Manual cervical traction reduces alpha-motoneuron excitability in normal subjects

The excitability of the Flexor Carpi Radialis alpha-motoneuron pool following manual cervical traction was assessed in twenty asymptomatic subjects, and compared to a hands only intervention. The excitability of the alpha-motoneuron pool was measured indirectly using the Hoffmann (H) reflex. H-reflex recruitment curves were taken to assess the number of alpha-motoneurons (alpha-motoneurons) firing in response to a given incremental increase in stimulation intensity. The rate of rise of the slope of the H-reflex recruitment curve (Hslp) was assessed using linear regression. Following manual cervical traction Hslp was significantly lower than pre-intervention trials. Manual cervical traction, therefore, reduced the excitability of the Flexor Carpi Radialis alpha-motoneuron pool. This effect was mediated by the central nervous system. There was no significant decrease in alpha-motoneuron excitability following the hands only intervention. Hslp was shown to be a more sensitive measure of changes in the H-reflex than the more traditional parameter of Hmax/Mmax ratio and should be used in future studies of this nature.     

Effect of voluntary contraction intensity on the H-reflex and V-wave responses

This study examined the evolution of H-reflex and V-wave responses of soleus muscle during maximal voluntary plantar-flexor contraction. We also investigated the relationship between the V response and force level and between V-wave during maximal voluntary contraction (MVC) and the maximal H reflex at rest. The H-reflex and the V-wave responses are measures of motoneuron excitability and also reflect the magnitude of presynaptic inhibition on Ia afferents and the magnitude of descending motor drive. Both may be influenced by postsynaptic inhibition. Twenty male subjects participated in the study and were assigned to one of two groups. The maximal M wave (Mmax) was evoked at rest in the 20 subjects, who then performed 10 maximal voluntary contraction. During MCV performance, a stimulus was delivered at supra-maximal intensity, which allowed us to record the superimposed M wave (Msup) and V wave of the soleus muscle. These parameters were also recorded during sub-maximal contractions (20, 40, 60, 80% of one MVC) in 10 subjects. The maximal H reflex (Hmax), was evoked at rest in the other 10 subjects. These subjects then performed 10 MVC and the Hsup (superimposed H, evoked by means of stimulus at Hmax intensity) was recorded. The results show that the amplitude of maximal M wave increased during MVC (gain 44.52 +/- 10.71%). No significant difference between Hmax/Mmax at rest and the Hsup/Msup ratios during MVC was observed, while an effect of force level on the V/Msup ratio was found. V/Msup and Hmax/Mmax were linearly correlated (r2 = 0.81), but V/Msup was significantly lower (P < 0.01) than Hmax/Mmax. In conclusion, the present study shows that maximal voluntary contractions potentiate some reflex responses. The V wave, which reflects motoneuron excitability presynaptic inhibition of Ia afferents and the magnitude of descending central motor drive to spinal motoneurons, may be a relatively simple method to analyse the modulation adaptive neural alterations at spinal and supraspinal level during voluntary contractions.     

Comparison of human triceps surae H-reflexes obtained from mid and distal recording sites

The purpose of this study was to determine whether a distal triceps surae recording site produced H-reflex responses similar to those obtained from the conventional soleus location in 10 healthy subjects at rest. H-reflex and M-response recruitment profiles were generated and the peak-to-peak amplitudes were measured from the two sites simultaneously. Following this, an H-reflex control signal was determined (peak-to-peak amplitude approximately equal to 30% Mmax) and 4 series of 10 trials were recorded at 5 minute intervals. A paired t-test showed non-significant (p greater than 0.05) differences in Hmax, Mmax, and Hmax: Mmax values recorded from the two sites. Linear regression analyses of recruitment profile H-reflexes and M-responses, obtained simultaneously from the two recording sites showed strong correlations (r greater than 0.90, p less than 0.001). To assess the stability of the reflex over time, intraclass correlation coefficients (ICC) were computed for each recording site (R = 0.99 in both instances). Thus the distal recording site, just proximal to the flare of the medial malleolus, is an acceptable alternative to the conventional recording location. This permits recording H-reflex amplitude changes, resulting from the application of a therapeutic intervention to the calf muscles, without having to remove and reposition the recording electrodes.     

