The relationship between the kinematics of passive movement, the stretch of extensor muscles of the leg and the change induced in the gain of the soleus H reflex in humans

Effectiveness of Osteopathic Treatment in Adults with Short Hamstring Syndrome: A Systematic Review

Background/Objective: Short hamstring syndrome is common in the general population and can lead to impaired balance, function, and posture, and increased risk of injuries. Local treatments have obtained controversial results, so it is necessary to evaluate the effectiveness of other types of therapy such as osteopathic treatment. To evaluate the efficacy of osteopathic techniques in increasing the elasticity of the hamstring musculature in short hamstring syndrome. Methods: A systematic review of randomised controlled trials was conducted in PubMed, Medline, Cinhal, Scopus, WOS, SPORTDiscuss, and PEDro. The PEDro scale was used to evaluate the methodological quality and the RoB2 for the evaluation of biases. Results: A total of eight articles were selected. Most of the participants were assessed with the Active Knee Extension or Straight Leg Raise tests. The osteopathic techniques used were the muscle energy technique, suboccipital inhibition, and vertebral mobilisations. As for the control interventions, they mainly included passive stretching and placebo. Conclusions: The results suggest that osteopathic techniques are more effective than placebo or other interventions in increasing flexibility in adult patients with short hamstring syndrome. This effect can be explained by neurophysiological (Golgi apparatus, neuromuscular spindle activity, and Hoffmann reflex) and structural factors (dura mater, posture, and myofascial chains). Nevertheless, the evidence suggests that it would be beneficial to incorporate this type of treatment into flexibility improvement programmes.

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

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

Robot controlled, continuous passive movement of the ankle reduces spinal cord excitability in participants with spasticity: a pilot study

Spasticity of the ankle reduces quality of life by impeding walking and other activities of daily living. Robot-driven continuous passive movement (CPM) is a strategy for lower limb spasticity management but effects on spasticity, walking ability and spinal cord excitability (SCE) are unknown. The objectives of this experiment were to evaluate (1) acute changes in SCE induced by 30 min of CPM at the ankle joint, in individuals without neurological impairment and those with lower limb spasticity; and, (2) the effects of 6 weeks of CPM training on SCE, spasticity and walking ability in those with lower limb spasticity. SCE was assessed using soleus Hoffmann (H-) reflexes, collected prior to and immediately after CPM for acute assessments, whereas a multiple baseline repeated measures design assessed changes following 18 CPM sessions. Spasticity and walking ability were assessed using the Modified Ashworth Scale, the 10 m Walk test, and the Timed Up and Go test. Twenty-one neurologically intact and nine participants with spasticity (various neurological conditions) were recruited. In the neurologically intact group, CPM caused bi-directional modulation of H-reflexes creating 'facilitation' and 'suppression' groups. In contrast, amongst participants with spasticity, acute CPM facilitated H-reflexes. After CPM training, H-reflex excitability on both the more-affected and less-affected sides was reduced; on the more affected side H@Thres, H@50 and H@100 all significantly decreased following CPM training by 96.5 ± 7.7%, 90.9 ± 9.2%, and 62.9 ± 21.1%, respectively. After training there were modest improvements in walking and clinical measures of spasticity for some participants. We conclude that CPM of the ankle can significantly alter SCE. The use of CPM in those with spasticity can provide a temporary period of improved walking, but efficacy of treatment remains unknown.

Robotic investigation on effect of stretch reflex and crossed inhibitory response on bipedal hopping

To maintain balance during dynamic locomotion, the effects of proprioceptive sensory feedback control (e.g. reflexive control) should not be ignored because of its simple sensation and fast reaction time. Scientists have identified the pathways of reflexes; however, it is difficult to investigate their effects during locomotion because locomotion is controlled by a complex neural system and current technology does not allow us to change the control pathways in living humans. To understand these effects, we construct a musculoskeletal bipedal robot, which has similar body structure and dynamics to those of a human. By conducting experiments on this robot, we investigate the effects of reflexes (stretch reflex and crossed inhibitory response) on posture during hopping, a simple and representative bouncing gait with complex dynamics. Through over 300 hopping trials, we confirm that both the stretch reflex and crossed response can contribute to reducing the lateral inclination during hopping. These reflexive pathways do not use any prior knowledge of the dynamic information of the body such as its inclination. Beyond improving the understanding of the human neural system, this study provides roboticists with biomimetic ideas for robot locomotion control.

Acute effects of centrally- and unilaterally-applied posterior-anterior mobilizations of the lumbar spine on lumbar range of motion, hamstring extensibility and muscle activation

BACKGROUND

Lumbar mobilizations are used to clinically treat the lumbar and hamstring region. However, evidence is limited regarding the effectiveness of specific mobilization methods.

OBJECTIVE

To compare central and unilateral posterior-anterior mobilizations (CPA, UPA) of the lumbar spine on lumbar and hamstring range of motion (ROM), and muscle activity (sEMG).

METHODS

Twenty participants received CPA, UPA, or no mobilization (CON) on separate occasions (crossover design). Post-treatment outcome measures were ROM during active lumbar flexion (ALF) and active knee extension (AKE), as well as sEMG of the Erector Spinae (ES) and Biceps Femoris (BF) during these movements.

RESULTS

sEMG was possibly to very likely lower following CPA (mean difference range =-5% to -21%) and UPA (-7% to -36%), while ROM was most likely greater (-12% to 25% and -17% to 24%, respectively). Most sEMG measures were possibly to likely lower following UPA versus CPA (-18% to -11%), while AKE ROM was possibly greater (-5.5%). Differences in ES sEMG (-2.5%) and ROM (-1.4%) during ALF were unclear and most likely trivial, respectively.

CONCLUSIONS

CPA and UPA mobilizations increase lumbar and hamstring ROM whilst reducing local muscle activity. These effects appear to be greater for UPA mobilizations when compared with CPA.

Immediate effects of thoracic spinal mobilisation on erector spinae muscle activity and pain in patients with thoracic spine pain: a preliminary randomised controlled trial

OBJECTIVES

To investigate the activity of the thoracic erector spinae muscles and perceived pain intensity immediately after central postero-anterior (PA) mobilisation of the thoracic spine.

DESIGN

Randomised, placebo-controlled, experimental design.

PARTICIPANTS AND INTERVENTIONS

Thirty-four participants with non-specific thoracic pain were randomised to the experimental group [grade III central PA mobilisation performed for 3minutes at the level of the seventh thoracic vertebra (T7)] or the placebo group (less than grade I central PA mobilisation performed for 3minutes at T7).

MAIN OUTCOME MEASURES

Before and immediately after PA mobilisation, surface electromyography (EMG) was recorded from the thoracic erector spinae muscles as the participants performed 10° spine extension from a prone position for 10seconds. Each participant rated their pain intensity as an investigator performed grade III central PA over the most symptomatic thoracic segment, and the pressure pain threshold (PPT) was evaluated bilaterally over the erector spinae muscles.

