at post-intervention (at 4 weeks), and at follow-up (at 8 weeks). Neuroimaging tests with diffusion tensor imaging for all participants will be evaluated at baseline and after the 4-week treatment.

Discussion

Based on the latest research progress on post-stroke spasticity, we innovatively propose a new neuromodulation target for improving post-stroke spasticity via cTBS. We expected that cTBS targeting cPMA would have significant immediate and short-term effects on spasticity and related neural pathways. The effect of cTBS-cPMA may be better than that of cTBS via conventional cM1. The results of our study will provide robust support for the application of cTBS neuromodulation in post-stroke spasticity after a severe stroke.

Clinical trial registration

This trial was registered with chictr.org.cn on June 13, 2022 (protocol version). http://www.chictr.org.cn/showproj.aspx?proj=171759.

Short-term arm immobilization modulates excitability of inhibitory circuits within, and between, primary motor cortices

Previous research has suggested that short-term immobilization of the arm may be a low-cost, non-invasive strategy to enhance the capacity for long-term potentiation (LTP)-like plasticity in primary motor cortex (M1). Short-term immobilization reduces corticospinal excitability (CSE) in the contralateral M1, and interhemispheric inhibition (IHI) from ipsi- onto contralateral M1 is increased. However, it is unclear whether reduced CSE and increased IHI are associated with changes in intracortical inhibition, which has been shown to be important for regulating neuroplasticity in M1. The current study used transcranial magnetic stimulation to evaluate the effects of short-term (6 h) arm immobilization on CSE, IHI, and intracortical inhibition measured bilaterally in 43 neurotypical young adults (23 immobilized). We replicated previous findings demonstrating that immobilization decreased CSE in, and increased IHI onto, the immobilized hemisphere, but a significant change in intracortical inhibition was not observed at the group level. Across individuals, decreased CSE was associated with a decreased short-interval intracortical inhibition, an index of GABAA -ergic inhibition, within the immobilized hemisphere only in the immobilization group. Previous research has demonstrated that decreases in GABAA -ergic inhibition are necessary for the induction of LTP-like plasticity in M1; therefore, decreased intracortical inhibition after short-term arm immobilization may provide a novel mechanism to enhance the capacity for LTP-like plasticity within M1 and may be a potential target for strategies to augment plasticity capacity to enhance motor learning in health and disease.

Exercise following immobility increases lower motor neuron excitability: F-wave and H-reflex studies

OBJECTIVES

The excitability of lower motor neurons can be explored non-invasively by several neurophysiological techniques, e.g., F-wave and H-reflex studies after a period of immobility and then after subsequent exercise. The aim of this study is to investigate the impact of exercise and high frequency repetitive nerve stimulation (RNS) following changes induced by 75 min of immobility.

METHODS

We studied 10 healthy subjects following 75 min lower limb immobility, then randomized to RNS or cycling on different days. The neurophysiological studies of M-response, F-wave latency, F/M amplitude ratio and persistence; H-reflex threshold and latency, H/M amplitude ratio, and homosynaptic depression were performed at baseline, after immobility and immediately following the intervention, using stimulation of posterior tibial and peroneal nerves.

RESULTS

After immobility F-wave latencies were delayed and homosynaptic depression at 2 Hz was increased (p < 0.025). RNS had no effect, but cycling exercise reduced H-reflex latencies (p = 0.025) and decreased homosynaptic depression at 2 Hz.

DISCUSSION

Our findings suggest that both proprioceptive stimulation and supraspinal pathways modulate intraspinal physiological changes after immobility. These observations suggest that specific exercise protocols may be useful in managing patients recovering from periods of immobility.

Reorganization of functional and directed corticomuscular connectivity during precision grip from childhood to adulthood

How does the neural control of fine movements develop from childhood to adulthood? Here, we investigated developmental differences in functional corticomuscular connectivity using coherence analyses in 111 individuals from four different age groups covering the age range 8–30 y. EEG and EMG were recorded while participants performed a uni-manual force-tracing task requiring fine control of force in a precision grip with both the dominant and non-dominant hand. Using beamforming methods, we located and reconstructed source activity from EEG data displaying peak coherence with the EMG activity of an intrinsic hand muscle during the task. Coherent cortical sources were found anterior and posterior to the central sulcus in the contralateral hemisphere. Undirected and directed corticomuscular coherence was quantified and compared between age groups. Our results revealed that coherence was greater in adults (20–30 yo) than in children (8–10 yo) and that this difference was driven by greater magnitudes of descending (cortex-to-muscle), rather than ascending (muscle-to-cortex), coherence. We speculate that the age-related differences reflect maturation of corticomuscular networks leading to increased functional connectivity with age. We interpret the greater magnitude of descending oscillatory coupling as reflecting a greater degree of feedforward control in adults compared to children. The findings provide a detailed characterization of differences in functional sensorimotor connectivity for individuals at different stages of typical ontogenetic development that may be related to the maturational refinement of dexterous motor control.

Motor imagery and action observation following immobilization-induced hypoactivity: a narrative review

BACKGROUND

In sports, the risk of pathology or event that leads to an injury, a cessation of practice or even to an immobilization is high. The subsequent reduction of physical activity, or hypoactivity, induces neural and muscular changes that adversely affect motor skills and functional motor rehabilitation. Because the implementation of physical practice is difficult, if not impossible, during and immediately following injury or immobilization, complementary techniques have been proposed to minimize the deleterious impact of hypoactivity on neuromuscular function.

OBJECTIVE

The current narrative review aimed to discuss the contributions of motor imagery and action observation, which enhance motor (re)learning and induce neural adaptations in both healthy individuals and injured athletes.

METHODS

Online literature research for studies of the effects of motor imagery, action observation and their combination on hypoactivity, extracting relevant publications within the last decade (2009-2020).

RESULTS

From published studies and the authors' knowledge of both motor imagery and action observation, some elements are provided for developing applied protocols during and after the immobilization period. Such interventions consist of associating congruent action observation with kinesthetic motor imagery of different movements, organized in increasing difficulty. The aim is to maintain motor functions and promote motor relearning by activating sensorimotor cortical areas and corticomotor pathways of the injured effector.

CONCLUSION

This narrative review supports the implementation of combined motor imagery and action observation protocols in the context of sports rehabilitation.

