Methodological implications of the post activation depression of the soleus H-reflex in man

Postcontraction influences on reaction time

The purpose of this study was to compare reaction time (RT) and fractionated RT components (premotor and motor times) between normal and postcontraction conditions. Twelve participants performed 20 trials each of control and postcontraction RT conditions. For the control condition, participants executed a learned, rapid, knee-extension contraction response to an auditory stimulus. The postcontraction condition was identical to the control condition except that the participants performed a 3-s isometric contraction of the knee extensor muscles prior to an auditory stimulus. Muscle activity was recorded from the quadriceps muscle group. Results indicated that the postcontraction condition was significantly faster than the control condition for the average RT, premotor time, and motor time. It was concluded that reaction time, processing time, and muscle contraction time for a learned task could be significantly reduced following an isometric contraction.

Neural adaptation to resistance training: changes in evoked V-wave and H-reflex responses

Combined V-wave and Hoffmann (H) reflex measurements were performed during maximal muscle contraction to examine the neural adaptation mechanisms induced by resistance training. The H-reflex can be used to assess the excitability of spinal alpha-motoneurons, while also reflecting transmission efficiency (i.e., presynaptic inhibition) in Ia afferent synapses. Furthermore, the V-wave reflects the overall magnitude of efferent motor output from the alpha-motoneuron pool because of activation from descending central pathways. Fourteen male subjects participated in 14 wk of resistance training that involved heavy weight-lifting exercises for the muscles of the leg. Evoked V-wave, H-reflex, and maximal M-wave (M(max)) responses were recorded before and after training in the soleus muscle during maximal isometric ramp contractions. Maximal isometric, concentric, and eccentric muscle strength was measured by use of isokinetic dynamometry. V-wave amplitude increased approximately 50% with training (P < 0.01) from 3.19 +/- 0.43 to 4.86 +/- 0.43 mV, or from 0.308 +/- 0.048 to 0.478 +/- 0.034 when expressed relative to M(max) (+/- SE). H-reflex amplitude increased approximately 20% (P < 0.05) from 5.37 +/- 0.41 to 6.24 +/- 0.49 mV, or from 0.514 +/- 0.032 to 0.609 +/- 0.025 when normalized to M(max). In contrast, resting H-reflex amplitude remained unchanged with training (0.503 +/- 0.059 vs. 0.499 +/- 0.063). Likewise, no change occurred in M(max) (10.78 +/- 0.86 vs. 10.21 +/- 0.66 mV). Maximal muscle strength increased 23-30% (P < 0.05). In conclusion, increases in evoked V-wave and H-reflex responses were observed during maximal muscle contraction after resistance training. Collectively, the present data suggest that the increase in motoneuronal output induced by resistance training may comprise both supraspinal and spinal adaptation mechanisms (i.e., increased central motor drive, elevated motoneuron excitability, reduced presynaptic inhibition).

Bilateral soleus H-reflexes in humans elicited by simultaneous trains of stimuli: symmetry, variability, and covariance

Experiments using electrical and mechanical activation of spinal reflexes have contributed important results toward the understanding of neuronal and synaptic dynamics involved in spinal neural circuits as well as their response to different inputs. In this work, data obtained from the simultaneous stimulation of both legs are analyzed to provide information on the degree of symmetry of the respective spinal reflex circuits and on the characteristics of reflex variability. H-reflexes recorded from relaxed muscles show a frequency-dependent amplitude depression when elicited by a train of stimuli. This effect has been attributed to homosynaptic depression. Soleus H-reflexes were recorded in response to trains of simultaneous stimuli applied to both legs in right-handed subjects that were sitting in a relaxed state. The first objective was to verify the existence of asymmetries in H-reflex parameters obtained from the two legs. We measured the mean, variance, and coefficient of variation of the depressed H-reflex amplitudes and the time constant of decay toward the depressed plateau. The second objective was the analysis of the time correlation of subsequent H-reflex amplitudes in a long train of responses recorded from a given leg. The statistical dependence of H-reflex amplitudes in the long trains recorded from both legs was also investigated. Data obtained from preliminary experiments showed that there was no effect of a given stimulus on the contralateral leg applied simultaneously or 1 s before, therefore validating the simultaneous stimulation paradigm. Paired t-tests indicated that several parameters measured bilaterally from soleus H-reflex trains of right-handed subjects were not statistically different in the overall, although individually there were statistically significant asymmetries, toward either the right or left leg. Sequences of H-reflex amplitudes, as measured by the auto-covariance, were either white or had a memory ranging from 2 up to 50 s. This indicates that the random fluctuations in presynaptic inhibition and/or postsynaptic inputs to motoneurons may have either fast or slow time courses. The average auto-covariance sequences of the right and left legs, computed from all subjects, were practically superposable. The cross-covariance between the bilateral H-reflex amplitudes showed a statistically significant peak at zero lag in some experiments, suggesting a correlation between the synaptic inputs to the Ia-motoneuron systems of the soleus muscles of both legs.