RESULTS

The EMG amplitude of thoracic erector spinae activity was reduced significantly after the intervention in the experimental group (P<0.05), but not in the placebo group. The difference between the groups was significant {pre-post change: placebo -14 [standard deviation (SD) 50]mV, experimental 28 (SD 48)mV; mean difference -42mV; 95% confidence interval of the difference -76 to 7; P<0.05} albeit small (Grissom=0.44). However, both groups showed a significant reduction in pain immediately after the intervention, and both groups showed a similar pre-post change in PPT.

CONCLUSION

These preliminary findings indicate that grade III central mobilisation over the most symptomatic thoracic segment reduces thoracic erector spinae activity during extension of the trunk in people with non-specific thoracic spine pain.

CLINICAL TRIAL REGISTRATION NUMBER

ISRCTN47601528.

The immediate and 24-hour follow-up effect of unilateral lumbar Z-joint mobilisation on posterior chain neurodynamics

Few studies have reported the effects of lumbar spine mobilization on neurodynamics. In a recent study, Szlezak et al. (2011) reported immediate improvement of posterior chain neurodynamics [range of passive straight leg raise (SLR)] following ipsilateral lumbar spine zygopophyseal (Z) joint mobilization. We re-duplicated the study with a 24 h follow-up measurement. Sixty healthy college students were assigned to two groups, mobilization and control. The mobilization group received ipsilateral grade 3 Maitland mobilizations to Z joint at a frequency of 2 MHz for 3 min and the control group received no treatment. The SLR was measured before and after the intervention for both the groups on the day of testing and 24-h later. Repeated measures ANOVA showed statistically significant pre to post improvement in SLR range after mobilization. The improvement was retained at 24-h. The results of the study are consistent with Szlezak et al. (2011).

Velocity-dependent suppression of the soleus H-reflex during robot-assisted passive stepping

The amplitude of the Hoffmann (H)-reflex in the soleus (Sol) muscle is known to be suppressed during passive stepping compared with during passive standing. The reduction of the H-reflex is not due to load-related afferent inputs, but rather to movement-related afferent inputs from the lower limbs. To elucidate the underlying neural mechanisms of this inhibition, we investigated the effects of the stepping velocity on the Sol H-reflex during robot-assisted passive stepping in 11 healthy subjects. The Sol H-reflexes were recorded during passive standing and stepping at five stepping velocities (stride frequencies: 14, 21, 28, 35, and 42 min(-1)) in the air. The Sol H-reflexes were significantly inhibited during passive stepping as compared with during passive standing, and reduced in size as the stepping velocity increased. These results indicate that the extent of H-reflex suppression increases with increasing movement-related afferent inputs from the lower limbs during passive stepping. The velocity dependence suggests that the Ia afferent inputs from lower-limb muscles around the hip and knee joints are most probably related to this inhibition.

Active-assisted cycling improves tremor and bradykinesia in Parkinson's disease

OBJECTIVES

To develop a rapid cadence cycling intervention (active-assisted cycling [AAC]) using a motorized bike and to examine physiological perimeters during these sessions in individuals with Parkinson's disease (PD). A secondary goal was to examine whether a single session of AAC at a high cadence would promote improvements in tremor and bradykinesia similar to the on medication state.

DESIGN

Before-after pilot trial with cross-over.

SETTING

University research laboratory.

PARTICIPANTS

Individuals with idiopathic PD (N=10, age 45-74y) in Hoehn and Yahr stages 1 to 3.

INTERVENTION

Forty minutes of AAC.

MAIN OUTCOME MEASURES

Heart rate, pedaling power, and rating of perceived exertion were recorded before, during, and after a bout of AAC. Functional assessments included tremor score during resting, postural, and kinetic tremor.

RESULTS

This AAC paradigm was well tolerated by individuals with PD without excessive fatigue, and most participants showed improvements in tremor and bradykinesia immediately after a single bout of cycling.

CONCLUSIONS

This paradigm could be used to examine changes in motor function in individuals with PD after bouts of high-intensity exercise.

Temporal depression of the soleus H-reflex during passive stretch

Synaptic efficacy associated with muscle spindle feedback is regulated via depression at the Ia-motoneurone synapse. The inhibitory effects of repetitive Ia afferent discharge on target motoneurones of different sizes were investigated during a passive stretch of ankle extensors in humans. H-reflex recruitment curves were collected from the soleus muscle for two conditions in ten subjects. H-reflexes were elicited during passive stretch at latencies of 50, 100, 300, and 500 ms after a slow (20°/s) dorsiflexion about the right ankle (from 100 to 90°). Control H-reflexes were recorded at corresponding static (without movement) ankle angles of 99, 98, 94, and 90° of flexion. The slope of the H-reflex recruitment curves (Hslp) was then calculated for both conditions. H-reflex values were similar for the static and passive stretch conditions prior to 50-100 ms, not showing the early facilitation typical of increased muscle spindle discharge rates. However, the H-reflex was significantly depressed by 300 ms and persisted through 500 ms. Furthermore, less than 300 ms into the stretch, there was significantly greater H-reflex depression with a lower stimulus intensity (20 % Mmax) versus a higher stimulus intensity (Hmax), though the effects begin to converge at later latencies (>300 ms). This suggests there is a distinct two-stage temporal process in the depression observed in the Ia afferent pathway for all motoneurones during a passive stretch. Additionally, there is not a single mechanism responsible for the depression, but rather both heterosynaptic presynaptic inhibition and homosynaptic post-activation depression are independently influencing the Ia-motoneurone pathway temporally during movement.

Acute effects of passive leg cycling on upper extremity tremor and bradykinesia in Parkinson's disease

BACKGROUND

Previous studies have shown that single bouts of high-rate active cycling (> 80 rpm) improve upper extremity motor function in individuals with Parkinson's disease (PD). It is unknown if passive leg cycling produces a similar effect on upper extremity function. This article examines whether passive leg cycling can promote immediate changes in upper tremor and bradykinesia in PD and if pedaling rates have variable effects.

METHODS

Twenty individuals with mild-to-moderate idiopathic PD completed 4 sessions, with each session taking place 1 week apart. In the second to fourth sessions, a motorized bicycle was set to passively rotate the subjects' legs at rates of 60, 70, or 80 rpm for 30 minutes. Quantitative upper extremity motor assessments were completed immediately before and after each session.

RESULTS

Passive leg cycling was shown to reduce tremor and bradykinesia in PD. However, the rate of passive cycling did not affect the degree of improvement in bradykinesia or tremor.

CONCLUSION

These findings suggest that lower extremity passive cycling can promote changes in upper extremity motor function in individuals with PD.