Birth hypoxia and spinal reflex in newborn babies

Birth hypoxia is detrimental to neuronal function. In this study, its effect on spinal monosynaptic reflex was investigated on two different age groups of human newborn babies using few non-invasive electrophysiological parameters. A total of 57 newborns (25 hypoxic + 32 non-hypoxic) were the study subject. Out of which, 31 newborns (11 hypoxic + 20 non-hypoxic) were examined within 48 hours of birth, and the rest 26 (14 hypoxic + 12 non-hypoxic) were examined between 48h and 120h (five days) of birth. H-reflex latency (HRL), distal motor latency (DL), and H-reflex conduction velocity (HRCV), were estimated for understanding the transmission of impulse in the monosynaptic reflex pathway. Moreover, Hmax, Mmax, and H/M ratio were observed for studying the spinal motor neuronal excitability. The injurious effect of hypoxia was detected on HRL, HRCV, Hmax and H/M ratio in babies who were examined within 48h of birth. The HRL was significantly increased and other parameters were significantly reduced. In the older group of babies, however, Hmax was the only parameter affected by hypoxia. The relatively older babies of hypoxic or non-hypoxic group had higher Hmax and H/M ratio compared to younger ones of their own group.     

Changes in central delay of soleus H reflex after facilitatory or inhibitory conditioning in humans

1. Central delay (CD) changes after facilitatory or inhibitory conditioning of the soleus H reflex have been investigated in a group of normal subjects as a function of the conditioning and test stimulus intensities and also as a function of the Hmax/Mmax ratio. Both facilitation and inhibition of the reflex response have been obtained by conditioning stimulation of the ipsilateral tibial nerve at suitable conditioning-test stimulus intervals. CD changes have been extrapolated from the variations of the time interval between afferent and efferent neural volleys underlying the H reflex, directly recorded from the sciatic nerve. 2. The CD was significantly decreased by facilitatory and increased by inhibitory conditioning. Facilitatory CD changes were positively related to test stimulus strength (at a given conditioning stimulus intensity) and negatively related to conditioning stimulus strength (at a given test stimulus intensity). Both trends were reversed after inhibitory conditioning. The effectiveness of facilitatory conditioning was positively related to the individual Hmax/Mmax ratio whereas a negative relationship could be observed after inhibitory conditioning. 3. Also, the "conditioning threshold" (the minimal conditioning stimulus strength affecting the reflex size) and the "maximal conditioning effect" (the conditioning stimulus intensities leading to either the saturation of the facilitatory effect or the suppression of the reflex response) were significantly related to the Hmax/Mmax ratio. 4. We suggest that temporal changes in the H reflex pathway after facilitatory or inhibitory conditioning stimuli depend both on the size of the motoneuronal pool underlying the reflex response, as determined by the test stimulus intensity, and on the individual excitability of the motoneuronal pool, as defined by the Hmax/Mmax ratio.(ABSTRACT TRUNCATED AT 250 WORDS)     

Is there any relationship between the Modified Ashworth Scale scores and alpha motoneuron excitability indicators?

The purpose of this study was to investigate the relationship between the modified Ashworth scale (MAS) scores and alpha motoneuron excitability indicators. Thirty-one post-stroke patients were assessed for this object. The main outcome measures were the MAS and electro physiologic assessments. The latter was performed using both conventional (Hmax/Mmax) and new (Hslope/Mslope) measures of spinal excitability. Data on thirty-one adult subjects with hemiplegia (twenty-five men and six women) were analysed. The soleus Hmax/Mmax ratio appeared to correlate directly with the MAS scores (r = 0.36; P < 0.05). Correlation between the MAS scores and either Hslope/Mslope ratio or H-reflex latency was not significant (P > 0.05). In seventeen patients whose H-reflex could be evoked bilaterally, spinal excitability indicators showed significant difference between the affected and non-affected sides (P < 0.05). Based on the results of this study, there is no relationship between the MAS scores and the preferred measure of alpha motoneuron excitability. This research suggests that the MAS could not distinguish between the reflexive and non-reflexive components of the hypertonicity in ankle plantar flexors.     