Modulation of H reflex of pretibial and soleus muscles during mastication in humans

A previous study in our laboratory demonstrated that the soleus H reflex was facilitated during mastication in humans. In the present study, we investigated whether there was any modulation of the magnitude of the pretibial H reflex during mastication in five healthy adult volunteers. The pretibial H reflex was significantly facilitated during mastication, and there was no significant difference in the facilitation between jaw-closing and jaw-opening phases; that is, the gain of the H reflex was modulated tonically but not in a phase-dependent manner during mastication. Furthermore, in the same subjects, we confirmed that the soleus H reflex was facilitated during mastication. Based on our findings, we conclude that the H reflexes in both the pretibial and soleus muscles undergo a nonreciprocal facilitation during mastication. It is suggested that mastication contributes to stabilization of postural stance in humans.

Postvibration depression of the H-reflex as a result of a dual mechanism: an experimental study in humans

Changes in amplitude of the soleus H (S(H))-reflex and its neurographic correlates (P(1) and P(2) waves) after vibration of the soleus muscle have been evaluated as a function of mechanical stimulation frequency, duration of the conditioning train, and test stimulus intensity. Additional experiments aimed at assessing the nervous system mechanisms underlying the postvibration depression (PVD) have been performed. In particular, homonymous (S(HMR) or S(H)) versus heteronymous (S(HTR)) soleus response, evoked respectively by tibial nerve and femoral nerve electrical stimulation, the effectiveness of sub-H threshold tibial nerve conditioning volleys on the S(HTR), and the respective effects of a brief passive stretching of the quadriceps and soleus muscles on the recovery of both the S(HMR) and S(HTR) after vibration of the homologous muscle were investigated under suitable experimental conditions. It was found that PVD occurs in the absence of changes in amplitude of the P(1) wave and the S(HTR), is paralleled by a reduced effectiveness of tibial nerve-conditioning volleys on the S(HTR) and is shortened consistently by brief passive stretching of the homologous muscle. It follows that PVD may be the result of a long-lasting reduction of the transmitter release from Ia presynaptic terminals depending, at least in part, on a protracted postvibration Ia afferent discharge caused by spindles thixotropy. These findings may provide a better understanding of the pathophysiologic mechanisms underlying spasticity in humans.

Large involuntary forces consistent with plateau-like behavior of human motoneurons

When electrical stimulation is applied over human muscle, the evoked force is generally considered to be of peripheral origin. However, in relaxed humans, stimulation (1 msec pulses, 100 Hz) over the muscles that plantarflex the ankle produced more than five times more force than could be accounted for by peripheral properties. This additional force was superimposed on the direct response to motor axon stimulation, produced up to 40% of the force generated during a maximal voluntary contraction, and was abolished during anesthesia of the tibial nerve proximal to the stimulation site. It therefore must have resulted from the activation of motoneurons within the spinal cord. The additional force could be initiated by stimulation of low-threshold afferents, distorted the classical relationship between force and stimulus frequency, and often outlasted the stimulation. The mean firing rate of 27 soleus motor units recorded during the sustained involuntary activity after the stimulation was 5.8 +/- 0.2 Hz. The additional force increments were not attributable to voluntary intervention because they were present in three sleeping subjects and in two subjects with lesions of the thoracic spinal cord. The phenomenon is consistent with activation of plateau potentials within motoneurons and, if so, the present findings imply that plateau potentials can make a large contribution to forces produced by the human nervous system.