Nonuniform loss of muscle strength and atrophy during bed rest: a systematic review

Muscle atrophy and decline in muscle strength appear very rapidly with prolonged disuse or mechanical unloading after acute hospitalization or experimental bed rest. The current study analyzed data from short-, medium-, and long-term bed rest (5-120 days) in a pooled sample of 318 healthy adults and modeled the mathematical relationship between muscle strength decline and atrophy. The results show a logarithmic disuse-induced loss of strength and muscle atrophy of the weight-bearing knee extensor muscles. The greatest rate of muscle strength decline and atrophy occurred in the earliest stages of bed rest, plateauing later, and likely contributed to the rapid neuromuscular loss of function in the early period. In addition, during the first 2 wk of bed rest, muscle strength decline is much faster than muscle atrophy: on day 5, the ratio of muscle atrophy to strength decline as a function of bed rest duration is 4.2, falls to 2.4 on day 14, and stabilizes to a value of 1.9 after ∼35 days of bed rest. Positive regression revealed that ∼79% of the muscle strength loss may be explained by muscle atrophy, while the remaining is most likely due to alterations in single fiber mechanical properties, excitation-contraction coupling, fiber architecture, tendon stiffness, muscle denervation, neuromuscular junction damage, and supraspinal changes. Future studies should focus on neural factors as well as muscular factors independent of atrophy (single fiber excitability and mechanical properties, architectural factors) and on the role of extracellular matrix changes. Bed rest results in nonuniform loss of isometric muscle strength and atrophy over time, where the magnitude of change was greater for muscle strength than for atrophy. Future research should focus on the loss of muscle function and the underlying mechanisms, which will aid in the development of countermeasures to mitigate or prevent the decline in neuromuscular efficiency.NEW & NOTEWORTHY Our study contributes to the characterization of muscle loss and weakness processes reflected by a logarithmic decline in muscle strength induced by chronic bed rest. Acute short-term hospitalization (≤5 days) associated with periods of disuse/immobilization/prolonged time in the supine position in the hospital bed is sufficient to significantly decrease muscle mass and size and induce functional changes related to weakness in maximal muscle strength. By bringing together integrated evaluation of muscle structure and function, this work identifies that 79% of the loss in muscle strength can be explained by muscle atrophy, leaving 21% of the functional loss unexplained. The outcomes of this study should be considered in the development of daily countermeasures for preserving neuromuscular integrity as well as preconditioning interventions to be implemented before clinical bed rest or chronic gravitational unloading (e.g., spaceflights).

Immobilization leads to reduced stretch reflexes but increased central reflex gain in the rat

Plastic adaptations are known to take place in muscles, tendons, joints, and the nervous system in response to changes in muscle activity. However, few studies have addressed how these plastic adaptations are related. Thus this study focuses on changes in the mechanical properties of the ankle plantarflexor muscle-tendon unit, stretch reflex activity, and spinal neuronal pathways in relation to cast immobilization. The left rat hindlimb from toes to hip was immobilized with a plaster cast for 1, 2, 4, or 8 wk followed by acute electrophysiological recordings to investigate muscle stiffness and stretch reflex torque. Moreover, additional acute experiments were performed after 4 wk of immobilization to investigate changes in the central gain of the stretch reflex. Monosynaptic reflexes (MSR) were recorded from the L4 and L5 ventral roots following stimulation of the corresponding dorsal roots. Rats developed reduced range of movement in the ankle joint 2 wk after immobilization. This was accompanied by significant increases in the stiffness of the muscle-tendon complex as well as an arthrosis at the ankle joint at 4 and 8 wk following immobilization. Stretch reflexes were significantly reduced at 4-8 wk following immobilization. This was associated with increased central gain of the stretch reflex. These data show that numerous interrelated plastic changes occur in muscles, connective tissue, and the central nervous system in response to changes in muscle use. The findings provide an understanding of coordinated adaptations in multiple tissues and have important implications for prevention and treatment of the negative consequences of immobilization following injuries of the nervous and musculoskeletal systems.NEW & NOTEWORTHY Immobilization leads to multiple simultaneous adaptive changes in muscle, connective tissue, and central nervous system.

Direct evidence for decreased presynaptic inhibition evoked by PBSt group I muscle afferents after chronic SCI and recovery with step-training in rats

KEY POINTS

Presynaptic inhibition is modulated by supraspinal centres and primary afferents in order to filter sensory information, adjust spinal reflex excitability, and ensure smooth movement. After spinal cord injury (SCI), the supraspinal control of primary afferent depolarization (PAD) interneurons is disengaged, suggesting an increased role for sensory afferents. While increased H-reflex excitability in spastic individuals indicates a possible decrease in presynaptic inhibition, it remains unclear whether a decrease in sensory-evoked PAD contributes to this effect. We investigated whether the PAD evoked by hindlimb afferents contributes to the change in presynaptic inhibition of the H-reflex in a decerebrated rat preparation. We found that chronic SCI decreases presynaptic inhibition of the plantar H-reflex through a reduction in PAD evoked by posterior biceps-semitendinosus (PBSt) muscle group I afferents. We further found that step-training restored presynaptic inhibition of the plantar H-reflex evoked by PBSt, suggesting the presence of activity-dependent plasticity of PAD pathways activated by flexor muscle group I afferents.

ABSTRACT

Spinal cord injury (SCI) results in the disruption of supraspinal control of spinal networks and an increase in the relative influence of afferent feedback to sublesional neural networks, both of which contribute to enhancing spinal reflex excitability. Hyperreflexia occurs in ∼75% of individuals with a chronic SCI and critically hinders functional recovery and quality of life. It is suggested that it results from an increase in motoneuronal excitability and a decrease in presynaptic and postsynaptic inhibitory mechanisms. In contrast, locomotor training decreases hyperreflexia by restoring presynaptic inhibition. Primary afferent depolarization (PAD) is a powerful presynaptic inhibitory mechanism that selectively gates primary afferent transmission to spinal neurons to adjust reflex excitability and ensure smooth movement. However, the effect of chronic SCI and step-training on the reorganization of presynaptic inhibition evoked by hindlimb afferents, and the contribution of PAD has never been demonstrated. The objective of this study is to directly measure changes in presynaptic inhibition through dorsal root potentials (DRPs) and its association with plantar H-reflex inhibition. We provide direct evidence that H-reflex hyperexcitability is associated with a decrease in transmission of PAD pathways activated by posterior biceps-semitendinosus (PBSt) afferents after chronic SCI. More precisely, we illustrate that the pattern of inhibition evoked by PBSt group I muscle afferents onto both L4-DRPs and plantar H-reflexes evoked by the distal tibial nerve is impaired after chronic SCI. These changes are not observed in step-trained animals, suggesting a role for activity-dependent plasticity to regulate PAD pathways activated by flexor muscle group I afferents.

Effect of Immobilisation on Neuromuscular Function In Vivo in Humans: A Systematic Review

BACKGROUND

Muscle strength loss following immobilisation has been predominantly attributed to rapid muscle atrophy. However, this cannot fully explain the magnitude of muscle strength loss, so changes in neuromuscular function (NMF) may be involved.

OBJECTIVES

We systematically reviewed literature that quantified changes in muscle strength, size and NMF following periods of limb immobilisation in vivo in humans.

METHODS

Studies were identified following systematic searches, assessed for inclusion, data extracted and quality appraised by two reviewers. Data were tabulated and reported narratively.

