Spinal reflexes in ankle flexor and extensor muscles after chronic central nervous system lesions and functional electrical stimulation

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.

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.

Impaired H-Reflex Adaptations Following Slope Walking in Individuals With Post-stroke Hemiparesis

Background and Purpose

Short term adaptations in the Ia afferent-motoneuron pathway, as measured using the H-reflex, in response to altered ground reaction forces (GRFs) applied at the feet during slope walking have been observed in the non-impaired nervous system. The ability of the stroke-impaired nervous system to adapt to altered GRFs have not been examined. The purpose of this study was to examine the acute effects of altered propulsive and braking forces applied at the feet, which naturally occurs when walking on different slopes, on adaptations of the H-reflex pathway in individuals with chronic post-stroke hemiparesis.

Methods

Twelve individuals chronically post-stroke and 10 age-similar non-neurologically impaired controls walked on an instrumented treadmill for 20 min under level, upslope and downslope conditions. GRFs were measured during walking and soleus H-reflexes were recorded prior to and immediately after walking. A 3 (limbs: paretic, non-paretic, and non-impaired) × 3 (slope: level, upslope, downslope) mixed factorial ANOVA was conducted on the propulsive and braking forces. A 2 (limb: paretic and non-impaired) × 2 (time: pre and post) × 3 (slope: level, upslope, and downslope) mixed factorial ANOVA was conducted to assess the soleus H-reflex amplitudes.

Results

In both post-stroke and non-impaired groups, during downslope walking, peak propulsive forces decreased, while peak braking forces increased. In contrast, during upslope walking, peak propulsive forces increased and peak braking forces decreased. We observed reduced soleus H-reflex amplitudes immediately following 20 min of level, downslope and upslope walking in non-impaired individuals but not in the paretic legs of individuals with chronic post-stroke hemiparesis.

Discussion and Conclusion

Similar pattern of change in peak propulsive and braking forces with respect to different slopes was observed in both individuals post-stroke and non-impaired individuals, but the magnitude of GRFs were smaller in individuals post-stroke due to the slower walking speed. Our results suggested that impaired modulation of the H-reflex pathway potentially underlies the lack of neuroadaptations in individuals with chronic post-stroke hemiparesis.

Modulation of soleus stretch reflexes during walking in people with chronic incomplete spinal cord injury

In people with spasticity due to chronic incomplete spinal cord injury (SCI), it has been presumed that the abnormal stretch reflex activity impairs gait. However, locomotor stretch reflexes across all phases of walking have not been investigated in people with SCI. Thus, to understand modulation of stretch reflex excitability during spastic gait, we investigated soleus stretch reflexes across the entire gait cycle in nine neurologically normal participants and nine participants with spasticity due to chronic incomplete SCI (2.5–11 year post-injury). While the participant walked on the treadmill at his/her preferred speed, unexpected ankle dorsiflexion perturbations (6° at 250°/s) were imposed every 4–6 steps. The soleus H-reflex was also examined. In participants without SCI, spinal short-latency “M1”, spinal medium latency “M2”, and long-latency “M3” were clearly modulated throughout the step cycle; the responses were largest in the mid-stance and almost completely suppressed during the stance-swing transition and swing phases. In participants with SCI, M1 and M2 were abnormally large in the mid–late-swing phase, while M3 modulation was similar to that in participants without SCI. The H-reflex was also large in the mid–late-swing phase. Elicitation of H-reflex and stretch reflexes in the late swing often triggered clonus and affected the soleus activity in the following stance. In individuals without SCI, moderate positive correlation was found between H-reflex and stretch reflex sizes across the step cycle, whereas in participants with SCI, such correlation was weak to non-existing, suggesting that H-reflex investigation would not substitute for stretch reflex investigation in individuals after SCI.

H-reflex conditioning during locomotion in people with spinal cord injury

Key points

In people or animals with incomplete spinal cord injury (SCI), changing a spinal reflex through an operant conditioning protocol can improve locomotion.

All previous studies conditioned the reflex during steady‐state maintenance of a specific posture. By contrast, the present study down‐conditioned the reflex during the swing‐phase of locomotion in people with hyperreflexia as a result of chronic incomplete SCI. The aim was to modify the functioning of the reflex in a specific phase of a dynamic movement.

This novel swing‐phase conditioning protocol decreased the reflex much faster and farther than did the steady‐state protocol in people or animals with or without SCI, and it also improved locomotion.

The reflex decrease persisted for at least 6 months after conditioning ended.

The results suggest that conditioning reflex function in a specific phase of a dynamic movement offers a new approach to enhancing and/or accelerating recovery after SCI or in other disorders.

Abstract

In animals and people with incomplete spinal cord injury, appropriate operant conditioning of a spinal reflex can improve impaired locomotion. In all previous conditioning studies, the reflex was conditioned during steady‐state maintenance of a stable posture; this steady‐state protocol aimed to change the excitability of the targeted reflex pathway; reflex size gradually changed over 8–10 weeks. The present study introduces a new protocol, comprising a dynamic protocol that aims to change the functioning of the reflex pathway during a specific phase of a complex movement. Specifically, we down‐conditioned the soleus H‐reflex during the swing‐phase of locomotion in people with hyperreflexia as a result of chronic incomplete SCI. The swing‐phase H‐reflex, which is absent or very small in neurologically normal individuals, is abnormally large in this patient population. The results were clear. With swing‐phase down‐conditioning, the H‐reflex decreased much faster and farther than did the H‐reflex in all previous animal or human studies with the steady‐state protocol, and the decrease persisted for at least 6 months after conditioning ended. The H‐reflex decrease was accompanied by improvements in walking speed and in the modulation of locomotor electromyograph activity in proximal and distal muscles of both legs. These results provide new insight into the factors controlling spinal reflex conditioning; they suggest that the conditioning protocols targeting reflex function in a specific movement phase provide a promising new opportunity to enhance functional recovery after SCI or in other disorders.

