Influences of neck receptors on soleus motoneuron excitability in man

Relationships between changes in lateral vestibulospinal tract excitability and postural control by dynamic balance intervention in healthy individuals: A preliminary study

Introduction

We conducted dynamic balance or static intervention on healthy young adults to examine the changes in lateral vestibulospinal tract (LVST) excitability and postural control that ensued following dynamic balance intervention and to investigate the correlation between these changes.

Methods

Twenty-eight healthy young adults were randomly assigned to either the dynamic balance group or the control group. They performed either a dynamic balance or static intervention for 10 trials of 30 s each and were assessed for head jerks during the intervention to confirm adaptation to the intervention. The dynamic balance intervention consisted of maintaining balance on a horizontally unstable surface, whereas the control intervention involved standing in the same foot position as the dynamic balance intervention on a stable surface while completing a maze task. LVST excitability and postural stability were assessed before and after the interventions. LVST excitability was assessed as the change rate in the soleus H-reflex amplitude with galvanic vestibular stimulation (GVSH). The velocity and area of the center of pressure (COP) were examined in the eyes closed/foam rubber condition.

Results

No significant main and interaction effects (task, time) were observed for GVSH and COP variables. In the dynamic balance intervention, head jerk significantly decreased, and GVSH-change and changes in head jerk and COP area were significantly negatively correlated.

Discussion

The LVST excitability change for the dynamic balance intervention varied among the participants, although increased LVST excitability may have been related to increased postural stability.

Reliability and laterality of the soleus H-reflex following galvanic vestibular stimulation in healthy individuals

The vestibulospinal tract (VST) plays an important role in the control of the ipsilateral antigravity muscles, and the balance of left and right VST excitability is important in human postural control. A method for measuring VST excitability is the application of galvanic vestibular stimulation (GVS) before tibial nerve stimulation that evokes the soleus H-reflex; the change rate of the H-reflex amplitude is then evaluated. Assessments of VST excitability and the left and right balance could be useful when determining the pathology for interventions in postural control impairments. However, the reliability and laterality of this assessment have not been clarified, nor has its relationship to postural control. We investigated the reliability, laterality and standing postural control in relation to the degree of facilitation of the H-reflex following GVS in 15 healthy adults. The assessments were performed in two sessions, one each for the left- and right-sides, in random order. The inter-session reliability of the short-interval assessments of an increase in the H-reflex following GVS on both sides were sufficient. The degree of H-reflex facilitation by GVS showed no significant difference between the left- and right-sides in any session. There was a moderate positive correlation between the mediolateral position of the center of pressure in the eyes-closed standing on foam condition and the left/right ratio of the degree of increased H-reflex in the first-session. We concluded that this method for evaluating the increase in the soleus H-reflex following GVS has high inter-session reliability in the short-interval that did not differ between sides.

Posture influences on vestibulospinal tract excitability

The human vestibulospinal tract has important roles in postural control, but it has been unknown whether vestibulospinal tract excitability is influenced by the body's postures. We investigated whether postures influence the vestibulospinal tract excitability by a neurophysiological method, i.e., applying galvanic vestibular stimulation (GVS) 100 ms before tibial nerve stimulation evoking the soleus H-reflex. GVS is a percutaneous stimulation, and it has not been clarified how the cutaneous input from GVS influences the facilitation effect of cathodal GVS on the soleus H-reflex amplitude. In Experiment 1, we evaluated the effects of GVS on the soleus H-reflex amplitude of subjects in the prone, supine, and sitting positions in random order to clarify the differences in the GVS effects among these postures. In Experiment 2, to determine whether the effects of GVS in the supine and sitting positions are due solely to cutaneous input from GVS, we provided GVS and cutaneous stimulations as conditioning stimuli and compared the effects in both postures. Interaction effects between postures and stimulus conditions were observed in both experiments. The facilitation rate of the maximum H-reflex amplitude by GVS in the sitting position was significantly higher than those in the prone and supine positions (Experiment 1). The facilitation rate of GVS was significantly larger than the cutaneous stimulation only in the sitting position (Experiment 2). These results indicate that vestibulospinal tract excitability may be higher in the sitting position than in either lying position (prone and supine), due mainly to the increased need for postural control.

Can neck muscle spindle afferents activate fusimotor neurons of the lower limb?

