Modulation of presynaptic inhibition and disynaptic reciprocal Ia inhibition during voluntary movement in spasticity

Evaluation of spasticity: IFCN Handbook Chapter

There is no generally accepted definition of spasticity, but hyperexcitable stretch reflexes, exaggerated tendon jerks, clonus, spasms, cramps, increased resistance to passive joint movement, sustained involuntary muscle activity and aberrant muscle activation, including co-contraction of antagonist muscles are all signs and symptoms which are usually associated clinically to the term spasticity. This review describes how biomechanical and electrophysiological techniques may be used to provide quantitative and objective measures of each of these signs and symptoms. The review further describes how neurophysiological techniques may be used to evaluate pathophysiological changes in spinal motor control mechanisms. It is emphasized that understanding the pathophysiology and distinguishing the specific signs and symptoms associated with spasticity, using objective, valid, and reproducible measurements, is essential for providing optimal therapy.

Neuromuscular electrical stimulation reduces spinal excitability in Multiple Sclerosis patients with spasticity symptoms

BACKGROUND

The use of neuromuscular electrical stimulation (NMES) has been recently proposed in patients with neurological diseases, such as spinal cord injuries and stroke, to improve symptoms of spasticity, resulting in both increased control of voluntary movements and improved functional ability in daily activities. Despite several authors suggest that these results could be related to a reduced spinal excitability, which is known to be higher in spastic patients, no previous studies investigated the neurophysiological mechanisms underlying the effect of NMES in reducing spasticity. In addition, there are no studies in the literature adopting NMES to improve spasticity in patients with Multiple Sclerosis (MS). Therefore, this study aims at comparing acute responses in spinal excitability, as measured by H-reflex, between MS patients with and without spasticity, following three experimental conditions: 1) isometric voluntary contraction (ISO) of the ankle plantar flexor muscles; 2) NMES passively applied (pNMES) to the ankle plantar flexor muscles; and 3) NMES superimposed onto isometric voluntary contraction (NMES+) of the same muscles.

METHODS

15 MS patients with spasticity (MS+) and 15 MS patients without spasticity (MS-) took part in a single experimental session, which consisted in the application of NMES to the ankle plantar-flexor muscles in the most spastic and compromised leg. Following the assessment of maximum voluntary isometric contraction (MVIC), participants were asked to perform 15 repetitions of 6 s at 20 % of MVIC, with 6 s of recovery between repetitions, during the three experimental conditions (ISO, pNMES, NMES+). Before and after each condition, soleus (SOL) H-reflex amplitudes were recorded by using surface electromyography (sEMG).

RESULTS

In MS+, H-reflex amplitude significantly decreased after both pNMES (p = 0.007) and NMES+ (p = 0.003), while it was unaltered after ISO (p = 0.829). In MS-, H-reflex amplitude did not change under any experimental condition (ISO: p = 0.383; pNMES: p = 0.328; NMES+: p = 0.087).

CONCLUSION

The reduction of H-reflex after pNMES and NMES+ can be attributed to a reduced spinal excitability in spastic MS patients, which may be attributed to presynaptic inhibition, recurrent inhibition, gamma-aminobutyric acid activity and persistent inward current. These results are highly relevant from both neurophysiological and clinical point of views, suggesting new approaches to manage spasticity symptoms in neurological patients.

Modulation of Primary Afferent Nerve Fiber (Ia) Reciprocal Inhibition Under Voluntary and Electrically Stimulated Muscle Conditions: Within-Subject Study Design

Background: Reciprocal inhibition (RI) is a spinal reflex that controls posture and movement. The modulation of spinal RI represented by the H-reflex has been studied, before and after voluntary contraction and electrical nerve stimulation but not during voluntary, electrically induced muscle contraction or a combination of voluntary and electrically induced muscle contractions. This study investigates the effects of the ongoing voluntary isometric contraction, the electrically induced isometric contraction, and the combination of voluntary with electrically induced isometric contraction of the Tibialis Anterior (TA) muscle on spinal RI represented by Soleus H-reflex. Methods: Eighteen healthy adults participated. Soleus H-reflex and M-response were measured during four different conditions as follows: (1) at rest, (2) electrically induced isometric contraction of the TA, (3) voluntary isometric contraction of the TA with a 1 kg force, and (4) combined voluntary and electrically induced isometric contraction of the TA with a 1 kg force. Results: The ANOVA clearly demonstrated significant differences in Soleus H-reflex amplitude across the four recording conditions (F3,16, 17.28, p < 0.001). The amplitude at rest was significantly higher than during electrically induced isometric contraction, voluntary isometric contraction, and the combined contraction conditions (p < 0.05). Furthermore, the amplitude recorded during the electrically induced isometric contraction condition significantly surpassed that of voluntary isometric contraction and the combined contraction conditions (p < 0.05). Moreover, there was no significant difference between Soleus H-reflex amplitude recorded during voluntary isometric contraction and the combined voluntary isometric contraction and electrically induced isometric contraction (p < 0.87). The combined voluntary isometric contraction and electrically induced isometric contraction condition had a higher inhibitory effect on the Soleus H-reflex with no significant differences from voluntary isometric contraction. Moreover, both were significantly better than electrically induced isometric contraction (p = 0.05). In terms of Soleus H-reflex latency, there was no significant difference among all four conditions (p > 0.05), meaning Soleus H-reflex latency was not influenced by the conditions. Conclusions: RI can be best modulated by combining voluntary with electrically induced isometric muscle contractions.

Neurophysiological effects of latent trigger point dry needling on spinal reflexes

Deep dry needling (DDN) is a method to treat muscle trigger points (TrPs) often found in persons with neuromuscular pain and spasticity. Currently, its neurophysiological actions are not well established. Thus, to understand how DDN affects spinal cord physiology, we investigated the effects of TrP DDN on spinal reflexes. In 17 adults with latent TrPs in the medial gastrocnemius (MG) without known neurological or orthopedic injuries, the H reflex, M wave, and reciprocal inhibition in the soleus, MG, and lateral gastrocnemius (LG) and passive ankle range of motion (ROM) were measured before and immediately, 90 min, and 72 h after a single bout of DDN at the MG TrPs. The MG maximum M wave (Mmax) amplitude was decreased immediately and 90 min post DDN (by -14% and -18%) and returned to pre-DDN level at 72 h post. LG and soleus Mmax did not change. The maximum H reflex (Hmax) amplitude did not change in any of the triceps surae. Soleus inhibition was increased significantly immediately (+30%) and 72 h (+36%) post DDN. ROM was increased by ≈4° immediately and ≈3° at 72 h post DDN. Temporary reduction of MG (but not soleus or LG) Mmax amplitude after DDN and its recovery at 72 h post indicate temporary and specific effects of DDN in the treated muscle. The immediate and 72 h post increases in the ROM and soleus inhibition with no changes in Hmax suggest complex effects of DDN at the spinal level.NEW & NOTEWORTHY In this study, we examined the effects of deep dry needling (DDN) on spinal reflexes in the triceps surae. We found that the H reflex (an excitatory reflex) did not change after DDN but soleus inhibition was increased immediately and 72 h after DDN, corresponding to increases in ankle range of motion. Differential effects of DDN on excitatory and inhibitory reflexes over the first 72 h may reflect its complex neurophysiological effects at the spinal level.

Transcutaneous spinal cord stimulation neuromodulates pre- and postsynaptic inhibition in the control of spinal spasticity

Aside from enabling voluntary control over paralyzed muscles, a key effect of spinal cord stimulation is the alleviation of spasticity. Dysfunction of spinal inhibitory circuits is considered a major cause of spasticity. These circuits are contacted by Ia muscle spindle afferents, which are also the primary targets of transcutaneous lumbar spinal cord stimulation (TSCS). We hypothesize that TSCS controls spasticity by transiently strengthening spinal inhibitory circuit function through their Ia-mediated activation. We show that 30 min of antispasticity TSCS improves activity in post- and presynaptic inhibitory circuits beyond the intervention in ten individuals with traumatic spinal cord injury to normative levels established in 20 neurologically intact individuals. These changes in circuit function correlate with improvements in muscle hypertonia, spasms, and clonus. Our study opens the black box of the carryover effects of antispasticity TSCS and underpins a causal role of deficient post- and presynaptic inhibitory circuits in spinal spasticity.