In people or animals with incomplete spinal cord injury (SCI), changing a spinal reflex through an operant conditioning protocol can improve locomotion.

All previous studies conditioned the reflex during steady‐state maintenance of a specific posture. By contrast, the present study down‐conditioned the reflex during the swing‐phase of locomotion in people with hyperreflexia as a result of chronic incomplete SCI. The aim was to modify the functioning of the reflex in a specific phase of a dynamic movement.

This novel swing‐phase conditioning protocol decreased the reflex much faster and farther than did the steady‐state protocol in people or animals with or without SCI, and it also improved locomotion.

The reflex decrease persisted for at least 6 months after conditioning ended.

The results suggest that conditioning reflex function in a specific phase of a dynamic movement offers a new approach to enhancing and/or accelerating recovery after SCI or in other disorders.

Operant conditioning of the motor-evoked potential and locomotion in people with and without chronic incomplete spinal cord injury

Foot drop is very common among people with chronic incomplete spinal cord injury (SCI) and likely stems from SCI that disturbs the corticospinal activation of the ankle dorsiflexor tibialis anterior (TA). Thus, if one can recover or increase the corticospinal excitability reduced by SCI, motor function recovery may be facilitated. Here, we hypothesized that in people suffering from weak dorsiflexion due to chronic incomplete SCI, increasing the TA motor-evoked potential (MEP) through operant up-conditioning can improve dorsiflexion during locomotion, while in people without any injuries, it would have little impact on already normal locomotion. Before and after 24 MEP conditioning or control sessions, locomotor electromyography (EMG) and kinematics were measured. This study reports the results of these locomotor assessments. In participants without SCI, locomotor EMG activity, soleus Hoffmann reflex modulation, and joint kinematics did not change, indicating that MEP up-conditioning or repeated single-pulse transcranial magnetic stimulation (i.e., control protocol) does not influence normal locomotion. In participants with SCI, MEP up-conditioning increased TA activity during the swing-to-swing stance transition phases and ankle joint motion during locomotion in the conditioned leg and increased walking speed consistently. In addition, the swing-phase TA activity and ankle joint motion also improved in the contralateral leg. The results are consistent with our hypothesis. Together with the previous operant conditioning studies in humans and rats, the present study suggests that operant conditioning can be a useful therapeutic tool for enhancing motor function recovery in people with SCI and other central nervous system disorders. NEW & NOTEWORTHY This study examined the functional impact of operant conditioning of motor-evoked potential (MEP) to transcranial magnetic stimulation that aimed to increase corticospinal excitability for the ankle dorsiflexor tibialis anterior (TA). In people with chronic incomplete spinal cord injury (SCI), MEP up-conditioning increased TA activity and improved dorsiflexion during locomotion, while in people without injuries, it had little impact on already normal locomotion. MEP conditioning may potentially be used to enhance motor function recovery after SCI.

Operant conditioning of the tibialis anterior motor evoked potential in people with and without chronic incomplete spinal cord injury

The activity of corticospinal pathways is important in movement control, and its plasticity is essential for motor skill learning and re-learning after central nervous system (CNS) injuries. Therefore, enhancing the corticospinal function may improve motor function recovery after CNS injuries. Operant conditioning of stimulus-induced muscle responses (e.g., reflexes) is known to induce the targeted plasticity in a targeted pathway. Thus, an operant conditioning protocol to target the corticospinal pathways may be able to enhance the corticospinal function. To test this possibility, we investigated whether operant conditioning of the tibialis anterior (TA) motor evoked potential (MEP) to transcranial magnetic stimulation can enhance corticospinal excitability in people with and without chronic incomplete spinal cord injury (SCI). The protocol consisted of 6 baseline and 24 up-conditioning/control sessions over 10 wk. In all sessions, TA MEPs were elicited at 10% above active MEP threshold while the sitting participant provided a fixed preset level of TA background electromyographic activity. During baseline sessions, MEPs were simply measured. During conditioning trials of the conditioning sessions, the participant was encouraged to increase MEP and was given immediate feedback indicating whether MEP size was above a criterion. In 5/8 participants without SCI and 9/10 with SCI, over 24 up-conditioning sessions, MEP size increased significantly to ~150% of the baseline value, whereas the silent period (SP) duration decreased by ~20%. In a control group of participants without SCI, neither MEP nor SP changed. These results indicate that MEP up-conditioning can facilitate corticospinal excitation, which is essential for enhancing motor function recovery after SCI. NEW & NOTEWORTHY We investigated whether operant conditioning of the motor evoked potential (MEP) to transcranial magnetic stimulation can systematically increase corticospinal excitability for the ankle dorsiflexor tibialis anterior (TA) in people with and without chronic incomplete spinal cord injury. We found that up-conditioning can increase the TA MEP while reducing the accompanying silent period (SP) duration. These findings suggest that MEP up-conditioning produces the facilitation of corticospinal excitation as targeted, whereas it suppresses inhibitory mechanisms reflected in SP.