Postcontraction depression of reciprocal inhibition in human forearm muscles

We tested whether a preceding muscle contraction changes reciprocal inhibition (RI) between forearm antagonists. RI was studied in 14 healthy subjects by assessing changes in the H reflex (evoked by median-nerve stimulation) in forearm flexor muscles after conditioning radial-nerve stimulation at 0- and 20-ms intervals. The maximum sizes of the M wave (M(max)) and H reflex (H(max)) were also measured. After a long-lasting maximum voluntary handgrip contraction (mean +/- SEM: 3.9 +/- 0.6 min) of ipsilateral forearm muscles, M(max) and H(max) were unchanged but RI was diminished. After contraction of the contralateral homologous muscles and after contraction elicited by ipsilateral muscle stimulation, RI remained unchanged. These results show that a preceding maximum voluntary contraction (lasting 30 s or more) reduces the activity of the spinal inhibitory interneurons mediating RI. This finding may imply the need to reinterpret results from RI studies in normal subjects and in patients with movement disorders.

The effect of baclofen on the transmission in spinal pathways in spastic multiple sclerosis patients

OBJECTIVES

To measure the effect of baclofen on the transmission in different spinal pathways to soleus motoneurones in spastic multiple sclerosis patients.

METHODS

Baclofen was administered orally in 14 and intrathecally in 8 patients. H(max)/M(max), presynaptic inhibition by biceps femoris tendon tap of femoral nerve stimulation, depression of the soleus H-reflex following previous activation of the Ia afferents from the soleus muscle (i.e. postactivation depression), disynaptic reciprocal Ia inhibition of the soleus H-reflex and the number of backpropagating action potentials in primary afferents, which may be a sign of presynaptic inhibition, were examined.

RESULTS

Baclofen depressed the soleus H(max)/M(max) ratio significantly following oral and intrathecal baclofen. None of the two tests of presynaptic inhibition, or the postactivation depression or the disynaptic reciprocal Ia inhibition of the soleus H-reflex were affected by baclofen administration. Also the action potentials of the primary afferents were unchanged during baclofen administration.

CONCLUSIONS

The antispastic effect of baclofen is not caused by an effect on the transmitter release from Ia afferents or on disynaptic reciprocal Ia inhibition. One possible explanation of the depression of the H-reflex by baclofen is suggested to be a direct depression of motoneuronal excitability.

Inhibition of the triceps surae stretch reflex by stimulation of the deep peroneal nerve in persons with spastic stroke

OBJECTIVE

To reduce the triceps surae stretch reflex by electrical stimulation of the deep peroneal nerve.

DESIGN

Intervention study.

SETTING

Research institution.

PARTICIPANTS

Sample of convenience of 10 spastic stroke individuals.

INTERVENTION

After the deep peroneal nerve was stimulated between 0.9 and 4 times tibialis anterior motor threshold, the triceps surae was stretched to elicit a reflex.

MAIN OUTCOME MEASURE

The triceps surae stretch reflex was quantified by the amplitude of the reflex electromyography (EMG) in soleus and medial gastrocnemius muscles and mean ankle moment. Paired t test and the Wilcoxon signed rank test (p < .05) were used to evaluate the effect of conditioning stimulation.

RESULTS

The soleus stretch reflex EMG was reduced significantly (p < .001) by stimulating the deep peroneal nerve to 25%+/-6% (standard error) of the unconditioned value (relaxed triceps surae). The optimal interval between stimulation and stretch was 141+/-15 msec. The velocity threshold increased significantly (p = .006) from a median value of 8 degrees per second to 33 degrees per second and the area under the stretch velocity/stretch reflex relation decreased significantly (p < .001) (soleus EMG).

CONCLUSIONS

The stretch reflex of relaxed triceps surae in persons with spastic stroke can be extensively reduced by stimulating the deep peroneal nerve at several times motor threshold of the tibialis anterior.

Low frequency depression of H-reflexes in humans with acute and chronic spinal-cord injury