Corticomotor excitability of wrist flexor and extensor muscles during active and passive movement

The excitability of the corticospinal projection to upper and lower limbs is constantly modulated during voluntary and passive movement; however a direct comparison during a comparable movement has not been reported. In the present study we used transcranial magnetic stimulation (TMS) to compare corticomotor excitability to the extensor and flexor carpi radialis (ECR/FCR) muscles of the forearm during voluntary rhythmic wrist movement (through 45 degrees of range), during a matched (for range and rhythm) passive movement of the wrist, and while the wrist was stationary (in mid-range). TMS was delivered when the wrist was in the neutral position. With passive and active movement, and for both FCR and ECR, corticomotor excitability was reduced during lengthening relative to shortening phases of movement. With active movement, this pattern was maintained and superimposed on an overall increase in excitability to both muscles that was greater for the ECR. The results favor a common pattern of excitability changes shared by extensor and flexor muscles as they undergo lengthening and shortening, which may be mediated by afferent input during both passive and active movement. This is combined with an overall increase in excitability associated with active movement that is greater for extensor muscles perhaps due to differences in the strength of the corticomotor projection to these muscles.

Effect of sensory inputs on the soleus H-reflex amplitude during robotic passive stepping in humans

We investigated the modulation of the soleus (Sol) Hoffmann (H-) reflex excitability by peripheral sensory inputs during passive stepping using a robotic-driven gait orthosis in healthy subjects and spinal cord-injured patients. The Sol H-reflex was evoked at standing and at six phases during passive stepping in 40 and 100% body weight unloaded conditions. The Sol H-reflex excitability was significantly inhibited during passive stepping when compared with standing posture at each unloaded condition. During passive stepping, the H-reflex amplitude was significantly smaller in the early- and mid-swing phases than in the stance phase, which was similar to the modulation pattern previously reported for normal walking. No significant differences were observed in the H-reflex amplitude between the two unloaded conditions during passive stepping. The reflex depression observed at the early part of the swing phase during passive stepping might be attributed to the sensory inputs elicited by flexion of the hip and knee joints. The present study provides evidence that peripheral sensory inputs have a significant role in phase-dependent modulation of the Sol H-reflex during walking, and that the Sol H-reflex excitability might be less affected by load-related afferents during walking.

MODULATION OF THE SOLEUS H-REFLEX DURING STATIC AND DYNAMIC IMPOSED HIP ANGLE CHANGES

The aim of the study was to investigate the modulation of the soleus H-reflex during static and dynamic imposed hip angle changes. Five healthy subjects participated. H-reflexes were measured during hip joint passively flexed and extended in the sagittal plane. In flexion phase, the soleus H-reflex during dynamic conditions was lower than the stationary controls. By contrast, it was conversely higher in extension phase. The findings suggest that the modulation of the soleus H-reflex from hip proprioceptors is a major factor in passive hip movement. Additionally, the central pattern generator might modulate the soleus H-reflex.

Comparison of surface electromyographic activity of erector spinae before and after the application of central posteroanterior mobilisation on the lumbar spine

Lumbar spine accessory movements, used by therapists in the treatment of patients with low back pain, is thought to decrease paravertebral muscular activity; however there is little research to support this suggestion. This study investigated the effects of lumbar spine accessory movements on surface electromyography (sEMG) activity of erector spinae. A condition randomised, placebo controlled, repeated measures design was used. sEMG measurements were recorded from 36 asymptomatic subjects following a control, placebo and central posteroanterior (PA) mobilisation to L3 each for 2min. The therapist stood on a force platform while applying the PA mobilisation to quantify the force used. The PA mobilisation applied to each subject had a mean maximum force of 103.3N, mean amplitude of force oscillation of 41.1N, and a frequency of 1.2Hz. Surface electromyographic data were recorded from the musculature adjacent to L3, L5 and T10. There were statistically significant reductions of 15.5% (95% CI: 8.0-22.5%) and 17.8% (95% CI: 12.9-22.4%) in mean sEMG values following mobilisation compared with the control and placebo, respectively. This study demonstrates that a central PA mobilisation to L3 results in a statistically significant decrease in the sEMG activity of erector spinae of an asymptomatic population.

Rhythmic arm cycling produces a non-specific signal that suppresses Soleus H-reflex amplitude in stationary legs

Rhythmic arm cycling significantly suppresses Hoffmann (H-) reflex amplitude in Soleus muscles of stationary legs. The specific parameters of arm cycling contributing to this suppression, however, are unknown. Between the arms or legs, movement results in suppression of the H-reflex that is specifically related to the phase of movement and the locus of limb movement. We speculated that the effects of arm movement features on H-reflexes in the leg would be similar and hypothesized that the Soleus H-reflex suppression evoked by arm movement would therefore be specifically related to: (1) phase of the movement; (2) the locus of the movement (i.e., ipsilateral or contralateral arm); (3) range of arm motion; and (4) frequency of arm cycling. Participants performed bilateral arm cycling at 1 and 2 Hz with short and long-crank lengths. Ipsilateral and contralateral arm cycling was also performed at 1 Hz with a long-crank length. Soleus H-reflexes were evoked at four equidistant phases and comparisons were made while maintaining similar evoked motor waves and Soleus activation. Our results show that comparable suppressive effects were seen at all phases of the arm movement: there was no phase-dependence. Further, bilateral or unilateral (whether ipsi- or contralateral arm) cycling yielded equivalent suppression of the H-reflex amplitude. Cycling at 2 Hz resulted in a significantly larger suppression than with 1 Hz cycling. We conclude that a general, rather than a specific, signal related to the command to produce rhythmic arm muscle activity mediates the suppression of Soleus H-reflex during arm cycling.

Spasticity-assessment: a review

STUDY DESIGN

Review of the literature on the validity and reliability of assessment of spasticity and spasms.

OBJECTIVES

Evaluate the most frequently used methods for assessment of spasticity and spasms, with particular focus on individuals with spinal cord lesions.

SETTING

Clinic for Spinal Cord Injuries, Rigshospitalet, University Hospital of Copenhagen, and Department of Medical Physiology, University of Copenhagen, Denmark.

METHODS

The assessment methods are grouped into clinical, biomechanical and electrophysiological, and the correlation between these is evaluated.

RESULTS

Clinical methods: For assessment of spasticity, the Ashworth and the modified Ashworth scales are commonly used. They provide a semiquantitative measure of the resistance to passive movement, but have limited interrater reliability. Guidelines for the testing procedures should be adhered to. Spasm frequency scales seem not to have been tested for reliability. Biomechanical methods such as isokinetic dynamometers are of value when an objective quantitative measure of the resistance to passive movement is necessary. They play a minor role in the daily clinical evaluation of spasticity. Electrophysiological methods: These techniques have provided valuable insight to the pathophysiological mechanisms involved in spasticity, but none of these techniques provide an easy and reliable assessment of spasticity for use in the daily clinic.

CONCLUSION

A combination of electrophysiological and biomechanical techniques shows some promise for a full characterization of the spastic syndrome. There is a need of simple instruments, which provide a reliable quantitative measure with a low interrater variability.