Operant Up-Conditioning of the Tibialis Anterior Motor-Evoked Potential in Multiple Sclerosis: Feasibility Case Studies

Damage to the corticospinal pathway often results in weak dorsiflexion of the ankle, thereby limiting the mobility of people with multiple sclerosis (MS). Thus, strengthening corticospinal connectivity may improve locomotion. Here, we investigated the feasibility of tibialis anterior (TA) motor-evoked potential (MEP) operant conditioning and whether it can enhance corticospinal excitability and alleviate locomotor problems in people with chronic stable MS. The protocol consisted of 6 baseline and 24 up-conditioning sessions over 10 weeks. In all sessions, TA MEPs were elicited at 10% above active threshold while the sitting subject provided 30–35% maximum voluntary contraction (MVC) level of TA background EMG. During baseline sessions, MEPs were simply measured. During conditioning trials of the conditioning sessions, the subject was encouraged to increase MEP and was given immediate feedback indicating whether MEP size was above a criterion. In 3/4 subjects, TA MEP increased 32–75%, MVC increased 28–52%, locomotor EMG modulation improved in multiple leg muscles, and foot drop became less severe. In one of them, MEP and MVC increases were maintained throughout 3 years of extensive follow-up sessions. These initial results support a therapeutic possibility of MEP operant conditioning for improving locomotion in people with MS or other CNS disorders, such as spinal cord injury and stroke.

Supraspinal Control Predicts Locomotor Function and Forecasts Responsiveness to Training after Spinal Cord Injury

Restoration of walking ability is an area of great interest in the rehabilitation of persons with spinal cord injury. Because many cortical, subcortical, and spinal neural centers contribute to locomotor function, it is important that intervention strategies be designed to target neural elements at all levels of the neuraxis that are important for walking ability. While to date most strategies have focused on activation of spinal circuits, more recent studies are investigating the value of engaging supraspinal circuits. Despite the apparent potential of pharmacological, biological, and genetic approaches, as yet none has proved more effective than physical therapeutic rehabilitation strategies. By making optimal use of the potential of the nervous system to respond to training, strategies can be developed that meet the unique needs of each person. To complement the development of optimal training interventions, it is valuable to have the ability to predict future walking function based on early clinical presentation, and to forecast responsiveness to training. A number of clinical prediction rules and association models based on common clinical measures have been developed with the intent, respectively, to predict future walking function based on early clinical presentation, and to delineate characteristics associated with responsiveness to training. Further, a number of variables that are correlated with walking function have been identified. Not surprisingly, most of these prediction rules, association models, and correlated variables incorporate measures of volitional lower extremity strength, illustrating the important influence of supraspinal centers in the production of walking behavior in humans.

The effects of anodal transcranial direct current stimulation and patterned electrical stimulation on spinal inhibitory interneurons and motor function in patients with spinal cord injury

Supraspinal excitability and sensory input may play an important role for the modulation of spinal inhibitory interneurons and functional recovery among patients with incomplete spinal cord injury (SCI). Here, we investigated the effects of anodal transcranial direct current stimulation (tDCS) combined with patterned electrical stimulation (PES) on spinal inhibitory interneurons in patients with chronic incomplete SCI and in healthy individuals. Eleven patients with incomplete SCI and ten healthy adults participated in a single-masked, sham-controlled crossover study. PES involved stimulating the common peroneal nerve with a train of ten 100 Hz pulses every 2 s for 20 min. Anodal tDCS (1 mA) was simultaneously applied to the primary motor cortex that controls the tibialis anterior muscle. We measured reciprocal inhibition and presynaptic inhibition of a soleus H-reflex by stimulating the common peroneal nerve prior to tibial nerve stimulation, which elicits the H-reflex. The inhibition was assessed before, immediately after, 10 min after and 20 min after the stimulation. Compared with baseline, simultaneous application of anodal tDCS with PES significantly increased changes in disynaptic reciprocal inhibition and long-latency presynaptic inhibition in both healthy and SCI groups for at least 20 min after the stimulation (all, p < 0.001). In patients with incomplete SCI, anodal tDCS with PES significantly increased the number of ankle movements in 10 s at 20 min after the stimulation (p = 0.004). In conclusion, anodal tDCS combined with PES could induce spinal plasticity and improve ankle movement in patients with incomplete SCI.

The impact of walking speed on the effects of functional electrical stimulation for foot drop in people with multiple sclerosis

PURPOSE

Functional electrical stimulation (FES) is effective in assisting people with multiple sclerosis (pwMS) with gait. Previous studies have investigated the effects of FES in pwMS with slow self-selected walking speeds (SSWS). This study reports on the effect of the Odstock Dropped Foot Stimulator (ODFS) on the speed and oxygen (O2) cost of gait in pwMS walking at a range of SSWS.

METHODS

Twenty pwMS (mean age 50.4 ± 7.3 years) currently using FES walked at their SSWS for 5 min with and without FES. O2 cost of gait was measured using a gas analysis system, and gait speed was calculated. Data were analysed for all participants, and comparisons were made between those with a SSWS < and >0.8 m/s (walking speed required for community ambulation).

RESULTS

Significant improvements in the speed and O2 cost of gait were seen using FES in the group with SSWS <0.8 m/s (n = 11, p = 0.005). When participants' SSWS >0.8 m/s, no difference in gait speed was noted, and a significant increase in O2 cost of gait using FES (n = 9, p = 0.004) was noted.

CONCLUSION

FES has a different effect on the speed and O2 cost of gait dependent on the SSWS of pwMS. This requires further investigation. Implications for Rehabilitation Functional electrical stimulation (FES) used for foot drop is effective in improving the speed and oxygen cost of walking in pwMS walking at SSWS <0.8 m/s. FES does not seem to have a beneficial effect on the speed and oxygen cost of walking in pwMS walking at SSWS >0.8 m/s. Further research is needed to understand the possible mechanisms involved so that FES for foot drop can be efficiently prescribed.