We measured low-frequency depression of soleus H-reflexes in individuals with acute (n=5) and chronic (n=7) spinal-cord injury and in able-bodied controls (n=7). In one acute subject, we monitored longitudinal changes in low-frequency depression of H-reflexes over 44 weeks and examined the relationship between H-reflex depression and soleus-muscle fatigue properties. Soleus H-reflexes were elicited at 0.1, 0.2, 1, 5, and 10 Hz. The mean peak-to-peak amplitude of ten reflexes at each frequency was calculated, and values obtained at each frequency were normalized to 0.1 Hz. H-reflex amplitude decreased with increasing stimulation frequency in all three groups, but H-reflex suppression was significantly larger in the able-bodied and acute groups than in the chronic group. The acute subject who was monitored longitudinally displayed reduced low-frequency depression with increasing time post injury. At 44 weeks post injury, the acute subject's H-reflex depression was similar to that of chronic subjects, and his soleus fatigue index (assessed with a modified Burke fatigue protocol) dropped substantially, consistent with transformation to faster muscle. There was a significant inverse correlation over the 44 weeks between the fatigue index and the mean normalized H-reflex amplitude at 1, 5, and 10 Hz. We conclude that: (1) the chronically paralyzed soleus muscle displays impaired low-frequency depression of H-reflexes, (2) attenuation of rate-sensitive depression in humans with spinal-cord injury occurs gradually, and (3) changes in H-reflex excitability are generally correlated with adaptation of the neuromuscular system. Possible mechanisms underlying changes in low-frequency depression and their association with neuromuscular adaptation are discussed.

Temporal changes in the excitability of the reciprocal inhibitory interneurones preceding cocontraction of the antagonistic muscle in the human wrist

The supraspinal control of reciprocal inhibitory interneurones preceding cocontraction of antagonistic muscles in the human wrist was investigated in 5 subjects. The method combined a reaction time task paradigm and a technique for conditioning the H-reflex. Unconditioned H-reflexes were elicited from flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles by using electrical stimulation to median and radial nerves. The conditioned H-reflex of the FCR muscle was obtained by simultaneous electrical stimulation to the median and the radial nerves. While subjects performed the bilateral isometric cocontraction task, excitabilities of the alpha motoneurone pools and that of the reciprocal inhibitory interneurones were assessed by the unconditioned and conditioned H-reflex, respectively. Alpha mononeuron pools of FCR and ECR muscles were facilitated starting from approximately 50 msec. preceeding EMG activities. Reciprocal inhibitory interneurones which suppressed FCR alpha motoneurones were disinhibited at almost same time. The present results suggested that supraspinal control of the reciprocal inhibitory interneurones preceding antagonist cocontraction is different from that of preceding agonist contraction.

Monitoring of head injury by myotatic reflex evaluation

OBJECTIVES

(1) To establish the feasibility of myotatic reflex measurement in patients with head injury. (2) To test the hypothesis that cerebral dysfunction after head injury causes myotatic reflex abnormalities through disordered descending control. These objectives arise from a proposal to use reflex measurements in monitoring patients with head injury.

METHODS

The phasic stretch reflex of biceps brachii was elicited by a servo-positioned tendon hammer. Antagonist inhibition was evoked by vibration to the triceps. Using surface EMG, the amplitude of the unconditioned biceps reflex and percentage antagonist inhibition were measured. After standardisation in 16 normal adult subjects, the technique was applied to 36 patients with head injury across the range of severity. Objective (1) was addressed by attempting a measurement on each patient without therapeutic paralysis; three patients were also measured under partial paralysis. Objective (2) was addressed by preceding each of the 36 unparalysed measurements with an assessment of cerebral function using the Glasgow coma scale (GCS); rank correlation was employed to test a null hypothesis that GCS and reflex indices are unrelated.

RESULTS

In normal subjects, unconditioned reflex amplitude exhibited a positive skew requiring logarithmic transformation. Antagonist inhibition had a prolonged time course suggesting presynaptic mechanisms; subsequent measurements were standardised at 80 ms conditioning test interval (index termed "TI(80)"). Measurements were obtained on all patients, even under therapeutic paralysis (objective (1)). The unconditioned reflex was absent in most patients with GCS less than 5; otherwise it varied little across the patient group. TI(80) fell progressively with lower GCS, although patients' individual GCS could not be inferred from single measurements. Both reflex indices correlated with GCS (p<0.01), thereby dismissing the null hypothesis (objective (2)).

CONCLUSION

Cerebral dysfunction in head injury is reflected in myotatic reflex abnormalities which can be measured at the bedside. With greater reproducibility, reflex measurements may assist monitoring of patients with head injury.