Cutaneous reflexes during rhythmic arm cycling are insensitive to asymmetrical changes in crank length

The neural control of a movement depends upon the motor task performed. To further understand the neural regulation of different variations of the same type of movement, we created three dissimilar bilateral rhythmic arm cycling tasks by unilaterally manipulating crank length (CL). Modulation in the amplitude and sign of cutaneous reflexes was used as an index of neural control. Neurologically intact subjects performed three bilateral cycling trials at approximately 1 Hz with the ipsilateral crank arm at one of three different lengths. Cutaneous reflexes were evoked during each trial with trains (5 x 1.0 ms pulses at 300 Hz) of electrical stimulation delivered to the superficial radial nerve at the ipsilateral wrist. EMG recordings were made bilaterally from muscles acting at the shoulder, elbow, and wrist. Analysis was conducted after phase-averaging contingent upon the timing of stimulation in the movement cycle. CL variation created an asymmetrical cycling pattern and produced significant changes in the range of motion at the ipsilateral shoulder and elbow. Background EMG amplitude in muscles of the contralateral arm generally increased significantly as CL decreased. Therefore at a given phase in the movement cycle, the background EMG was different between the three cycling trials. In contrast, cutaneous reflex amplitudes in muscles of both arms were similar at each phase of the movement cycle between the different CLs trials at both early and middle latencies. This was particularly evident in muscles ipsilateral to nerve stimulation. We suggest that variations of arm cycling that primarily yield significant changes in the amplitude of muscle activity do not require significant task-specific change in neural control.

Amplitude of muscle stretch modulates corticomotor gain during passive movement

Previous studies have shown that the excitability of corticomotor projections to forearm muscles exhibit phasic modulation during passive movement (flexion-extension) about the wrist joint. We examined the stimulus-response properties of flexor carpi radialis (FCR) and extensor carpi radialis (ECR) to transcranial magnetic stimulation (TMS) applied over the contralateral motor cortex while the wrist was moved passively at two different sinusoidal frequency-amplitude relationships. Movement velocity (and therefore, the rate of change in muscle length) at the time of stimulation was held constant. Motor evoked potential (MEP) amplitudes were facilitated during passive muscle shortening and suppressed during passive muscle lengthening with suppression being more evident at higher stimulation intensities. For both FCR and ECR, during the shortening phase, responses were facilitated during the large amplitude movement relative to the small amplitude movement. It is suggested that the altered gain may be related to the thixotropic properties of muscle.

Temporal aspects of passive movement-related corticomotor inhibition

We have previously shown that during rhythmic passive movement of the index finger, the amplitude of the motor evoke potential (MEP) of the first dorsal interosseous muscle (FDI) as the index finger moved through mid-range adduction, is significantly reduced compared to rest [Edwards, D. J., Thickbroom, G. W., Byrnes, M. L., Ghosh, S., & Mastaglia, F. L. (2002). Reduced corticomotor excitability with passive movement: A study using Transcranial Magnetic Stimulation. Human Movement Science 21, 533-540]. In the present study we have investigated the time-course of this phenomenon. We found that MEP amplitude was significantly reduced at the mid-range position in the first cycle of movement (50+/-6% of resting baseline values), and did not vary across subsequent cycles (10 cycles in 50 s), but that MEP amplitude returned to baseline values within 1s of cessation of movement. The results suggest that the pattern of afferent discharge set up by the kinematics of the movement acting at spinal or supraspinal levels underlies the inhibition observed, rather than an effect of central origin or a cumulative effect of ongoing cyclic movement.

Changes in the gain of the soleus H-reflex with changes in the motor recruitment level and/or movement speed

The behaviour of the soleus H-reflex is considered to be motor task-dependent. However, the speed of movements as well as the motor recruitment level is altered when motor tasks are changed. Therefore it is ambiguous to what extent the motor task-dependent changes found between walking and running, for example, are simply due to changes in these two parameters. The purpose of this study was to investigate how movement speed and motor recruitment level separately influence the soleus H-reflex behaviour when the motor task is unchanged. Soleus H-reflexes were elicited during pedalling at different cadences and crank loads, by which movement speed and muscle recruitment level were modified separately. The H-reflex gain was expressed as the slope of the linear relation between the reflex amplitudes and the background electromyelogram (EMG), and the reflex threshold was expressed by the intercept. The results showed a decrease in reflex gain by 54% ( P=0.001) when the speed of movement was doubled from 40 to 80 rpm (repetitions per minute) without changes in the level of soleus EMG activity. Reflex gain decreased 40% ( P=0.002) when the soleus EMG level was increased by 47% without changing the speed of movement. No significant changes were found in the reflex threshold. We conclude that as speed of movement and motor recruitment level influence the gain of the soleus H-reflex, it is significant that these two parameters are comparable before changes in H-reflexes are stated to be task-dependent.

The H-reflex as a tool in neurophysiology: its limitations and uses in understanding nervous system function

The Hoffmann reflex (H-reflex) is extensively used as both a research and clinical tool. The ease with which this reflex can be elicited in several muscles throughout the body makes it an attractive tool. This review discusses some of the important limitations in using the H-reflex. In particular, the inaccurate but widely held assumptions that the H-reflex (1). represents the monosynaptic reflex of the Ia afferent onto homonymous motoneurons, and (2). can be used to measure motoneuronal excitability are addressed. The second part of this review explores the utility of the H-reflex as a neural probe in neurophysiology and motor control research. Applications ranging from the investigation of the functional organization of neural circuitry to the study of adaptive plasticity in spinal structures in health and disease suggest that the H-reflex will continue to be an extensively used tool in motor control neurophysiology.

Neural control of rhythmic human arm movement: phase dependence and task modulation of hoffmann reflexes in forearm muscles

Although we move our arms rhythmically during walking, running, and swimming, we know little about the neural control of such movements. Our working hypothesis is that neural mechanisms controlling rhythmic movements are similar in the human lumbar and cervical spinal cord. Thus reflex modulation during rhythmic arm movement should be similar to that seen during leg movement. Our main experimental hypotheses were that the amplitude of H-reflexes in the forearm muscles would be modulated during arm movement (i.e., phase-dependent) and would be inhibited during cycling compared with static contraction (i.e., task-dependent). Furthermore, to determine the locus of any modulation, we tested the effect that active and passive movement of the ipsilateral (relative to stimulated arm) and contralateral arm had on H-reflex amplitude. Subjects performed rhythmic arm cycling on a custom-made hydraulic ergometer in which the two arms could be constrained to move together (180 degrees out of phase) or could rotate independently. Position of the stimulated limb in the movement cycle is described with respect to the clock face. H-reflexes were evoked at 12, 3, 6, and 9 o'clock positions during static contraction as well as during rhythmic arm movements. Reflex amplitudes were compared between tasks at equal M wave amplitudes and similar levels of electromyographic (EMG) activity in the target muscle. Surface EMG recordings were obtained bilaterally from flexor carpi radialis as well as from other muscles controlling the wrist, elbow, and shoulder. Compared with reflexes evoked during static contractions, movement of the stimulated limb attenuated H-reflexes by 50.8% (P < 0.005), 65.3% (P < 0.001), and 52.6% (P < 0.001) for bilateral, active ipsilateral, and passive ipsilateral movements, respectively. In contrast, movement of the contralateral limb did not significantly alter H-reflex amplitude. H-reflexes were also modulated by limb position (P < 0.005). Thus task- and phase-dependent modulation were observed in the arm as previously demonstrated in the leg. The data support the hypothesis that neural mechanisms regulating reflex pathways in the moving limb are similar in the human upper and lower limbs. However, the inhibition of H-reflex amplitude induced by contralateral leg movement is absent in the arms. This may reflect the greater extent to which the arms can be used independently.