Targeted neuroplasticity for rehabilitation

An operant-conditioning protocol that bases reward on the electromyographic response produced by a specific CNS pathway can change that pathway. For example, in both animals and people, an operant-conditioning protocol can increase or decrease the spinal stretch reflex or its electrical analog, the H-reflex. Reflex change is associated with plasticity in the pathway of the reflex as well as elsewhere in the spinal cord and brain. Because these pathways serve many different behaviors, the plasticity produced by this conditioning can change other behaviors. Thus, in animals or people with partial spinal cord injuries, appropriate reflex conditioning can improve locomotion. Furthermore, in people with spinal cord injuries, appropriate reflex conditioning can trigger widespread beneficial plasticity. This wider plasticity appears to reflect an iterative process through which the multiple behaviors in the individual's repertoire negotiate the properties of the spinal neurons and synapses that they all use. Operant-conditioning protocols are a promising new therapeutic method that could complement other rehabilitation methods and enhance functional recovery. Their successful use requires strict adherence to appropriately designed procedures, as well as close attention to accommodating and engaging the individual subject in the conditioning process.

The simplest motor skill: mechanisms and applications of reflex operant conditioning

Operant conditioning protocols can change spinal reflexes gradually, which are the simplest behaviors. This article summarizes the evidence supporting two propositions: that these protocols provide excellent models for defining the substrates of learning and that they can induce and guide plasticity to help restore skills, such as locomotion, that have been impaired by spinal cord injury or other disorders.

Effects of functional electrical stimulation on gait recovery post-neurological injury during inpatient rehabilitation

This stage 2 trial investigated the therapeutic effect of single channel, peroneal functional electrical stimulation (FES) for improving gait and muscle activity in people with neurological injuries who were enrolled in an inpatient rehabilitation program. Twenty-six patients (16 male; M age = 51.3 yr., SD = 16.2; 2-33 days post-injury) completed the study. Participants were randomly assigned to an experimental group (n = 13) or control group (n = 13). The experimental group received FES and the control group received sensory stimulation during 45-min. gait training sessions three times a week for the duration of their stay in a rehabilitation facility (average of four sessions for both groups). Changes in gait speed, tibialis anterior muscle electromyography (EMG), and FIM™ locomotion scores were compared between groups. No significant differences were found, as both groups demonstrated similar improvements. The current results with this small sample suggest a low dose of gait training with single channel FES did not augment gait nor EMG activity beyond gait training with sensory stimulation; therefore, clinicians will likely be better served using a larger dose of FES or multichannel FES in this clinical population.

Poststroke muscle architectural parameters of the tibialis anterior and the potential implications for rehabilitation of foot drop

Poststroke dorsiflexor weakness and paretic limb foot drop increase the risk of stumbling and falling and decrease overall functional mobility. It is of interest whether dorsiflexor muscle weakness is primarily neurological in origin or whether morphological differences also contribute to the impairment. Ten poststroke hemiparetic individuals were imaged bilaterally using noninvasive medical imaging techniques. Magnetic resonance imaging was used to identify changes in tibialis anterior muscle volume and muscle belly length. Ultrasonography was used to measure fascicle length and pennation angle in a neutral position. We found no clinically meaningful bilateral differences in any architectural parameter across all subjects, which indicates that these subjects have the muscular capacity to dorsiflex their foot. Therefore, poststroke dorsiflexor weakness is primarily neural in origin and likely due to muscle activation failure or increased spasticity of the plantar flexors. The current finding suggests that electrical stimulation methods or additional neuromuscular retraining may be more beneficial than targeting muscle strength (i.e., increasing muscle mass).

Restoring walking after spinal cord injury: operant conditioning of spinal reflexes can help

People with incomplete spinal cord injury (SCI) frequently suffer motor disabilities due to spasticity and poor muscle control, even after conventional therapy. Abnormal spinal reflex activity often contributes to these problems. Operant conditioning of spinal reflexes, which can target plasticity to specific reflex pathways, can enhance recovery. In rats in which a right lateral column lesion had weakened right stance and produced an asymmetrical gait, up-conditioning of the right soleus H-reflex, which increased muscle spindle afferent excitation of soleus, strengthened right stance and eliminated the asymmetry. In people with hyperreflexia due to incomplete SCI, down-conditioning of the soleus H-reflex improved walking speed and symmetry. Furthermore, modulation of electromyographic activity during walking improved bilaterally, indicating that a protocol that targets plasticity to a specific pathway can trigger widespread plasticity that improves recovery far beyond that attributable to the change in the targeted pathway. These improvements were apparent to people in their daily lives. They reported walking faster and farther, and noted less spasticity and better balance. Operant conditioning protocols could be developed to modify other spinal reflexes or corticospinal connections; and could be combined with other therapies to enhance recovery in people with SCI or other neuromuscular disorders.

Disturbances of motor unit rate modulation are prevalent in muscles of spastic-paretic stroke survivors

Stroke survivors often exhibit abnormally low motor unit firing rates during voluntary muscle activation. Our purpose was to assess the prevalence of saturation in motor unit firing rates in the spastic-paretic biceps brachii muscle of stroke survivors. To achieve this objective, we recorded the incidence and duration of impaired lower- and higher-threshold motor unit firing rate modulation in spastic-paretic, contralateral, and healthy control muscle during increases in isometric force generated by the elbow flexor muscles. Impaired firing was considered to have occurred when firing rate became constant (i.e., saturated), despite increasing force. The duration of impaired firing rate modulation in the lower-threshold unit was longer for spastic-paretic (3.9 ± 2.2 s) than for contralateral (1.4 ± 0.9 s; P < 0.001) and control (1.1 ± 1.0 s; P = 0.005) muscles. The duration of impaired firing rate modulation in the higher-threshold unit was also longer for the spastic-paretic (1.7 ± 1.6 s) than contralateral (0.3 ± 0.3 s; P = 0.007) and control (0.1 ± 0.2 s; P = 0.009) muscles. This impaired firing rate of the lower-threshold unit arose, despite an increase in the overall descending command, as shown by the recruitment of the higher-threshold unit during the time that the lower-threshold unit was saturating, and by the continuous increase in averages of the rectified EMG of the biceps brachii muscle throughout the rising phase of the contraction. These results suggest that impairments in firing rate modulation are prevalent in motor units of spastic-paretic muscle, even when the overall descending command to the muscle is increasing.