Transcutaneous spinal cord stimulation phase-dependently modulates spinal reciprocal inhibition induced by pedaling in healthy individuals

Reciprocal inhibition (RI) between leg muscles is crucial for smooth movement. Pedaling is a rhythmic movement that can increase RI in healthy individuals. Transcutaneous spinal cord stimulation (tSCS) stimulates spinal neural circuits by targeting the afferent fibers. Pedaling with simultaneous tSCS may modulate the plasticity of the spinal neural circuit and alter neural activity based on movement and muscle engagement. This study investigated the RI changes after pedaling and tSCS and determined the phase of pedaling in which tSCS should be applied for optimal RI modulation in healthy individuals. Eleven subjects underwent three interventions: pedaling combined with tSCS during the early phase of lower extension (phase 1), pedaling combined with tSCS during the late phase of lower flexion (phase 4) of the pedaling cycle, and pedaling combined with sham tSCS. The RI from the tibialis anterior to the soleus muscle was assessed before, immediately after, 15 min, and 30 min after the intervention. RI increased immediately after phase 4 and pedaling combined with sham tSCS, whereas no changes were observed after phase 1. These results demonstrate that tSCS modulates RI changes induced by pedaling in a stimulus phase-dependent manner in healthy individuals. However, the mechanism involved in this intervention needs to be explored to achieve higher efficacy.

Trans-spinal magnetic stimulation combined with kinesiotherapy as a new method for enhancing functional recovery in patients with spinal cord injury due to neuromyelitis optica: a case report

BACKGROUND

Experimental studies have shown that repetitive trans-spinal magnetic stimulation (TsMS) decreases demyelination and enables recovery after spinal cord injury (SCI). However, the usefulness of TsMS in humans with SCI remains unclear. Therefore, the main objective of this study is to evaluate the effects of TsMS combined with kinesiotherapy on SCI symptoms. We describe a protocol treatment with TsMS and kinesiotherapy in a patient with SCI due to neuromyelitis optica (NMO)-associated transverse myelitis.

CASE PRESENTATION

A 23-year-old white male with NMO spectrum disorders started symptoms in 2014 and included lumbar pain evolving into a mild loss of strength and sensitivity in both lower limbs. Five months later, the symptoms improved spontaneously, and there were no sensorimotor deficits. Two years later, in 2016, the symptoms recurred with a total loss of strength and sensitivity in both lower limbs. Initially, physiotherapy was provided in 15 sessions with goals of motor-sensory recovery and improving balance and functional mobility. Subsequently, TsMS (10 Hz, 600 pulses, 20-seconds inter-trains interval, at 90% of resting motor threshold of the paravertebral muscle) was applied at the 10th thoracic vertebral spinous process before physiotherapy in 12 sessions. Outcomes were assessed at three time points: prior to physiotherapy alone (T-1), before the first session of TsMS combined with kinesiotherapy (T0), and after 12 sessions of TsMS combined with kinesiotherapy (T1). The patient showed a 25% improvement in walking independence, a 125% improvement in balance, and an 18.8% improvement in functional mobility. The Patient Global Impression of Change Scale assessed the patient's global impression of change as 'much improved'.

CONCLUSION

TsMS combined with kinesiotherapy may safely and effectively improve balance, walking independence, and functional mobility of patients with SCI due to NMO-associated transverse myelitis.

Effects of Different Intensities of Repetitive Peripheral Magnetic Stimulation on Spinal Reciprocal Inhibition in Healthy Persons

Objectives

This study aimed to assess the effect of the spinal circuit of repetitive magnetic stimulation (rPMS) on the soleus muscle among healthy subjects.

Methods

Nineteen healthy adults were included in this study. Intermittent rPMS was applied to the left soleus muscle for 20 minutes. We applied different intensity rPMS (high-intensity, low-intensity, and non-stimulation) in different three days. RI (reciprocal inhibition) from the tibialis anterior to the soleus muscle with an inter-stimulus interval (ISI) of 2ms and 20ms was assessed before, immediately after and 30 minutes at each session.

Results

Two factor repeated measure ANOVA test showed a significant interaction (F2,33 = 9.688, p < 0.001) between tasks and time in the RI ratio 2ms. Post-hoc analysis showed that RI ratio 2ms significantly differed from those immediately after, and 30 min after high-intensity rPMS (p = 0.001 and p = 0.003, respectively). A significant difference was observed between high-intensity rPMS and non-stimulation immediately after the stimulation (p = 0.003). However, no significant difference was found in the RI ratio 20ms between all the intensities (p > 0.05).

Conclusion

This study demonstrates that high-intensity rPMS can effectively modulate spinal circuits, as evidenced by the decreased RI in healthy individuals. This suggests the potential use of rPMS as a therapeutic intervention for patients with muscle weakness. Disinhibition of the RI may lead to a more effective contraction of the target muscle. This effect could be expected to strengthen the muscles and alleviate paralysis, making it a promising avenue for future research and clinical applications in the field of rehabilitation. Further investigation is warranted to explore the precise mechanisms underlying the observed effects and to optimize the parameters of rPMS for specific clinical populations.

A new methodological approach to characterize selective motor control in children with cerebral palsy

Introduction

Despite being a primary impairment in individuals with cerebral palsy (CP), selective motor control (SMC) is not routinely measured. Personalized treatment approaches in CP will be unattainable without the ability to precisely characterize the types and degrees of impairments in motor control. The objective of this study is to report the development and feasibility of a new methodological approach measuring muscle activation patterns during single-joint tasks to characterize obligatory muscle co-activation patterns that may underly impaired SMC.

Methods

Muscle activation patterns were recorded during sub-maximal voluntary isometric contraction (sub-MVIC) tasks at the hip, knee, and ankle with an interactive feedback game to standardize effort across participants. We calculated indices of co-activation, synergistic movement, mirror movement, and overflow (indices range 0-2, greater scores equal to greater impairment in SMC) for each isolated joint task in 15 children - 8 with typical development (TD) (mean age 4.7 ± 1.0 SD years) and 7 with CP (mean age 5.8 ± 0.7 SD years). Indices were compared with Mann-Whitney tests. The relationships between the indices and gross motor function (GMFM-66) were examined with Pearson's r.

Results

Mean indices were higher in the CP vs. the TD group for each of the six tasks, with mean differences ranging from 0.05 (abduction and plantarflexion) to 0.44 (dorsiflexion). There was great inter-subject variability in the CP group such that significant group differences were detected for knee flexion mirroring (p = 0.029), dorsiflexion coactivation (p = 0.021), and dorsiflexion overflow (p = 0.014). Significant negative linear relations to gross motor function were found in all four indices for knee extension (r = -0.56 to -0.75), three of the indices for ankle dorsiflexion (r = -0.68 to -0.78) and in two of the indices for knee flexion (r = -0.66 to -0.67), and ankle plantarflexion (r = -0.53 to -0.60).

Discussion

Indices of coactivation, mirror movement, synergy, and overflow during single-joint lower limb tasks may quantify the type and degree of impairment in SMC. Preliminary concurrent validity between several of the indices of SMC and gross motor function was observed. Our findings established the feasibility of a new methodological approach that quantifies muscle activation patterns using electromyography paired with biofeedback during single-joint movement.