The monosynaptic reflex: a tool to investigate motor control in humans. Interest and limits

The principle of the monosynaptic reflex used as a tool to explore the excitability of the motoneurones (MNs) is explained and the general methodology of the H reflex is described. The different drawbacks inherent in the technique are then considered: mechanisms other than the monosynaptic la excitation of MNs contributing to the H reflex size (limitation of the H reflex size by disynaptic IPSPs, presynaptic inhibition of la terminals, post-activation depression); non-linearity and changes in the 'recruitment gain' in the MN pool; and poor time resolution of the method. Despite these drawbacks, it is emphasized that the H reflex is the only available technique enabling one to investigate changes in transmission in spinal pathways during motor tasks.

Impaired response of human motoneurones to corticospinal stimulation after voluntary exercise

1. Activation of descending corticospinal tracts with transmastoid electrical stimuli has been used to assess changes in the behaviour of motoneurones after voluntary contractions. Stimuli were delivered before and after maximal voluntary isometric contractions (MVCs) of the elbow flexor muscles. 2. Following a sustained MVC of the elbow flexors lasting 5-120 s there was an immediate reduction of the response to transmastoid stimulation to about half of the control value. The response recovered to control levels after about 2 min. This was evident even when the size of the responses was adjusted to accommodate changes in the maximal muscle action potential (assessed with supramaximal stimuli at the brachial plexus). 3. To determine whether the post-contraction depression required activity in descending motor paths, motoneurones were activated by supramaximal tetanic stimulation of the musculocutaneous nerve for 10 s. This did not depress the response to transmastoid stimulation. 4. Following a sustained MVC of 120 s duration, the response to transcranial magnetic stimulation of the motor cortex gradually declined to a minimal level by about 2 min and remained depressed for more than 10 min. 5. Additional studies were performed to check that the activation of descending tracts by transmastoid stimulation was likely to involve excitation of direct corticospinal paths. When magnetic cortical stimuli and transmastoid stimuli were timed appropriately, the response to magnetic cortical stimulation could be largely occluded. 6. This study describes a novel depression of effectiveness of corticospinal actions on human motoneurones. This depression may involve the corticomotoneuronal synapse.

Ia presynaptic inhibition in human wrist extensor muscles: effects of motor task and cutaneous afferent activity

The task-dependence of the presynaptic inhibition of the muscle spindle primary afferents in human forearm muscles was studied, focusing in particular on the modulation associated with the co-contraction of antagonist muscles and the activation of cutaneous afferents. The changes known to affect the motoneuron proprioceptive assistance during antagonist muscle co-activation in human leg and arm muscles were compared. The evidence available so far that these changes might reflect changes in the presynaptic inhibition of the muscle spindle afferent is briefly reviewed. The possible reasons for changes in presynaptic inhibition during the antagonist muscle co-contraction are discussed. Some new experiments on the wrist extensor muscles are briefly described. The results showed that the changes in the Ia presynaptic inhibition occurring during the co-contraction of the wrist flexor and extensor muscles while the hand cutaneous receptors were being activated (the subject's hand was clenched around a manipulandum) could be mimicked by contracting the wrist extensor muscles alone while applying extraneous stimulation to the hand cutaneous receptors. It is concluded that besides the possible contribution of inputs generated by the co-contraction of antagonist muscles and by supraspinal pathways, cutaneous inputs may play a major role in modulating the proprioceptive assistance during manipulatory movements.

Presynaptic inhibition in humans: a comparison between normal and spastic patients

During the last 40 years, several studies in man have been devoted to the pathophysiological mechanisms underlying spasticity. Spasticity is characterised by a velocity dependent increase in muscle tone. Many spinal pathways control stretch reflex excitability and a malfunction in any one of them could theoretically produce the exaggeration of the stretch reflex. Delwaide showed that the vibration-induced inhibition of Ia fibres is reduced in spastic patients. However, the relation between a decrease in presynaptic Ia inhibition and the pathophysiology of spasticity has been recently questioned since it was argued that homosynaptic depression (or post-activation depression) also contributes to the vibratory-induced depression of monosynaptic reflexes. This paper is thus devoted to a review of the methods recently developed to study selectively presynaptic Ia inhibition in man and to a reevaluation of the relations between modifications in presynaptic Ia inhibition and spasticity in hemiplegic and spinal spastic patients.