Reduced corticomotor excitability with cyclic passive movement: a study using transcranial magnetic stimulation

Human voluntary movement involves the integration of kinaesthetic information with efferent motor activity during the planning and execution stages of movement. While much is known of the inhibitory and excitatory effects resulting from activation of specific kinaesthetic sensory receptors, in the present study we employed cyclic passive movement of the index finger in order to activate a range of kinaesthetic receptors in a manner that was intended to correspond to how these receptors might be active during a comparable voluntary movement. We intended to identify how this passive movement protocol might affect the excitability of the corticomotor pathway. During 1 Hz cyclic passive movement of the index finger there was an approximately 60% reduction in the amplitude of the motor evoked response from the first dorsal interosseous muscle. The results of the present study demonstrate that passive movement can have a profound effect on the excitability of the corticomotor pathway.

Reflex effects of induced muscle contraction in normal and spinal cord injured subjects

The modulation of the soleus H reflex in response to functional electrical stimulation (FES) of the rectus femoris (RF) muscle and its overlying skin was examined in 11 normal adults and 6 patients with a clinically defined complete spinal cord injury (SCI). Stimulation of RF at twice motor threshold (MT) resulted in a long-lasting (>1,000 ms) and significant reduction (50-70% of control) in the size of the soleus H reflex in all normal subjects tested. For five of the SCI subjects, 2MT stimulation of RF induced a 55-60% reduction in the soleus H reflex that was also long-lasting (>160 ms). In the remaining SCI subject, 2MT stimulation resulted in an initial period of significant H-reflex facilitation (0-14 ms) that was followed by a longer-lasting inhibition commencing 60 ms after the cessation of the conditioning stimulation. Decreasing the strength of stimulation to below that required to generate a clear contraction in RF resulted in mixed facilitatory and inhibitory actions that were subject dependent. The changes in H-reflex excitability resulting from FES highlight the potential use of FES in the management of hypertonicity in SCI but also suggest that the central actions of FES need to be considered when FES gait restoration programs are designed.

Disinhibition in the human motor cortex is enhanced by synchronous upper limb movements

The phasic modulation of wrist flexor corticomotor disinhibition has previously been demonstrated during the flexion phase of rhythmical passive flexion-extension of the human wrist. Here we ask if rhythmical bimanual flexion-extension movements of the wrists of neurologically intact subjects, modulate inhibitory activity in the motor cortex. In the first experiment intracortical inhibition was assessed when one wrist was passively flexed and extended on its own, with the addition of the opposite limb voluntarily moving synchronously in a mirror symmetric pattern, and also in a near-symmetric asynchronous pattern. Two subsequent experiments investigated firstly the modulation of spinal reflex pathway activity during the same three movement conditions, and secondly the effect of contralateral wrist movement alone on the excitability of corticomotoneuronal pathways to a static test limb. When the wrist flexors of both upper limbs were shortening simultaneously (i.e. synchronously), intracortical inhibition associated with flexor representations was suppressed to a greater extent than when the two muscles were shortening asynchronously. The results of the three experiments indicate that modulation of inhibitory activity was taking place at the cortical level. These findings may have further application in the study of rehabilitation procedures where the effects of simultaneous activation of affected and unaffected upper limbs in hemiparetic patients are to be investigated.

Modulations in corticomotor excitability during passive upper-limb movement: is there a cortical influence?

Modulations in the excitability of corticomotor pathways to forearm musculature have previously been demonstrated during passive wrist movement [Brain Res. 900 (2001) 282]. Investigations were conducted to determine the level of the neuroaxis at which these modulations arise, and to establish the influence of proprioceptive task constraints on pathway excitability. Forearm motor evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) were examined during passive wrist movement while subjects maintained a low-level muscle activation, thus stabilising the excitability of the motoneuron pool. Modulations in response amplitude during movement were evident in both forearm flexor and extensor muscles. The pattern of modulation generally mirrored that seen in quiescent musculature during movement, with responses potentiated during the phases where the muscle was in a shortened position. Variations in MEP amplitude were not detected while the wrist was constrained statically at various joint angles. This suggests a dynamic influence of movement, most likely mediated by spindle receptors, arising at a supraspinal level. We also investigated the influence of a kinesthetic tracking task on corticomotor excitability during passive movement of the wrist joint. MEPs were recorded from the target driven limb while the contralateral limb was stationary, while the contralateral limb actively tracked the movements of the target limb, and while the contralateral limb moved actively in time with a metronome. The results revealed no differences in MEP characteristics in the driven limb between the three conditions. Placing the movement elicited afferent information in an active movement context does not appear to enhance the modulations in cortical excitability.

The acquisition of movement skills: practice enhances the dynamic stability of bimanual coordination

During bimanual movements, two relatively stable "inherent" patterns of coordination (in-phase and anti-phase) are displayed (e.g., Kelso, Am. J. Physiol. 246 (1984) R1000). Recent research has shown that new patterns of coordination can be learned. For example, following practice a 90 degrees out-of-phase pattern can emerge as an additional, relatively stable, state (e.g., Zanone & Kelso, J. Exp. Psychol.: Human Performance and Perception 18 (1992) 403). On this basis, it has been concluded that practice leads to the evolution and stabilisation of the newly learned pattern and that this process of learning changes the entire attractor layout of the dynamic system. A general feature of such research has been to observe the changes of the targeted pattern's stability characteristics during training at a single movement frequency. The present study was designed to examine how practice affects the maintenance of a coordinated pattern as the movement frequency is scaled. Eleven volunteers were asked to perform a bimanual forearm pronation-supination task. Time to transition onset was used as an index of the subjects' ability to maintain two symmetrically opposite coordinated patterns (target task - 90 degrees out-of-phase - transfer task - 270 degrees out-of-phase). Their ability to maintain the target task and the transfer task were examined again after five practice sessions each consisting of 15 trials of only the 90 degrees out-of-phase pattern. Concurrent performance feedback (a Lissajous figure) was available to the participants during each practice trial. A comparison of the time to transition onset showed that the target task was more stable after practice (p=0.025). These changes were still observed one week (p=0.05) and two months (p=0.075) after the practice period. Changes in the stability of the transfer task were not observed until two months after practice (p=0.025). Notably, following practice, transitions from the 90 degrees pattern were generally to the anti-phase (180 degrees ) pattern, whereas, transitions from the 270 degrees pattern were to the 90 degrees pattern. These results suggest that practice does improve the stability of a 90 degrees pattern, and that such improvements are transferable to the performance of the unpractised 270 degrees pattern. In addition, the anti-phase pattern remained more stable than the practised 90 degrees pattern throughout.