Neuromuscular Electrical Stimulation Does Not Influence Spinal Excitability in Multiple Sclerosis Patients

(1) Background: Neuromuscular electrical stimulation (NMES) has beneficial effects on physical functions in Multiple sclerosis (MS) patients. However, the neurophysiological mechanisms underlying these functional improvements are still unclear. This study aims at comparing acute responses in spinal excitability, as measured by soleus Hoffmann reflex (H-reflex), between MS patients and healthy individuals, under three experimental conditions involving the ankle planta flexor muscles: (1) passive NMES (pNMES); (2) NMES superimposed onto isometric voluntary contraction (NMES+); and (3) isometric voluntary contraction (ISO). (2) Methods: In total, 20 MS patients (MS) and 20 healthy individuals as the control group (CG) took part in a single experimental session. Under each condition, participants performed 15 repetitions of 6 s at 20% of maximal voluntary isometric contraction, with 6 s of recovery between repetitions. Before and after each condition, H-reflex amplitudes were recorded. (3) Results: In MS, H-reflex amplitude did not change under any experimental condition (ISO: p = 0.506; pNMES: p = 0.068; NMES+: p = 0.126). In CG, H-reflex amplitude significantly increased under NMES+ (p = 0.01), decreased under pNMES (p < 0.000) and was unaltered under ISO (p = 0.829). (4) Conclusions: The different H-reflex responses between MS and CG might reflect a reduced ability of MS patients in modulating spinal excitability.

Sensory Stimulation of the Triceps Surae Muscle Complex Modulates Spinal Reflex Responses-A Comparison between Tapotement Massage and Repetitive Peripheral Magnetic Stimulation (rPMS)

BACKGROUND

The reduction of muscular hypertonia is important in the treatment of various diseases or rehabilitation. This study aims to test the efficacy of a 5 Hz mechanical muscle stimulation (tapotement massage) in comparison to a 5 Hz repetitive peripheral magnetic stimulation (rPMS) on the neuromuscular reflex response.

METHODS

In a randomized control trial, 15 healthy volunteers were administered with either 5 Hz rPMS, tapotement massage, or rPMS sham stimulation. The posterior tibial nerve was stimulated with rPMS and sham stimulation. The Achilles tendon was exposed to a mechanically applied high-amplitude 5 Hz repetitive tendon tapotement massage (rTTM). The tendon reflex (TR) was measured for the spinal response of the soleus muscle.

RESULTS

After rPMS, there was a reduction of the TR response (-9.8%, p ≤ 0.034) with no significant changes after sham stimulation. Likewise, TR decreased significantly (-17.4%, p ≤ 0.002) after Achilles tendon tapotement intervention.

CONCLUSIONS

These findings support the hypothesis that both afferent 5 Hz sensory stimulations contributed to a modulation within the spinal and/or supraspinal circuits, which resulted in a reduction of the spinal reflex excitability. The effects could be beneficial for patients with muscle hypertonia and could improve the functional results of rehabilitation programs.

Inhibitory Effects of Prolonged Focal Muscle Vibration on Maximal Grip Strength and Muscle Activity of Wrist and Extrinsic Finger Flexor Muscles

Objective

The objective of this study was to identify effective stimulus time by quantifying the inhibitory effects of focal muscle vibration (FMV) on maximal grip strength and muscle activities of the wrist and extrinsic finger flexors.

Methods

A randomized repeated-measures design was used in this study. A total of 22 healthy volunteers (mean age, 20.9 years) participated. An FMV of 86 Hz was applied to the anterior surface of the distal forearm under the following 3 conditions: no FMV (control), 5-minute FMV, and 10-minute FMV. Maximal grip strength was measured before and after FMV. The muscle activities of the flexor digitorum superficialis, flexor digitorum profundus (FDP), and flexor carpi ulnaris were simultaneously recorded using surface electromyography. Discomfort and complications following FMV were also assessed.

Results

Compared with the control group, a significant decrease in muscle activity was observed in both the flexor digitorum superficialis and flexor carpi ulnaris after 5 and 10 minutes of FMV. In contrast, there was no significant decrease in the maximal grip strength or FDP muscle activity after either FMV condition. The discomfort was significantly higher immediately after both FMV conditions than in the control group, but it decreased 15 minutes after FMV, indicating no significant difference among the 3 conditions. Redness and/or swelling were observed in 13.6% and 36.3% of the participants after 5 and 10 minutes of FMV, respectively.

Conclusion

Five-minute FMV to the distal forearm could be a useful therapeutic method with few complications. However, the FMV in this area alone was not sufficient to suppress the muscle activity of the FDP located in the deep layer.

Altered cutaneous reflexes to non-noxious stimuli in the triceps surae of people with chronic incomplete spinal cord injury

Following spinal cord injury (SCI) task-dependent modulation of spinal reflexes are often impaired. To gain insight into the state of the spinal interneuronal pathways following injury, we studied the amplitude modulation of triceps surae cutaneous reflexes to non-noxious stimuli during standing and early-to-mid stance phase of walking in participants with and without chronic incomplete SCI. Reflex eliciting nerve stimulation was delivered to the superficial peroneal, sural, and distal tibial nerves about the ankle. Reflexes were analyzed in the short (SLR, 50-80 ms post stimulation onset) and the medium (MLR, 80-120 ms) latency response windows. Further, the relation between cutaneous and H-reflexes was also examined during standing. In participants without injuries the soleus SLR was modulated task-dependently with nerve specificity, and the soleus and medial gastrocnemius MLRs were modulated task-dependently. In contrast, participants with SCI, no task-dependent or nerve-specific modulation of triceps cutaneous reflexes was observed. The triceps surae cutaneous and H-reflexes were not correlated in either group (r = 0.01-0.37). The presence of cutaneous reflexes but the absence of significant amplitude modulation may suggest impaired function of spinal interneuronal pathways in this population. The lack of correlation between the cutaneous and H-reflexes may suggest that interneurons that are involved in H-reflex modulation and cutaneous reflex modulation do not receive common input, or the impact of the common input is outweighed by other input. Present findings highlight the importance of examining multiple spinal reflexes to better understanding spinal interneuronal pathways that affect motor control in people after SCI.

Modulation of spinal circuits following phase-dependent electrical stimulation of afferent pathways

Objective.Peripheral electrical stimulation (PES) of afferent pathways is a tool commonly used to induce neural adaptations in some neural disorders such as pathological tremor or stroke. However, the neuromodulatory effects of stimulation interventions synchronized with physiological activity (closed-loop strategies) have been scarcely researched in the upper-limb. Here, the short-term spinal effects of a 20-minute stimulation protocol where afferent pathways were stimulated with a closed-loop strategy named selective and adaptive timely stimulation (SATS) were explored in 11 healthy subjects.Approach. SATS was applied to the radial nerve in-phase (INP) or out-of-phase (OOP) with respect to the muscle activity of the extensor carpi radialis (ECR). The neural adaptations at the spinal cord level were assessed for the flexor carpi radialis (FCR) by measuring disynaptic Group I inhibition, Ia presynaptic inhibition, Ib facilitation from the H-reflex and estimation of the neural drive before, immediately after, and 30 minutes after the intervention.Main results.SATS strategy delivered electrical stimulation synchronized with the real-time muscle activity measured, with an average delay of 17 ± 8 ms. SATS-INP induced increased disynaptic Group I inhibition (77 ± 23% of baseline conditioned FCR H-reflex), while SATS-OOP elicited the opposite effect (125 ± 46% of baseline conditioned FCR H-reflex). Some of the subjects maintained the changes after 30 minutes. No other significant changes were found for the rest of measurements.Significance.These results suggest that the short-term modulatory effects of phase-dependent PES occur at specific targeted spinal pathways for the wrist muscles in healthy individuals. Importantly, timely recruitment of afferent pathways synchronized with specific muscle activity is a fundamental principle that shall be considered when tailoring PES protocols to modulate specific neural circuits. (NCT number 04501133).

