Movement modulation of a short latency reflex linking the lower leg and the knee extensor muscles in humans.

Movement modulation of a heteronymous short latency spinal reflex was investigated with transcutaneous, low threshold, stimuli to the common peroneal nerve, during leg pedalling in humans. Electromyographic (EMG) electrodes on the skin over the quadriceps muscles revealed an excitatory response at a minimum latency of 22.8 msec. Samples were obtained at 5 phases in the cycle of pedal crank rotation, and for stimulus intensities to evoke both maximal, and submaximal, reflexes in quadriceps. Over the pedal cycle, the peak-to-peak magnitude of the reflex, in the 3 quadriceps muscles studied, was deeply modulated, including an areflexive phase. The reflex magnitude was linearly dependent on the ongoing EMG activity in quadriceps. It could be fully re-initiated if contractions were produced out of phase in pedalling. The slopes of the regression lines of reflex magnitude (as % maximum reflex) on contraction level (% maximum voluntary contraction) when pedalling (1.05 (r = 0.86, P less than 0.01], and with isometric contractions in the same subjects seated (1.03 (r = 0.69, P less than 0.01], were very similar, with similar thresholds. This contrasts with the inhibition of soleus H reflexes during human gait. The present heteronymous reflex, acting between limb segments, is modulated coincident with ongoing contraction level in the target muscle.     

Short latency heteronymous excitatory and inhibitory reflexes between antagonist and heteronymous muscles of the human shoulder and upper limb

The aims were (i) to investigate heteronymous excitatory and inhibitory Group Ia reflexes linking agonist/antagonist muscle pairs acting at the shoulder and elbow; clavicular pectoralis major (Pmajor) and posterior deltoid (Pdeltoid); biceps brachii (Bi) and Tri brachii (Tri), and linking muscles acting at the elbow (Bi and Tri) with muscles acting at the shoulder (Pmajor and Pdeltoid). (ii) To test the hypothesis that the excitability of the reflexes would vary between different tasks in a functionally relevant manner. The study was performed on 45 adults. Reflexes were recorded in the surface EMG when the target muscle was contracting at 10% maximum voluntary contraction. Reflexes were recorded in Bi and Tri with the elbow joint in one of three positions: 105 degrees, 80 degrees, or 55 degrees from full extension. Group Ia reflexes were evoked using a small, brief tap to the tendon of the muscle being stimulated. Reflexes were recorded by cross-correlation of the surface EMG and pseudo-random series of taps. All subjects demonstrated short latency inhibition and excitation between agonist/antagonist muscle pairs; inhibition was significantly more frequent than excitation. Excitation and inhibition occurred with equal frequency between muscle pairs acting between elbow and shoulder. Minimum central delays for excitatory reflexes were 1 ms, consistent with monosynaptic projections and for inhibitory responses were 2 ms consistent with disynaptic linkage. Later excitatory and inhibitory reflexes with central delays of up to 15 ms also occurred. The probability of evoking excitation or inhibition in Tri or Bi changed with the different elbow positions.     

H-reflex modulations during voluntary and automatic movements following upper motor neuron damage

OBJECTIVES: It is not known whether similar mechanisms account for the impairments of voluntary movement and automatic postural responses of individuals with spasticity secondary to damage to the sensorimotor cortex and its projections (i.e. upper motor neuron syndrome (UMN)). METHODS: The present study examined changes in soleus H-reflexes preceding and during voluntary tibialis anterior (TA) muscle contraction of standing subjects and during balance platform induced postural perturbations that elicited similar TA muscle contractions. Twenty-two subjects (12 non-disabled; 4 with spastic-type cerebral palsy; 6 with adult-onset cerebral vascular accident) participated in the study. Data were analyzed using ANOVAs and Tukey HSD post-hoc comparison tests to assess the timing and magnitude of soleus H-reflex amplitude changes relative to the onset of TA muscle activation. RESULTS: Results indicated that, regardless of the level of TA activation, soleus H-reflexes of subjects with UMN involvement did not demonstrate inhibition either. during voluntary movements or during automatic postural perturbations. CONCLUSIONS: These findings indicate that postural reflexes, as well as volitional movements, are impaired following UMN damage and that deficits in neural pathways subserving reciprocal inhibition contribute to the impairments.     

