Training-induced adaptive plasticity in human somatosensory reflex pathways

M-wave and H-reflex recruitment curves in boys and men

The aim of the present study was to check whether the M-wave and H-reflex recruitment curves differ between prepubertal boys and men. Eleven boys (9-11 yr) and eleven men (18-35 yr) were magnetically stimulated at the tibial nerve in a prone position. M-wave and H-reflex maximal amplitudes (H_max; M_max ; H_max /M_max ), thresholds, regression slopes (H_slp ; M_slp ; H_slp /M_slp ) were extracted from M-wave and H-reflex recruitment curves and compared between the two age groups. Overall, no significant difference in M-wave and H-reflex recruitment curve parameters was found between the two populations. Nevertheless, the size of the M-wave associated with maximal H-reflex amplitude was lower in boys as compared to men when expressed relative to maximal M-wave amplitude (M_Hmax /M_max : 0.18 ± 0.06 vs. 0.31 ± 0.13; p < .05). This result suggests that the development of peripheral nerve was completed in 9 to 11-year-old boys and did not affect the M-wave and H-reflex recruitment curves parameters. In neuromuscular function studies, it implies that H_max /M_max and H_slp /M_slp could be used indifferently to compare spinal motoneuron excitability between 9-11-year-old boys and men. Conversely, evoking H-reflexes at a given percentage of M_max may bias the comparison between boys and men.

Repeated and patterned stimulation of cutaneous reflex pathways amplifies spinal cord excitability

Priming with patterned stimulation of antagonist muscle afferents induces modulation of spinal cord excitability as evidenced by changes in group Ia reciprocal inhibition. When assessed transiently with a condition-test pulse paradigm, stimulating cutaneous afferents innervating the foot reduces Ia presynaptic inhibition and facilitates soleus Hoffmann (H)-reflex amplitudes. Modulatory effects (i.e., priming) of longer lasting sensory stimulation of cutaneous afferents innervating the foot have yet to be examined. As a first step, we examined how priming with 20 min of patterned and alternating stimulation between the left and right foot affects spinal cord excitability. During priming, stimulus trains (550 ms; consisting of twenty-eight 1-ms pulses at 51 Hz, 1.2 times the radiating threshold) were applied simultaneously to the sural and plantar nerves of the ankle. Stimulation to the left and right ankle was out of phase by 500 ms. We evoked soleus H-reflexes and muscle compound action potentials (M waves) before and following priming stimulation to provide a proxy measure of spinal cord excitability. H-reflex and M-wave recruitment curves were recorded at rest, during brief (<2 min) arm cycling, and with sural conditioning [train of five 1-ms pulses at 2 times the radiating threshold (RT) with a condition-test interval (C-T) = 80 ms]. Data indicate an increase in H-reflex excitability following priming via patterned sensory stimulation. Transient sural conditioning was less effective following priming, indicating that the increased excitability of the H-reflex is partially attributable to reductions in group Ia presynaptic inhibition. Sensory stimulation to cutaneous afferents, which enhances spinal cord excitability, may prove useful in both rehabilitation and performance settings.NEW & NOTEWORTHY Priming via patterned stimulation of the nervous system induces neuroplasticity. Yet, accessing previously known cutaneous reflex pathways to alter muscle reflex excitability has not yet been examined. Here, we show that sensory stimulation of the cutaneous afferents that innervate the foot sole can amplify spinal cord excitability, which, in this case, is attributed to reductions in presynaptic inhibition.

Plantarflexion force is amplified with sensory stimulation during ramping submaximal isometric contractions

Stimulating cutaneous nerves, causing tactile sensations, reduces the perceived heaviness of an object, suggesting that either descending commands are facilitated or the perception of effort is reduced when tactile sensation is enhanced. Sensory stimulation can also mitigate decrements in motor output and spinal cord excitability that occur with fatigue. The effects of sensory stimulation applied with coincident timing of voluntary force output, however, are yet to be examined. Therefore, the purpose of this study was to examine effects of sensory enhancement to nerves innervating opposed skin areas of the foot (top or bottom) on force production during voluntary plantarflexion or dorsiflexion contractions. Stimulation trains were applied for 2 s at either a uniform 150 Hz or a modulated frequency that increased linearly from 50 to 150 Hz and were delivered at the initiation of the contraction. Participants were instructed to perform a ramp contraction [~10% maximal voluntary contraction (MVC)/s] to ~20% MVC and then to hold ~20% MVC for 2 s while receiving real-time visual feedback. Cutaneous reflexes were evoked 75 ms after initiating the hold (75 ms after sensory enhancement ended). Force output was greater for all sensory-enhanced conditions compared with control during plantarflexion; however, force output was not amplified during dorsiflexion. Cutaneous reflexes evoked after sensory enhancement were unaltered. These results indicate that sensory enhancement can amplify plantarflexion but not dorsiflexion, likely as a result of differences in neuroanatomical projections to the flexor and extensor motor pools. Further work is required to elucidate the mechanisms of enhanced force during cutaneous stimulation.NEW & NOTEWORTHY The efficacy of behaviorally timed sensory stimulation to enhance sensations and amplify force output has not been examined. Here we show cutaneous nerve sensory stimulation can amplify plantarflexion force output. This amplification in force occurs irrespective of whether the cutaneous field that is stimulated resides on the surface that is producing the force or the opposing surface. This information may provide insights for the development of technologies to improve performance and/or rehabilitation training.