H-reflex modulation during passive lengthening and shortening of the human triceps surae

1. The present study investigated the effects of lengthening and shortening actions on H-reflex amplitude. H-reflexes were evoked in the soleus (SOL) and medial gastrocnemius (MG) of human subjects during passive isometric, lengthening and shortening actions performed at angular velocities of 0, +/-2, +/-5 and +/-15 deg s(-1). 2. H-reflex amplitudes in both SOL and MG were significantly depressed during passive lengthening actions and facilitated during passive shortening actions, when compared with the isometric H-reflex amplitude. 3. Four experiments were performed in which the latencies from the onset of movement to delivery of the stimulus were altered. Passive H-reflex modulation during lengthening actions was found to begin at latencies of less than 60 ms suggesting that this inhibition was due to peripheral and/or spinal mechanisms. 4. It is postulated that the H-reflex modulation seen in the present study is related to the tonic discharge of muscle spindle afferents and the consequent effects of transmission within the Ia pathway. Inhibition of the H-reflex at less than 60 ms after the onset of muscle lengthening may be attributed to several mechanisms, which cannot be distinguished using the current protocol. These may include the inability to evoke volleys in Ia fibres that are refractory following muscle spindle discharge during rapid muscle lengthening, a reduced probability of transmitter release from the presynaptic terminal (homosynaptic post-activation depression) and presynaptic inhibition of Ia afferents from plantar flexor agonists. Short latency facilitation of the H-reflex may be attributed to temporal summation of excitatory postsynaptic potentials arising from muscle spindle afferents during rapid muscle lengthening. At longer latencies, presynaptic inhibition of Ia afferents cannot be excluded as a potential inhibitory mechanism.

Phasic modulation of corticomotor excitability during passive movement of the upper limb: effects of movement frequency and muscle specificity

Modulations in the excitability of spinal reflex pathways during passive rhythmic movements of the lower limb have been demonstrated by a number of previous studies [4]. Less emphasis has been placed on the role of supraspinal pathways during passive movement, and on tasks involving the upper limb. In the present study, transcranial magnetic stimulation (TMS) was delivered to subjects while undergoing passive flexion-extension movements of the contralateral wrist. Motor evoked potentials (MEPs) of flexor carpi radialis (FCR) and abductor pollicus brevis (APB) muscles were recorded. Stimuli were delivered in eight phases of the movement cycle during three different frequencies of movement. Evidence of marked modulations in pathway excitability was found in the MEP amplitudes of the FCR muscle, with responses inhibited and facilitated from static values in the extension and flexion phases, respectively. The results indicated that at higher frequencies of movement there was greater modulation in pathway excitability. Paired-pulse TMS (sub-threshold conditioning) at short interstimulus intervals revealed modulations in the extent of inhibition in MEP amplitude at high movement frequencies. In the APB muscle, there was some evidence of phasic modulations of response amplitude, although the effects were less marked than those observed in FCR. It is speculated that these modulatory effects are mediated via Ia afferent pathways and arise as a consequence of the induced forearm muscle shortening and lengthening. Although the level at which this input influences the corticomotoneuronal pathway is difficult to discern, a contribution from cortical regions is suggested.

Differential regulation of cutaneous and H-reflexes during leg cycling in humans

Reflexes undergo modulation according to task and timing during standing, walking, running, and leg cycling in humans. Both cutaneous and Hoffman (H-) reflexes are modulated by movement and task. However, recent evidence suggests that the modulation pattern for cutaneous and H-reflexes may be different. We sought to clarify this issue by reducing the effect of movement phase and altering the level of background muscle activation (low and high) in static and dynamic (leg cycling) conditions. Electromyography was recorded from the ankle extensors soleus and medial gastrocnemius (MG) and the knee extensor vastus lateralis (VL). Reflexes were evoked during the downstroke of stationary leg cycling. Cutaneous reflexes were evoked with trains of 5 x 1.0 ms pulses at 300 Hz delivered to the distal tibial nerve, whereas H-reflexes were evoked in soleus by stimulation with single 1.0-ms pulses. There were two main observations in this study: 1) middle latency cutaneous reflexes were facilitatory during static contraction but were dramatically attenuated or reversed to suppressive responses during cycling (task-dependent modulation); 2) soleus H-reflexes were larger in the high muscle activation condition but were unaffected by task (no task-dependent modulation). Thus opposite results were obtained in the two reflex pathways. It is concluded that cutaneous and H-reflexes are modulated by different mechanisms during active locomotor-like movements.

Soleus Hoffmann-reflex modulation during walking in healthy elderly and young adults

Soleus Hoffmann-reflex (H-reflex) modulation during walking was examined in 7 young and 13 elderly adults. H-reflex size was measured in 16 equal time divisions (phases) of the step cycle. In both the elderly and the young groups, the H reflex was minimal at the time of heel contact, rose to a maximum shortly after midstance, decreased rapidly as toe-off neared, then was minimal during swing. There was a significant interaction between age group and step cycle phase (p < .05). During midstance of walking, the elderly participants had a smaller H-reflex size during two of the 16 time phases of the step cycle (p < .05), despite no significant difference in H-reflex size between the age groups while standing. The smaller H-reflex size during the stance phase of walking may reflect changes in central reflex mechanisms that may impact stretch reflex contribution to ankle extensor neural drive and ankle stiffness in elderly persons during walking.

Modulation of human cutaneous reflexes during rhythmic cyclical arm movement

The organization and pattern of cutaneous reflex modulation is unknown during rhythmic cyclical movements of the human upper limbs. On the assumption that these cyclic arm movements are central pattern generator (CPG) driven as has been suggested for leg movements such as walking, we hypothesized that cutaneous reflex amplitude would be independent of electromyographic (EMG) muscle activation level during rhythmic arm movement (phase-dependent modulation, as is often the case in the lower limb during locomotion). EMG was recorded from eight muscles crossing the human shoulder, elbow, and wrist joints while whole arm rhythmic cyclical movements were performed. Cutaneous reflexes were evoked with trains of electrical stimulation delivered at non-noxious intensities (approximately 2 x threshold for radiating paresthesia) to the superficial radial nerve innervating the lateral portion of the back of the hand. Phasic bursts of rhythmic muscle activity occurred throughout the movement cycle. Rhythmic EMG and kinematic patterns were similar to what has been seen in the human lower limb during locomotor activities such as cycling or walking: there were extensive periods of reciprocal activation of antagonist muscles. For most muscles, cutaneous reflexes were modulated with the movement cycle and were strongly correlated with the movement-related background EMG amplitude. It is concluded that cutaneous reflexes are primarily modulated by the background muscle activity during rhythmic human upper limb movements, with only some muscles showing phase-dependent modulation.