Treatment of spasticity

Spasticity is characterized by an enhanced size and reduced threshold for activation of stretch reflexes and is associated with "positive signs" such as clonus and spasms, as well as "negative features" such as paresis and a loss of automatic postural responses. Spasticity develops over time after a lesion and can be associated with reduced speed of movement, cocontraction, abnormal synergies, and pain. Spasticity is caused by a combination of damage to descending tracts, reductions in inhibitory activity within spinal cord circuits, and adaptive changes within motoneurons. Increased tone, hypertonia, can also be caused by changes in passive stiffness due to, for example, increase in connective tissue and reduction in muscle fascicle length. Understanding the cause of hypertonia is important for determining the management strategy as nonneural, passive causes of stiffness will be more amenable to physical rather than pharmacological interventions. The management of spasticity is determined by the views and goals of the patient, family, and carers, which should be integral to the multidisciplinary assessment. An assessment, and treatment, of trigger factors such as infection and skin breakdown should be made especially in people with a recent change in tone. The choice of management strategies for an individual will vary depending on the severity of spasticity, the distribution of spasticity (i.e., whether it affects multiple muscle groups or is more prominent in one or two groups), the type of lesion, and the potential for recovery. Management options include physical therapy, oral agents; focal therapies such as botulinum injections; and peripheral nerve blocks. Intrathecal baclofen can lead to a reduction in required oral antispasticity medications. When spasticity is severe intrathecal phenol may be an option. Surgical interventions, largely used in the pediatric population, include muscle transfers and lengthening and selective dorsal root rhizotomy.

Altered regulation of Ia afferent input during voluntary contraction in humans with spinal cord injury

Sensory input converging on the spinal cord contributes to the control of movement. Although sensory pathways reorganize following spinal cord injury (SCI), the extent to which sensory input from Ia afferents is regulated during voluntary contraction after the injury remains largely unknown. To address this question, the soleus H-reflex and conditioning of the H-reflex by stimulating homonymous [depression of the soleus H-reflex evoked by common peroneal nerve (CPN) stimulation, D1 inhibition] and heteronymous (d), [monosynaptic Ia facilitation of the soleus H-reflex evoked by femoral nerve stimulation (FN facilitation)] nerves were tested at rest, and during tonic voluntary contraction in humans with and without chronic incomplete SCI. The soleus H-reflex size increased in both groups during voluntary contraction compared with rest, but to a lesser extent in SCI participants. Compared with rest, the D1 inhibition decreased during voluntary contraction in controls but it was still present in SCI participants. Further, the FN facilitation increased in controls but remained unchanged in SCI participants during voluntary contraction compared with rest. Changes in the D1 inhibition and FN facilitation were correlated with changes in the H-reflex during voluntary contraction, suggesting an association between outcomes. These findings provide the first demonstration that the regulation of Ia afferent input from homonymous and heteronymous nerves is altered during voluntary contraction in humans with SCI, resulting in lesser facilitatory effect on motor neurons.

Effects of whole-body vibration on neuropathic pain and the relationship between pain and spasticity in persons with spinal cord injury

OBJECTIVE

Whole-body vibration (WBV) appears to modulate reflex hyperexcitability and spasticity. Due to common underlying neural mechanisms between spasticity and neuropathic pain, WBV may also reduce chronic pain after spinal cord injury (SCI). Our objective was to determine whether there are dose-related changes in pain following WBV and to examine the relationships between neuropathic pain and reflex excitability.

STUDY DESIGN

Secondary analysis of a sub-population (participants with neuropathic pain, n = 16) from a larger trial comparing the effects of two different doses of WBV on spasticity in persons with SCI.

SETTING

Hospital/Rehabilitation Center in Atlanta, GA, USA.

METHODS

Participants were randomized to 8-bout or 16-bout WBV groups. Both groups received ten sessions of sham intervention, followed by ten sessions of WBV. Primary measures included the Neuropathic Pain Symptom Inventory (NPSI) for pain symptom severity and H-reflex paired-pulse depression (PPD) for reflex excitability.

RESULTS

Mean change in NPSI scores were not significantly different between the groups (7 ± 6; p = 0.29; ES = 0.57); however, 8-bouts of WBV were consistently beneficial for participants with high neuropathic pain symptom severity (NPSI total score >30), while 16-bouts of WBV appeared to increase pain in some individuals with high NPSI scores. A baseline NPSI cut score of 30 predicted PPD response (sensitivity = 1.0, specificity = 0.83), with higher NPSI scores associated with decreased PPD in response to WBV.

CONCLUSIONS

WBV in moderate doses appears to decrease neuropathic pain symptoms and improve reflex modulation. However, at higher doses neuropathic pain symptoms may be aggravated. Lower baseline NPSI scores were associated with improved reflex modulation.

Influence of Electrotherapy with Task-Oriented Training on Spasticity, Hand Function, Upper Limb Function, and Activities of Daily Living in Patients with Subacute Stroke: A Double-Blinded, Randomized, Controlled Trial

The effects of electrotherapy with task-oriented training on upper limb function in subacute stroke patients are unclear. This study investigated the influence of transcutaneous electrical nerve stimulation (TENS) with task-oriented training on spasticity, hand function, upper limb function, and activities of daily living in patients with subacute stroke. Forty-eight patients with subacute stroke were randomly assigned to either the TENS group (n = 22) or the placebo-TENS group (n = 21). High-frequency (100 Hz) TENS with below-motor threshold intensity or placebo-TENS was applied for 30 min/day, five times a week, for 4 weeks. The two groups also received task-oriented training after TENS. The Modified Ashworth Scale (MAS), Jebsen–Taylor Hand Function Test (JTHFT), Manual Function Test (MFT), and Modified Barthel Index (MBI) were used to assess spasticity, hand function, upper limb function, and activities of daily living, respectively. There was a significant time–group interaction with the MFT (p = 0.003). The TENS group showed significantly improved MAS (p = 0.003), JTHFT (p < 0.001), MFT (p < 0.001), and MBI (p < 0.001) scores after the intervention. The placebo-TENS group showed significantly improved JTHFT (p < 0.001), MFT (p = 0.001), and MBI scores (p < 0.001). There was a significant correlation between the MFT and MBI scores (p = 0.025). These results suggest that electrotherapy with task-oriented training can be used to improve upper limb function in patients with subacute stroke.

Soleus responses to Achilles tendon stimuli are suppressed by heel and enhanced by metatarsal cutaneous stimuli during standing

KEY POINTS

We examined the influence of cutaneous feedback from the heel and metatarsal regions of the foot sole on the soleus stretch reflex pathway during standing. We found that heel electrical stimuli suppressed and metatarsal stimuli enhanced the soleus vibration response. Follow-up experiments indicated that the interaction between foot sole cutaneous feedback and the soleus vibration response was likely not mediated by presynaptic inhibition and was contingent upon a modulation at the ⍺-motoneuron pool level. The spatially organized interaction between cutaneous feedback from the foot sole and the soleus vibration response provides information about how somatosensory information is combined to appropriately respond to perturbations during standing.

ABSTRACT

Cutaneous feedback from the foot sole provides balance-relevant information and has the potential to interact with spinal reflex pathways. In this study, we examined how cutaneous feedback from the foot sole (heel and metatarsals) influenced the soleus response to proprioceptive stimuli during standing. We delivered noisy vibration (10-115 Hz) to the right Achilles tendon while we intermittently applied electrical pulse trains (five 1-ms pulses at 200 Hz, every 0.8-1.0 s) to the skin under either the heel or the metatarsals of the ipsilateral foot sole. We analysed time-dependent (referenced to cutaneous stimuli) coherence and cross-correlations between the vibration acceleration and rectified soleus EMG. Vibration-EMG coherence was observed across a bandwidth of ∼10-80 Hz, and coherence was suppressed by heel but enhanced by metatarsal cutaneous stimuli. Cross-correlations showed soleus EMG was correlated with the vibration (∼40 ms lag) and cross-correlations were also suppressed by heel (from 104-155 ms) but enhanced by metatarsal (from 76-128 ms) stimuli. To examine the neural mechanisms mediating this reflex interaction, we conducted two further experiments to probe potential contributions from (1) presynaptic inhibition, and (2) modulations at the ⍺- and γ-motoneuron pools. Results suggest the cutaneous interactions with the stretch reflex pathway required a modulation at the ⍺-motoneuron pool and were likely not mediated by presynaptic inhibition. These findings demonstrate that foot sole cutaneous information functionally tunes the stretch reflex pathway during the control of upright posture and balance.