RESULTS

Forty eligible studies were included, 22 immobilised lower and 18 immobilised upper limbs. Limb immobilisation ranged from 12 h to 56 days. Isometric muscle strength and muscle size declined following immobilisation; however, change magnitude was greater for strength than size. Evoked resting twitch force decreased for lower but increased for upper limbs. Rate of force development either remained unchanged or slowed for lower and typically slowed for upper limbs. Twitch relaxation rate slowed for both lower and upper limbs. Central motor drive typically decreased for both locations, while electromyography amplitude during maximum voluntary contractions decreased for the lower and presented mixed findings for the upper limbs. Trends imply faster rates of NMF loss relative to size earlier in immobilisation periods for all outcomes.

CONCLUSIONS

Limb immobilisation results in non-uniform loss of isometric muscle strength, size and NMF over time. Different outcomes between upper and lower limbs could be attributed to higher degrees of central neural control of upper limb musculature. Future research should focus on muscle function losses and mechanisms following acute immobilisation.

REGISTRATION

PROSPERO reference: CRD42016033692.

Corticomuscular interactions during different movement periods in a multi-joint compound movement

While much is known about motor control during simple movements, corticomuscular communication profiles during compound movement control remain largely unexplored. Here, we aimed at examining frequency band related interactions between brain and muscles during different movement periods of a bipedal squat (BpS) task utilizing regression corticomuscular coherence (rCMC), as well as partial directed coherence (PDC) analyses. Participants performed 40 squats, divided into three successive movement periods (Eccentric (ECC), Isometric (ISO) and Concentric (CON)) in a standardized manner. EEG was recorded from 32 channels specifically-tailored to cover bilateral sensorimotor areas while bilateral EMG was recorded from four main muscles of BpS. We found both significant CMC and PDC (in beta and gamma bands) during BpS execution, where CMC was significantly elevated during ECC and CON when compared to ISO. Further, the dominant direction of information flow (DIF) was most prominent in EEG-EMG direction for CON and EMG-EEG direction for ECC. Collectively, we provide novel evidence that motor control during BpS is potentially achieved through central motor commands driven by a combination of directed inputs spanning across multiple frequency bands. These results serve as an important step toward a better understanding of brain-muscle relationships during multi joint compound movements.

Hand rest and wrist support are effective in preventing fatigue during prolonged typing

STUDY DESIGN

Case series (longitudinal).

INTRODUCTION

Only few reports concerning the efficacy of commonly used strategies for preventing upper limb occupational disorders associated with prolonged typing exist.

PURPOSE OF THE STUDY

We aimed to investigate whether the duration of typing and the use of 2 strategies (hand rest and wrist support) changes muscle physiological response and therefore the electromyography records.

METHODS

We enrolled 25 volunteers, who were unfamiliar with the task and did not have musculoskeletal disorders. The subjects underwent 3 prolonged typing protocols to investigate the efficacy of the 2 adopted strategies in reducing the trapezius, biceps brachii, and extensor digitorum communis fatigue.

RESULTS

Typing for 1 hour induced muscular fatigue (60%-67% of the subjects). The extensor digitorum communis muscle exhibited the highest percentage of fatigue (72%-84%) after 1 and 4 hours of typing (1 hour, P = .04; 4 hours, P = .02). Fatigue levels in this muscle were significantly reduced (24%) with the use of pause typing (4 hours, P = .045), whereas biceps brachii muscle fatigue was reduced (32%) only with the use of wrist supports (P = .02, after 4 hours). Trapezius muscle fatigue was unaffected by the tested occupational strategies (1 hour, P = .62; 4 hours, P = .85).

DISCUSSION

Despite presenting an overall tendency for fatigue detected during the application of the protocols, the assessed muscles exhibited different behavior patterns, depending on both the preventive strategy applied and the muscle mechanical role during the task.

CONCLUSION

Hand rest and wrist support can successfully reduce muscle fatigue in specific upper limb muscles during prolonged typing, leading to a muscle-selective reduction in the occurrence of fatigue and thus provide direct evidence that they may prevent work-related musculoskeletal disorders.

LEVEL OF EVIDENCE

N/A.

Neuromuscular Properties of the Human Wrist Flexors as a Function of the Wrist Joint Angle

The joint angle dependence of voluntary activation and twitch properties has been investigated for several human skeletal muscles. However, although they play a key role for hand function and possess a unique neural control compared to muscles surrounding other joint complexes, little is known about the wrist flexors innervated by the median nerve. Therefore, isometric voluntary and electrically evoked contractions of the wrist flexors were analyzed at three wrist joint angles (extension: -30°, neutral: 0°, flexion: 30°) to quantify the joint angle dependence of (i) voluntary activation (assessed via peripheral nerve stimulation and electromyography [EMG]), (ii) unpotentiated twitch torques, and (iii) potentiated twitch torques. Maximum voluntary torque was lower in extension compared to neutral and flexion. Although voluntary activation was generally high, data indicate that voluntary activation of the wrist flexors innervated by the median nerve was lower and the antagonist·agonist^-1 EMG ratio was higher with the wrist joint in flexion compared to extension. Peak twitch torque, rate of twitch torque development, and twitch half-relaxation time increased, whereas electromechanical delay decreased from flexion to extension for the unpotentiated twitch torques. Activity-induced potentiation partly abolished these differences and was higher in short than long wrist flexors. Different angle-dependent excitatory and inhibitory inputs to spinal and supraspinal centers might be responsible for the altered activation of the investigated wrist muscles. Potential mechanisms were discussed and might have operated conjointly to increase stiffness of the flexed wrist joint. Differences in twitch torque properties were probably related to angle-dependent alterations in series elastic properties, actin-myosin interaction, Ca²⁺ sensitivity, and phosphorylation of myosin regulatory light chains. The results of the present study provide valuable information about the contribution of neural and muscular properties to changes in strength capabilities of the wrist flexors at different wrist joint angles. These data could help to understand normal wrist function, which is a first step in determining mechanisms underlying musculoskeletal disorders and in giving recommendations for the restoration of musculoskeletal function after traumatic or overuse injuries.

Effects of short-term immobilization of the upper limb on the somatosensory pathway: a study of short-latency somatosensory evoked potentials

[Purpose] Previous studies have reported that the nervous system is influenced during short-term cast immobilization. However, the effects of short-term inactivity on somatosensory information processing systems are not well understood. This study investigated the effect of 10 h of upper limb immobilization on the somatosensory pathway using short-latency somatosensory evoked potentials. [Participants and Methods] Twenty right-handed healthy participants (mean age 21.7 years) were enrolled in this study. The participants’ left hands and forearms were wrapped in a soft bandage at a 90° elbow flexed position. The participants were instructed not to move their left hand for 10 h. To obtain short-latency somatosensory evoked potentials, we used a multimodal evoked potential system. The left median nerve was electrically stimulated at a rate of 5 Hz for a duration of 0.2 ms. The intensity of the stimulus was adjusted to induce mild twitches of the thumb. The amplitudes and latencies of the short-latency somatosensory evoked potential components (N9, N13, and N20) were measured before and after immobilization. [Results] The amplitude of the N9 component significantly increased after immobilization. [Conclusion] Our results indicated that the changes in the excitability of the peripheral somatosensory nerve were due to 10 h of inactivity.