Facilitation of corticospinal connections in able-bodied people and people with central nervous system disorders using eight interventions

BACKGROUND

Voluntary contractions (VOL), functional electrical stimulation (FES), and transcranial magnetic stimulation (TMS) can facilitate corticospinal connections.

OBJECTIVE

To find the best methods for increasing corticospinal excitability by testing eight combinations: (1) VOL, (2) FES, (3) FES + VOL, (4) TMS, (5) TMS + VOL, (6) paired associative stimulation (PAS) consisting of FES + TMS, (7) PAS + VOL, and (8) double-pulse TMS + VOL.

METHODS

Interventions were applied for 3 × 10 minutes in 15 able-bodied subjects, 14 subjects with stable central nervous system lesions (e.g., chronic stroke, and incomplete spinal cord injury) and 16 subjects with progressive central nervous system conditions (e.g., secondary progressive multiple sclerosis). Motor-evoked potentials (MEP), M-waves, and H-reflexes were monitored over a 1-hour period.

RESULTS

Three interventions (PAS, PAS + VOL, and double-pulse TMS + VOL) caused 15% to 20% increases (P < 0.05) in the MEP at a stimulus level that initially produced a half-maximal response (MEP(half)) during a contraction. Interventions were less effective in both clinical groups than in the able-bodied group. Interventions with VOL were more effective in increasing the MEP(half) than those without (P = 0.022). When more modalities were combined, the MEP increases were larger (P = 0.022).

CONCLUSIONS

(1) Short-term application of FES, TMS, and VOL can facilitate corticospinal pathways, particularly when methods are combined. (2) The effects may depend on the total activation of neural pathways, which is reduced in central nervous system disorders.

Operant conditioning to increase ankle control or decrease reflex excitability improves reflex modulation and walking function in chronic spinal cord injury

Ankle clonus is common after spinal cord injury (SCI) and is attributed to loss of supraspinally mediated inhibition of soleus stretch reflexes and maladaptive reorganization of spinal reflex pathways. The maladaptive reorganization underlying ankle clonus is associated with other abnormalities, such as coactivation and reciprocal facilitation of tibialis anterior (TA) and soleus (SOL), which contribute to impaired walking ability in individuals with motor-incomplete SCI. Operant conditioning can increase muscle activation and decrease stretch reflexes in individuals with SCI. We compared two operant conditioning-based interventions in individuals with ankle clonus and impaired walking ability due to SCI. Training included either voluntary TA activation (TA↑) to enhance supraspinal drive or SOL H-reflex suppression (SOL↓) to modulate reflex pathways at the spinal cord level. We measured clonus duration, plantar flexor reflex threshold angle, timed toe tapping, dorsiflexion (DF) active range of motion, lower extremity motor scores (LEMS), walking foot clearance, speed and distance, SOL H-reflex amplitude modulation as an index of reciprocal inhibition, presynaptic inhibition, low-frequency depression, and SOL-to-TA clonus coactivation ratio. TA↑ decreased plantar flexor reflex threshold angle (-4.33°) and DF active range-of-motion angle (-4.32°) and increased LEMS of DF (+0.8 points), total LEMS of the training leg (+2.2 points), and nontraining leg (+0.8 points), and increased walking foot clearance (+ 4.8 mm) and distance (+12.09 m). SOL↓ decreased SOL-to-TA coactivation ratio (-0.21), increased nontraining leg LEMS (+1.8 points), walking speed (+0.02 m/s), and distance (+6.25 m). In sum, we found increased voluntary control associated with TA↑ outcomes and decreased reflex excitability associated with SOL↓ outcomes.

Differential modulation of reciprocal inhibition in ankle muscles during rhythmic arm cycling

Interlimb neural linkages relay activity related to rhythmic arm movement to the lumbar spinal cord. This is detected by modulated reflex amplitudes in muscles remote from the rhythmic movement. Improved understanding of modulation in ankle flexor and extensor muscles due to rhythmic arm movement can be gained using modulation of spinal excitability as a probe. The modulatory effect of rhythmic arm movement on Ia reciprocal inhibition (RI) between functional antagonists at the ankle has not been studied. We investigated the influence of rhythmic arm cycling on short latency (∼55ms post-stimulus) RI between ankle flexor (tibialis anterior, TA) and extensor (soleus, SOL) muscles at varying (0.9, 1.0, 1.2, 1.5 and 2.0× motor threshold (MT)) stimulus intensities. We hypothesized that arm cycling would increase RI between antagonists, but that movement conditioning would vary depending on stimulus intensity used to evoke the RI response. Amplitude of RI deduced from suppression of ongoing EMG activity was compared in static and arm cycling conditions. Arm cycling significantly (p<0.05) increased RI in SOL at 1.0×MT, but had no effect in TA at any stimulus intensity (p>0.05). Descending signals arising from rhythmic arm movement significantly alter transmission in RI pathways between ankle flexor and extensor muscles differentially. This may be due to differences in descending supraspinal inputs to ankle flexors vs. extensors, and could be related to functional requirements during locomotion.