Simultaneous Cervical and Lumbar Spinal Cord Stimulation Induces Facilitation of Both Spinal and Corticospinal Circuitry in Humans

Coupling between cervical and lumbar spinal networks (cervico-lumbar coupling) is vital during human locomotion. Impaired cervico-lumbar coupling after neural injuries or diseases can be reengaged via simultaneous arm and leg cycling training. Sensorimotor circuitry including cervico-lumbar coupling may further be enhanced by non-invasive modulation of spinal circuity using transcutaneous spinal cord stimulation (tSCS). This project aimed to determine the effect of cervical, lumbar, or combined tSCS on spinal reflex (Hoffmann [H-]) and corticospinal (motor evoked potential [MEP]) excitability during a static or cycling cervico-lumbar coupling task. Fourteen neurologically intact study participants were seated in a recumbent leg cycling system. H-reflex and MEP amplitudes were assessed in the left flexor carpi radialis (FCR) muscle during two tasks (Static and Cycling) and four conditions: (1) No tSCS, (2) tSCS applied to the cervical enlargement (Cervical); (3) tSCS applied to the lumbar enlargement (Lumbar); (4) simultaneous cervical and lumbar tSCS (Combined). While cervical tSCS did not alter FCR H-reflex amplitude relative to No tSCS, lumbar tSCS significantly facilitated H-reflex amplitude by 11.1%, and combined cervical and lumbar tSCS significantly enhanced the facilitation to 19.6%. Neither cervical nor lumbar tSCS altered MEP amplitude alone (+4.9 and 1.8% relative to legs static, No tSCS); however, combined tSCS significantly increased MEP amplitude by 19.7% compared to No tSCS. Leg cycling alone significantly suppressed the FCR H-reflex relative to static, No tSCS by 13.6%, while facilitating MEP amplitude by 18.6%. When combined with leg cycling, tSCS was unable to alter excitability for any condition. This indicates that in neurologically intact individuals where interlimb coordination and corticospinal tract are intact, the effect of leg cycling on cervico-lumbar coupling and corticospinal drive was not impacted significantly with the tSCS intensity used. This study demonstrates, for the first time, that tonic activation of spinal cord networks through multiple sites of tSCS provides a facilitation of both spinal reflex and corticospinal pathways. It remains vital to determine if combined tSCS can influence interlimb coupling after neural injury or disease when cervico-lumbar connectivity is impaired.

The changes in spinal reciprocal inhibition during motor imagery in lower extremity

Motor imagery (MI) is known to improve motor function through enhancement of motor cortex activity. Spinal reciprocal inhibition (RI) is modulated by motor cortex activity, and, therefore, MI may change RI. The aim of this study was to examine the changes in RI during MI involving the lower extremity. Spinal RI was measured from the tibialis anterior (TA) to the soleus (SOL). Eleven healthy adults participated in experiment 1. All participants performed the following three conditions, and RI was assessed during each condition: (1) resting condition; (2) MI of ankle dorsiflexion condition (MI-DF); and (3) MI of ankle plantarflexion condition (MI-PF). Twelve healthy adults participated in experiment 2. All participants performed the following two conditions, and RI was assessed before and after MI practice for 10 min: (1) resting condition and (2) MI-DF. The interval between the conditioning and test stimulus (inter-stimulus interval; ISI) was set at 0, 1, 2, or 3 ms and 20 ms. In experiment 1, RI during MI-PF was significantly decreased compared with that during resting with both stimulus intervals. RI during MI-DF showed no significant change compared with that during resting with both ISIs. In experiment 2, the difference between the rest condition and the MI-DF condition after the MI task with ISI of 20 ms was significantly higher than before the MI task. Our findings suggest that real-time changes in RI during MI involving the lower extremity may vary depending on the direction of motion and MI practice.

Post-activation depression of primary afferents reevaluated in humans

Amplitude variation of Hoffmann Reflex (H-reflex) was used as a tool to investigate many neuronal networks. However, H-reflex itself is a subject to intrinsic changes including post-activation depression (P-AD). We aimed to investigate P-AD and its implication on motor control in humans. Upon tibial nerve stimulation in 23 healthy participants, peak-to-peak amplitude change of H-reflex was investigated using surface electromyography (SEMG) of soleus muscle. Variety of stimulus intensities, interstimulus intervals (ISIs), voluntary contraction levels/types and force recording were used to investigate the nature of P-AD. We have shown that P-AD was significantly stronger in the shorter ISIs. The only exception was the ISI of 200 msecs which had a weaker P-AD than some of the longer ISIs. Sudden muscle relaxation, on the other hand, further increased the effectiveness of the ongoing P-AD. Moreover, P-AD displayed its full effect with the first stimulus when there was no muscle contraction and was efficient to reduce the muscle force output by about 30%. These findings provide insight about the variations and mechanism of P-AD and could lead to improvements in diagnostic tools in neurological diseases.

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.

Can Operant Conditioning of EMG-Evoked Responses Help to Target Corticospinal Plasticity for Improving Motor Function in People With Multiple Sclerosis?

Corticospinal pathway and its function are essential in motor control and motor rehabilitation. Multiple sclerosis (MS) causes damage to the brain and descending connections, and often diminishes corticospinal function. In people with MS, neural plasticity is available, although it does not necessarily remain stable over the course of disease progress. Thus, inducing plasticity to the corticospinal pathway so as to improve its function may lead to motor control improvements, which impact one's mobility, health, and wellness. In order to harness plasticity in people with MS, over the past two decades, non-invasive brain stimulation techniques have been examined for addressing common symptoms, such as cognitive deficits, fatigue, and spasticity. While these methods appear promising, when it comes to motor rehabilitation, just inducing plasticity or having a capacity for it does not guarantee generation of better motor functions. Targeting plasticity to a key pathway, such as the corticospinal pathway, could change what limits one's motor control and improve function. One of such neural training methods is operant conditioning of the motor-evoked potential that aims to train the behavior of the corticospinal-motoneuron pathway. Through up-conditioning training, the person learns to produce the rewarded neuronal behavior/state of increased corticospinal excitability, and through iterative training, the rewarded behavior/state becomes one's habitual, daily motor behavior. This minireview introduces operant conditioning approach for people with MS. Guiding beneficial CNS plasticity on top of continuous disease progress may help to prolong the duration of maintained motor function and quality of life in people living with MS.

Interlimb conditioning of lumbosacral spinally evoked motor responses after spinal cord injury

OBJECTIVE

The importance of subcortical pathways to functional motor recovery after spinal cord injury (SCI) has been demonstrated in multiple animal models. The current study evaluated descending interlimb influence on lumbosacral motor excitability after chronic SCI in humans.

METHODS

Ulnar nerve stimulation and transcutaneous electrical spinal stimulation were used in a condition-test paradigm to evaluate the presence of interlimb connections linking the cervical and lumbosacral spinal segments in non-injured (n=15) and spinal cord injured (SCI) (n=18) participants.

RESULTS

Potentiation of spinally evoked motor responses (sEMRs) by ulnar nerve conditioning was observed in 7/7 SCI participants with volitional leg muscle activation, and in 6/11 SCI participants with no volitional activation. Of these six, conditioning of sEMRs was present only when the neurological level of injury was rostral to the ulnar innervation entry zones.

CONCLUSIONS

Descending modulation of lumbosacral motor pools via interlimb projections may exist in SCI participants despite the absence of volitional leg muscle activation.

SIGNIFICANCE

Evaluation of sub-clinical, spared pathways within the spinal cord after SCI may provide an improved understanding of both the contributions of different pathways to residual function, and the mechanisms of plasticity and functional motor recovery following rehabilitation..