Clinical and electrophysiological investigation of spastic muscle overactivity in patients with disorders of consciousness following severe brain injury

OBJECTIVE

The clinical and electrophysiological profile of spastic muscle overactivity (SMO) is poorly documented in patients with disorders of consciousness (DOC) following severe cortical and subcortical injury. We aim at investigating the link between the clinical observations of SMO and the electrophysiological spastic over-reactivity in patients with prolonged DOC.

METHODS

We prospectively enrolled adult patients with DOC at least 3 months post traumatic or non-traumatic brain injury. The spastic profile was investigated using the Modified Ashworth Scale and the Hmax/Mmax ratio. T1 MRI data and impact of medication were analyzed as well.

RESULTS

21 patients were included (mean age: 41 ± 11 years; time since injury: 4 ± 5 years; 9 women; 10 traumatic etiologies). Eighteen patients presented signs of SMO and 11 had an increased ratio. Eight patients presented signs of SMO but no increased ratio. We did not find any significant correlation between the ratio and the MAS score for each limb (all ps > 0.05). The presence of medication was not significantly associated with a reduction in MAS scores or Hmax/Mmax ratios.

CONCLUSIONS

In this preliminary study, the Hmax/Mmax ratio does not seem to reflect the clinical MAS scores in patients with DOC. This supports the fact they do not only present spasticity but other forms of SMO and contracture.

SIGNIFICANCE

Patients with DOC are still in need of optimized tools to evaluate their spastic profile and therapeutic approaches should be adapted accordingly.

Interplay between hypoactivity, muscle properties and motor command: How to escape the vicious deconditioning circle?

Activity-dependent processes addressing the central nervous system (CNS) and musculoskeletal structures are critical for maintaining motor performance. Chronic reduction in activity, whether due to a sedentary lifestyle or extended bed rest, results in impaired performance in motor tasks and thus decreased quality of life. In the first part of this paper, we give a narrative review of the effects of hypoactivity on the neuromuscular system and behavioral outcomes. Motor impairments arise from a combination of factors including altered muscle properties, impaired afferent input, and plastic changes in neural structure and function throughout the nervous system. There is a reciprocal interplay between the CNS and muscle properties, and these sensorimotor loops are essential for controlling posture and movement. As a result, patients under hypoactivity experience a self-perpetuating cycle, in with sedentarity leading to decreased motor activity and thus a progressive worsening of a situation, and finally deconditioning. Various rehabilitation strategies have been studied to slow down or reverse muscle alteration and altered motor performance. In the second part of the paper, we review representative protocols directed toward the muscle, the sensory input and/or the cerebral cortex. Improving an understanding of the loss of motor function under conditions of disuse (such as extended bed rest) as well as identifying means to slow this decline may lead to therapeutic strategies to preserve quality of life for a range of individuals. The most efficient strategies seem multifactorial, using a combination of approaches targeting different levels of the neuromuscular system.

Contralateral effects of unilateral training: sparing of muscle strength and size after immobilization

The contralateral effects of unilateral strength training, known as cross-education of strength, date back well over a century. In the last decade, a limited number of studies have emerged demonstrating the preservation or "sparing" effects of cross-education during immobilization. Recently published evidence reveals that the sparing effects of cross-education show muscle site specificity and involve preservation of muscle cross-sectional area. The new research also demonstrates utility of training with eccentric contractions as a potent stimulus to preserve immobilized limb strength across multiple modes of contraction. The cumulative data in nonclinical settings suggest that cross-education can completely abolish expected declines in strength and muscle size in the range of ∼13% and ∼4%, respectively, after 3-4 weeks of immobilization of a healthy arm. The evidence hints towards the possibility that unique mechanisms may be involved in preservation effects of cross-education, as compared with those that lead to functional improvements under normal conditions. Cross-education effects after strength training appear to be larger in clinical settings, but there is still only 1 randomized clinical trial demonstrating the potential utility of cross-education in addition to standard treatment. More work is necessary in both controlled and clinical settings to understand the potential interaction of neural and muscle adaptations involved in the observed sparing effects, but there is growing evidence to advocate for the clinical utility of cross-education.

Nervous system excitability and joint stiffness following short-term dynamic ankle immobilization

Joint immobilization has been demonstrated to modify neural excitability in subsets of healthy populations, leading to disinhibition of cortical and reflexive pathways. However, these findings may have limited clinical application as most models have investigated casting and rigid immobilization, while many musculoskeletal injuries often utilize dynamic immobilization devices such as boot immobilizers and pneumatic splints that allow for modified ambulation. We therefore aimed to determine the short-term effects of ambulation in ankle immobilization devices on nervous system excitability and stiffness in able-bodied individuals. A repeated-measures design was implemented where 12 healthy individuals were tested for cortical excitability to the ankle musculature using transcranial magnetic stimulation, reflexive excitability using the Hoffmann reflex, and ankle joint stiffness using arthrometry before and after 30min of ambulation with a boot immobilizer, pneumatic leg splint, or barefoot. Motor evoked potential (MEP), cortical silent period (CSP), H_max to M_max ratio, and ankle joint displacement were extracted as dependent variables. Results indicated that despite the novel motor demands of walking in immobilization devices, no significant changes in cortical excitability (F≥0.335, P≥0.169), reflexive excitability (F≥0.027, P≥0.083), or joint stiffness (F≥0.558, P≥0.169) occurred. These findings indicate that short-term ambulation in dynamic immobilization devices does not modify neural excitability despite forced constraints on the sensorimotor system. We may therefore conclude that modifications to neural excitability in previous immobilization models are mediated by long-term nervous system plasticity rather than acute mechanisms, and there appear to be no robust changes in corticomotor or spinal excitability acutely posed by ambulation with immobilization devices.

How plastic are human spinal cord motor circuitries?

Human and animal studies have documented that neural circuitries in the spinal cord show adaptive changes caused by altered supraspinal and/or afferent input to the spinal circuitry in relation to learning, immobilization, injury and neurorehabilitation. Reversible adaptations following, e.g. the acquisition or refinement of a motor skill rely heavily on the functional integration between supraspinal and sensory inputs to the spinal cord networks. Accordingly, what is frequently conceived as a change in the spinal circuitry may be a change in either descending or afferent input or in the relative integration of these, i.e. a change in the neuronal weighting. This is evident from findings documenting only task-specific functional changes after periods of altered inputs whereas resting responses remain unaffected. In fact, the proximity of the spinal circuitry to the outer world may demand a more rigid organization compared to the highly flexible cortical circuits. The understanding of all of this is important for the planning and execution of neurorehabilitation.