Auditory startle (audio-spinal) reaction in normal man: EMG responses and H reflex changes in antagonistic lower limb muscles

The audiospinal reaction (ASR) to a 30 msec tone of 90 dB has been studied in 66 seated healthy volunteers. Electromyographic bursts induced in trapezius (Tra), soleus (Sol) and tibialis anterior (TA) have been looked for. Facilitation of Sol H reflex has been measured in 21 subjects in terms of delays from delivery of the sound. Among the subjects, 11 had a stable TA H reflex whose facilitation was compared to that of Sol H. Effects of selective isometric voluntary contraction of either Sol or TA were assessed both on EMG responses and H reflex facilitation.At rest, only 36% of subjects exhibited a response in Sol at a mean latency of 123 msec and 39% in TA at a latency of 119 msec. Responses were seen in antagonist muscles in 74% of subjects. Incidence in Tra was 96%. During voluntary contraction, the results were not significantly changed either in the contracted muscle or its antagonist. H reflex facilitation of both TA and Sol started 50 msec after the sound to peak after 75–125 msec and returned to baseline values after 250 msec. Extent of Sol H reflex facilitation remained similar during voluntary contraction of Sol and TA. It was observed that EMG responses were more frequent in subjects with brisk reflexes but not necessarily in those who exhibited the largest H reflex facilitation. The results are in agreement with assumptions that ASR is mediated through reticulo-spinal pathways but do not support the view that it corresponds to a flexor reaction. In addition to providing quantitative data for comparisons in pathological cases, they suggest differences in the mode of activation of motoneurones by the motor cortex and subcortical nuclei.     

Antagonist inhibition during rest and precontraction.

The excitability of antagonist soleus motoneurons was tested during fast voluntary contractions of tibialis anterior (TA). Contractions of TA started either from zero or higher levels of tonic contraction of ankle extensors. Results showed that H reflex depression under 3 different conditions: 25% (R25) or 50% (R50) of the maximal isometric dorsiflexion from the resting state and the sequential isometric response (SW) appeared about 20-40 msec prior to the EMG onset of TA. In particular, H reflex depression was clear in the SW response, and the stronger the prior contraction of extensors the greater the H reflex depression. There was a significant difference in the integrated EMG between R25 and SW but none between R50 and SW. The different amounts of inhibition found for dorsiflexion from the resting state and sequential movement is most probably explained by presynaptic inhibition. If presynaptic inhibition were increased throughout the sequential movement, in comparison with the resting state, change in the H reflex gain would occur.     

Impaired “natural reciprocal inhibition” in patients with spasticity due to incomplete spinal cord injury

Experiments were performed to compare the ability of normal subjects and patients with spinal spasticity to suppress antagonist H reflexes during isometric ankle contractions. Soleus H reflex suppression was examined during tonic pretibial muscle contractions in which the torque levels were constant and during dynamic pretibial muscle contractions in which the torque followed a predetermined ramp. As well, subjects were instructed to alternately contract ankle plantarflexors and dorsiflexors at various frequencies to examine patterns of EMG activity during rhythmically alternating isometric contractions in antagonist muscles. Patients with incomplete spinal cord injury demonstrated reduced ability to suppress soleus H reflexes during pretibial muscle contraction. At slow speeds of alternating contraction, spinal cord injured patients retained the ability to perform alternating isometric pretibial/soleus muscle contractions. The patients demonstrated abnormal coactivation in soleus muscle during faster alternating isometric ankle muscle contractions. Furthermore, the patients who demonstrated the greatest impairment in natural reciprocal inhibition, also displayed the largest amount of coactivation. In general, the results would suggest that impairment of natural reciprocal inhibition is correlated with an increase in the amount of antagonist muscle coactivation seen during alternating isometric muscle contractions.     