The role of spinal reciprocal inhibition and intracortical inhibition in functional recovery from stroke

Spinal reciprocal inhibition (RI) and intracortical inhibition are important physiological mechanisms for voluntary movement control and functional recovery of voluntary movement in patients with stroke. Spasticity, which impairs motor performance, is one of the major manifestations of stroke. RI may be involved in reducing spasticity. This might allow finger extension, and, therefore, better hand function by reducing co-contraction with finger extensors. One potential mechanism of functional reorganization of the motor cortex is that pre-existing masking pathways are unmasked by decreased intracortical inhibition. The inhibitory neurotransmitter GABA plays an important role in this process. Changes in RI might be mediated through unmasking of cortical pathways through decreased inhibition, with the neurotransmitter GABA. These changes can be assessed using short-latency intracortical inhibition (SICI) and RI. Functional recovery in the chronic phase of stroke induced by rehabilitation was accompanied by SICI and spinal RI changes. Cortical reorganization and spinal plasticity might play important roles in functional recovery induced by rehabilitation, even in patients with chronic severe hemiparesis. This review aims to provide a focused overview of neuroplasticity of spinal RI and intracortical inhibition associated with functional motor recovery from stroke.

Short-term effect of a close-fitting type of walking assistive device on spinal cord reciprocal inhibition

One of the major problems with walking encountered by patients with spastic hemiplegia is diminished toe clearance due to spasticity of their leg muscles. To improve their walking, a specialized robot assist for ankle movements (RE-Gait) has been utilized. The present study examined the neurophysiological effects whether spinal cord reciprocal Ia inhibition (RI) in the leg was altered by using RE-Gait. Sixteen patients with a clinical diagnosis of stroke were divided into the two groups, RE-Gait walking group (Group R) and normal (controlled) walking group (Group C). In each group, they walked on a flat floor for 15 min with or without RE-Gait. The depression of soleus (Sol) H-reflexes conditioned by common peroneal nerve stimuli with the conditioning-test (C-T) intervals of 1, 2, 3, and 4 ms were assessed before and immediately after each walking session. After the intervention, the LSM (SE) of Sol H-reflex amplitude with 1, 2 and 3 ms C-T interval conditions were significantly decreased in group R (1 ms: 88.15 (4.60), 2 ms: 86.37 (4.60), 3 ms: 89.68 (4.62)) compared to group C (1 ms: 105.57 (4.56), 2 ms: 100.89 (4.58), 3 ms: 107.72 (4.58)) [1 ms: p = 0.012, 2 ms: p = 0.035, 3 ms: p = 0.011]. Walking assistive robot that targets ankle movements might be a new rehabilitation tool for regulating spinal cord excitability.

Interrogating interneurone function using threshold tracking of the H reflex in healthy subjects and patients with motor neurone disease

OBJECTIVE

The excitability of the lower motoneurone pool is traditionally tested using the H reflex and a constant-stimulus paradigm, which measures changes in the amplitude of the reflex response. This technique has limitations because reflex responses of different size must involve the recruitment or inhibition of different motoneurones. The threshold-tracking technique ensures that the changes in excitability occur for an identical population of motoneurones. We aimed to assess this technique and then apply it in patients with motor neurone disease (MND).

METHODS

The threshold-tracking approach was assessed in 17 healthy subjects and 11 patients with MND. The soleus H reflex was conditioned by deep peroneal nerve stimulation producing reciprocal Ia and so-called D1 and D2 inhibitions, which are believed to reflect presynaptic inhibition of soleus Ia afferents.

RESULTS

Threshold tracking was quicker than the constant-stimulus technique and reliable, properties that may be advantageous for clinical studies. D1 inhibition was significantly reduced in patients with MND.

CONCLUSIONS

Threshold tracking is useful and may be preferable under some conditions for studying the excitability of the motoneurone pool. The decreased D1 inhibition in the patients suggests that presynaptic inhibition may be reduced in MND.

SIGNIFICANCE

Reduced presynaptic inhibition could be evidence of an interneuronopathy in MND. It is possible that the hyperreflexia is a spinal pre-motoneuronal disorder, and not definitive evidence of corticospinal involvement in MND.

Spastic co-contraction is directly associated with altered cortical beta oscillations after stroke

OBJECTIVE

Spastic co-contraction is a motor-disabling form of muscle overactivity occurring after a stroke, contributing to a limitation in active movement and a certain level of motor impairment. The cortical mechanisms underlying spastic co-contraction remain to be more fully elucidated, the present study aimed to investigate the role of the cortical beta oscillations in spastic co-contraction after a stroke.

METHOD

We recruited fifteen post-stroke participants and nine healthy controls. The participants were asked to perform active elbow extensions. In the study, multimodal analysis was performed to combine the evaluation of three-dimensional elbow kinematics, the elbow muscles electromyographic activations, and the cortical oscillatory activity.

RESULTS

The movement-related beta desynchronization was significantly decreased in post-stroke participants compared to healthy participants. We found a significant correlation between the movement-related beta desynchronization and the elbow flexors activation during the active elbow extension in post-stroke participants. When compared to healthy participants, post-stroke participants exhibited significant alterations in the elbow kinematics and greater muscle activation levels.

CONCLUSIONS

Cortical beta oscillation alterations may reflect an important neural mechanism underlying spastic co-contraction after a stroke.

SIGNIFICANCE

Measuring the cortical oscillatory activity could be useful to further characterize neuromuscular plasticity induced by recovery or therapeutic interventions.

Presynaptic inhibition in restless legs syndrome

Introduction: Restless legs syndrome (RLS) is a condition that particularly urges at night in resting and causes the need to move the legs. Although the pathophysiology has not yet been clarified, dopamine and iron metabolism and spinal cord pathologies are blamed for causing the condition. There are few studies on spinal reflex mechanisms on RLS. In the present study, we aimed to investigate the role of presynaptic inhibition (PreI) in the spinal cord in RLS.Methods: Fourteen patients with RLS and 14 controls with similar demographic characteristics were included in the study. Soleus muscle H-reflex (Ht) investigation was performed for subjects whose electrophysiologic investigation was normal. The Ht response was conditioned to the stimulation of the common peroneal nerve (CPN) (Hc). The test and conditioned stimulation intervals were kept between 10 ms, 20 ms, 30 ms, 40 ms, 50 ms, 75 ms, 100 ms, 150 ms and 200 ms. In each inter-stimulus interval, nonparametric repeat measurement evaluations were conducted with the percentage value of Hc/Ht. The Hc/Ht values of the study and control groups in the same intervals were compared separately.Results: A significant decrease was detected in Hc values in the control group in the repeat measurement values at 20 ms and 100 ms inter-stimulus intervals; however, there was not decrease in any intervals in the patient's group.Conclusion: The absence of any decrease in Hc reflexes for 20-100 ms intervals revealed that discernible PreI was vanished in RLS patients.

Effect of Ankle Angles on the Soleus H-Reflex Excitability During Standing

This study investigated effects of ankle joint angle on the Hoffman's reflex (H-reflex) excitability during loaded (weight borne with both legs) and unloaded (full body weight borne with the contralateral leg) standing in people without neurological injuries. Soleus H-reflex/M-wave recruitment curves were examined during upright standing on three different slopes that imposed plantar flexion (-15°), dorsiflexion (+15°), and neutral (0°) angles at the ankle, with the test leg loaded and unloaded. With the leg loaded and unloaded, maximum H-reflex/maximum M-wave ratio of -15° was significantly larger than those of 0° and +15° conditions. The maximum H-reflex/maximum M-wave ratios were 51%, 43%, and 41% with loaded and 56%, 46%, and 44% with unloaded for -15°, 0°, and +15° slope conditions, respectively. Thus, limb loading/unloading had limited impact on the extent of influence that ankle angles exert on the H-reflex excitability. This suggests that task-dependent central nervous system control of reflex excitability may regulate the influence of sensory input on the spinal reflex during standing.