We Are Upright-Walking Cats: Human Limbs as Sensory Antennae During Locomotion

Humans and cats share many characteristics pertaining to the neural control of locomotion, which has enabled the comprehensive study of cutaneous feedback during locomotion. Feedback from discrete skin regions on both surfaces of the human foot has revealed that neuromechanical responses are highly topographically organized and contribute to "sensory guidance" of our limbs during locomotion.

Effects of enhanced cutaneous sensory input on interlimb strength transfer of the wrist extensors

The relative contribution of cutaneous sensory feedback to interlimb strength transfer remains unexplored. Therefore, this study aimed to determine the relative contribution of cutaneous afferent pathways as a substrate for cross-education by directly assessing how "enhanced" cutaneous stimulation alters ipsilateral and contralateral strength gains in the forearm. Twenty-seven right-handed participants were randomly assigned to 1-of-3 training groups and completed 6 sets of 8 repetitions 3x/week for 5 weeks. Voluntary training (TRAIN) included unilateral maximal voluntary contractions (MVCs) of the wrist extensors. Cutaneous stimulation (STIM), a sham training condition, included cutaneous stimulation (2x radiating threshold; 3sec; 50Hz) of the superficial radial (SR) nerve at the wrist. TRAIN + STIM training included MVCs of the wrist extensors with simultaneous SR stimulation. Two pre- and one posttraining session assessed the relative increase in force output during MVCs of isometric wrist extension, wrist flexion, and handgrip. Maximal voluntary muscle activation was simultaneously recorded from the flexor and extensor carpi radialis. Cutaneous reflex pathways were evaluated through stimulation of the SR nerve during graded ipsilateral contractions. Results indicate TRAIN increased force output compared with STIM in both trained (85.0 ± 6.2 Nm vs. 59.8 ± 6.1 Nm) and untrained wrist extensors (73.9 ± 3.5 Nm vs. 58.8 Nm). Providing 'enhanced' sensory input during training (TRAIN + STIM) also led to increases in strength in the trained limb compared with STIM (79.3 ± 6.3 Nm vs. 59.8 ± 6.1 Nm). However, in the untrained limb no difference occurred between TRAIN + STIM and STIM (63.0 ± 3.7 Nm vs. 58.8 Nm). This suggests when 'enhanced' input was provided independent of timing with active muscle contraction, interlimb strength transfer to the untrained wrist extensors was blocked. This indicates that the sensory volley may have interfered with the integration of appropriate sensorimotor cues required to facilitate an interlimb transfer, highlighting the importance of appropriately timed cutaneous feedback.

TRPM8-mediated cutaneous stimulation modulates motor neuron activity during treadmill stepping in mice

Motor units are generally recruited from the smallest to the largest following the size principle, while cutaneous stimulation has the potential to affect spinal motor control. We aimed to examine the effects of stimulating transient receptor potential channel sub-family M8 (TRPM8) combined with exercise on the modulation of spinal motor neuron (MN) excitability. Mice were topically administrated 1.5% icilin on the hindlimbs, followed by treadmill stepping. Spinal cord sections were immunostained with antibodies against c-fos and choline acetyltransferase. Icilin stimulation did not change the number of c-fos+ MNs, but increased the average soma size of the c-fos+ MNs during low-speed treadmill stepping. Furthermore, icilin stimulation combined with stepping increased c-fos+ cholinergic interneurons near the central canal, which are thought to modulate MN excitability. These findings suggest that TRPM8-mediated cutaneous stimulation with low-load exercise promotes preferential recruitment of large MNs and is potentially useful as a new training method for rehabilitation.

Sensory enhancement amplifies interlimb cutaneous reflexes in wrist extensor muscles

Interlimb neural connections support motor tasks such as locomotion and cross-education strength training. Somatosensory pathways that can be assessed with cutaneous reflex paradigms assist in subserving these connections. Many studies show that stimulation of cutaneous nerves elicits reflexes in muscles widespread across the body and induces neural plasticity after training. Sensory enhancement, such as long-duration trains of transcutaneous stimulation, facilitates performance during rehabilitation training or fatiguing motor tasks. Performance improvements due to sensory stimulation may be caused by altered spinal and corticospinal excitability. However, how enhanced sensory input regulates the excitability of interlimb cutaneous reflex pathways has not been studied. Our purpose was to investigate the effects of sensory enhancement on interlimb cutaneous reflexes in wrist extensor muscles. Stimulation to provide sensory enhancement (2-s trains at 150 Hz to median or superficial radial nerves) or evoke cutaneous reflexes (15-ms trains at 300 Hz to superficial radial nerve) was applied in different arms while participants (n = 13) performed graded isometric wrist extension. Wrist extensor electromyography and cutaneous reflexes were measured bilaterally. We found amplified inhibitory reflexes in the arm receiving superficial radial and median nerve sensory enhancement with net reflex amplitudes decreased by 709.5% and 695.3% repetitively. This suggests sensory input alters neuronal excitabilities in the interlimb cutaneous pathways. These findings have potential application in facilitating motor function recovery through alterations in spinal cord excitability enhancing sensory input during targeted rehabilitation and sports training.NEW & NOTEWORTHY We show that sensory enhancement increases excitability in interlimb cutaneous pathways and that these effects are not influenced by descending motor drive on the contralateral side. These findings confirm the role of sensory input and cutaneous pathways in regulating interlimb movements. In targeted motor function training or rehabilitation, sensory enhancement may be applied to facilitate outcomes.