Muscle history, fusimotor activity and the human stretch reflex

1. The previous history of contraction and length changes of a muscle influences the size of the stretch reflex and H reflex. Here we ask, is this dependence due to changes in mechanical properties of extrafusal fibres, intrafusal fibres of spindles, or both? 2. The soleus muscle of human subjects was conditioned using either a voluntary contraction or a contraction evoked by low-strength electrical stimulation, in the range 0-25 % of maximum. Following conditioning, reflexes were increased by more than twofold above the no-contraction value by a voluntary contraction of 5 % of maximum, or more, but not by electrical stimulation which presumably did not contract the intrafusal fibres of spindles. 3. When the muscle was conditioned with a contraction at a length shorter than the test length, rather than at the test length, a depressing effect on reflexes was attributed to both the burst of impulses generated in spindles when the muscle was stretched back to the test length and to a reduced stretch sensitivity of muscle spindles. 4. The experiments demonstrate the importance of keeping the muscle and its spindles in a defined mechanical state when measuring reflexes. They also point to the powerful facilitating influences of conditioning muscle contractions provided they recruit the intrafusal fibres of spindles.

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.

Sensitivity of H-reflexes and stretch reflexes to presynaptic inhibition in humans

The sensitivity of soleus H-reflexes, T-reflexes, and short-latency stretch reflexes (M1) to presynaptic inhibition evoked by a weak tap applied to the biceps femoris tendon or stimulation of the common peroneal nerve (CPN) was compared in 17 healthy human subjects. The H-reflex was strongly depressed for a period lasting up to 300-400 ms (depression to 48 +/- 23%, mean +/- SD, of control at a conditioning test interval of 70 ms) by the biceps femoris tendon tap. In contrast, the short-latency soleus stretch reflex elicited by a quick passive dorsiflexion of the ankle joint was not depressed. The soleus T-reflex elicited by an Achilles tendon tap was only weakly depressed (92 +/- 8%). The H-reflex was also significantly more depressed than the T-reflex at long intervals (>15 ms) after stimulation of CPN (H-reflex 63 +/- 14%, T-reflex 91 +/- 13%; P < 0. 01). However, the short-latency (2 ms) disynaptic reciprocal Ia inhibition evoked by stimulation of CPN was equally strong for H- and T-reflexes (H-reflex 72 +/- 10%, T-reflex 67 +/- 13%; P = 0.07). Peaks in the poststimulus time histogram (PSTH) of the discharge probability of single soleus motor units (n = 53) elicited by an Achilles tendon tap had a longer duration than peaks evoked by electrical stimulation of the tibial nerve (on average 5.0 ms as compared with 2.7 ms). All parts of the electrically evoked peaks were depressed by the conditioning biceps femoris tendon tap (average depression to 55 +/- 27% of control; P < 0.001). A similar depression was observed for the initial 2 ms of the peaks evoked by the Achilles tendon tap (69 +/- 48%; P < 0.001), but the last 2 ms were not depressed. Conditioning stimulation of the CPN at long intervals (>15 ms) also depressed all parts of the electrically evoked PSTH peaks (n = 34; average 65%; P < 0.001) but had only a significant effect on the initial 2 ms of the peaks evoked by the Achilles tendon tap (85%; P < 0.001). We suggest that the different sensitivity of mechanically and electrically evoked reflexes to presynaptic inhibition is caused by a difference in the shape and composition of the excitatory postsynaptic potentials underlying the two reflexes. This difference may be explained by a different composition and/or temporal dispersion of the afferent volleys evoked by electrical and mechanical stimuli. We conclude that it is not straightforward to predict the modulation of stretch reflexes based on observations of H-reflex modulation.

Postexercise potentiation of the H-reflex in humans

Post-muscle activation effects on segmental reflexes reveal divergent results dependent upon the manner in which the muscle is activated. Electrically activating triceps surae invokes a potentiation of the Achilles' tendon reflex and the soleus (S) H-reflex termed posttetanic potentiation. In contrast, brief volitional activation produces a subsequent potentiation of tendon reflexes, whereas H-reflexes become depressed.

PURPOSE

The present investigation explored the effect of an intense bout of volitional resistance exercise on the S and lateral gastrocnemius (LG) H-reflexes to determine if a potentiation of the H-reflex could be induced with physiological stimuli.

METHODS

LG and S H-reflexes were obtained from 10 college age men and women before and after a vigorous bout (eight sets of 10 repetitions) of concentric-eccentric triceps surae exercise.