INTRODUCTION

We investigated whether vibratory stimulation of the dorsal neck muscles activates fusimotor neurons of lower limb muscles in relaxed human subjects.

METHODS

The triceps surae (TS) muscles of seated subjects (n = 15) were conditioned to leave their muscle spindles in either an insensitive (hold-long) or sensitive (hold-short) state. A vibrator (80 HZ) was applied to the dorsal neck muscles for 10 seconds. The tendon jerk was evoked from the right TS immediately before (during) or 5 seconds after (interposed) the offset of vibration.

RESULTS

The size of the reflex after hold-long muscle conditioning and after neck vibration was significantly smaller than the control hold-short reflex (P < 0.001). However, after hold-short conditioning, neck vibration significantly increased tendon jerk amplitude, both during (P = 0.001) and interposed (P = 0.026).

CONCLUSION

Dorsal neck vibration increases spinal reflex excitability of the TS in relaxed and seated subjects, but not through fusimotor excitation.

Cyclic modulation of H-reflex depression in ipsilateral and contralateral soleus muscles during rhythmic arm swing

The objective of the present study was to investigate the effects of rhythmic arm swing on ipsilateral and contralateral soleus motoneuron pool excitability. Ten healthy human subjects participated in this study. Soleus H-reflexes were recorded from the ipsilateral and contralateral soleus muscles while the subject swung the right arm anteroposteriorly as if during gait. The soleus H-reflex was depressed throughout the whole arm swing cycle except in the ipsilateral leg during the onset of the backward arm swing, and in the contralateral leg during the last half of the backward arm swing and the onset of the forward arm swing. The depression was cyclically modulated in accordance with the time course of the arm swing periods, and the pattern of the modulation was reciprocal between the ipsilateral and contralateral legs. This cyclical and reciprocal modulation may be related to the regulation of soleus motoneuron pool excitability during gait.

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.

Responses to whole head-and-body tilts with and without passive ankle dorsiflexion in the absence of visual feedback

OBJECTIVES

We have investigated lower limb responses in seven blindfolded healthy subjects to well controlled tilts in the standing position. Our aims were (1) to determine the effect of head acceleration magnitude on responses evoked by whole head-and-body tilts, and (2) to establish whether tilt-evoked responses are modifiable by passive ankle dorsiflexion. Whole head-and-body tilts evoked responses in the biceps femoris, medial gastrocnemius and tibialis anterior muscles.

METHODS

Seven young healthy subjects stood on a spring-activated tilting apparatus and underwent sudden whole head-and-body tilts of about 15 degrees from the vertical position, with or without passive ankle dorsiflexion. Head acceleration was recorded with a linear accelerometer and ankle angular displacement with a potentiometer. Surface EMG signals were recorded in the right biceps femoris, medial gastrocnemius and tibialis anterior muscles.

RESULTS

As the peak of head acceleration was increased from 0.5 g to 1.8 g, the frequency of occurrence of tilt-evoked responses increased from 7% to 60% of trials in the biceps femoris muscle during whole head-and-body tilts. In general, the more proximal muscle (biceps femoris) was activated before the more distal muscle (medial gastrocnemius) during whole head-and-body tilts, while the opposite pattern was found during tilt with dorsiflexion.

CONCLUSIONS

Our results indicate that the occurrence of tilt-evoked responses increases with an increase in the amplitude of tilting acceleration. This suggests that tilt-evoked responses are dependent, at least in part, on vestibular stimulation. In addition, the spatio-temporal pattern of biceps femoris and medial gastrocnemius muscle activation was opposite during whole head-and-body tilts and tilts with dorsiflexion. This finding suggests that foot/ankle somatosensory inputs can modify tilt-evoked responses.

Interaction effects of galvanic vestibular stimulation and head position on the soleus H reflex in humans

OBJECTIVES

The objective of this study is to measure the influence of galvanic vestibular stimulation and head-on-body alignment on the soleus H-reflex in prone human subjects.

METHODS

We studied changes in the amplitude of the right soleus H-reflex to monopolar monaural galvanic stimulation in 10 healthy prone lying subjects. Trials were randomly administered according to head position (left or right) and stimulus polarity (anode or cathode). We also investigated the influence of the conditioning stimuli by examining the amplitude of the H-reflex based on head position without galvanic stimulation. A one-way and two-way repeated measures analysis of variance were used to compare the mean amplitudes of the test and conditioned H-reflexes.