The effect of Bobath approach on the excitability of the spinal alpha motor neurones in stroke patients with muscle spasticity

A clinical study was performed to evaluate the efficacy of the Bobath approach on the excitability of the spinal alpha motor neurones in patients with poststroke spasticity. Ten subjects ranging in age from 37 through 76 years (average 60 years) with ankle plantarflexor spasticity secondary to a stroke were recruited and completed the trial. They had physiotherapy according to Bobath concept for ten treatment sessions, three days per week. Two repeated measures, one before and another after treatment, were taken to quantify clinical efficacy. The effect of this type of therapy on the excitability of alpha motor neurones (aMN) was assessed by measuring the latency of the Hoffmann reflex (H-reflex) and the Hmax/Mmax ratio. The original Ashworth scale and ankle range of motion were also measured. The mean HmaxlMmax ratio on the affected side at baseline was high in the study patients. However, there were no statistically significant differences in the HmaxlMmax ratio or in the H-reflex latency between the baseline values and those recorded after therapy intervention. Before treatment, the HmaxlMmax ratio was significantly higher in the affected side than in the unaffected side. However, it was similar at both sides after treatment. Following treatment, the significant reduction in spasticity was clinically detected as measured with the original Ashworth scale. The ankle joint active and passive range of motion was significantly increased. In conclusion, Bobath therapy had a statistically significant effect on the excitability of the aMN in the affected side compared to the unaffected side in stroke patients with muscle spasticity.     

Changes in hip position modulate soleus H-reflex excitability in man

The effects of hip flexion and extension on the ipsilateral soleus Hoffmann (H) reflex recruitment curve were studied in 11 healthy subjects. Hip flexion (50 degrees), but not hip extension (15-20 degrees), produced changes in the H-reflex. A maintained facilitation, peaking at intensities of stimulation producing a maximal H-reflex (Hmax), was observed in 6/18 sessions. Inhibition, peaking at intensities submaximal for Hmax, was seen in 7/18 sessions. In some of the latter experiments, there was also a facilitation at high intensities of stimulation (greater than Hmax). The remaining experiments were classified as showing no effect: 3 were unmodulated but 2 showed a facilitation at high intensities of stimulation (greater than Hmax). Since the knee was extended in the test position, a second series of experiments (n = 7) were carried out to determine the possible influence of stretch of the biarticular hamstrings muscle group on the soleus H-reflex by comparing the effects of hip flexion with the knee extended with those obtained when the knee was flexed, thereby relaxing the hamstrings. The results provided no evidence that the variability could be explained by differences in the relative degree of stretch on the hamstrings muscle group. There were, however, systematic variations in the shape of the corresponding control H-reflex recruitment curves between subjects: the mean slope of the rising limb of the recruitment curve was highest in those experiments showing an inhibition, intermediate in the ineffective experiments and lowest in those showing a maintained facilitation. These observations indicate that the reflex output studied was different in the three groups, possibly reflecting differences in the relative proportions of slow- and fast-twitch motor units contributing to the reflex response.     

Characteristics of H- and M-waves recorded from rat forelimbs

The Hoffman reflex (H-reflex) is a useful tool for studying the functional aspects of the spinal cord without anesthesia and/or damage to the body. H-reflex studies are performed mainly in the hindlimbs. The purpose of the present study was to evaluate the characteristics of the H-reflex in the forelimbs and hindlimbs in rats anesthetized with ketamine–HCl. H- and M-waves were recorded from the interosseous muscles after electrical stimulation of the n. lateral plantar of the hindlimb and n. medialis of the forelimb. Hmax/Mmax values were significantly smaller in the forelimbs than in the hindlimbs. Furthermore, paired-pulse attenuation tended to be stronger in the forelimbs than in the hindlimbs. These findings suggest that control by descending and/or propriospinal pathways is stronger in the forelimbs than in the hindlimbs in rats.     