Surface EMG recording of heteronymous reflex excitation of semitendinosus motoneurones by group II afferents.

OBJECTIVE: A potent stretch reflex originates in muscle spindle secondary endings and is mediated by group II muscle afferents. Electrical stimulation of peripheral nerve could also induce group II facilitation, as found in single motor unit recordings for various nerve-muscle combinations in man. The aim of the present investigation was to use surface electromyogram (EMG) to record and to quantify heteronymous excitation of semitendinosus (ST) motoneurones by group II afferents of the tibial nerve. METHODS: This study included 20 healthy subjects. The conditioning stimulation of the tibial nerve was delivered at the popliteal fossa at two intensity levels and for two levels of ST contraction. EMG activity was recorded by surface electrodes placed over the ST muscle, and the averaged and rectified ST EMG signal was analyzed. RESULTS: We observed 4 periods of interest: a first period of prestimulus background EMG activity; a second period of early reinforcement of ST EMG activity starting 30 ms poststimulus and lasting for 35-45 ms; a third period of relative inhibition of ST contraction; a fourth period of late reinforcement of poststimulus ST EMG activity. CONCLUSIONS: We hypothesized that the early period of facilitation was mainly related to heteronymous excitation of ST motoneurones by group II afferents on the basis of the following observations: (i) the amount of facilitation increased for a high stimulus intensity level, which was suitable for group II recruitment; (ii) the onset latency of this facilitation was consistent with the theoretically calculated latency of a group II-mediated reflex. SIGNIFICANCE: Surface EMG recording offers various advantages compared to single motor unit recording to study group II facilitation in the ST muscle. This technique could be applied in the future to confirm that heteronymous reflex excitation by group II afferents is enhanced in spastics and plays an important role in the development of lower limb spasticity.     

Presynaptic and postsynaptic mechanisms underlying H-reflex changes produced by a selective voluntary contraction

Concurrent recordings of (i) the soleus H reflex and (ii) the underlying afferent (P₁) and efferent (P₂) neural volleys were performed during a protracted, moderate, isometric, voluntary contraction of the soleus (S) muscle, and the subsequent release period. Besides the expected enhancement of the H reflex, muscular contraction caused a significant reduction in the corresponding central delay (as extrapolated from variations of P₁–P₂ interval), while the opposite trend occurred during the release phase. Control experiments, based on (a) neural blockade below the stimulation site, (b) muscle stretching at the end of the muscular contraction, (c) changes in amplitude of homonymous and heteronymous S responses, and (d) variations in effectiveness of homonymous and heteronymous conditioning volleys on the S motoneuronal pool, showed that both voluntary contraction and the subsequent release period are associated with a reduced effectiveness of Ia afferents, while postsynaptic motoneuronal responsiveness is significantly modified only during the actual contraction time. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:439–453, 1998.     

Effect of intrathecal baclofen on the monosynaptic reflex in humans: evidence for a postsynaptic action.

Intrathecal baclofen is a very powerful antispastic agent. Its mechanism of action on the monosynaptic H-reflex in spinal patients was investigated. It could inhibit rapidly and profoundly monosynaptic reflexes in lower limbs, but did not modify Ia vibratory inhibition of the soleus H-reflex. To assess more precisely its effect on Ia afferents, an experimental paradigm using Ia heteronymous facilitation of the soleus H-reflex was used. Intrathecal baclofen did not modify the amount of monosynaptic facilitation of the soleus H-reflex brought about by stimulation of the femoral nerve. This demonstrates that the main part of the inhibitory effect of baclofen on the H-reflex in spinal patients is not due to a presynaptic effect, suggesting a postsynaptic site of action.     