A pilot study to measure upper extremity H-reflexes following neuromuscular electrical stimulation therapy after stroke

Upper extremity (UE) hemiparesis persists after stroke, limiting hand function. Neuromuscular electrical stimulation (NMES) is an effective intervention to improve UE recovery, although the underlying mechanisms are not fully understood. Our objective was to establish a reliable protocol to measure UE agonist-antagonist forearm monosynaptic reflexes in a pilot study to determine if NMES improves wrist function after stroke. We established the between-day reliability of the H-reflex in the extensor carpi radialis longus (ECRL) and flexor carpi radialis (FCR) musculature for individuals with prior stroke (n=18). The same-day generation of ECRL/FCR H-reflex recruitment curves was well tolerated, regardless of age or UE spasticity. The between-day reliability of the ECRL H-reflex was enhanced above FCR, similar to healthy subjects [20], with the Hmax the most reliable parameter quantified in both muscles. H-reflex and functional measures following NMES show the potential for NMES-induced increases in ECRL Hmax, but confirmation requires a larger clinical study. Our initial results support the safe, easy, and efficacious use of in-home NMES, and establish a potential method to measure UE monosynaptic reflexes after stroke.

Soleus H-reflex operant conditioning changes the H-reflex recruitment curve

INTRODUCTION

Operant conditioning can gradually change the human soleus H-reflex. The protocol conditions the reflex near M-wave threshold. In this study we examine its impact on the reflexes at other stimulus strengths.

METHODS

H-reflex recruitment curves were obtained before and after a 24-session exposure to an up-conditioning (HRup) or a down-conditioning (HRdown) protocol and were compared.

RESULTS

In both HRup and HRdown subjects, conditioning affected the entire H-reflex recruitment curve. In 5 of 6 HRup and 3 of 6 HRdown subjects, conditioning elevated (HRup) or depressed (HRdown), respectively, the entire curve. In the other HRup subject or the other 3 HRdown subjects, the curve was shifted to the left or to the right, respectively.

CONCLUSIONS

H-reflex conditioning does not simply change the H-reflex to a stimulus of particular strength; it also changes the H-reflexes to stimuli of different strengths. Thus, it is likely to affect many actions in which this pathway participates.

High-intensity unilateral dorsiflexor resistance training results in bilateral neuromuscular plasticity after stroke

Hemiparesis after stroke decreases ability to dorsiflex the more-affected ankle during walking. Increased strength would be beneficial, but the more-affected limb is often too weak to be trained. In neurologically intact participants, training one limb induces strength gains in the contralateral, untrained limb. This approach remains unexplored post-stroke. The aim of this study was to test the hypothesis that unilateral dorsiflexor high-intensity resistance training on the less-affected side increases strength and motor output bilaterally following stroke. 19 participants (84.1 ± 77.6 months post-infarct) performed 6 weeks of maximal isometric dorsiflexion training using the less-affected leg. Voluntary isometric strength (dorsiflexion torque, muscle activation), reciprocal inhibition (RI), walking ability (gait speed, kinematics, EMG patterns), and clinical function were measured within 1 week before and 4 days following training. Post-intervention, dorsiflexion torque increased by ~31 % (p < 0.05) in the more-affected (untrained) and by ~34 % (p < 0.05) in the less-affected (trained) legs. Muscle activation significantly increased bilaterally, by ~59 and ~20 % in the trained and untrained legs, respectively. Notably, 4 participants who were unable to generate functional dorsiflexion on the more-affected side before training could do so post-intervention. Significant correlations between muscle activation and size of RI were noted across muscle groups before and after training, and the relation between size of RI and level of muscle activation in the more-affected tibialis anterior muscle was significantly altered by training. Thus, significant gains in voluntary strength and muscle activation on the untrained, more-affected side after stroke can be invoked through training the opposite limb. We demonstrate residual plasticity existing many years post-stroke and suggest clinical application of the cross-education effect where training the more-affected limb is not initially possible.

A novel modulation strategy to increase stimulation duration in neuromuscular electrical stimulation

INTRODUCTION

Neuromuscular electrical stimulation (NMES) has been shown to be an effective treatment for muscular dysfunction. Yet, a fundamental barrier to NMES treatments is the rapid onset of muscle fatigue. The purpose of this study is to examine the effect of feedback-based frequency modulation on the closed-loop performance of the quadriceps during repeated dynamic contractions.

METHODS

In the first experiment, subjects completed four different frequency modulation NMES protocols utilizing the same amplitude modulation control to compare the successful run times (SRTs). A second experiment was performed to determine the change in muscle response to high- and low-frequency stimulation.

RESULTS

Compared with constant-frequency stimulation, results indicate that using an error-driven strategy to vary the stimulation frequency during amplitude modulation increases the number of successful contractions during non-isometric conditions.

CONCLUSION

Simultaneous frequency and amplitude modulation increases the SRT during closed-loop NMES control.