Therapeutic ultrasound in the treatment of ankle plantarflexor spasticity in a unilateral stroke population: a randomized, single-blind, placebo-controlled trial

Spasticity is a common and disabling symptom associated with Upper Motor Neuron Syndrome. The current methods of treatment for muscle spasticity have been claimed as unsatisfactory. Therapeutic ultrasound (US) is a common therapeutic modality in physiotherapy used for treating a wide variety of disorders. The aim of present study was to quantify the effects of continuous US on ankle plantarflexor spasticity in a randomized, single-blind, placebo-controlled trial. Twelve stroke patients (6 women and 6 men) randomly allocated to ultrasound (n = 6) or placebo (n = 6) groups. The patients were treated with either US or sham US for three days per week, every other day for 15 treatment sessions. The primary efficacy measures were the H-reflex and the Ashworth Scale (AS). In experimental group there was a significant reduction in the Hmax/Mmax ratio (p = 0.03). The Hmax/Mmax ratio was increased in the placebo group patients (p > 0.05). The difference between two groups was statistically significant (p = 0. 02). In both groups there was a reduction in the posttreatment AS scores. The Ashworth scores demonstrated statistically significant changes in the US group (p = 0.04). There was no statistical difference, however, when these two groups were compared (p > 0.05). Results from the present study show that treatment with US can reduce HmaxlMmax ratio as a measure of alpha motoneuron excitability and spasticity measure of AS in stroke patients with ankle plantarflexor spasticity.     

Effect of voluntary contraction with electrical stimulation to antagonist muscle on agonist H-reflex

OBJECTIVE: Hybrid exercise (HE) was designed to use the force generated by an electrically stimulated antagonist to provide resistance to a volitionally contracting agonist. The purpose of this study was to measure and compare the soleus H-reflex before and after HE or conventional resistance exercise (CRE). METHODS: The experiments were carried out in 18 healthy subjects (5 men and 13 women; 19-30 yr), who were divided into 2 groups of 9 for each protocol (HE or CRE). The exercise sessions lasted for 15 consecutive minutes. The soleus Hmax/Mmax was measured before and after the HE or the CRE. RESULTS: In the HE group, although there was no significant difference, the soleus Hmax/Mmax after the exercise increased compared with before the exercise (54.7 +/- 10.2% to 59.0 +/- 14.5%). On the other hand, the soleus Hmax/Mmax decreased in the CRE group (61.8 +/- 14.9% to 55.7 +/- 16.1%). In the rate of change of the soleus Hmax/Mmax, the result for the HE group was significantly higher than in the CRE group (108.0 +/- 11.7% and 89.1 +/- 8.0%, respectively) (p < 0.001). CONCLUSION: Our results show a clear difference of the neurophysiological mechanism between HE and CRE. Thus, HE might not be an alternative method for CRE.     

Changes in alpha-motoneuron excitability with positions that tension neural tissue

The slump test assesses the contribution of neural tissue to the referred symptoms associated with spinal pain and musculo-skeletal injuries of the lower limb. The limitation to full range of movement in performing this test has, in the past, been attributed to a mechanical restriction in mobility of neural tissue. Recent literature suggests that the limitation may be caused by protective reflex muscle action. The purpose of this study was to establish whether the slump test was associated with an increase or a decrease in excitability of alpha-motoneurons and, therefore, an alteration in muscle activity at the end of the range of movement of the test. Forty-three normal subjects and eight subjects with abnormal neural tension participated in this study. Changes in alpha-motoneuron excitability in neck flexion, moderate slump, and maximum slump positions were assessed by observing changes in H-reflex recruitment curves. Linear regression analysis on the rising portion of the H-reflex recruitment curve enabled calculation of the dependent variable Hslp for statistical analysis. Normal subjects in the moderate and maximum slump positions demonstrated a significant decrease (p < 0.05) in the slope of the H-reflex recruitment curve. Subjects with abnormal neural tension showed a non-significant increase in slope when in these positions. Subject flexibility had a significant influence on motoneuron excitability in the moderate neural tension position with inflexible subjects demonstrating a significant inhibition of motoneurons. The difference between the flexible or moderately flexible subjects and inflexible subjects was not significant in the maximum neural tension position. These findings have important implications for the rationale for treatment selection and success of treatment outcomes in the clinical setting.     

Inhibition of human motoneurones, probably of Renshaw origin, elicited by an orthodromic motor discharge

1. The pattern of variations of a test H-reflex after a conditioning H-reflex was investigated in human subjects by an experimental design in which both reflexes involved the same soleus motoneurones. This was made possible by using a method based upon a collision in the motor axons between the orthodromic conditioning reflex volley and the antidromic volley elicited by a test stimulus supramaximal for the motor axons.