Modulation of human short latency reflexes between standing and walking

Inhibition of the magnitude of soleus muscle homonymous (H) reflexes occurs in humans when walking, compared to standing. The current study asked, (1) was the task modulation of Ia reflexes limited to soleus muscle, (2) was there support for attributing a presynaptic source to the inhibition in humans and (3) did an oligosynaptic short latency reflex show similar task modulation? In 3 subjects, H reflexes were evoked in vastus medialis and soleus, at 4 levels of contraction in the target muscle, with constant stimulus intensity when walking and standing. The reflex magnitudes in both muscles were significantly inhibited during the contractions for walking, compared to standing. Such inhibition also occurred in H reflexes of tibialis anterior muscle. An excitatory oligosynaptic reflex was then evoked in vastus medialis, through low intensity stimulation of the common peroneal nerve during walking and standing. The mean amplitudes of this reflex were not significantly different (P less than 0.05) between the two conditions, at any contraction level. The depression of quadriceps H reflexes, compared to the oligosynaptic reflexes through the same quadriceps motoneuronal pool in the same task, strongly suggested that the inhibition of H reflexes arose at other sites besides the motoneuronal cell body and proximal dendrites. We conclude that Ia H reflexes of various leg muscles of humans are inhibited when walking but that this does not generalize to the oligosynaptic short latency reflex between the anterior shank and thigh.     

Clinical and electrophysiological appraisal of the significance of radicular injury in back pain.

Clinical electrophysiological studies were analysed in 60 consecutive patients with back pain with or without other evidence for a radiculopathy. These studies included needle EMG of relevant limb and paraspinal muscles as well as F responses and H reflexes recorded from the soleus muscle. Segmental denervation was found in 29 of the 60 patients. In 57 patients, abnormal slowing of the F response was present in 27, either unilaterally (25) or bilaterally (two). In 18 of 47 patients with H reflex studies, the H reflex was either unilaterally absent (12), asymmetrically prolonged (five), or bilaterally prolonged (one). Statistically significant (P less than 0.05) associations were found between (1) abnormalities of H reflexes and F responses, (2) F response slowing and radicular injury shown by EMG, (3) segmentally consistent radiographic defects and abnormalities of both H reflexes and F responses, and (4) depressed Achilles reflexes as well as sensory loss and abnormal H reflexes. No significant association, however, was present between abnormalities of EMG, F responses, or H reflexes and pain radiation by history or positive straight leg-raising tests. These data suggest that pain in radicular syndromes is related to the functioning of smaller afferent fibres.     

Modular organization of excitatory and inhibitory reflex receptive fields elicited by electrical stimulation of the foot sole in man.

OBJECTIVES: The present study aimed to investigate how the inhibitory and excitatory reflex components of the human (polysynaptic) withdrawal reflex are organized depending on the stimulation site. The reflexes were elicited during a voluntary pre-contraction (between 10 and 20% of maximum voluntary contraction) of two antagonistic muscles. METHODS: Inhibitory and excitatory reflex receptive fields to tibialis anterior (TA) and soleus (SO) were mapped in 14 healthy subjects using randomized electrical stimulation at 16 sites of the foot sole. Low, non-painful (3x perception threshold), and high, painful (1.5x pain threshold), stimulus intensities were used. RESULTS: The inhibitory reflex receptive fields were organized in a highly functional manner supporting the action of the excitatory reflex. Together the two reflexes result in an optimal withdrawal from the stimulus. Low stimulation intensity was found sufficient to elicit the inhibitory reflex. High stimulation intensity caused a reversal of the inhibition to excitation in tibialis anterior. In soleus the inhibition was facilitated for stronger intensities. CONCLUSION: In conclusion, findings in animals of a modular organization of inhibitory reflexes are reproduced in humans.     

Relief of hemiparetic spasticity by TENS is associated with improvement in reflex and voluntary motor functions.