Phase-dependent inhibition of H-reflexes during walking in humans is independent of reduction in knee angular velocity

The purpose of this investigation was to investigate whether reduction in impulses arising from stretch of the quadriceps by restricting rapid knee flexion in early swing would affect inhibition of the H-reflex during swing. The contribution of afferent input arising from knee angular velocity to phase-dependent modulation of short-latency responses in the soleus was studied by simultaneously measuring joint velocity and soleus H-reflex responses at midstance and midswing phases of treadmill walking in 15 normal subjects. Stimulus strength was varied so that both maximal M and H waves were identified in each subject at midswing and midstance with the knee unrestricted (UK) and with knee movement restricted (RK), using a full leg bivalved cast to immobilize the knee joint. All subjects exhibited short-latency reflex responses in the soleus muscle. The H/M ratio at midswing was significantly reduced compared with midstance under both UK and RK walking conditions (P < 0.0001). When compared with UK walking, knee joint angular velocity during RK walking was significantly reduced at midswing (P < 0.001) and midstance (P < 0.005) compared with UK. There were, however, no significant differences in H/M ratios at midswing and midstance between UK and RK walking tests. Inhibition of the H-reflex in the soleus muscle during swing was not affected by significant reduction in knee angular velocity. These results indicate that the sensory input from changes in angular velocity at the knee does not lay the inhibitory foundation of phase-related reflex modulation in the ankle extensors during walking as suggested by Brooke and colleagues.

Cutaneous reflexes of the human leg during passive movement

1. Four experiments tested the hypothesis that movement-induced discharge of somatosensory receptors attenuates cutaneous reflexes in the human lower limb. In the first experiment, cutaneous reflexes were evoked in the isometrically contracting tibialis anterior muscle (TA) by a train of stimuli to the tibial nerve at the ankle. The constancy of stimulus amplitudes was indirectly verified by monitoring M waves elicited in the abductor hallucis muscle. There was a small increase in the reflex excitation (early latency, EL) during passive cycling movement of the leg compared with when the leg was stationary, a result opposite to that hypothesized. There was no significant effect on the magnitude of the subsequent inhibitory reflex component (middle latency, ML), even with increased rate of movement, or on the latency of any of the reflex components. 2. In the second experiment, the two reflex components (EL and ML) elicited in TA at four positions in the movement cycle were compared with corresponding reflexes elicited with the limb stationary at those positions. Despite the markedly different degree of stretch of the leg muscles, movement phase exerted no statistically significant effect on EL or ML reflex magnitudes. 3. In the third experiment, taps to the quadriceps tendon, to elicit muscle spindle discharge, had no effect on the magnitude of ML in TA muscle. The conditioning attenuated EL magnitude for the first 110 ms. Tendon tap to the skin over the tibia revealed similar attenuation of EL. 4. The sural nerve was stimulated at the ankle in the fourth experiment. TA EMG reflex excitatory and inhibitory responses still showed no significant attenuation with passive movement. Initial somatosensory evoked potentials (SEPs), measured from scalp electrodes, were attenuated by movement. 5. The results indicate that there is separate control of transmission in Ia and cutaneous pathways during leg movement. This suggests that modulation of the cutaneous reflex during locomotion is not the result of inhibition arising from motion-related sensory receptor discharge.

Modulation of stretch reflexes during imposed walking movements of the human ankle

Our overall objectives were to examine the role of peripheral afferents from the ankle in modulating stretch reflexes during imposed walking movements and to assess the mechanical consequences of this reflex activity. Specifically we sought to define the changes in the electromyographic (EMG) and mechanical responses to a stretch as a function of the phase of the step cycle. We recorded the ankle position of a normal subject walking on a treadmill at 3 km/h and used a hydraulic actuator to impose the same movements on supine subjects generating a constant level of ankle torque. Small pulse displacements, superimposed on the simulated walking movement, evoked stretch reflexes at different phases of the cycle. Three major findings resulted: 1) soleus reflex EMG responses were influenced strongly by imposed walking movements. The response amplitude was substantially smaller than that observed during steady-state conditions and was modulated throughout the step cycle. This modulation was qualitatively similar to that observed during active walking. Because central factors were held constant during the imposed walking experiments, we conclude that peripheral mechanisms were capable of both reducing the amplitude of the reflex EMG and producing its modulation throughout the movement. 2) Pulse disturbances applied from early to midstance of the imposed walking cycle generated large reflex torques, suggesting that the stretch reflex could help to resist unexpected perturbations during this phase of walking. In contrast, pulses applied during late stance and swing phase generated little reflex torque. 3) Reflex EMG and reflex torque were modulated differently throughout the imposed walking cycle. In fact, at the time when the reflex EMG response was largest, the corresponding reflex torque was negligible. Thus movement not only changes the reflex EMG but greatly modifies the mechanical output that results.

Prefrontal cortex regulates inhibition and excitation in distributed neural networks

Prefrontal cortex provides both inhibitory and excitatory input to distributed neural circuits required to support performance in diverse tasks. Neurological patients with prefrontal damage are impaired in their ability to inhibit task-irrelevant information during behavioral tasks requiring performance over a delay. The observed enhancements of primary auditory and somatosensory cortical responses to task-irrelevant distractors suggest that prefrontal damage disrupts inhibitory modulation of inputs to primary sensory cortex, perhaps through abnormalities in a prefrontal-thalamic sensory gating system. Failure to suppress irrelevant sensory information results in increased neural noise, contributing to the deficits in decision making routinely observed in these patients. In addition to a critical role in inhibitory control of sensory flow to primary cortical regions, and tertiary prefrontal cortex also exerts excitatory input to activity in multiple sub-regions of secondary association cortex. Unilateral prefrontal damage results in multi-modal decreases in neural activity in posterior association cortex in the hemisphere ipsilateral to damage. This excitatory modulation is necessary to sustain neural activity during working memory. Thus, prefrontal cortex is able to sculpt behavior through parallel inhibitory and excitatory regulation of neural activity in distributed neural networks.

Movement-induced modulation of soleus H reflexes with altered length of biarticular muscles

Passive pedaling movements of the leg results in the phasic modulation of the soleus H reflex of that leg. In contrast, the H reflex of the contralateral leg is attenuated tonically. The phasic modulation of the reflex ipsilaterally can be attributed to the afferent discharge associated with the cyclic lengthening of the extensor muscles. We hypothesized that the tonic attenuation of the contralateral reflex could be explained if the afferent feedback arising from the lengthening of the biarticular muscles had an increased importance in regulating the amplitude of the contralateral reflex. To test this, the passive pedaling movements were reduced to those about either the knee or hip alone. Despite the alteration in the pattern of stretching of the biarticular muscles, the contralateral soleus H reflex was tonically attenuated during both forms of single joint movements. We suggest that the same phasic afferent discharge responsible for the modulation of the ipsilateral soleus H reflex initiates the tonic attenuation contralaterally, but that the signal undergoes a complex transformation in crossing the cord. These results do not rule out the possibility that the stretching of the biarticular muscles contributes to the attenuation of the ipsilateral soleus H reflex, which is subsequently masked by a powerful influence from the stretching of the uniarticular extensor muscles. To test this possibility, a second experiment manipulated the lengths of the muscles of the leg by altering the positions of the static joints during isolated rotation of either the knee or hip and measuring the amplitude of the ipsilateral soleus H reflex. From the results, it was clear that stretching the uniarticular extensor muscles produced the most dramatic effects. However, the stretch of the biarticular muscles yielded mild inhibitory influences if these muscles were near their maximal lengths.