2. The variations of the test reflex amplitude seen when increasing the conditioning reflex discharge were studied. This was made possible by facilitating the conditioning reflex without changing the strength of the afferent volley. This facilitation was obtained through a soleus stretch elicited by a stimulation of the plantar nerves.

3. The amplitude of the test reflex depended only on the size of the conditioning reflex discharge.

4. As long as the conditioning reflex was of low amplitude, all the motoneurones responsible for the conditioning response could be activated by the test volley, even though these motoneurones were undergoing after-hyperpolarization. This indicates that, in man, the after-hyperpolarization of the most excitable motoneurones can be completely overcome by a large Ia afferent volley.

5. Increasing the conditioning reflex beyond a specific value resulted in an absolute decrease in the number of motoneurones involved in the test reflex. The amount of this decrease was related only to the amplitude of the conditioning reflex.

6. This inhibition decreased progressively as the time interval separating the test stimulus from the conditioning stimulus increased. The time course of this inhibition was studied with conditioning reflexes of different amplitudes. The duration of the inhibition increased with the size of the conditioning reflex.

7. These results strongly suggest that Renshaw cells excited by the conditioning reflex are responsible for this inhibition. The results are in agreement with observations made in animals on recurrent inhibition.

     

Comparison of linear regression and probit analysis for detecting H-reflex threshold in individuals with and without spinal cord injury

A major challenge to understanding spinal reflex organization in health and disease is identifying sensitive measures of reflex excitability. The purpose of this study was to determine whether linear regression or probit analysis techniques are more sensitive for detecting H-reflex and M-wave threshold and for identifying differences in H-reflex threshold in individuals with and without spinal cord injury (SCI). Soleus H-reflex recruitment curves were generated in 9 individuals with SCI and 20 able-bodied individuals. H-reflex and M-wave threshold was estimated using three different methods, two that used linear regression of H-reflex peak-to-peak amplitude and one that used probit analysis of quantal H-reflexes. Results indicate that in both groups all three techniques were equally sensitive for detecting H-reflex but not M-wave threshold. When H-reflex threshold was normalized to M-wave threshold, different techniques provided different estimates of H-reflex threshold. However, between-group differences (SCI vs. able-body) in H-reflex and M-wave threshold were not affected by the measurement techniques that were compared in this study. We conclude that these techniques provide equally sensitive estimates of H-reflex but not M-wave threshold in persons with and without SCI. Hence, caution should be used when interpreting normalized and non-normalized values of H-reflex threshold.     

Contribution of M-waves and H-reflexes to contractions evoked by tetanic nerve stimulation in humans

Tetanic neuromuscular stimulation evokes contractions by depolarizing motor axons beneath the stimulating electrodes. However, we have shown that extra torque can develop due to the discharge of spinal neurons recruited by the evoked sensory volley. The present experiments investigated whether extra torque in the ankle plantar- and dorsiflexors was associated with enhanced H-reflexes. The tibial and common peroneal nerves were stimulated using 7-s trains (20 Hz for 2 s, 100 Hz for 2 s, 20 Hz for 3 s). Extra torque was defined as significantly more torque during 20-Hz stimulation after the 100-Hz burst (time2) than before it (time1). In 9 of 11 subjects, extra plantarflexion torque developed during stimulation just above motor threshold. In these nine subjects, torque increased from 8 to 13% MVC (time1 to time2), the soleus H-reflex increased from 13 to 19% Mmax and the M-wave of approximately 2% Mmax did not change significantly. To evoke extra dorsiflexion torque, greater stimulation intensities were required. In 6 of 13 subjects, extra torque developed at intensities that evoked an M-wave of 5-20% Mmax at time1. In these six subjects, torque doubled from 2 to 4% MVC (time1 to time2), whereas tibialis anterior (TA) H-reflexes and M-waves did not change significantly (H-reflex from 0.8 to 2% Mmax; M-wave from 12 to 14% Mmax). In 7 of 13 subjects, extra torque developed at higher stimulation intensities (35-65% Mmax). In these seven subjects, torque increased from 13 to 20% MVC, whereas TA H-reflexes and M-waves were not significantly different (H-reflex from 0.7 to 1% Mmax; M-wave from 49 to 54% Mmax). Thus enhanced H-reflexes contributed to extra plantarflexion, however, other factors generated extra dorsiflexion.     

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.     