Motoneuron activity is required for enhancements in functional recovery after peripheral nerve injury in exercised female mice

Inhibitory luminopsins (iLMO2) integrate opto- and chemo-genetic approaches and allow for cell-type specific inhibition of neuronal activity. When exposed to a Renilla luciferase substrate, Coelenterazine (CTZ), iLMO2 generates bioluminescence-mediated activation of its amino-terminal halorhodopsin, resulting in neuronal inhibition. Moderate daily exercise in the form of interval treadmill-training (IT) applied following a peripheral nerve injury results in enhanced motor axon regeneration and muscle fiber reinnervation in female mice. We hypothesized that iLMO2 mediated inhibition of motoneuron activity during IT would block this enhancement. Unilateral intramuscular injections of Cre-dependent AAV2/9-EF1a-DIO-iLMO2 (∼8.5 x 10¹³ vg/ml) were made into the gastrocnemius and tibialis anterior muscles of young female ChAT-IRES-Cre mice, thereby limiting iLMO2 expression specifically to their motoneurons. Four to six weeks were allowed for retrograde viral transduction after which a unilateral sciatic nerve transection (Tx) and repair was performed. Animals were randomized into four groups: IT only, IT + CTZ, CTZ only, and untreated (UT). Three weeks post Tx-repair, the maximal amplitude direct muscle responses (M-max) in both muscles in the IT only group were significantly greater than in UT mice, consistent with the enhancing effects of this exercise regimen. Inhibiting motoneuron activity during exercise by a single injection of CTZ, administered 30 minutes prior to exercise, completely blocked the enhancing effect of exercise. Similar treatments with CTZ in mice without iLMO2 had no effect on regeneration. Neuronal activity is required for successful enhancement of motor axon regeneration by exercise.

Acute and chronic neuromuscular adaptations to local vibration training

Vibratory stimuli are thought to have the potential to promote neural and/or muscular (re)conditioning. This has been well described for whole-body vibration (WBV), which is commonly used as a training method to improve strength and/or functional abilities. Yet, this technique may present some limitations, especially in clinical settings where patients are unable to maintain an active position during the vibration exposure. Thus, a local vibration (LV) technique, which consists of applying portable vibrators directly over the tendon or muscle belly without active contribution from the participant, may present an alternative to WBV. The purpose of this narrative review is (1) to provide a comprehensive overview of the literature related to the acute and chronic neuromuscular changes associated with LV, and (2) to show that LV training may be an innovative and efficient alternative method to the 'classic' training programs, including in the context of muscle deconditioning prevention or rehabilitation. An acute LV application (one bout of 20-60 min) may be considered as a significant neuromuscular workload, as demonstrated by an impairment of force generating capacity and LV-induced neural changes. Accordingly, it has been reported that a training period of LV is efficient in improving muscular performance over a wide range of training (duration, number of session) and vibration (frequency, amplitude, site of application) parameters. The functional improvements are principally triggered by adaptations within the central nervous system. A model illustrating the current research on LV-induced adaptations is provided.

Muscle disuse caused by botulinum toxin injection leads to increased central gain of the stretch reflex in the rat

Botulinum toxin (Btx) is used in children with cerebral palsy and in other neurological patients to diminish spasticity and reduce the risk of development of contractures. We investigated changes in the central gain of the stretch reflex circuitry in response to Btx injection in the triceps surae muscle in rats. Experiments were performed in 21 rats. Eight rats were a control group, and 13 rats were injected with 6 IU of Btx in the left triceps surae muscle. Two weeks after Btx injection, larger monosynaptic reflexes (MSR) were recorded from the left (injected) than the right (noninjected) L4 + L5 ventral roots following stimulation of the corresponding dorsal roots. A similar increase on the left side was observed in response to stimulation of descending motor tracts, suggesting that increased excitability of spinal motor neurons may at least partly explain the increased reflexes. However, significant changes were also observed in postactivation depression of the MSR, suggesting that plastic changes in transmission from Ia afferent to the motor neurons also may be involved. The data demonstrate that muscle paralysis induced by Btx injection is accompanied by plastic adaptations in the central stretch reflex circuitry, which counteract the antispastic effect of Btx.NEW & NOTEWORTHY Injection of botulinum toxin into ankle muscles causes increased gain of stretch reflex. This is caused by adaptive changes in regulation of transmitter release from Ia afferents and increased excitability of spinal motor neurons.

The effects of cervical transcutaneous spinal direct current stimulation on motor pathways supplying the upper limb in humans

Non-invasive, weak direct current stimulation can induce changes in excitability of underlying neural tissue. Many studies have used transcranial direct current stimulation to induce changes in the brain, however more recently a number of studies have used transcutaneous spinal direct current stimulation to induce changes in the spinal cord. This study further characterises the effects following cervical transcutaneous spinal direct current stimulation on motor pathways supplying the upper limb. In Study 1, on two separate days, participants (n = 12, 5 F) received 20 minutes of either real or sham direct current stimulation at 3 mA through electrodes placed in an anterior-posterior configuration over the neck (anode anterior). Biceps brachii, flexor carpi radialis and first dorsal interosseous responses to transcranial magnetic stimulation (motor evoked potentials) and cervicomedullary stimulation (cervicomedullary motor evoked potentials) were measured before and after real or sham stimulation. In Study 2, on two separate days, participants (n = 12, 7 F) received either real or sham direct current stimulation in the same way as for Study 1. Before and after real or sham stimulation, median nerve stimulation elicited M waves and H reflexes in the flexor carpi radialis. H-reflex recruitment curves and homosynaptic depression of the H reflex were assessed. Results show that the effects of real and sham direct current stimulation did not differ for motor evoked potentials or cervicomedullary motor evoked potentials for any muscle, nor for H-reflex recruitment curve parameters or homosynaptic depression. Cervical transcutaneous spinal direct current stimulation with the parameters described here does not modify motor responses to corticospinal stimulation nor does it modify H reflexes of the upper limb. These results are important for the emerging field of transcutaneous spinal direct current stimulation.

Characterization of Involuntary Contractions after Spinal Cord Injury Reveals Associations between Physiological and Self-Reported Measures of Spasticity

Correlations between physiological, clinical and self-reported assessments of spasticity are often weak. Our aims were to quantify functional, self-reported and physiological indices of spasticity in individuals with thoracic spinal cord injury (SCI; 3 women, 9 men; 19–52 years), and to compare the strength and direction of associations between these measures. The functional measure we introduced involved recording involuntary electromyographic activity during a transfer from wheelchair to bed which is a daily task necessary for function. High soleus (SL) and tibialis anterior (TA) F-wave/M-wave area ratios were the only physiological measures that distinguished injured participants from the uninjured (6 women, 13 men, 19–67 years). Hyporeflexia (decreased SL H/M ratio) was unexpectedly present in older participants after injury. During transfers, the duration and intensity of involuntary electromyographic activity varied across muscles and participants, but coactivity was common. Wide inter-participant variability was seen for self-reported spasm frequency, severity, pain and interference with function, as well as tone (resistance to imposed joint movement). Our recordings of involuntary electromyographic activity during transfers provided evidence of significant associations between physiological and self-reported measures of spasticity. Reduced low frequency H-reflex depression in SL and high F-wave/M-wave area ratios in TA, physiological indicators of reduced inhibition and greater motoneuron excitability, respectively, were associated with long duration SL and biceps femoris (BF) electromyographic activity during transfers. In turn, participants reported high spasm frequency when transfers involved short duration TA EMG, decreased co-activation between SL and TA, as well as between rectus femoris (RF) vs. BF. Thus, the duration of muscle activity and/or the time of agonist-antagonist muscle coactivity may be used by injured individuals to count spasms. Intense electromyographic activity and high tone related closely (possibly from joint stabilization), while intense electromyographic activity in one muscle of an agonist-antagonist pair (especially in TA vs. SL, and RF vs. BF) likely induced joint movement and was associated with severe spasms. These data support the idea that individuals with SCI describe their spasticity by both the duration and intensity of involuntary agonist-antagonist muscle coactivity during everyday tasks.