The H-reflex in the passive human soleus muscle is modulated faster than predicted from post-activation depression

The purpose of the present study was to investigate the influence of afferent activity (mainly homonymous Ia-afferent activity) on the modulation (post-activation depression) of the soleus H-reflex during isolated and passive sinusoidal ankle joint rotations at a speed and amplitude comparable to slow walking. The H-reflex modulation was measured in the relaxed soleus muscle on human subjects during different imposed patterns of 20 degrees haversine ankle joint rotations (0.5-0.6 Hz) while they were sitting comfortably in a chair. Eighteen healthy males and four male patients with clinically complete spinal cord lesion above the soleus motoneuron pool participated in the study. During a single dorsi-plantar flexion rotation the H-reflex was depressed to 27+/-7% (mean+/-S.E.M.) of the initial level within 600 ms. The course of this depression was reversed when the dorsi-flexion velocity started to decrease. At the end of the dorsi-flexion movement the depression was already relieved to a level of 73+/-6% of the initial level. The H-reflex returned more slowly to the initial level within 2 s after the end of the movement cycle. During two consecutive ankle joint rotations and continuous ankle joint rotations both at 0.5 Hz the H-reflex was modulated but also generally depressed while the movement was imposed. The reflex only returned to the reference level after the movements were stopped. These observations indicate the action of a fast and a slow mechanism in the post-activation depression of the soleus H-reflex. The H-reflex modulations observed in the spinal cord injured patients were comparable to the reflex modulations observed in the healthy subjects, except the depressions were smaller. This suggests that a major part of the amplitude of the H-reflex modulation observed in healthy subjects was caused by peripheral and spinal influences. The fast 500 ms recovery of the H-reflex had a time course comparable to presynaptic inhibition. The slow 2 s recovery after the end of a given imposed movement may be explained by a change in the probability of transmitter release from the homonymous soleus Ia-afferent synaptic terminals after repeated activations.

Crossed inhibition of the soleus H reflex during passive pedalling movement

We hypothesized that sensory input from the moving leg induces presynaptic inhibition of the soleus H reflex pathway in the contralateral stationary leg. The results showed a crossed inhibition during passive pedalling movement of the leg, which was not removed by low levels of tonic contraction of soleus in the stationary leg. The inhibition was correlated exponentially to the rate of the movement (R2 = 0.934, P < 0.05) and was not dependent on the quadrants through which the moving leg was passing. Static flexion of the stationary leg caused ipsilateral inhibition of the reflexes (t = 5.590, P < 0.05), independent of the orientations of the other leg. We concluded that sensory inflow from the moving leg induces presynaptic inhibition in the stationary leg, that a complex transformation of the sensory input in the spinal cord or brain underlies the tonic crossed inhibition and phasic ipsilateral inhibition, and that descending motor commands exert a powerful control over these sensorimotor modulatory mechanisms.

Stretch of quadriceps inhibits the soleus H reflex during locomotion in decerebrate cats

Previously, it has been demonstrated that afferent signals from the quadriceps muscles can suppress H reflexes in humans during passive movements of the leg. To establish whether afferent input from quadriceps contributes to the modulation of the soleus H reflex during locomotion, the soleus H reflex was conditioned with stretches of the quadriceps muscle during bouts of spontaneous treadmill locomotion in decerebrate cats. We hypothesized that 1) in the absence of locomotion such conditioning would lead to suppression of the soleus H reflex and 2) this would be retained during periods of locomotor activity. In the absence of locomotion, slow sinusoidal stretches (0.2 Hz, 8 mm) of quadriceps cyclically modulated the amplitude of the soleus H reflex. The H reflex amplitude was least during the lengthening of the quadriceps and greatest as quadriceps shortened. Further, low-amplitude vibrations (48-78 micron) applied to the patellar tendon suppressed the reflex, indicating that the muscle spindle primaries were the receptor eliciting the effect. During bouts of locomotion, ramp stretches of quadriceps were applied during the extensor phase of the locomotor rhythm. Soleus H reflexes sampled at two points during the stance phase were reduced compared with phase-matched controls. The background level of the soleus electromyographic activity was not influenced by the applied stretches to quadriceps, either during locomotion or in the absence of locomotion. This indicates that the excitability of the soleus motoneuron pool was not influenced by the stretching of quadriceps, and that the inhibition of the soleus H reflex is due to presynaptic inhibition. We conclude that group Ia afferent feedback from quadriceps contributes to the regulation of the soleus H reflex during the stance phase of locomotion in decerebrate cats. This afferent mediated source of regulation of the H reflex, or monosynaptic stretch reflex, would allow for rapid alterations in reflex gain according to the dynamic needs of the animal. During early stance, this source of regulation might suppress the soleus stretch reflex to allow adequate yielding at the ankle and facilitate the movement of the body over the foot.

Modulation of H reflexes in human tibialis anterior muscle with passive movement

Significant movement-induced gain changes in H reflexes have been observed in soleus muscle following passive movement of the lower limb. Hypotheses from these concepts were tested on magnitudes of H reflexes in tonically contracted tibialis anterior. From eleven subjects at rates of 20 and 60 r.p.m. passive leg movement, statistically significant attenuation from controls and phasic modulation occurred. The results make more general the conclusions from soleus H reflexes. However, the functional effect should be much smaller, as tibialis anterior H reflexes are smaller compared to those in soleus.

Sensori-sensory afferent conditioning with leg movement: gain control in spinal reflex and ascending paths

Studies are reviewed, predominantly involving healthy humans, on gain changes in spinal reflexes and supraspinal ascending paths during passive and active leg movement. The passive movement research shows that the pathways of H reflexes of the leg and foot are down-regulated as a consequence of movement-elicited discharge from somatosensory receptors, likely muscle spindle primary endings, both ipsi- and contralaterally. Discharge from the conditioning receptors in extensor muscles of the knee and hip appears to lead to presynaptic inhibition evoked over a spinal path, and to long-lasting attenuation when movement stops. The ipsilateral modulation is similar in phase to that seen with active movement. The contralateral conditioning does not phase modulate with passive movement and modulates to the phase of active ipsilateral movement. There are also centrifugal effects onto these pathways during movement. The pathways of the cutaneous reflexes of the human leg also are gain-modulated during active movement. The review summarizes the effects across muscles, across nociceptive and non-nociceptive stimuli and over time elapsed after the stimulus. Some of the gain changes in such reflexes have been associated with central pattern generators. However, the centripetal effect of movement-induced proprioceptive drive awaits exploration in these pathways. Scalp-recorded evoked potentials from rapidly conducting pathways that ascend to the human somatosensory cortex from stimulation sites in the leg also are gain-attenuated in relation to passive movement-elicited discharge of the extensor muscle spindle primary endings. Centrifugal influences due to a requirement for accurate active movement can partially lift the attenuation on the ascending path, both during and before movement. We suggest that a significant role for muscle spindle discharge is to control the gain in Ia pathways from the legs, consequent or prior to their movement. This control can reduce the strength of synaptic input onto target neurons from these kinesthetic receptors, which are powerfully activated by the movement, perhaps to retain the opportunity for target neuron modulation from other control sources.