Exploiting cervicolumbar connections enhances short-term spinal cord plasticity induced by rhythmic movement

Arm cycling causes suppression of soleus (SOL) Hoffmann (H-) reflex that outlasts the activity period. Arm cycling presumably activates propriospinal networks that modulate Ia presynaptic inhibition. Interlimb pathways are thought to relate to the control of quadrupedal locomotion, allowing for smooth, coordinated movement of the arms and legs. We examined whether the number of active limb pairs affects the amount and duration of activity-dependent plasticity of the SOL H-reflex. On separate days, 14 participants completed 4 randomly ordered 30 min experimental sessions: (1) quiet sitting (CTRL); (2) arm cycling (ARM); (3) leg cycling (LEG); and (4) arm and leg cycling (A&L) on an ergometer. SOL H-reflex and M-wave were evoked via electrical stimulation of the tibial nerve. M-wave and H-reflex recruitment curves were recorded, while the participants sat quietly prior to, 10 and 20 min into, immediately after, and at 2.5, 5, 7.5, 10, 15, 20, 25, and 30 min after each experimental session. Normalized maximal H-reflexes were unchanged in CTRL, but were suppressed by > 30% during the ARM, LEG, and A&L. H-reflex suppression outlasted activity duration for ARM (≤ 2.5 mins), LEG (≤ 5 mins), and A&L (≤ 30 mins). The duration of reflex suppression after A&L was greater than the algebraic summation of ARM and LEG. This non-linear summation suggests that using the arms and legs simultaneously-as in typical locomotor synergies-amplifies networks responsible for the short-term plasticity of lumbar spinal cord excitability. Enhanced activity of spinal networks may have important implications for the implementation of locomotor training for targeted rehabilitation.

Effects of a compression garment on sensory feedback transmission in the human upper limb

Compression apparel is popular in both medical and sport performance settings. Perceived benefits are suggested to include changes in sensory feedback transmission caused by activation of mechanoreceptors. However, little is known about effects of compression apparel on sensorimotor control. Our purpose was to mechanistically examine whether compression apparel modulates sensory feedback transmission and reaching accuracy in the upper limb. Two experiments were completed under CONTROL and COMPRESSION (sleeve applied across the elbow joint) conditions. M-waves and H-reflexes were elicited by stimulating the median nerve and were recorded via surface electromyography (EMG). In experiment 1, H-reflexes and M-H recruitment curves were assessed at REST, during wrist flexion (10% EMG_max), and during a cutaneous conditioning of the superficial radial (SR) or distal median (MED) nerve. Cutaneous reflexes were elicited during 10% wrist flexion via stimulation of SR or MED. In experiment 2, unconditioned H-reflex measures were assessed at rest, during arm cycling, and during a discrete reaching task. Results indicate that compression apparel modulates spinal cord excitability across multiple sensory pathways and movement tasks. Interestingly, there was a significant improvement in reaching accuracy while wearing the compression sleeve. Taken together, the compression sleeve appears to increase precision and sensitivity around the joint where the sleeve is applied. Compression apparel may function as a "filter" of irrelevant mechanoreceptor information allowing for optimal task-related sensory information to enhance proprioception. NEW & NOTEWORTHY Wearing a customized compression sleeve was shown to alter the excitability of multiple pathways within the central nervous system regardless of conditioning input or movement task and was accompanied by improved accuracy of reaching movements and determination of movement end point. Compression apparel may assist as a type of "filter function" of tonic and nonspecific mechanoreceptor information leading to increased precision and movement sensitivity around the joint where compression is applied.

Sherlock Holmes and the curious case of the human locomotor central pattern generator

Evidence first described in reduced animal models over 100 years ago led to deductions about the control of locomotion through spinal locomotor central pattern-generating (CPG) networks. These discoveries in nature were contemporaneous with another form of deductive reasoning found in popular culture, that of Arthur Conan Doyle's detective, Sherlock Holmes. Because the invasive methods used in reduced nonhuman animal preparations are not amenable to study in humans, we are left instead with deducing from other measures and observations. Using the deductive reasoning approach of Sherlock Holmes as a metaphor for framing research into human CPGs, we speculate and weigh the evidence that should be observable in humans based on knowledge from other species. This review summarizes indirect inference to assess "observable evidence" of pattern-generating activity that leads to the logical deduction of CPG contributions to arm and leg activity during locomotion in humans. The question of where a CPG may be housed in the human nervous system remains incompletely resolved at this time. Ongoing understanding, elaboration, and application of functioning locomotor CPGs in humans is important for gait rehabilitation strategies in those with neurological injuries.

Long-Term Plasticity in Reflex Excitability Induced by Five Weeks of Arm and Leg Cycling Training after Stroke

Neural connections remain partially viable after stroke, and access to these residual connections provides a substrate for training-induced plasticity. The objective of this project was to test if reflex excitability could be modified with arm and leg (A &amp; L) cycling training. Nineteen individuals with chronic stroke (more than six months postlesion) performed 30 min of A &amp; L cycling training three times a week for five weeks. Changes in reflex excitability were inferred from modulation of cutaneous and stretch reflexes. A multiple baseline (three pretests) within-subject control design was used. Plasticity in reflex excitability was determined as an increase in the conditioning effect of arm cycling on soleus stretch reflex amplitude on the more affected side, by the index of modulation, and by the modulation ratio between sides for cutaneous reflexes. In general, A &amp; L cycling training induces plasticity and modifies reflex excitability after stroke.