Long-term progressive motor skill training enhances corticospinal excitability for the ipsilateral hemisphere and motor performance of the untrained hand

It is well established that unilateral motor practice can lead to increased performance in the opposite non-trained hand. Here, we test the hypothesis that progressively increasing task difficulty during long-term skill training with the dominant right hand increase performance and corticomotor excitability of the left non-trained hand. Subjects practiced a visuomotor tracking task engaging right digit V for 6 weeks with either progressively increasing task difficulty (PT) or no progression (NPT). Corticospinal excitability (CSE) was evaluated from the resting motor threshold (rMT) and recruitment curve parameters following application of transcranial magnetic stimulation (TMS) to the ipsilateral primary motor cortex (iM1) hotspot of the left abductor digiti minimi muscle (ADM). PT led to significant improvements in left-hand motor performance immediately after 6 weeks of training (63 ± 18%, P < 0.001) and 8 days later (76 ± 14%, P < 0.001). In addition, PT led to better task performance compared to NPT (19 ± 15%, P = 0.024 and 27 ± 15%, P = 0.016). Following the initial training session, CSE increased across all subjects. After 6 weeks of training and 8 days later, only PT was accompanied by increased CSE demonstrated by a left and upwards shift in the recruitment curves, e.g. indicated by increased MEP_max (P = 0.012). Eight days after training similar effects were observed, but 14 months later motor performance and CSE were similar between groups. We suggest that progressively adjusting demands for timing and accuracy to individual proficiency promotes motor skill learning and drives the iM1-CSE resulting in enhanced performance of the non-trained hand. The results underline the importance of increasing task difficulty progressively and individually in skill learning and rehabilitation training.

Botulinum toxin injection causes hyper-reflexia and increased muscle stiffness of the triceps surae muscle in the rat

Botulinum toxin is used with the intention of diminishing spasticity and reducing the risk of development of contractures. Here, we investigated changes in muscle stiffness caused by reflex activity or elastic muscle properties following botulinum toxin injection in the triceps surae muscle in rats. Forty-four rats received injection of botulinum toxin in the left triceps surae muscle. Control measurements were performed on the noninjected contralateral side in all rats. Acute experiments were performed, 1, 2, 4, and 8 wk following injection. The triceps surae muscle was dissected free, and the Achilles tendon was cut and attached to a muscle puller. The resistance of the muscle to stretches of different amplitudes and velocities was systematically investigated. Reflex-mediated torque was normalized to the maximal muscle force evoked by supramaximal stimulation of the tibial nerve. Botulinum toxin injection caused severe atrophy of the triceps surae muscle at all time points. The force generated by stretch reflex activity was also strongly diminished but not to the same extent as the maximal muscle force at 2 and 4 wk, signifying a relative reflex hyperexcitability. Passive muscle stiffness was unaltered at 1 wk but increased at 2, 4, and 8 wk (P < 0.01). These data demonstrate that botulinum toxin causes a relative increase in reflex stiffness, which is likely caused by compensatory neuroplastic changes. The stiffness of elastic elements in the muscles also increased. The data are not consistent with the ideas that botulinum toxin is an efficient antispastic medication or that it may prevent development of contractures.

Comparison of the effects of remote after-effects of static contractions for different upper-extremity positions and pinch-force strengths in patients with restricted wrist flexion range of motion

The objective of the study was to examine the after-effects of static contractions of upper extremity muscles in different shoulder joint positions and at different pinch-force strengths on the maximal active range of motion (MAROM) and wrist agonist/antagonist IEMG activities for patients with restricted wrist flexion range of motion (ROM) due to upper limb pain and dysfunction. The subjects were 10 outpatients (3 males, 7 females) with restricted wrist joints. These subjects performed four static contractions of upper extremity muscles in neutral and diagonal shoulder joint positions and with weak and strong pinch-force strengths in random order. Two-way repeated measures analysis of variance showed that the change in MAROM was significantly larger (P < 0.05) after diagonal-strong static contractions than after neutral-weak static contractions. There were no significant correlations between changes in MAROM and IEMG activities. These results indicate that shoulder joint position and pinch-force strength should be considered for effective induction of remote after-effects of static contractions for increasing MAROM for restricted wrist flexion ROM.

The mechanical strength of side-to-side tendon repair with mismatched tendon size and shape

Tendon transfers frequently require coaptation of two mismatched tendons. In this cadaver study, ultimate load, stiffness, and Young's modulus were measured in tendon-to-tendon attachments with mismatched donor and recipient tendons, using pronator teres (PT) to extensor carpi radialis brevis (ECRB) and flexor carpi ulnaris (FCU) to extensor digitorum communis (EDC). FCU-to-EDC attachments failed at higher loads than PT-to-ECRB attachments, but they had similar modulus and stiffness values. Ultimate tensile strength of the tendon attachments exceeded the maximum predicted contraction force of any of the transferred muscles, with safety factors of four-fold for the FCU-to-EDC and two-fold for the PT-to-ECRB transfers. This implies that size and shape mismatches should not be contraindications to tendon attachment in transfers. The strength safety factors suggest that postoperative immobilization of these transfers is unnecessary.

The power of the mind: the cortex as a critical determinant of muscle strength/weakness

We tested the hypothesis that the nervous system, and the cortex in particular, is a critical determinant of muscle strength/weakness and that a high level of corticospinal inhibition is an important neurophysiological factor regulating force generation. A group of healthy individuals underwent 4 wk of wrist-hand immobilization to induce weakness. Another group also underwent 4 wk of immobilization, but they also performed mental imagery of strong muscle contractions 5 days/wk. Mental imagery has been shown to activate several cortical areas that are involved with actual motor behaviors, including premotor and M1 regions. A control group, who underwent no interventions, also participated in this study. Before, immediately after, and 1 wk following immobilization, we measured wrist flexor strength, voluntary activation (VA), and the cortical silent period (SP; a measure that reflect corticospinal inhibition quantified via transcranial magnetic stimulation). Immobilization decreased strength 45.1 ± 5.0%, impaired VA 23.2 ± 5.8%, and prolonged the SP 13.5 ± 2.6%. Mental imagery training, however, attenuated the loss of strength and VA by ∼50% (23.8 ± 5.6% and 12.9 ± 3.2% reductions, respectively) and eliminated prolongation of the SP (4.8 ± 2.8% reduction). Significant associations were observed between the changes in muscle strength and VA (r = 0.56) and SP (r = -0.39). These findings suggest neurological mechanisms, most likely at the cortical level, contribute significantly to disuse-induced weakness, and that regular activation of the cortical regions via imagery attenuates weakness and VA by maintaining normal levels of inhibition.