Plantarflexor stretch training increases reciprocal inhibition measured during voluntary dorsiflexion

Agonist-mediated reciprocal inhibition (RI) in distal skeletal muscles is an important neurophysiological phenomenon leading to improved movement coordination and efficiency. It has been shown to be reduced in aged and clinical populations, so the development of interventions augmenting RI is an important research goal. We examined the efficacy of using chronic passive muscle stretching to augment RI. The influence of 3 wk of plantarflexor stretching (4 × 30 s, two times/day) on RI of soleus and gastrocnemius initiated by tonic, voluntary dorsiflexion contractions [20% of maximum voluntary contraction (MVC)] was examined in 11 healthy men who performed stretch training and in nine nontraining controls. Hoffmann's reflexes (H-reflexes) were elicited by tibial nerve stimulation during both weak isometric (2% MVC) plantarflexions and dorsiflexion contractions at 20% MVC. Changes were examined at three joint angles, normalized to each subject's range of motion (ROM; plantarflexed = 10 ± 0°, neutral = −3.3 ± 2.9°, dorsiflexed = −16.5 ± 5.6°). No changes were detected in controls. A 20% increase in ROM in the stretch subjects was associated with a significant decrease in maximum H-reflex (Hmax): maximum evoked potential (Mmax), measured during 2% plantarflexion at the plantarflexed and neutral angles in soleus and at the plantarflexed angle in gastrocnemius ( P < 0.05–0.01). By contrast, decreases in Hmax:Mmax during 20% dorsiflexion contract were also seen at each angle in soleus and at the dorsiflexed angle in gastrocnemius. However, a greater decrease in Hmax:Mmax measured during voluntary dorsiflexion rather than during plantarflexion, which indicates a specific change in RI, was detected only at the dorsiflexed angle (−30.7 ± 9.4% and −35.8 ± 6.8% for soleus and gastrocnemius, respectively). These results demonstrate the efficacy of soleus-gastrocnemius stretch training in increasing agonist-mediated RI from tibialis anterior onto soleus-gastrocnemius in young, healthy individuals at dorsiflexed, but not plantarflexed, joint angles.

Operant conditioning of rat soleus H-reflex oppositely affects another H-reflex and changes locomotor kinematics

H-reflex conditioning is a model for studying the plasticity associated with a new motor skill. We are exploring its effects on other reflexes and on locomotion. Rats were implanted with EMG electrodes in both solei (SOL(R) and SOL(L)) and right quadriceps (QD(R)), and stimulating cuffs on both posterior tibial (PT) nerves and right posterior femoral nerve. When SOL(R) EMG remained in a defined range, PT(R) stimulation just above M-response threshold elicited the SOL(R) H-reflex. Analogous procedures elicited the QD(R) and SOL(L) H-reflexes. After a control period, each rat was exposed for 50 d to a protocol that rewarded SOL(R) H-reflexes that were above (HRup rats) or below (HRdown rats) a criterion. HRup conditioning increased the SOL(R) H-reflex to 214 ± 37% (mean ± SEM) of control (p = 0.02) and decreased the QD(R) H-reflex to 71 ± 26% (p = 0.06). HRdown conditioning decreased the SOL(R) H-reflex to 69 ± 2% (p < 0.001) and increased the QD(R) H-reflex to 121 ± 7% (p = 0.02). These changes remained during locomotion. The SOL(L) H-reflex did not change. During the stance phase of locomotion, ankle plantarflexion increased in HRup rats and decreased in HRdown rats, hip extension did the opposite, and hip height did not change. The plasticity that changes the QD(R) H-reflex and locomotor kinematics may be inevitable (i.e., reactive) due to the ubiquity of activity-dependent CNS plasticity, and/or necessary (i.e., compensatory) to preserve other behaviors (e.g., locomotion) that would otherwise be disturbed by the change in the SOL(R) H-reflex pathway. The changes in joint angles, coupled with the preservation of hip height, suggest that compensatory plasticity did occur.

Repetitive common peroneal nerve stimulation increases ankle dorsiflexor motor evoked potentials in incomplete spinal cord lesions

Plasticity of corticospinal tract (CST) activity likely plays a key role in motor function recovery after central nervous system (CNS) lesions. In non-injured adults, 30 min of repetitive common peroneal nerve stimulation (rCPnS) increases CST excitability by 40-50% and the effect persists for at least 30 min. The present study evaluated with transcranial magnetic stimulation (TMS) the changes in CST excitability after 30 min of rCPnS in people with foot drop due to incomplete SCI. Suprathreshold rCPnS (25 Hz, alternating 1 s on 1 s off stimulation cycle) was given for two 15-min periods, while the subject sat at rest with ankle and knee joints fixed. Before, between, and after the periods of stimulation, the tibialis anterior (TA) motor evoked potentials (MEPs) to TMS were measured at a TMS intensity that originally produced a half-maximum MEP (typically 10-20% above threshold) while the sitting subject provided 25-30% maximum voluntary TA contraction. In 10 subjects with SCI, the peak-to-peak TA MEP increased by 14 ± 3% after rCPnS and the peak increase (+21 ± 7%) occurred 15 min after the cessation of rCPnS. The TA H-reflex, measured in separate experiments in 7 subjects, did not increase after rCPnS. The results indicate that rCPnS can increase CST excitability for the TA in people with incomplete SCI, although its effects appear smaller and shorter lasting than those found in non-injured control subjects. Such short-term plasticity in the CST excitability induced by rCPnS may contribute to long-term therapeutic effects of functional electrical stimulation previously reported in people with CNS lesions.

Phase-dependent modulation of percutaneously elicited multisegmental muscle responses after spinal cord injury

Phase-dependent modulation of monosynaptic reflexes has been reported for several muscles of the lower limb of uninjured rats and humans. To assess whether this step-phase-dependent modulation can be mediated at the level of the human spinal cord, we compared the modulation of responses evoked simultaneously in multiple motor pools in clinically complete spinal cord injury (SCI) compared with noninjured (NI) individuals. We induced multisegmental responses of the soleus, medial gastrocnemius, tibialis anterior, medial hamstring, and vastus lateralis muscles in response to percutaneous spinal cord stimulation over the Th11-Th12 vertebrae during standing and stepping on a treadmill. Individuals with SCI stepped on a treadmill with partial body-weight support and manual assistance of leg movements. The NI group demonstrated phase-dependent modulation of evoked potentials in all recorded muscles with the modulation of the response amplitude corresponding with changes in EMG amplitude in the same muscle. The SCI group demonstrated more variation in the pattern of modulation across the step cycle and same individuals in the SCI group could display responses with a magnitude as great as that of modulation observed in the NI group. The relationship between modulation and EMG activity during the step cycle varied from noncorrelated to highly correlated patterns. These findings demonstrate that the human lumbosacral spinal cord can phase-dependently modulate motor neuron excitability in the absence of functional supraspinal influence, although with much less consistency than that in NI individuals.