Ejaculatory training lengthens the ejaculation latency and facilitates the functioning of the spinal generator for ejaculation of rats with rapid ejaculation

A spinal pattern generator controls the ejaculatory response. Central pattern generators (CPGs) may be entrained to improve the motor patterns under their control. In the present study we tested the hypothesis that training of the spinal generator for ejaculation (SGE) by daily copulation until ejaculation, could promote substantive changes in its functioning permitting a better SGE control of the genital motor pattern of ejaculation (GMPE) and, as a consequence, a normalization of the ejaculation latency of rats with rapid ejaculation. To that aim, we evaluated in sexually experienced male rats with rapid ejaculation (1) the effects of daily copulation to ejaculation, following different entrainment schedules, on their ejaculation latencies, (2) the impact of these different ejaculatory entrainment schedules upon the parameters of the GMPE and (3) the possible emergence of persistent changes in the functioning of the SGE associated to the daily ejaculation entrainment schedules. The data obtained show that intense ejaculatory training of rats with rapid ejaculation lengthens the ejaculation latency during copulation and augments the ejaculatory capacity of the SGE in this population when spinalized. Thus, present data reveal that like other CPGs, the SGE can be trained and put forward that training of the SGE by daily copulation to ejaculation might be a promising alternative that should be taken into consideration for the treatment of premature ejaculation.

Training-Specific Neural Plasticity in Spinal Reflexes after Incomplete Spinal Cord Injury

The neural plasticity of spinal reflexes after two contrasting forms of walking training was determined in individuals with chronic, motor-incomplete spinal cord injury (SCI). Endurance Training involved treadmill walking for as long as possible, and Precision Training involved walking precisely over obstacles and onto targets overground. Twenty participants started either Endurance or Precision Training for 2 months and then crossed over after a 2-month rest period to the other form of training for 2 months. Measures were taken before and after each phase of training and rest. The cutaneomuscular reflex (CMR) during walking was evoked in the soleus (SOL) and tibialis anterior muscles by stimulating the posterior tibial nerve at the ankle. Clonus was estimated from the EMG power in the SOL during unperturbed walking. The inhibitory component of the SOL CMR was enhanced after Endurance but not Precision Training. Clonus did not change after either form of training. Participants with lower reflex excitability tended to be better walkers (i.e., faster walking speeds) prior to training, and the reduction in clonus was significantly correlated with the improvement in walking speed and distance. Thus, reflex excitability responded in a training-specific way, with the reduction in reflex excitability related to improvements in walking function. Trial registration number is NCT01765153.

The effect of a combined strength and proprioceptive training on muscle strength and postural balance in boys with intellectual disability: An exploratory study

The aim of our study was to investigate the effect of a combined strength and proprioception training (CSPT) program on muscle strength and postural balance in children with intellectual disability (ID). The maximum voluntary contraction (MVC) and postural parameters (CoPVm, CoPLX, CoPLY) of 20 children with ID were recorded before and after 8 weeks of a CSPT program. The participants were divided into two groups: an experimental group who attended a CSPT program and a control group who continued with daily activities. In the trained group, the MVC increased significantly (p<0.001) after the training period and the postural parameters decreased significantly in Double-Leg Stance (DLS) and One-Leg Stance (OLS) during the firm surface condition as well as in the DLS during the foam surface condition; in both eyes open (EO) and eyes closed (EC) conditions. A CSPT program improves postural balance in children with ID could be due to the enhancement in muscle strength and proprioceptive input integration.

Strength training-induced responses in older adults: attenuation of descending neural drive with age

Although reductions in resting H-reflex responses and maximal firing frequency suggest that reduced efferent drive may limit muscle strength in elderly, there are currently no reports of V-wave measurements in elderly, reflecting the magnitude of efferent output to the muscle during maximal contraction. Furthermore, it is uncertain whether potential age-related neural deficiencies can be restored by resistance training. We assessed evoked reflex recordings in the triceps surae muscles during rest and maximal voluntary contraction (MVC), rate of force development (RFD), and muscle mass in seven elderly (74 ± 6 years) males before and after 8 weeks of heavy resistance training, contrasted by seven young (24 ± 4 years) male controls. At baseline, m. soleus (SOL) V/M ratio (0.124 ± 0.082 vs. 0.465 ± 0.197, p < 0.05) and H/M ratio (0.379 ± 0.044 vs. 0.486 ± 0.101 p = 0.07) were attenuated in elderly compared to young. Also, SOL H-reflex latency (33.29 ± 2.41 vs. 30.29 ± 0.67 ms, p < 0.05) was longer in elderly. The reduced neural drive was, despite similar leg muscle mass (10.7 ± 1.2 vs. 11.5 ± 1.4 kg), mirrored by lower MVC (158 ± 48 vs. 240 ± 54 Nm, p < 0.05) and RFD (294 ± 126 vs. 533 ± 123 Nm s(-1), p < 0.05) in elderly. In response to training SOL V/M ratio (0.184 ± 0.092, p < 0.05) increased in the elderly, yet only to a level ~40 % of the young. This was accompanied by increased MVC (190 ± 70 Nm, p < 0.05) and RFD (401 ± 147 Nm[Symbol: see text]s(-1), p < 0.05) to levels of ~80 % and ~75 % of the young. H/M ratio remained unchanged. These findings suggest that changes in supraspinal activation play a significant role in the age-related changes in muscle strength. Furthermore, this motor system impairment can to some extent be improved by heavy resistance training.