Changes in corticospinal transmission following 8weeks of ankle joint immobilization

OBJECTIVES

Joint immobilization has previously been shown to modulate corticospinal excitability. The present study investigated changes in the excitability of distinct fractions of the corticospinal pathway by means of conditioning the H-reflex with transcranial magnetic stimulation (TMS) of the primary motor cortex (Hcond). This method allows assessment of transmission in fast (monosynaptic) and slow(er) (polysynaptic) corticospinal pathways.

METHODS

9 subjects underwent 8weeks of unilateral ankle joint immobilization during daytime, 7 subjects served as controls. The measures obtained before and after immobilization included stretch- and H-reflexes assessing excitability of the spinal reflex circuitries, TMS recruitment curves estimating overall changes in corticospinal excitability, and Hcond.

RESULTS

TMS recruitment curves showed an overall increase in corticospinal excitability following immobilization. Importantly, Hcond revealed significant facilitation of conditioned reflexes, but only for longer conditioning intervals, suggesting that immobilization increased excitability only of slower, indirect corticospinal pathways. No changes were observed in the control group. Immobilization had no significant effects on spinal reflex measures.

CONCLUSIONS

8weeks of ankle joint immobilization was accompanied by pathway-specific modulation of corticospinal transmission.

SIGNIFICANCE

It is particularly interesting that fast corticospinal projections were unaffected as these are involved in controlling many, if not most, movements in humans.

High-density surface electromyography improves the identification of oscillatory synaptic inputs to motoneurons

Many studies have addressed corticomuscular coherence (CMC), but broad applications are limited by low coherence values and the variability across subjects and recordings. Here, we investigated how the use of high-density surface electromyography (HDsEMG) can improve the detection of CMC. Sixteen healthy subjects performed isometric contractions at six low-force levels using a pinch-grip, while HDsEMG of the adductor pollicis transversus and flexor and abductor pollicis brevis and whole-head magnetoencephalography were recorded. Different configurations were constructed from the HDsEMG grid, such as a bipolar and Laplacian montage, as well as a montage based on principal component analysis (PCA). CMC was estimated for each configuration, and the strength of coherence was compared across configurations. As expected, performance of the precision-grip task resulted in significant CMC in the β-frequency band (16-26 Hz). Compared with a bipolar EMG montage, all multichannel configurations obtained from the HDsEMG grid revealed a significant increase in CMC. The configuration, based on PCA, showed the largest (37%) increase. HDsEMG did not reduce the between-subject variability; rather, many configurations showed an increased coefficient of variation. Increased CMC presumably reflects the ability of HDsEMG to counteract inherent EMG signal factors-such as amplitude cancellation-which impact the detection of oscillatory inputs. In contrast, the between-subject variability of CMC most likely has a cortical origin.

Hypo-activity induced skeletal muscle atrophy and potential nutritional interventions: A review

Periods of hypo-activity result in profound changes in skeletal muscle morphology and strength. This review primarily addresses the differential impact of de-training, bed-rest, limb immobilisation and unilateral lower limb suspension on muscle morphology, strength and fatigability. The degree of muscle atrophy differs depending on the hypo-activity model and the muscles in question, with the leg and postural muscles being the most susceptible to atrophy. Hypo-activity also results in the dramatic loss of strength that often surpasses the loss of muscle mass, and consequently, the nervous system and contractile properties adapt to adjust for this excessive loss of strength. In addition, the degree of muscle strength loss is different depending on the hypo-activity model, with immobilisation appearing to have a greater impact on strength than unloaded models. There is a step-wise difference in the magnitude of muscle loss so that, even after accounting for differential durations of interventions immobilisation ≥ unilateral lower limb suspension ≥ bed-rest ≥ de-training. Muscle fatigability varies between hypo-activity models but the results are equivocal and this may be due to task-specific adaptations. This review also addresses potential nutritional interventions for attenuating hypo-activity induced muscle atrophy and strength declines, in the absence of exercise. Essential amino acid supplementation stands as a strong candidate but other supplements are good contenders for attenuating hypo-activity induced atrophy and strength losses. Several potential nutritional supplements are highlighted that could be used to combat muscle atrophy but extensive research is needed to determine the most effective.

BDNF Val66Met polymorphism alters spinal DC stimulation-induced plasticity in humans

The brain-derived neurotrophic factor gene (BDNF) is one of many genes thought to influence neuronal survival, synaptic plasticity, and neurogenesis. A common single nucleotide polymorphism (SNP) of the BDNF gene due to valine-to-methionine substitution at codon 66 (BDNF Val66Met) in the normal population has been associated with complex neuronal phenotype, including differences in brain morphology, episodic memory, or cortical plasticity following brain stimulation and is believed to influence synaptic changes following motor learning task. However, the effect of this polymorphism on spinal plasticity remains largely unknown. Here, we used anodal transcutaneous spinal direct current stimulation (tsDCS), a novel noninvasive technique that induces plasticity of spinal neuronal circuits in healthy subjects. To investigate whether the susceptibility of tsDCS probes of spinal plasticity is significantly influenced by BDNF polymorphism, we collected stimulus-response curves of the soleus (Sol) H reflex before, during, at current offset, and 15 min after anodal tsDCS delivered at Th11 (2.5 mA, 15 min, 0.071 mA/cm2, and 64 mC/cm2) in 17 healthy, Met allele carriers and 17 Val homozygotes who were matched for age and sex. Anodal tsDCS induced a progressive leftward shift of recruitment curve of the H reflex during the stimulation that persisted for at least 15 min after current offset in Val/Val individuals. In contrast, this shift was not observed in Met allele carriers. Our findings demonstrate for the first time that the BDNF Val66Met genotype impacts spinal plasticity in humans, as assessed by tsDCS, and may be one factor influencing the natural response of the spinal cord to injury or disease.