Recovery cycles of posterior root-muscle reflexes evoked by transcutaneous spinal cord stimulation and of the H reflex in individuals with intact and injured spinal cord

Posterior root-muscle (PRM) reflexes are short-latency spinal reflexes evoked by epidural or transcutaneous spinal cord stimulation (SCS) in clinical and physiological studies. PRM reflexes share key physiological characteristics with the H reflex elicited by electrical stimulation of large-diameter muscle spindle afferents in the tibial nerve. Here, we compared the H reflex and the PRM reflex of soleus in response to transcutaneous stimulation by studying their recovery cycles in ten neurologically intact volunteers and ten individuals with traumatic, chronic spinal cord injury (SCI). The recovery cycles of the reflexes, i.e., the time course of their excitability changes, were assessed by paired pulses with conditioning-test intervals of 20–5000 ms. Between the subject groups, no statistical difference was found for the recovery cycles of the H reflexes, yet those of the PRM reflexes differed significantly, with a striking suppression in the intact group. When comparing the reflex types, they did not differ in the SCI group, while the PRM reflexes were more strongly depressed in the intact group for durations characteristic for presynaptic inhibition. These differences may arise from the concomitant stimulation of several posterior roots containing afferent fibers of various lower extremity nerves by transcutaneous SCS, producing multi-source heteronymous presynaptic inhibition, and the collective dysfunction of inhibitory mechanisms after SCI contributing to spasticity. PRM-reflex recovery cycles additionally obtained for bilateral rectus femoris, biceps femoris, tibialis anterior, and soleus all demonstrated a stronger suppression in the intact group. Within both subject groups, the thigh muscles showed a stronger recovery than the lower leg muscles, which may reflect a characteristic difference in motor control of diverse muscles. Based on the substantial difference between intact and SCI individuals, PRM-reflex depression tested with paired pulses could become a sensitive measure for spasticity and motor recovery.

Exploratory Randomized Double-Blind Placebo-Controlled Trial of Botulinum Therapy on Grasp Release After Stroke (PrOMBiS)

Background. OnabotulinumtoxinA injections improve upper-limb spasticity after stroke, but their effect on arm function remains uncertain. Objective. To determine whether a single treatment with onabotulinumtoxinA injections combined with upper-limb physiotherapy improves grasp release compared with physiotherapy alone after stroke. Methods. A total of 28 patients, at least 1 month poststroke, were randomized to receive either onabotulinumtoxinA or placebo injections to the affected upper limb followed by standardized upper-limb physiotherapy (10 sessions over 4 weeks). The primary outcome was time to release grasp during a functionally relevant standardized task. Secondary outcomes included measures of wrist and finger spasticity and strength using a customized servomotor, clinical assessments of stiffness (modified Ashworth Scale), arm function (Action Research Arm Test [ARAT], Nine Hole Peg Test), arm use (Arm Measure of Activity), Goal Attainment Scale, and quality of life (EQ5D). Results. There was no significant difference between treatment groups in grasp release time 5 weeks post injection (placebo median = 3.0 s, treatment median = 2.0 s; t(24) = 1.20; P = .24; treatment effect = -0.44, 95% CI = -1.19 to 0.31). None of the secondary measures passed significance after correcting for multiple comparisons. Both groups achieved their treatment goals (placebo = 65%; treatment = 71%), and made improvements on the ARAT (placebo +3, treatment +5) and in active wrist extension (placebo +9°, treatment +11°). Conclusions. In this group of stroke patients with mild to moderate spastic hemiparesis, a single treatment with onabotulinumtoxinA did not augment the improvements seen in grasp release time after a standardized upper-limb physiotherapy program.

Effects of downslope walking on Soleus H-reflexes and walking function in individuals with multiple sclerosis: A preliminary study

BACKGROUND

Downslope walking (DSW) is an eccentric-based exercise intervention that promotes neuroplasticity of spinal reflex circuitry by inducing depression of Soleus Hoffman (H)-reflexes in young, neurologically unimpaired adults.

OBJECTIVE

The objective of the study was to evaluate the effects of DSW on spinal excitability (SE) and walking function (WF) in people with multiple sclerosis (PwMS).

METHODS

Our study comprised two experiments on 12 PwMS (11 women; 45.3±11.8 years). Experiment 1 evaluated acute effects of a single 20-minute session of treadmill walking at three different walking grades on SE, 0% or level walking (LW), - 7.5% DSW, and - 15% DSW. Experiment 2 evaluated the effects of 6 sessions of DSW, at - 7.5% DSW (with second session being - 15% DSW) on SE and WF.

RESULTS

Experiment 1 showed significantly greater acute % H-reflex depression following - 15% DSW compared to LW (p = 0.02) and - 7.5% DSW (p = 0.05). Experiment 2 demonstrated significant improvements in WF. PwMS who showed greater acute H-reflex depression during the - 15% DSW session also demonstrated greater physical activity, long-distance WF, and the ability to have greater H-reflex depression after DSW training. Significant changes were not observed in regards to SE.

CONCLUSIONS

Though significant changes were not observed in SE after DSW training, we observed an improvement in WF which merits further investigation of DSW in PwMS.

Improving Sexual Function by Using Focal Vibrations in Men with Spinal Cord Injury: Encouraging Findings from a Feasibility Study

Erectile dysfunction (ED) is a frequent and disabling condition in patients with spinal cord injury (SCI). Spasticity can negatively affect sexual intercourse, as it may interfere with positioning, mobility, and muscle activation and strength, leading to ED. The aim of our study was to evaluate the feasibility and efficacy of muscle vibration (MV) applied to the pelvic muscles in improving ED in men with SCI. Ten adult men with traumatic SCI were submitted to 15 sessions of MV, applied on the perineum and the suprapubic and sacrococcygeal areas, using a pneumatic vibrator. MV was performed three times a week for five consecutive weeks, each session lasting 30 min. Muscle tone and sexual function were assessed before and after MV using the Modified Ashworth Scale (MAS) and International Index of Erectile Function (IIEF). We assessed the cremasteric and bulbocavernosus reflexes, as well as the electrophysiological bulbocavernosus reflex (eBCR) and pudendal nerve somatosensory-evoked potential (PSEP). MV was safe and well tolerated. All the patients reported an improvement in MAS and IIEF, with better reflexive responses, and a significant increase in eBCR and PSEP amplitude. In conclusion, MV of the pelvic floor is a promising method to reduce segmental spasticity and improve ED in men with incomplete SCI. However, our findings require confirmation through a randomized clinical trial with a larger sample size and longer trial period to examine long-term after effects.

Nociceptor-dependent locomotor dysfunction after clinically-modeled hindlimb muscle stretching in adult rats with spinal cord injury

In the course of investigating how common clinical treatments and adaptive technologies affect recovery after spinal cord injury (SCI), we discovered that a clinically-modeled hindlimb stretching protocol dramatically, but transiently, reduces locomotor function. Nociceptive sensory input is capable of altering motor output at the spinal level, and nociceptive neurons are sensitized after SCI. Here we tested the hypotheses that stretch-induced locomotor deficits are dependent on nociceptive afferents by depleting TRPV1+ sensory afferents using capsaicin injections in neonatal rats. Following maturation, animals received 25g-cm contusive SCI at T10. After plateau of locomotor recovery at 6 weeks, daily stretching was performed for 3 weeks, followed by 2 weeks without stretch, and again for two additional weeks. Animals were sacrificed 2 h after the last stretching session for histological assessments. Consistent with previous findings, stretch-induced drops in locomotor function were observed in nociceptor-intact animals but were nearly absent in nociceptor-depleted animals. These functional changes were accompanied by corresponding increases in the number of c-Fos+ nuclei throughout the lumbar enlargement. As expected, nociceptor-depleted animals had very little CGRP+ axonal innervation of the dorsal horn. Nociceptor-intact stretched animals had significantly higher levels of CGRP+ as compared to non-stretched SCI rats, suggesting that stretching promoted intraspinal CGRP+ sprouting. These results indicate that stretch-induced locomotor dysfunction in animals with incomplete SCI involves C-fibers, adding a negative post-SCI role to their adaptive roles (e.g., bladder control), and suggesting that the clinical use of muscle stretching to combat contractures and spasticity may be unintentionally detrimental to locomotor function.