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.

Changes in H reflex and neuromechanical properties of the trapezius muscle after 5 weeks of eccentric training: a randomized controlled trial

Trapezius muscle Hoffman (H) reflexes were obtained to investigate the neural adaptations induced by a 5-wk strength training regimen, based solely on eccentric contractions of the shoulder muscles. Twenty-nine healthy subjects were randomized into an eccentric training group ( n = 15) and a reference group ( n = 14). The eccentric training program consisted of nine training sessions of eccentric exercise performed over a 5-wk period. H-reflex recruitment curves, the maximal M wave (Mmax), maximal voluntary contraction (MVC) force, rate of force development (RFD), and electromyographic (EMG) voluntary activity were recorded before and after training. H reflexes were recorded from the middle part of the trapezius muscle by electrical stimulation of the C3/4 cervical nerves; Mmax was measured by electrical stimulation of the accessory nerve. Eccentric strength training resulted in significant increases in the maximal trapezius muscle H reflex (Hmax) (21.4% [5.5–37.3]; P = 0.01), MVC force (26.4% [15.0–37.7]; P < 0.01), and RFD (24.6% [3.2–46.0]; P = 0.025), while no significant changes were observed in the reference group. Mmax remained unchanged in both groups. A significant positive correlation was found between the change in MVC force and the change in EMG voluntary activity in the training group ( r = 0.57; P = 0.03). These results indicate that the net excitability of the trapezius muscle H-reflex pathway increased after 5 wk of eccentric training. This is the first study to investigate and document changes in the trapezius muscle H reflex following eccentric strength training.

Operant conditioning of spinal reflexes: from basic science to clinical therapy

New appreciation of the adaptive capabilities of the nervous system, recent recognition that most spinal cord injuries are incomplete, and progress in enabling regeneration are generating growing interest in novel rehabilitation therapies. Here we review the 35-year evolution of one promising new approach, operant conditioning of spinal reflexes. This work began in the late 1970’s as basic science; its purpose was to develop and exploit a uniquely accessible model for studying the acquisition and maintenance of a simple behavior in the mammalian central nervous system (CNS). The model was developed first in monkeys and then in rats, mice, and humans. Studies with it showed that the ostensibly simple behavior (i.e., a larger or smaller reflex) rests on a complex hierarchy of brain and spinal cord plasticity; and current investigations are delineating this plasticity and its interactions with the plasticity that supports other behaviors. In the last decade, the possible therapeutic uses of reflex conditioning have come under study, first in rats and then in humans. The initial results are very exciting, and they are spurring further studies. At the same time, the original basic science purpose and the new clinical purpose are enabling and illuminating each other in unexpected ways. The long course and current state of this work illustrate the practical importance of basic research and the valuable synergy that can develop between basic science questions and clinical needs.

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.

Acute neuromuscular adaptation at the spinal level following middle cerebral artery occlusion-reperfusion in the rat

The purpose of the study was to highlight the acute motor reflex adaptation and to deepen functional deficits following a middle cerebral artery occlusion-reperfusion (MCAO-r). Thirty-six Sprague-Dawley rats were included in this study. The middle cerebral artery occlusion (MCAO; 120 min) was performed on 16 rats studied at 1 and 7 days, respectively (MCAO-D1 and MCAO-D7, n = 8 for each group). The other animals were divided into 3 groups: SHAM-D1 (n = 6), SHAM-D7 (n = 6) and Control (n = 8). Rats performed 4 behavioral tests (the elevated body swing test, the beam balance test, the ladder-climbing test and the forelimb grip force) before the surgery and daily after MCAO-r. H-reflex on triceps brachii was measured before and after isometric exercise. Infarction size and cerebral edema were respectively assessed by histological (Cresyl violet) and MRI measurements at the same time points than H-reflex recordings. Animals with cerebral ischemia showed persistent functional deficits during the first week post-MCAO-r. H-reflex was not decreased in response to isometric exercise one day after the cerebral ischemia contrary to the other groups. The motor reflex regulation was recovered 7 days post-MCAO-r. This result reflects an acute sensorimotor adaptation at the spinal level after MCAO-r.

Operant conditioning of a spinal reflex can improve locomotion after spinal cord injury in humans

Operant conditioning protocols can modify the activity of specific spinal cord pathways and can thereby affect behaviors that use these pathways. To explore the therapeutic application of these protocols, we studied the impact of down-conditioning the soleus H-reflex in people with impaired locomotion caused by chronic incomplete spinal cord injury. After a baseline period in which soleus H-reflex size was measured and locomotion was assessed, subjects completed either 30 H-reflex down-conditioning sessions (DC subjects) or 30 sessions in which the H-reflex was simply measured [unconditioned (UC) subjects], and locomotion was reassessed. Over the 30 sessions, the soleus H-reflex decreased in two-thirds of the DC subjects (a success rate similar to that in normal subjects) and remained smaller several months later. In these subjects, locomotion became faster and more symmetrical, and the modulation of EMG activity across the step cycle increased bilaterally. Furthermore, beginning about halfway through the conditioning sessions, all of these subjects commented spontaneously that they were walking faster and farther in their daily lives, and several noted less clonus, easier stepping, and/or other improvements. The H-reflex did not decrease in the other DC subjects or in any of the UC subjects; and their locomotion did not improve. These results suggest that reflex-conditioning protocols can enhance recovery of function after incomplete spinal cord injuries and possibly in other disorders as well. Because they are able to target specific spinal pathways, these protocols could be designed to address each individual's particular deficits, and might thereby complement other rehabilitation methods.