Effect of rhythmic arm movement on reflexes in the legs: modulation of soleus H-reflexes and somatosensory conditioning

During locomotor tasks such as walking, running, and swimming, the arms move rhythmically with the legs. It has been suggested that connections between the cervical and lumbosacral spinal cord may mediate some of this interlimb coordination. However, it is unclear how these interlimb pathways modulate reflex excitability during movement. We hypothesized that rhythmic arm movement would alter the gain of reflex pathways in the stationary leg. Soleus H-reflexes recorded during arm cycling were compared with those recorded at similar positions with the arms stationary. Nerve stimulation was delivered with the right arm at approximately 70 degrees shoulder flexion or 10 degrees shoulder extension. H-reflexes were evoked alone (unconditioned) or with sural or common peroneal nerve (CP) conditioning to decrease or increase soleus IA presynaptic inhibition, respectively. Both conditioning stimuli were also delivered with no H-reflex stimulation. H-reflex amplitudes were compared at similar M-wave amplitudes and activation levels of the soleus. Arm cycling significantly reduced (P < 0.05) unconditioned soleus H-reflexes at shoulder flexion by 21.7% and at shoulder extension by 8.8% compared with static controls. The results demonstrate a task-dependent modulation of soleus H-reflexes between arm cycling and stationary trials. Sural nerve stimulation facilitated H-reflexes at shoulder extension but not at shoulder flexion during static and cycling trials. CP nerve stimulation significantly reduced H-reflex amplitude in all conditions. Reflexes in soleus when sural and CP nerve stimulation were delivered alone, were not different between cycling and static trials; thus the task-dependent change in H reflex amplitude was not due to changes in motoneuron excitability. Therefore modulation occurred at a pre-motoneuronal level, probably by presynaptic inhibition of the IA afferent volley. Results indicate that neural networks coupling the cervical and lumbosacral spinal cord in humans are activated during rhythmic arm movement. It is proposed that activation of these networks may assist in reflex linkages between the arms and legs during locomotor tasks.     

Effects of intrathecal clonidine injection on spinal reflexes and human locomotion in incomplete paraplegic subjects

We studied the effect of the intrathecal (i.t.) injection of clonidine (30, 60 and 90 μg) on the polysynaptic spinal reflexes (PSR) elicited by electrical stimulation of flexor reflex afferents (FRA), monosynaptic reflex and gait of 11 subjects with spinal cord injuries. The effect of clonidine administration on gait velocity, stride amplitude and duration was measured in eight subjects who were able to walk. Five subjects were able to walk after intrathecal injection of clonidine and three were not able to stand up. Three subjects improved their gait velocity after clonidine administration; one (S6) increased his stride amplitude; the two others decreased their cycle durations. The tibialis anterior seemed to be more regularly activated during gait. Spasticity was reduced dramatically (P<0.0001) after i.t. clonidine injection, but there was no statistically significant difference in the soleus H reflex (no effect on Hmax/Mmax). Clonidine administration decreased the amplitude of the early PSR (90–120 ms, N=4) and the threshold and maximal integrated EMG corresponding to the late response (140–450 ms, N=7). This effect was dose dependent (30, 60 and 90 μg). Placebo injection (N=4) caused no change. The changes in spinal reflexes, with a large reduction in spasticity, no change in motoneurone excitability and a large decrease in PSR, suggest that clonidine acts at a premotoneuronal level, possibly by presynaptic inhibition of group II fibres. The increase in gait velocity in three subjects could have been due to reduced spasticity or activation of spinal circuitry. Received: 30 June 1998 / Accepted: 1 June 1999     

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.     

Changes in the excitability of the H-reflex in wrist flexors related to the prone or supine position of the forearm in man

The effect of the forearm position, prone vs. supine, on the excitability of the H-reflex in flexor carpi radialis (FCR) muscle was tested in nine adult volunteers by comparing the recruitment profiles of the H and M waves. The H-reflex size, normalized to the maximal M response, was lower when the forearm was supine than when it was prone, with an average reduction of about 50% over most of the H-recruitment curve. In three wrist positions, intermediate between prone and supine, the amount of reflex attenuation was related to the prono-supination angle. Control experiments excluded that the changes in the H reflex excitability were due to displacements of the stimulating or recording electrodes.