Effects of Leg Motor Imagery Combined With Electrical Stimulation on Plasticity of Corticospinal Excitability and Spinal Reciprocal Inhibition

Motor imagery (MI) combined with electrical stimulation (ES) enhances upper-limb corticospinal excitability. However, its after-effects on both lower limb corticospinal excitability and spinal reciprocal inhibition remain unknown. We aimed to investigate the effects of MI combined with peripheral nerve ES (MI + ES) on the plasticity of lower limb corticospinal excitability and spinal reciprocal inhibition. Seventeen healthy individuals performed the following three tasks on different days, in a random order: (1) MI alone; (2) ES alone; and (3) MI + ES. The MI task consisted of repetitive right ankle dorsiflexion for 20 min. ES was percutaneously applied to the common peroneal nerve at a frequency of 100 Hz and intensity of 120% of the sensory threshold of the tibialis anterior (TA) muscle. We examined changes in motor-evoked potential (MEP) of the TA (task-related muscle) and soleus muscle (SOL; task-unrelated muscle). We also examined disynaptic reciprocal inhibition before, immediately after, and 10, 20, and 30 min after the task. MI + ES significantly increased TA MEPs immediately and 10 min after the task compared with baseline, but did not change the task-unrelated muscle (SOL) MEPs. MI + ES resulted in a significant increase in the magnitude of reciprocal inhibition immediately and 10 min after the task compared with baseline. MI and ES alone did not affect TA MEPs or reciprocal inhibition. MI combined with ES is effective in inducing plastic changes in lower limb corticospinal excitability and reciprocal Ia inhibition.

Effects of Reciprocal Ia Inhibition on Contraction Intensity of Co-contraction

Introduction: Excessive co-contraction interferes with smooth joint movement. One mechanism is the failure of reciprocal inhibition against antagonists during joint movement. Reciprocal inhibition has been investigated using joint torque as an index of intensity during co-contraction. However, contraction intensity as an index of co-contraction intensity has not been investigated. In this study, we aimed to evaluate the influence of changes in contraction intensity during co-contraction on reciprocal inhibition.

Methods: We established eight stimulus conditions in 20 healthy adult males to investigate the influence of changes in contraction intensity during co-contraction on reciprocal inhibition. These stimulus conditions comprised a conditioning stimulus-test stimulation interval (C–T interval) of -2, 0, 1, 2, 3, 4, or 5 ms plus a test stimulus without a conditioning stimulus (single). Co-contraction of the tibialis anterior and soleus muscles at the same as contraction intensity was examined at rest and at 5, 15, and 30% maximal voluntary contraction (MVC).

Results: At 5 and 15% MVC in the co-contraction task, the H-reflex amplitude was significantly decreased compared with single stimulation at a 2-ms C–T interval. At 30% MVC, there was no significant difference compared with single stimulation at a 2-ms C–T interval. At a 5-ms C–T interval, the H-reflex amplitude at 30% MVC was significantly reduced compared with that at rest.

Discussion: The findings indicated that during co-contraction, reciprocal Ia inhibition worked at 5 and 15% MVC. Contrary inhibition of reciprocal Ia inhibition did not apparently work at 30% MVC, and presynaptic inhibition (D1 inhibition) might work.

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.

The neurophysiology of deforming spastic paresis: A revised taxonomy

This paper revisits the taxonomy of the neurophysiological consequences of a persistent impairment of motor command execution in the classic environment of sensorimotor restriction and muscle hypo-mobilization in short position. Around each joint, the syndrome involves 2 disorders, muscular and neurologic. The muscular disorder is promoted by muscle hypo-mobilization in short position in the context of paresis, in the hours and days after paresis onset: this genetically mediated, evolving myopathy, is called spastic myopathy. The clinician may suspect it by feeling extensibility loss in a resting muscle, although long after the actual onset of the disease. The neurologic disorder, promoted by sensorimotor restriction in the context of paresis and by the muscle disorder itself, comprises 4 main components, mostly affecting antagonists to desired movements: the first is spastic dystonia, an unwanted, involuntary muscle activation at rest, in the absence of stretch or voluntary effort; spastic dystonia superimposes on spastic myopathy to cause visible, gradually increasing body deformities; the second is spastic cocontraction, an unwanted, involuntary antagonist muscle activation during voluntary effort directed to the agonist, aggravated by antagonist stretch; it is primarily due to misdirection of the supraspinal descending drive and contributes to reducing movement amplitude; and the third is spasticity, one form of hyperreflexia, defined by an enhancement of the velocity-dependent responses to phasic stretch, detected and measured at rest (another form of hyperreflexia is "nociceptive spasms", following flexor reflex afferent stimulation, particularly after spinal cord lesions). The 3 main forms of overactivity, spastic dystonia, spastic cocontraction and spasticity, share the same motor neuron hyperexcitability as a contributing factor, all being predominant in the muscles that are more affected by spastic myopathy. The fourth component of the neurologic disorder affects the agonist: it is stretch-sensitive paresis, which is a decreased access of the central command to the agonist, aggravated by antagonist stretch. Improved understanding of the pathophysiology of deforming spastic paresis should help clinicians select meaningful assessments and refined treatments, including the utmost need to preserve muscle tissue integrity as soon as paresis sets in.

The Short-Term Effect of Slope Walking on Soleus H-Reflexes in People with Multiple Sclerosis

Downslope walking (DSW) causes H-reflex depression in healthy adults, and thus may hold promise for inducing spinal reflex plasticity in people with Multiple Sclerosis (PwMS). The study purpose was to test the hypothesis that DSW will cause acute depression of spinal excitability in PwMS. Soleus H-reflexes were measured in PwMS (n = 18) before and after 20 min of treadmill walking during three visits. Participants walked on a different slope each visit [level: 0% level walking (LW), upslope: +7.5% treadmill walking with an upslope (USW) or downslope: -7.5% (DSW)]. The soleus H_max/M_max ratio was used to measure spinal excitability. Heart rate and ratings of perceived exertion (RPE) were measured during walking. DSW induced the largest change in spinal excitability (a 26.7% reduction in soleus H_max/M_max (p = 0.001)), although LW also reduced H_max/M_max (-5.3%, p = 0.05). Heart rate (p < 0.001) was lowest for DSW, and RPE for DSW did not exceed "Fairly light". DSW evokes short-term spinal plasticity in PwMS, while requiring no greater effort than LW. Our results suggest that PwMS retain the capacity for DSW-induced short-term spinal reflex modulation previously found in healthy adults. These results may provide a foundation for further investigation of long-term effects of DSW on spinal reflex plasticity and functional ability in PwMS.

Impaired Ability to Suppress Excitability of Antagonist Motoneurons at Onset of Dorsiflexion in Adults with Cerebral Palsy

We recently showed that impaired gait function in adults with cerebral palsy (CP) is associated with reduced rate of force development in ankle dorsiflexors. Here, we explore potential mechanisms. We investigated the suppression of antagonist excitability, calculated as the amount of soleus H-reflex depression at the onset of ankle dorsiflexion compared to rest, in 24 adults with CP (34.3 years, range 18–57; GMFCS 1.95, range 1–3) and 15 healthy, age-matched controls. Furthermore, the central common drive to dorsiflexor motoneurons during a static contraction in the two groups was examined by coherence analyses. The H-reflex was significantly reduced by 37% at the onset of dorsiflexion compared to rest in healthy adults (P < 0.001) but unchanged in adults with CP (P = 0.91). Also, the adults with CP had significantly less coherence. These findings suggest that the ability to suppress antagonist motoneuronal excitability at movement onset is impaired and that the central common drive during static contractions is reduced in adults with CP.