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.

Wearing of complete dentures reduces slow fibre and enhances hybrid fibre fraction in masseter muscle

Edentulous conditions and use of complete dentures alter the function of jaw muscles, which is presumably reflected in the myosin heavy chain (MyHC) isoform composition. This study is the first dealing with MyHC isoforms expression in edentulous persons with the aim to clarify to which extent the decreased functional load following teeth loss contributes to the changed muscle phenotype during ageing. We analysed MyHC expression in old masseter muscle at decreased and full functional load by comparing age-matched edentulous and dentate subjects. Edentulous subjects had upper and lower complete dentures. Dentate subjects had at least 24 natural teeth in continuous dental arches with two molars present in each quadrant and normal intermaxillary relationship. The adaptive response to the reduced masticatory load was lower numerical and area proportion of MyHC-1 expressing fibres and higher numerical proportion of hybrid fibres in edentulous compared with dentate subjects with no significant difference in the proportion of MyHC-neo-expressing fibres between both groups. We conclude that the observed differences in the proportion of fibre types between denture wearers and dentate subjects cannot be ascribed to degenerative changes intrinsic to the ageing muscle, but to functional differences in muscle activity and to morphological alterations of stomatognathic system accompanying the complete teeth loss.

Effects of fast functional exercise on muscle activity after stroke

BACKGROUND

In stroke rehabilitation, considerable emphasis is placed on improving muscle strength with less focus on the speed of movement. Muscle power (product of force and velocity) is essential for balance and mobility but velocity of movement is impaired after stroke.

OBJECTIVE

The purpose of this efficacy study is to determine if a single session of fast functional movements can increase muscle activation and the speed of movement in participants with a subacute stroke.

METHODS

In total, 32 individuals poststroke and 32 age- and sex-matched controls performed a single session of 50 fast squats and steps. Electromyographic (EMG) activity was measured bilaterally in the rectus femoris (RF), biceps femoris (BF), tibialis anterior (TA), and soleus muscles. The average EMG area and the movement speed were calculated over 10 trials. The effect of exercise was determined as the change from the second set (Start) to the last set (End) of 10 trials.

RESULTS

The stroke group had significant increases in EMG area of the TA, BF, and RF during the squatting exercise. There was an increase in EMG area of the RF and BF when the paretic leg was stepping. Improvements in EMG area of the soleus and RF when the paretic leg was in stance accompanied increases in EMG area when the nonparetic leg was stepping. There was a trend for improved movement speed for both exercises.

CONCLUSION

A single session of exercises emphasizing speed of movement can be used to improve muscle activation in persons with mild to moderately severe strokes.

Asymmetrical neural adaptation in lower leg muscles as a consequence of stereotypical motor training

Despite well-authorized facts regarding asymmetrical architectural changes between different limbs after persistent participation in particular motor training, no studies have addressed the neural aspects to the present. The authors undertook the study to elucidate the possibility of neural adaptation on a limb-by-limb basis after repetitive engagement in a particular motor training routine. We investigated lower leg muscles in endurance-trained track runners who have been trained by routinely running on a track in counterclockwise direction on curved paths. Stretch reflex responses in the plantarflexor muscles (soleus [SOL], medial [MG], and lateral head of the gastrocnemius muscle) were evaluated bilaterally with participants sitting at rest. Comparisons were made between homonymous muscles of the right (corresponding to outside leg for track running) and left leg (inside leg, likewise) and with a group of nontrained controls. The result clearly demonstrated that the responses were prominently different between the legs (thus, asymmetrical) in the MG muscles and partially in the SOL muscles in the trained group. In contrast, no such differences were obtained in the nontrained control group. The result demonstrated that neural adaptation took place asymmetrically and that could be attributable to their repetitive engagement in the stereotypical motor task.

Changes in H reflex and V wave following short-term endurance and strength training

This study examined the effects of 3 wk of either endurance or strength training on plasticity of the neural mechanisms involved in the soleus H reflex and V wave. Twenty-five sedentary healthy subjects were randomized into an endurance group ( n = 13) or strength group ( n = 12). Evoked V-wave, H-reflex, and M-wave recruitment curves, maximal voluntary contraction (MVC), and time-to-task-failure (isometric contraction at 40% MVC) of the plantar flexors were recorded before and after training. Following strength training, MVC of the plantar flexors increased by 14.4 ± 5.2% in the strength group ( P < 0.001), whereas time-to-task-failure was prolonged in the endurance group (22.7 ± 17.1%; P < 0.05). The V wave-to-maximal M wave (V/Mmax) ratio increased significantly (55.1 ± 28.3%; P < 0.001) following strength training, but the maximal H wave-to-maximal M wave (Hmax/Mmax) ratio remained unchanged. Conversely, in the endurance group the V/Mmax ratio was not altered, whereas the Hmax/Mmax ratio increased by 30.8 ± 21.7% ( P < 0.05). The endurance training group also displayed a reduction in the H-reflex excitability threshold while the H-reflex amplitude on the ascending limb of the recruitment curve increased. Strength training only elicited a significant decrease in H-reflex excitability threshold, while H-reflex amplitudes over the ascending limb remained unchanged. These observations indicate that the H-reflex pathway is strongly involved in the enhanced endurance resistance that occurs following endurance training. On the contrary, the improvements in MVC following strength training are likely attributed to increased descending drive and/or modulation in afferents other than Ia afferents.