RESULTS

The greatest facilitation was observed when the head was turned to the left and coupled with cathodal stimulation. The largest inhibitory effect was found when the head was turned to the right and paired with anodal stimulation. However, when head left was paired with anodal stimulation or head right was coupled with cathodal stimulation, only minimal changes in amplitude were observed.

CONCLUSIONS

The results demonstrate that there is an interaction between head position and galvanic stimulus polarity when measuring the excitability of the soleus motoneuron pool in prone lying subjects.

Comparison of tibial nerve H-reflex excitability after cervical and lumbar spine manipulation

BACKGROUND

Previous investigations indicate that spinal manipulation leads to short-term attenuation of alpha-motoneuron excitability, when assessed by means of the Hoffmann reflex. Past studies, however, are limited to regional effects, such as lumbar manipulation effects on lumbar alpha-motoneuron activity.

OBJECTIVE

This study compared and contrasted the effects of cervical and lumbar spine manipulation on the excitability of the lumbar alpha-motoneuronal pool in human subjects without low back pain, and compared the effects of cervical (nonregional) and lumbar (regional) spinal manipulation on lumbar alpha-motoneuron pool excitability in healthy subjects. The specific aim of this study was to determine if the inhibitory effects on the lumbar alpha-motoneuron pool associated with spinal manipulation are limited to the specific region in which the manipulative procedure is applied, or if rostral (cervical) manipulation can also influence caudal (lumbar) motoneuron excitability.

METHOD

Thirty-six nonpatient human subjects were used to study the effect of cervical and lumbar spinal manipulation on the amplitude of the tibial nerve Hoffmann reflex, recorded from the gastrocnemius muscle. The Hoffmann reflex (H-reflex) technique allows for an indirect index of motoneuron pool excitability by means of peripheral nerve Ia-afferent fiber stimulation. Reflexes were recorded before and after spinal manipulative procedures.

RESULTS

Lumbar spinal manipulation, as measured by amplitude changes of the tibial nerve H-reflex, attenuated lumbar alpha-motoneuronal activity. Suppression of motoneuronal excitability was significant (P <.05) but transient, with a return to baseline within 60 seconds after manipulation. Cervical spinal manipulation had no significant effect on lumbar motoneuron activity.

CONCLUSION

These data indicate that the inhibitory effects of spinal manipulation on motoneuronal excitability are regional, rather than global.

Reflex Interactions during Whole Head-and-Body Tilts are Modified by Age in Humans

We have demonstrated that lower limb extensor muscle response to tibial nerve stimulation is significantly facilitated by whole head-and-body tilt in the forward direction. Our aim was to examine whether this reflex interaction is influenced by age. Reflexes were elicited in lower limb muscles by electrical stimulation (ES) of the right tibial nerve. We compared reflexes during supported stance (ESalone) and sudden forward tilting of the whole head-and-body (Tilt+ES) between 10 healthy subjects aged 66 ± 4 years and 13 subjects aged 27 ± 3 years. In young subjects the area of response evoked in the ipsilateral vastus lateralis (iVL) was significantly increased by 74% during Tilt+ES as compared with ESalone (p < 0.05). Moreover, the latency of contralateral VL and soleus muscle responses (cVL and/or cSO) was significantly shorter by 61 ms during Tilt+ES than ESalone (p < 0.01). In contrast, older subjects showed no significant increase in the excitability of iVL muscle response and cVL and/or cSO muscle responses during Tilt+ES as compared with ESalone despite the application of similar intensity of ES and head acceleration as in young subjects. Our findings showed that the interaction between ES-evoked and tilt-evoked responses in lower limb extensor muscles is modified with age, which suggests modifications of sensorimotor integration involved in balance control.

The effects of cervical traction on the soleus h reflex in stroke patients

The purpose of this study was to investigate the effects of cervical traction on the soleus H reflex amplitude in stroke patients. Cervical traction with intensity of 3 kgw was performed on six stroke patients. The soleus H reflex was evoked before, during, and after the cervical traction. All subjects showed smaller H reflex amplitudes in the second and third minutes after traction than before the traction. The results suggested that cervical traction with intensity of 3 kgw inhibited the soleus H reflex amplitude several minutes after traction in stroke patients.