Does Functional Electrical Stimulation for Foot Drop Strengthen Corticospinal Connections?

Background. Long-term use of a foot-drop stimulator applying functional electrical stimulation (FES) to the common peroneal nerve improves walking performance even when the stimulator is off. This “therapeutic” effect might result from neuroplastic changes. Objective. To determine the effect of long-term use of a foot-drop stimulator on residual corticospinal connections in people with central nervous system disorders. Methods. Ten people with nonprogressive disorders (eg, stroke) and 26 with progressive disorders (eg, multiple sclerosis) used a foot-drop stimulator for 3 to 12 months while walking in the community. Walking performance and electrophysiological variables were measured before and after FES use. From the surface electromyogram of the tibialis anterior muscle, we measured the following: (1) motor-evoked potential (MEP) from transcranial magnetic stimulation over the motor cortex, (2) maximum voluntary contraction (MVC), and (3) maximum motor wave (Mmax) from stimulating the common peroneal nerve. Results. After using FES, MEP and MVC increased significantly by comparable amounts, 50% and 48%, respectively, in the nonprogressive group and 27% and 17% in the progressive group; the changes were positively correlated ( R2 = .35; P < .001). Walking speed increased with the stimulator off (therapeutic effect) by 24% ( P = .008) and 7% ( P = .014) in the nonprogressive and progressive groups, respectively. The changes in Mmax were small and not correlated with changes in MEP. Conclusions. The large increases in MVC and MEP suggest that regular use of a foot-drop stimulator strengthens activation of motor cortical areas and their residual descending connections, which may explain the therapeutic effect on walking speed.

Long-term therapeutic and orthotic effects of a foot drop stimulator on walking performance in progressive and nonprogressive neurological disorders

BACKGROUND

Stimulators applying functional electrical stimulation (FES) to the common peroneal nerve improve walking with a foot drop, which occurs in several disorders.

OBJECTIVE

To compare the orthotic and therapeutic effects of a foot drop stimulator on walking performance of subjects with chronic nonprogressive (eg, stroke) and progressive (eg, multiple sclerosis) disorders.

METHODS

Subjects with nonprogressive (41) and progressive (32) conditions used a foot drop stimulator for 3 to 12 months while walking in the community. Walking speed was measured with a 10-m test and a 4-minute figure-8 test; physiological cost index (PCI) and device usage were also measured. The subjects were tested with FES on and off (orthotic effect) before and after (therapeutic effect) stimulator use.

RESULTS

After 3 months of FES use, the nonprogressive and progressive groups had a similar, significant orthotic effect (5.0% and 5.7%, respectively, P < .003; percentage change in mean values) and therapeutic effect with FES off (17.8% and 9.1%, respectively, P < .005) on figure-8 walking speed. Overall, PCI showed a decreasing trend (P = .031). The therapeutic effect on figure-8 speed diverged later between both groups to 28.0% (P < .001) and 7.9% at 11 months. The combined therapeutic plus orthotic effect on figure-8 speed at 11 months was, respectively, 37.8% (P < .001) and 13.1% (P = .012); PCI decreased 18.2% (P = .038) and 6.5%, respectively.

CONCLUSIONS

Subjects with progressive and nonprogressive disorders had an orthotic benefit from FES up to 11 months. The therapeutic effect increased for 11 months in nonprogressive disorders but only for 3 months in progressive disorders. The combined effect remained significant and clinically relevant.

Origins of abnormal excitability in biceps brachii motoneurons of spastic-paretic stroke survivors

Stroke survivors often exhibit abnormal motoneuron excitability, manifested clinically as spasticity with exaggerated stretch reflexes in resting muscles. We examined whether this abnormal excitability is a result of increased activation of intrinsic voltage-dependent persistent inward currents (PICs) or whether it is a result of enhanced synaptic inputs to the motoneuron. This distinction was made by recording firing rate profiles of pairs of motor units during isometric contractions of elbow flexor muscles. To estimate PIC amplitude, the discharge of the lower-threshold (reporter) motor unit of the pair was used to estimate the synaptic input to the higher-threshold (test) motor unit. The estimated synaptic input required to recruit the test unit was compared with the synaptic input when the test unit was derecruited (DeltaF) and this served as an estimate of the intrinsic (PIC) contribution to motoneuron firing. We found that PIC estimates were not larger in spastic-paretic motoneurons (DeltaF = 4.0 +/- 1.6 pps) compared with contralateral (4.6 +/- 1.4 pps) and age-matched healthy control motoneurons (3.8 +/- 1.7, all P > 0.1). Instead, following the voluntary contractions, the majority of lower-threshold motor units in spastic-paretic muscles (83%) exhibited spontaneous discharge, compared with 14% of contralateral and 0% of control motor units. Furthermore, there was strong co-modulation of simultaneously active units in spastic muscle. The presence of ongoing, correlated unit activity at "rest," coupled with firing behavior at recruitment unique to lower-threshold motor units in spastic muscles, suggested that firing changes are likely a result of a low-level depolarizing synaptic drive to the resting motoneuron pool.