Relating plastic changes of short latency human soleus stretch reflex to changes in task performance induced by training

Recent findings in the field of neurophysiology showed that operant conditioning on the human H-Reflex is possible. This leads to many possible clinical applications as well as possible sophisticated training methods for athletes. Although stretch reflexes have been subject to extensive literature, knowledge about the influence of short latency stretch reflexes on task performance is lacking. Within this study an ankle control task was designed where perturbations in the magnitude of functional relevance were applied. Results analyzing angle over time after perturbation confirm previous findings which used to analyze the EMG and force response to ankle perturbations. Further it was found that after training the response to perturbations shifted from initially containing latencies which indicate conscious support by transcortical pathways to latencies which could only origin from unconscious stretch reflex responses. The trend of the short latency response to shift towards the long latency response and to diminish, while pre-defined performance criteria improved, denote a functional relevance of the short latency stretch reflex to task performance. Whereas short latency reflexes have any importance at all or if improvements emerge only out of enhancements in the long latency response future work making use of operant conditioning on the short latency H-Reflex will have to unravel.

Persistence of locomotor-related interlimb reflex networks during walking after stroke

OBJECTIVE

Cutaneous nerve stimulation evokes coordinated and phase-modulated reflex output widely distributed to muscles of all four limbs during walking. Accessibility to this distributed network after stroke offers insight into the pathological changes and suggests utility for therapeutic applications. Here we examined muscles in both the more (MA) and less affected (LA) legs evoked by stimulation at the ankle and wrist during walking in chronic (>6 months post CVA) stroke.

METHODS

Stroke and control participants walked on a treadmill with a harness support system. Reflexes were evoked with trains of electrical stimuli delivered separately to the cutaneous superficial peroneal (SP; at the ankle) and superficial radial (SR; at the wrist) nerves. Background locomotor and reflex EMG were phase-averaged across the gait cycle and analyzed off line.

RESULTS

Locomotor background muscle activation patterns were altered bilaterally in stroke, as compared with control. Phase-dependent modulation of interlimb cutaneous reflexes was found in both stroke and control subjects with stimulation of each nerve, but responses were blunted in stroke. Reflex reversal in tibialis anterior (TA) at heel strike with SP nerve stimulation was present in both groups. Notably, SR nerve stimulation produced facilitation during the swing-to-stance transition in the TA and suppression of MG in the MA leg during stance.

CONCLUSIONS

Interlimb cutaneous inputs may access coordinated reflex pathways in the MA limb during walking after stroke. Importantly activation in these pathways could provoke responses to counter foot drop during swing phase of walking. Additionally, our data support the perspective that there is no "unaffected" side after stroke and that caution should be used when interpreting the LA side as "control" after stroke.

SIGNIFICANCE

The presence of functionally-relevant interlimb cutaneous reflexes in the MA leg presents a substrate that may be strengthened by rehabilitation.

Modulation of the spinal excitability by muscle metabosensitive afferent fibers

The aim of this study was to identify the effect of chemical activation of muscle metabosensitive afferent fibers from groups III and IV on Hoffmann (H-) reflex modulation in the vastus medialis muscle. The experiment was conducted in rats and was divided into two experiments. The first experiment consisted of recording the metabosensitive afferent activity from femoral nerve in rats in response to KCl intraarterial injections in nontreated adults and adults treated neonatally with capsaicin. Thus, the dose-response curve was determined. The second experiment consisted of eliciting the H- and M-waves before and after KCl injection in nontreated adult animals and those treated neonatally with capsaicin. Thus, the H(max)/M(max) ratio was measured. Results indicated that, 1) in nontreated animals, afferent fibers peak discharge was found after 10 mM KCl injection; 2) no significant increase in afferent discharge rate was found in capsaicin-treated animal after KCl injections, confirming that capsaicin is an excitotoxic agent that had destroyed the thin metabosensitive nerve fibers; 3) in nontreated animals, H(max)/M(max) ratio was significantly attenuated after a 10 mM KCl injection activating metabosensitive afferent fibers; and 4) in capsaicin-treated animals, no significant change in H(max)/M(max) ratio was observed after the KCl injection. These results reinforce the hypothesis that the spinal reflex response was influenced by metabosensitive muscle fibers and provide direct evidence that activation of these fibers could partially explain the reported decrease in H-reflex when metabolites are released in muscle.