Responses to dynamic head-and-body tilts are enhanced in Parkinson's disease

BACKGROUND

Previous studies demonstrated that destabilizing responses to slow perturbations were enhanced in patients with Parkinson's disease (PD). Our objectives were to investigate the influence of PD on responses to faster whole head-and-body tilts in the standing position, and to establish whether any modification of tilt-evoked responses in PD patients was related to possible changes in the modulation of soleus (SO) H-reflex.

METHODS

Ten PD patients and 10 age-matched normal subjects assumed a standing position on an L-shaped tilting apparatus. Their head and shoulders were firmly attached to the back support of the apparatus, while their feet were fixated to the standing platform. With their vision occluded, the subjects's whole head-and-body was suddenly tilted forward to 20 degrees, at a peak head acceleration of 0.7 g +/- 0.1 g. Tilt-evoked responses were recorded from the lower limb muscles bilaterally. In addition, 40 H-reflexes were elicited in the SO muscle at 30-190 ms intervals after the onset of head acceleration. The M response amplitude was kept within +/- 15% of its control value.

RESULTS

PD patients demonstrated an abnormally high responsiveness to whole head-and-body tilts in comparison with age-matched normal subjects. This was shown by the significantly larger proportion of PD patients manifesting responses in the SO, biceps femoris and vastus lateralis muscles (p < 0.05), as well as their significantly larger SO response area (413%; p < 0.01). In contrast, the amplitude of the SO H-reflex was significantly increased by only 14% (p < 0.05) in these patients, and only at 30-70 ms after head acceleration onset.

CONCLUSIONS

The overexcitable tilt-evoked responses of PD patients could originate from a reduced ability to suppress responses when the body is supported. This enhanced excitability of tilt-evoked responses was probably not due to motoneuronal hyperexcitability or decreased presynaptic inhibition of the group Ia terminals involved in the mainly monosynaptic H-reflex pathway. Thus, we hypothesize that the control of spinal interneurons involved in the tilt-evoked responses may be defective in PD.

Extensor muscle responses to tibial nerve stimulation are enhanced during dynamic tilts in standing humans

We studied the interaction between muscle responses evoked in standing by electrical stimulation (ES) of the tibial nerve and dynamic tilts of the head-and-body in 13 young healthy subjects. Subjects were attached to an L-shaped tilting apparatus and underwent sudden forward tilting of their head-and-body as a whole, without ankle rotation. During such tilts, the area of response evoked in the ipsilateral vastus lateralis (iVL) muscle by the ES was significantly increased by 74% as compared to quiet supported stance (P = 0.01). The response latency of the contralateral VL and soleus muscles i.e. the crossed extension reflex, was significantly shorter during tilt (54 +/- 22 ms) than during quiet supported stance (115 +/- 13 ms, P < 0.01). The increased excitability of extensor muscles activated by ES during tilt seems appropriate to maintain stance during a forward perturbation of the body.

The Effect of Postsurgical Edema of the Knee Joint on Reflex Inhibition of the Quadriceps Femoris

The purpose of this case study was to investigate reflex inhibition of the quadriceps femoris in a subject with postsurgical edema of the left knee. The subject was a 45-year-old male with a traumatic knee injury with resultant edema who underwent elective arthroscopic surgery. Reflex inhibition was assessed by H-reflex elicitation in the femoral nerve and surface electromyography of the quadriceps. To assess the degree of edema, direct circumferential measurements were taken. On the first presurgical visit, the left knee demonstrated mild edema with a decrease in H-reflex amplitudes. Two days after surgery, a further reduction in amplitudes and more swelling were demonstrated followed by an increase in amplitudes and a reduction in edema on the 28th postoperative day. These findings document a relationship between reflex inhibition and joint swelling that was previously described in experimental models where joint edema was simulated.