Ipsi- and contralateral H-reflexes and V-waves after unilateral chronic Achilles tendon vibration

Chronic Achilles tendon vibration has previously shown its effectiveness in improving plantar flexor's strength and activation capacities. The present study investigated the related neural mechanisms by analyzing H-reflexes and V-waves of the soleus (SOL) and gastrocnemii (GM gastrocnemius medialis; GL gastrocnemius lateralis) muscles under maximal isometric plantar flexion. Moreover, recordings were conducted bilaterally to address potential crossed effects. 11 subjects were engaged in this study. Maximal voluntary contraction and superimposed H-reflexes and V-waves were quantified in both legs at baseline (PRE) and 2 weeks later to verify repeatability of data (CON). Then, subjects were retested after 14 days of daily unilateral Achilles tendon vibration (VIB; 1 h per day; frequency: 50 Hz). No changes were reported between PRE and CON data. In the VIB condition, there was an increase in MVC for both the vibrated (+9.1 %; p = 0.016) and non-vibrated (+10.2 %; p = 0.009) legs. The H-reflex increased by a mean 25 % in the vibrated SOL (p < 0.001), while it remained unchanged for the contralateral side (p = 0.531). The SOL V-wave also increased in the vibrated limb (+43.3 %; p < 0.001), as well as in the non-vibrated one (+41.9 %; p = 0.006). Furthermore, the GM V-wave increased by 37.8 % (p = 0.081) in the vibrated side and by 39.4 % (p = 0.03) in the non-vibrated side. However, no changes were reported for the GL muscles. While the present study confirmed the strength gains induced by chronic Achilles tendon vibration, the results indicated a cross-education phenomenon with differences in neural adaptations between the vibrated leg and non-vibrated leg.

Inter-limb transfer of ballistic motor skill following non-dominant limb training in young and older adults

We recently reported considerably less inter-limb transfer in older, compared to young, adults following dominant (right) hand motor training (Hinder et al. in J Appl Physiol 110:166-175, 2011). This occurred despite the fact that both age groups exhibited similar performance improvements in the trained limb. However, asymmetries can exist with respect to the degree of transfer observed in some tasks, depending upon which limb undertakes the training. Accordingly, here we investigated inter-limb transfer following left hand ballistic motor training in young (n = 15; mean age 21.2 years) and older (n = 15; mean age 70.3 years) right handers. Following motor training that required participants to maximally abduct the left index finger, both groups exhibited significant performance improvements in the trained left hand. Moreover, the extent of inter-limb transfer was substantial and indistinguishable between the two age groups. Transcranial magnetic stimulation revealed that both age groups exhibited bilateral increases in cortical excitability following unilateral training, indicating that unilateral training affects both the trained and untrained hemisphere. However, only for young adults was the extent of the performance gain in the trained hand able to predict the degree of transfer. These findings suggest that different mechanisms may mediate inter-limb transfer of ballistic motor tasks for older and young adults. Because such tasks evoke similar neural responses to those observed following strength training (Selvanayagam et al. in J Appl Physiol 111:367-375, 2011; Carroll et al. in Acta Physiol 202:119-140, 2011), our findings have important implications for rehabilitation paradigms following stroke or limb immobilisation due to injury.

Nutritional strategies to attenuate muscle disuse atrophy

Situations such as recovery from injury or illness require otherwise healthy humans to undergo periods of disuse, which lead to considerable losses of skeletal muscle mass and, subsequently, numerous negative health consequences. It has been established that prolonged disuse (>10 days) leads to a decline in basal and postprandial rates of muscle protein synthesis, without an apparent change in muscle protein breakdown. It also seems, however, that an early and transient (1-5 days) increase in basal muscle protein breakdown may also contribute to disuse atrophy. A period of disuse reduces energy requirements and appetite. Consequently, food intake generally declines, resulting in an inadequate dietary protein consumption to allow proper muscle mass maintenance. Evidence suggests that maintaining protein intake during a period of disuse attenuates disuse atrophy. Furthermore, supplementation with dietary protein and/or essential amino acids can be applied to further aid in muscle mass preservation during disuse. Such strategies are of particular relevance to the older patient at risk of developing sarcopenia. More work is required to elucidate the impact of disuse on basal and postprandial rates of muscle protein synthesis and breakdown. Such information will provide novel targets for nutritional interventions to further attenuate muscle disuse atrophy and, as such, support healthy aging.

Distal forelimb representations in primary motor cortex are redistributed after forelimb restriction: a longitudinal study in adult squirrel monkeys

Primary motor cortex (M1) movement representations reflect acquired motor skills. Representations of muscles and joints used in a skilled task expand. However, it is unknown whether motor restriction in healthy individuals results in complementary reductions in M1 representations. With the use of intracortical microstimulation techniques in squirrel monkeys, detailed maps of movement representations in M1 were derived before and up to 35 wk after restriction of the preferred distal forelimb (DFL) by use of a soft cast. Although total DFL area and movement threshold remained constant, casting resulted in a redistribution of digit and wrist/forearm representations. Digit representations progressively decreased, whereas wrist/forearm representations progressively increased in areal extent. In three of four monkeys, hand preference returned to normal by the end of the postcast recovery period, and postrecovery maps demonstrated reversal of restriction-induced changes. However, in one monkey, a chronic motor impairment occurred in the casted limb. Rehabilitation via a forced-use paradigm resulted in recovery in use and skill of the impaired limb, as well as restoration of normal motor maps. These results demonstrate that plasticity in motor representations can be induced by training or restricting movements of the limb. Physiological changes induced by restriction appear to be reversible, even in the case of adverse motor outcomes. The respective contributions of both disuse and lost motor skills are discussed. These results have relevance for clinical conditions requiring forelimb casting as well as interpreting the differential effects of injury and disuse that are necessarily intertwined after cortical injury, as occurs in stroke.

Reduction of spinal sensory transmission by facilitation of 5-HT1B/D receptors in noninjured and spinal cord-injured humans

Activation of receptors by serotonin (5-HT1) and norepinephrine (α2) on primary afferent terminals and excitatory interneurons reduces transmission in spinal sensory pathways. Loss or reduction of descending sources of serotonin and norepinephrine after spinal cord injury (SCI) and the subsequent reduction of 5-HT1/α2 receptor activity contributes, in part, to the emergence of excessive motoneuron activation from sensory afferent pathways and the uncontrolled triggering of persistent inward currents that depolarize motoneurons during muscle spasms. We tested in a double-blind, placebo-controlled study whether facilitating 5-HT1B/D receptors with the agonist zolmitriptan reduces the sensory activation of motoneurons during an H-reflex in both noninjured control and spinal cord-injured participants. In both groups zolmitriptan, but not placebo, reduced the size of the maximum soleus H-reflex with a peak decrease to 59% (noninjured) and 62% (SCI) of predrug values. In SCI participants we also examined the effects of zolmitriptan on the cutaneomuscular reflex evoked in tibialis anterior from stimulation to the medial arch of the foot. Zolmitriptan, but not placebo, reduced the long-latency, polysynaptic component of the cutaneomuscular reflex (first 200 ms of reflex) by ∼50%. This ultimately reduced the triggering of the long-lasting component of the reflex (500 ms poststimulation to end of reflex) known to be mediated by persistent inward currents in the motoneuron. These results demonstrate that facilitation of 5-HT1B/D receptors reduces sensory transmission in both monosynaptic and polysynaptic reflex pathways to ultimately reduce long-lasting reflexes (spasms) after SCI.