Our previous studies showed that a single 45 min application of transcutaneous electrical nerve stimulation (TENS) prolonged soleus H and stretch reflex latencies in hemiparetic subjects. In addition, 9 daily 30 min TENS applications enhanced vibratory inhibition of the H reflex and tended to decrease hyperactive stretch reflexes. These findings suggested that longer-term TENS may be effective in reducing hemiparetic spasticity. Our present objectives were 2-fold: to determine whether longer-term repetitive TENS stimulation would lead to a reduction in clinical spasticity in hemiparetic subjects, and whether such a reduction could be associated with a decrease in stretch reflex excitability and an improvement in voluntary motor function. We compared the effects of 15 daily 60 min TENS treatments over a 3 week period, with those of placebo stimulation applied to the common peroneal nerve of the affected leg in similar groups of spastic hemiparetic subjects. Our test battery consisted of 5 measurements which assessed (1) clinical spasticity scores, (2) maximal H reflex to M response ratios, (3) vibratory inhibition of H reflex, (4) stretch reflexes, and (5) maximal voluntary isometric plantarflexion and dorsiflexion, in standing. In contrast to placebo stimulation which produced no significant effects, repeated applications of TENS over time decreased clinical spasticity (P less than 0.05), and increased vibratory inhibition of the soleus H reflex (P = 0.02) after 2 weeks. These changes occurred with a substantial improvement in voluntary dorsiflexing force up to 820%, but not plantarflexing force. They were followed by a reduction in the magnitude of stretch reflexes (P = 0.05) in the spastic ankle plantarflexor, concomitant with a decrease in the EMG co-contraction ratios after a further week of stimulation. Our results thus indicated that repeated applications of TENS can reduce clinical spasticity and improve control of reflex and motor functions in hemiparetic subjects. Furthermore, the underlying mechanisms may be due partly to an enhancement in presynaptic inhibition of the spastic plantarflexor, and partly to a possible "disinhibition" of descending voluntary commands to the paretic dorsiflexor motoneurons.     

Coexistent Hoffmann reflexes in human leg muscles are commonly due to volume conduction

The existence of "concomitant" (coexistent) electromyographic reflex responses in soleus and tibialis anterior muscles, produced by posterior tibial nerve stimulation, has been cited as evidence for "reciprocal excitation" between these antagonistic muscles normally reflexly linked by reciprocal inhibition. Using the Hoffmann reflex procedure and posterior tibial nerve stimulation, the existence of true reciprocal excitation was tested in six subjects with no neuromuscular impairment. Coexistent EMG responses were observed in all subjects. In no instance, however, could the tibialis anterior EMG volley to posterior tibial nerve stimulation of the soleus muscle be antidromically blocked by common peroneal nerve stimulation applied at 10 to 20 ms offset latencies. A second stimulation pulse applied to the common peroneal nerve at similar offset latencies did antidromically block a tibialis anterior reflex response to common peroneal nerve stimulation. Therefore, volume conduction of reflex activity from the posterior tibial compartment to the anterior tibial compartment was a common observance. We suggest that coexistent EMG reflex responses, presumed to reflect reciprocal excitation, should be tested by the procedure described to reject the possibility of EMG cross-talk as a confounding variable or as the actual variable under investigation.     

Stance control is not affected by paresis and reflex hyperexcitability: the case of spastic patients

OBJECTIVES: Spastic patients were studied to understand whether stance unsteadiness is associated with changes in the control of voluntary force, muscle tone, or reflex excitability, rather than to abnormal posture connected to the motor deficit itself. METHODS: Twenty four normal subjects, 12 patients affected by amyotrophic lateral sclerosis (ALS), seven by spastic paraparesis, and 14 by hemiparesis were studied. All patients featured various degrees of spasticity and paresis but were free from clinically evident sensory deficits. Body sway during quiet upright stance was assessed through a stabilometric platform under both eyes open (EO) and eyes closed (EC) conditions. The sudden rotation of a supporting platform, in a toe up and toe down direction respectively, evoked short (SLR) and medium latency (MLR) reflex responses to stretch of the soleus or the tibialis anterior (TA) muscle. RESULTS: No relation was found between clinical findings (tone, muscle strength, tendon reflexes, plantar response, and duration of disease) and body sway. On average, all patient groups exhibited a forward shift of the centre of foot pressure (CFP) with respect to normal subjects; in addition, paraparetic and to a much larger extent hemiparetic patients showed a lateral shift of CFP. Body sway area was significantly increased only in the hemiparetic patients. No relation was found between position of the CFP and sway within any patient group. Soleus SLR was increased in all patients with respect to normal subjects. TA SLR was often seen in both patients with ALS and paraparetic patients, but only rarely in normal subjects and hemiparetic patients. However, no relation was found between amplitude of soleus or TA SLRs and stabilometric variables. The frequency and size of soleus MLR and TA MLR were decreased in all patients. These responses were decreased in size and not modulated by background EMG in the affected leg of hemiparetic patients, suggesting a disturbed control of spinal reflexes fed by spindle group II afferent fibres. CONCLUSIONS: It is proposed that body posture, paresis, or monosynaptic reflex hyperexcitability do not affect the control of equilibrium during quiet upright stance. In hemiparetic patients, the decreased amplitude of MLRs might be the main cause of the large postural instability. The results are congruent with the hypothesis of a role for group II afferent input in the reflex control of equilibrium.     