Reciprocal inhibition in writer's cramp

We studied the inhibition of median H-reflexes by conditioning stimuli on the radial nerve in 13 patients with writer's cramp, eight of the simple type and five of the dystonic type, and in 14 normal volunteers. The patients and controls were right-handed, and their right arms were studied. Asymptomatic left arms were also studied in nine of 13 patients. In the control group we identified three periods of inhibition, with maximum peaks at conditioning-test intervals of 0 ms (41 +/- 17%), 20 ms (40 +/- 13%), and 100 ms (36 +/- 20%). In the patient group, the amplitudes of inhibition of these three periods in both arms were significantly less than those in the control group. However, there were no significant differences in the amplitudes of inhibition of these three periods between symptomatic and asymptomatic arms. There were also no significant differences between simple and dystonic writer's cramps. Our results indicate that the attenuation of reciprocal inhibition was present not only in symptomatic arms but also in asymptomatic arms of patients with writer's cramp. The defect of reciprocal inhibition in the asymptomatic hand has never been documented. We suggest that the preexistent electrophysiological abnormality may provide an explanation for the development of hand cramp after shifted writing.

Evidence for soleus H-reflex depression induced by ballistic head rotation

In this study, we described a sequence of changes of the electromyographic (EMG) and depression of the soleus (Sol) H-reflex that precede and follow the onset of ballistic head rotation in freely standing human subjects. During head rotation, a sequence of EMG changes occurred simultaneously in Sol and in biceps femoris muscle (BF). Specifically, changes to Sol EMG included deactivations (EMG silent period) and H-reflex depression coincident with BF excitation. Thus, in the case of ballistic head movement, control commands for anticipatory postural adjustment appear to excite BF and inhibit Sol in a similar way to that previously described for the onset of arm movement.

Reconsidering the 90 degrees head-rotation paradigm used in neuropsychological research: are there reflexive rather than hemispatial effects?

The 90 degrees head-rotation paradigm has often been used in neuropsychological studies to manipulate external hemispace (circumcorporeal space) relative to the head. Under the 90 degrees head-rotation paradigm, the performance of limb and hand movements carried out within the left or right hemispace as defined by head positions relative to the body is likely to be affected by the reflexive effect due to the neck and vestibular afferent inputs elicited by the head rotations, as well as by the hemispatial effect. Using the H-reflex technique, the present study examined whether the reflexive effect on the spinal motoneuron excitability occurred with head rotations under the 90 degrees head-rotation paradigm. The results showed that the amplitudes of H-reflexes evoked on both the thumb flexor and soleus muscles were not affected by head rotations, indicating no reflexive change in the spinal motoneuron excitability for both the thumb and soleus muscles. This finding suggests that the reflexive effect due to neck and vestibular afferent inputs can be ruled out from possible causal factors influencing the motor performance of limb and hand movements performed within the left or right hemispace as manipulated by the 90 degrees head-rotation paradigm.

Respiratory and Head Position Influences on Late Stretch Reflexes

The purpose of this study was to determine the relative effects of the asymmetric tonic neck reflex (ATNR) and of respiration on the upper limb, as measured by their influence on the stretch reflexes. Long latency stretch reflexes (M2 and M3) were induced in elbow muscles of human subjects. The subject's right forearm was strapped into a manipulandum. The forearm was perturbed with a torque test pulse at a fixed interval prior to an intended forearm movement. It was found that both the M2 and M3 reflexes, in triceps or brachialis muscles were significantly increased when perturbations were delivered during inspiration as compared to expiration. Rotated head positions to the right or left could also significantly alter reflex magnitude but there was no consistent pattern among subjects.

The clinical implications of these findings are discussed.

The influence of tonic neck reflexes on voluntary fatiguing elbow movements in humans

Tonic neck reflexes (TNR) are often assumed to be included in the building of voluntary motor programmes. The present study was designed to test the importance of TNR in the performance by healthy human adults of high level dynamic exercise. The subjects were placed in an experimental situation similar to the original one used by previous authors, but in a fixed restrained posture. They repeatedly lifted weights by extending the elbow, with the head rotated either towards the side of the active upper limb or towards the opposite side. The rate of movement was 0.5 Hz, and the initial amplitude 60 degrees. Successive series of 15 movements separated by an equivalent period of rest were made by each subject until exhaustion. The mechanical work performed during each series of movements was calculated. Surface electromyograms (EMG) from triceps brachii, anconeus, and biceps brachii muscles were recorded simultaneously with the elbow rotation displacement and then integrated. The EMG from the sterno-cleïdo-mastoidei muscles were also recorded. With successive series of lifting, movement amplitude decreased progressively as a result of fatigue. Thus there was a decrease in the work performed. In agreement with the experimental data of the previous authors as well as with the expected effect of TNR, the decrease in work was less marked when the subjects kept their heads turned towards their moving limb. However, in contrast with the results from other authors, the facilitation was very low. From these results, we concluded that TNR can slightly influence the amount of work that a subject can produce.(ABSTRACT TRUNCATED AT 250 WORDS)