Bilateral neuromuscular plasticity from unilateral training of the ankle dorsiflexors

Training a muscle group in one limb yields strength gains bilaterally-the so-called cross-education effect. However, to date there has been little study of the targeted application of this phenomenon in a manner relevant to clinical rehabilitation. For example, it may be applicable post-stroke, where hemiparesis leads to ankle flexor weakness. The purpose of this study was to examine the effects of high-intensity unilateral dorsiflexion resistance training on agonist (tibialis anterior, TA) and antagonist (plantarflexor soleus, SOL) muscular strength and H-reflex excitability in the trained and untrained limbs. Ankle flexor and extensor torque, as well as SOL and TA H-reflexes evoked during low-level contraction, were measured before and after 5 weeks of dorsiflexion training (n = 19). As a result of the intervention, dorsiflexor maximal voluntary isometric contraction force (MVIC) significantly increased (P < 0.05) in both the trained and untrained limbs by 14.7 and 8.4%, respectively. No changes in plantarflexor MVIC force were observed in either limb. Significant changes in H-reflex excitability threshold were also detected: H(@thresh) significantly increased in the trained TA and SOL; and H(@max) decreased in both SOL muscles. These findings reveal that muscular crossed effects can be obtained in the ankle dorsiflexor muscles and provide novel information on agonist and antagonist spinal adaptations that accompany unilateral training. It is possible that the ability to strengthen the ankle dorsiflexors bilaterally could be applied in post-stroke rehabilitation, where ankle flexor weakness could be counteracted via dorsiflexor training in the less-affected limb.

Motor unit behavior during submaximal contractions following six weeks of either endurance or strength training

The study investigated changes in motor output and motor unit behavior following 6 wk of either strength or endurance training programs commonly used in conditioning and rehabilitation. Twenty-seven sedentary healthy men (age, 26.1 ± 3.9 yr; mean ± SD) were randomly assigned to strength training (ST; n = 9), endurance training (ET; n = 10), or a control group (CT; n = 8). Maximum voluntary contraction (MVC), time to task failure (isometric contraction at 30% MVC), and rate of force development (RFD) of the quadriceps were measured before ( week 0), during ( week 3), and after a training program of 6 wk. In each experimental session, surface and intramuscular EMG signals were recorded from the vastus medialis obliquus and vastus lateralis muscles during isometric knee extension at 10 and 30% MVC. After 6 wk of training, MVC and RFD increased in the ST group (17.5 ± 7.5 and 33.3 ± 15.9%, respectively; P < 0.05), whereas time to task failure was prolonged in the ET group (29.7 ± 13.4%; P < 0.05). The surface EMG amplitude at 30% MVC force increased with training in both groups, but the training-induced changes in motor unit discharge rates differed between groups. After endurance training, the motor unit discharge rate at 30% MVC decreased from 11.3 ± 1.3 to 10.1 ± 1.1 pulses per second (pps; P < 0.05) in the vasti muscles, whereas after strength training it increased from 11.4 ± 1.2 to 12.7 ± 1.3 pps ( P < 0.05). Finally, motor unit conduction velocity during the contractions at 30% MVC increased for both the ST and ET groups, but only after 6 wk of training ( P < 0.05). In conclusion, these strength and endurance training programs elicit opposite adjustments in motor unit discharge rates but similar changes in muscle fiber conduction velocity.

Reflexive limb selection and control of reach direction to moving targets in cats, monkeys, and humans

When we reach for an object, we have to decide which arm to use and the direction in which to move. According to the established view, this is voluntarily controlled and programmed in advance in time-consuming and elaborate computations. Here, we systematically tested the motor strategy used by cats, monkeys, and humans when catching an object moving at high velocity to the left or right. In all species, targets moving to the right selectively initiated movement of the right forelimb and vice versa for targets moving to the left. Movements were from the start directed toward a prospective target position. In humans, the earliest onset of electromyographic activity from start of motion of the target ranged from 90 to 110 ms in different subjects. This indicates that the selection of the arm and specification of movement direction did not result from the subject's voluntary decision, but were determined in a reflex-like manner by the parameters of the target motion. As a whole the data suggest that control of goal-directed arm movement relies largely on an innate neuronal network that, when activated by the visual signal from the target, automatically guides the arm throughout the entire movement toward the target. In the view of the present data, parametric programming of reaching in advance seems to be superfluous.

Reproducible Measurement of Human Motoneuron Excitability With Magnetic Stimulation of the Corticospinal Tract

It is difficult to test responses of human motoneurons in a controlled way or to make longitudinal assessments of adaptive changes at the motoneuron level. These studies assessed the reliability of responses produced by magnetic stimulation of the corticospinal tract. Cervicomedullary motor evoked potentials (CMEPs) were recorded in the first dorsal interosseus (FDI) on 2 separate days. On each day, four sets of stimuli were delivered at the maximal output of the stimulator, with the final two sets ≥10 min after the initial sets. Sets of stimuli were also delivered at different stimulus intensities to obtain stimulus-response curves. In addition, on the second day, responses at different stimulus intensities were evoked during weak voluntary contractions. Responses were normalized to the maximal muscle compound action potential ( Mmax). CMEPs evoked in the relaxed FDI were small, even when stimulus intensity was maximal (3.6 ± 2.5% Mmax) but much larger during a weak contraction (e.g., 26.2 ± 10.2% Mmax). CMEPs evoked in the relaxed muscle at the maximal output of the stimulator were highly reproducible both within (ICC = 0.83, session 1; ICC = 0.87, session 2) and between sessions (ICC = 0.87). ICCs for parameters of the input-output curves, which included measures of motor threshold, slope, and maximal response size, ranged between 0.87 and 0.62. These results suggest that responses to magnetic stimulation of the corticospinal tract can be assessed in relaxation and contraction and can be reliably obtained for longitudinal studies of motoneuronal excitability.