Characteristics of parkinsonian and ataxic gaits: a study using surface electromyograms, angular displacements and floor reaction forces

To clarify the characteristics of parkinsonian and ataxic gaits, we analyzed electromyograms (EMGs) of the thigh and leg muscles, angular displacements of the hip and leg joints, and floor reaction forces during free walking for each gait phase in 16 patients with Parkinson's disease (PD) and 14 ataxic patients with cerebellar degenerations. We studied 17 healthy elderly subjects whose walking speed was similar to that of patients with moderate disease. Free walking by PD patients was characterized by low maximum activity of the gastrocnemius/soleus (GC) and tibialis anterior (TA) muscles. Ataxic patients showed high activity of GC and TA during the period when these muscles were not active in normal walking. The ratio of changes of EMG of the distal muscles to changes in angular displacement of the ankle (DeltaEMG/Deltaangle) was reduced in GC of PD patients in ankle dorsiflexion, whereas it was high in GC and TA of ataxic patients in ankle dorsiflexion and plantarflexion, respectively. Changes in DeltaEMG/Deltaangle coincided with those in proprioceptive reflexes reported previously. Our results showed that measurement of EMG for each phase revealed disease-specific factors, and that of DeltaEMG/Deltaangle might be a conventional clue for estimation of reflexes for these gait disorders.     

Axial perturbations evoke increased postural reflexes in Parkinson’s disease with postural instability

ObjectiveTo measure axially-evoked postural reflexes in 11 Parkinson’s disease (PD) subjects, both stable and unstable, and to compare these with 13 age-matched controls. Methods: We measured the short-latency electromyography (EMG) reflex effects of brief impulsive displacements applied to the upper sternum or C7 for tibialis anterior (TA) and soleus. Our subjects were studied standing normally and when leaning both forwards and backwards.ResultsThe initial mechanical effects of the stimuli were similar but the reflex responses for the unstable PD group were increased, even after allowing for the increased levels of tonic activation. For TA, unstable PD subjects had significantly larger responses than the stable PD group whose responses were in turn significantly larger than controls. For soleus, unstable PD subjects had significantly greater responses than controls.ConclusionsThese findings are consistent with previous evidence that exaggerated postural responses are characteristic of unstable PD subjects.SignificanceIncreased postural reflexes are characteristic of unstable PD subjects and may contribute to the instability seen for these patients in response to larger perturbations.     

Ankle position and voluntary contraction alter maximal M waves in soleus and tibialis anterior