Vestibular-evoked postural reactions in man and modulation of transmission in spinal reflex pathways

1. The effects of galvanic stimulation of the vestibular apparatus (with electrodes on the mastoid processes) have been studied in standing human subjects. With the head turned to one side, subjects swayed towards the anode. 2. Forwards sway was preceded by electromyographic (EMG) activity in quadriceps and tibialis anterior muscles. Backwards sway was preceded by EMG activity in soleus and hamstring muscles. 3. Using the method of H reflex conditioning, forward sway was found to be preceded by inhibition of soleus motoneurones. 4. Interaction between the vestibular-evoked inhibition of soleus motoneurones preceding forwards sway and peripheral reflex inhibition was examined by a spatial facilitation method. 5. Interaction was found between vestibular-evoked inhibition and Ia reciprocal, group I non-reciprocal and group Ia-Ia presynaptic inhibitory pathways. It is concluded that vestibular signals converge on spinal interneurones subserving these inhibitory actions. 6. A 'decoupling' of soleus motoneurons and soleus-coupled Renshaw cells was found in the period of soleus activation preceding backwards sway.

Modulation of soleus H reflex by lateral tilting in man

Static vestibular influences on extensor tone of the lower limbs in man were studied by analyzing the changes in right soleus H-reflex (RSHR) area in relation to lateral tiltings. Eight normal adult volunteers were tested in an experimental situation designed to minimize all afferent inputs, except the vestibular ones. Each subject was seated on a chair which could be tilted laterally from the vertical to both sides. Lateral tiltings were applied at a random order from the vertical (0 degree, control position) to 4 degrees, 8 degrees, 12 degrees, 16 degrees, and 20 degrees of both sides (test positions). The results showed inhibition in SHR area of the leg ipsilateral to the tilting and facilitation of the contralateral SHR. These data indicate that, in man, as in the decerebrate cat, tonic labyrinth reflexes act asymmetrically and that, in static condition, the vestibular system modulates muscle tone of the lower limbs adequately to counteract lateral perturbation of upright position.

Interaction of tonic labyrinth and neck reflexes in man

Interaction of tonic labyrinth and neck reflexes was studied in 3 healthy volunteers by analyzing changes in Soleus H-Reflex (SHR) area in relation to both lateral tiltings and neck rotations. By using a Kermath chair each subject was tilted laterally from the vertical to the left and to the right up 15 degrees in steps of 5 degrees and at the same time the longitudinal body axis, keeping the head fixed, was rotated to the right and to the left up to 15 degrees in steps of 5 degrees. All combinations of lateral tiltings and neck rotations were tested. Each test position was followed by a return to 0 degree for both rotation and tilting (control position). Twelve H-reflexes of right soleus muscle were recorded in each test and control position and the changes in RSHR area were expressed as percentage variations from the mean value absorbed in the pretest and post-test control position. Our data indicate that in man, as in animals, labyrinth and neck reflexes act in the opposite direction, and that in the static condition their contribution to postural stabilization is equal.

Modulation of flexor carpi radialis H reflex by lateral tilts in man

Static vestibular influences on upper limb flexor tone were studied in man by analyzing the changes in flexor carpi radialis H reflex area with lateral tilting of the longitudinal body axis. Ten healthy volunteers and 2 labyrinthine defective patients were tested in an experimental situation designed to minimize all afferent inputs except vestibular ones. Each subject was seated on a chair which could be tilted laterally to the left or the right from the vertical. Head and trunk were fixed upright, upper and lower limbs in half-flexed position and forearm in an intermediate position between supination and pronation. Lateral tilting was applied at random from the vertical (0 degrees control position) to left and right (4 degrees, 8 degrees, 12 degrees, 16 degrees, 20 degrees test positions). Each test position was followed by a return to 0 degrees and in each control and test position 20 consecutive H reflexes were recorded. The data observed in the normal subjects showed flexor tone inhibition in the arm which was tilted downwards and facilitation in the contralateral arm. These findings suggest that in man, like in animals, labyrinth reflexes act asymmetrically and in the opposite direction to neck reflexes.