Compound muscle action potentials (CMAPs) recorded using surface electrodes are often used to assess the excitability of neural pathways to skeletal muscle. However, the amplitude of CMAPs can be influenced by changes at the recording site, independent of mechanisms within the central nervous system. We quantified how joint angle and background contraction influenced CMAP amplitude. In seven subjects CMAPs evoked by supramaximal transcutaneous electrical stimulation of motor axons (M(max)) were recorded using surface electrodes from soleus and tibialis anterior (TA) at static positions over the full range of ankle movement at 5 degrees intervals. Across subjects the peak-to-peak amplitude of M(max) was 155% and 159% larger at the shortest than longest muscle lengths for soleus and TA, respectively. In five subjects the effect of ankle position and voluntary contraction on M-wave/H-reflex recruitment curves was assessed in the soleus. Both ankle position and level of contraction significantly influenced M(max), H(max), and the H(max) to M(max) ratio, but there were no interactions between the two parameters. These peripheral changes that influence M(max) will also impact other CMAPs such as submaximal M-waves, H-reflexes, and responses to transcranial magnetic stimulation. As such, during experimental studies CMAPs evoked at a given joint angle and contraction level should be normalized to M(max) recorded at similar joint angle and contraction strength.     

Voluntary teeth clenching facilitates human motor system excitability.

OBJECTIVE: Voluntary contraction of the teeth is a common maneuver used to facilitate peripheral monosynaptic reflexes. It was the aim of this study to determine the site along the neuraxis where this effect occurs. METHODS: Focal transcranial magnetic stimulation (TMS) was used to measure recruitment curves, motor thresholds and intracortical inhibition and facilitation from the right first dorsal interosseus (FDI) and tibialis anterior (TA) muscles in seven normal volunteers. Changes in excitability in subcortical structures during teeth clenching were studied using F waves, H reflexes, and brainstem magnetic stimulation. RESULTS: Recruitment curves of FDI and TA showed significant facilitation during voluntary teeth clenching indicating an overall enhancement in the motor system excitability. Teeth clenching additionally resulted in decreased intracortical inhibition in the FDI but not in TA, pointing to an intracortical site of enhancement for the hand. Motor evoked potentials (MEPs) following stimulation at the brainstem level and F waves in FDI and soleus H reflex amplitude were also facilitated by teeth clenching, indicating a subcortical site for this effect for the upper and lower extremity. M wave amplitudes did not change. CONCLUSIONS: The teeth clenching maneuver had a similar facilitatory effect on upper and lower extremities. Cortical and subcortical sites contribute to this effect in a hand muscle while only subcortical sites were identified in this facilitatory effect on the lower extremity.     

Tendon vibration-induced inhibition of human and cat triceps surae group I reflexes: evidence of selective Ib afferent fiber activation

In humans, prolonged vibration of the Achilles tendon produced transient depression or abolition of the soleus H-reflex. Recovery of the electrical reflex threshold to previbration values at a constant lower stimulus intensity usually occurred between 10 to 55 min. Electrical stimulation at higher multiples of the reflex threshold produced reflex EMG amplitudes more immediately comparable to previbration controls. When postvibration H-reflexes were completely abolished, poststimulus averaging of voluntarily maintained tonic EMG activity showed evidence of inhibition at a 46-ms latency in contrast to a 32-ms previbration H-reflex latency. In cat, observation of H-reflexes were rare, but stimulus-evoked changes in EMG activity mimicked the postvibration depression seen in humans. Ventral root postvibration reflexes from triceps surae varied in magnitude but were usually depressed or abolished at 1.0 to 1.2 times the electrical reflex threshold. These responses returned to previbration control amplitudes within 20 to 35 min. Magnitude of depression and time to recovery were dependent on the intensity of the electrical stimulus. In five experiments, depression of postvibration reflex activity and recovery were accompanied by gradual recovery in amplitude of the group I volley to previbration amplitudes. Elevated group Ia axonal electrical thresholds, monitored from seven isolated units, were observed to recover to previbration values in parallel with postvibration reflex recovery to control amplitudes. At electrical stimulus intensities greater than 1.4 times the reflex threshold, postvibration reflex responses were often potentiated, probably reflecting posttetanic potentiation of group Ia pathways activated at their higher axonal thresholds. In two observations, postvibration Ib axonal electrical thresholds did not change. Overall, the findings supported the proposal that postvibration depression of soleus H-reflexes in humans or cats is caused by both disfacilitation and autogenetic inhibition due to withdrawal of Ia afferent activation and increased selectivity of Ib afferent fiber stimulation, respectively.