RESULTS

Every subject displayed an initial depression of the LG (P < 0.01) and S H-reflex (P < 0.05) immediately postexercise, consistent with postactivation depression. As a group, there was a significant (P > 0.01) potentiation of the LG H/M ratio following the depression. Five of 10 subjects demonstrated this potentiation, which often lasted 10 min postexercise. The other five subjects displayed a longer and more profound early depression followed by a return to control levels.

CONCLUSION

The data suggest that at least two overlapping processes are occurring, a brief depression followed by or superimposed over a longer lasting potentiation. Possible neural mechanisms and implications to strength training are discussed.

Reassessment of H-reflex recovery curve using the double stimulation procedure

We conducted two types of experiments to assess the validity of the H-reflex recovery test, using double stimulation to test soleus motoneuron pool excitability in healthy and spastic subjects. One type dealt with the mechanical effect of the conditioning H reflex on the ankle joint; the other type with the effect of change in reflex size. The mechanical effect was tested both with the ankle joint fixed (FX) and free to move (FR). Differences between FX and FR conditions commenced with relaxation of soleus muscle contraction by the conditioning H reflex. In the FR condition, abrupt facilitation occurred, and changed to marked depression. We conclude that specific facilitation and inhibition in the FR condition were secondary effects of group Ia inflows caused by the ankle extensor muscle stretching on relaxation. In some spastic patients as well as in controls, facilitation due to the mechanical effect in the FR condition was observed despite the FX condition. The effects of systematic changes on soleus H-reflex size were investigated at conditioning-test intervals of 80 ms, so as to avoid mechanical effects. When conditioning and test reflexes were the same size, the amount of recovery increased as the H-reflex size increased. Comparison of the relation between amount of recovery and H-reflex size, expressed as a percentage of Mmax, showed no significant difference between the two groups. We speculate that the stronger recovery of spasticity mentioned in previous literature may have resulted from the fact that relatively greater H reflexes were tested in those studies. In conclusion, the present study indicates that double stimulation is not appropriate for assessing spinal motoneuron pool "excitability increase" in spasticity.

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

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

H-reflexes of different sizes exhibit differential sensitivity to low frequency depression

The amplitude of the H-reflex declines when activated repetitively. The magnitude of decline is greater when the amplitude of the H-reflex is small. To explore whether pre- or postsynaptic factors contribute to the differences observed in H-reflexes of different sizes, changes in the amplitude of H-reflexes of different sizes were measured during a train of stimulation in 10 normal subjects. Amplitudes of different sizes were obtained using differing stimulus intensities or during superimposed contraction, two manipulations which differently affect the number of active afferents and the excitation of the motoneuron pool. Small amplitude H-reflexes depressed to a lower plateau than larger H-reflexes and superimposed contraction did not alleviate the depression during each train. Nearly all the decline in larger amplitude H-reflexes occurred in a component that was in common with smaller amplitude H-reflexes. This suggests that the depressibility of the earliest activated units is greater than later activated units in H-reflexes and that the magnitude of decline is affected by prior activity as well as size.

Long-lasting depression of soleus motoneurons excitability following repetitive magnetic stimuli of the spinal cord in multiple sclerosis patients

The effect of repetitive magnetic stimulation at the spinal level on the soleus H-reflex amplitude was evaluated in II MS patients with lower limb spasticity and in nine healthy subjects. In MS patients stimulation with a train of 16 stimuli at 25 Hz induced a decrease in amplitude to 61.2 +/- 25.7% of the unconditioned H-reflex amplitude at interstimulus interval (ISI) of 10-1000 ms (P < 0.01). The amount of decrease in H-reflex amplitude was highly dependent on the stimulation intensity and the placement of the coil, and to a lesser extent influenced by the stimulation frequency. No decrease in motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation was seen following trains of 16 stimuli at mid-thoracic in contrast to the post-stimulation depression in H-reflex amplitude which could imply that mechanisms acting at presynaptic level were involved. In response to repetitive magnetic stimuli for 5 min, a long-lasting decrease in H-reflex amplitude to a level of about 70% of the pre-stimulation H-reflex amplitude occurred in MS patients (P < 0.01). A similar although not significant decrease was observed in healthy subjects. We propose that long-lasting depression of the soleus H-reflex amplitude after repetitive magnetic stimuli is due to long-term depression of the synaptic transmission.

The influence of muscle spindle discharge on the human H reflex and the monosynaptic reflex in the cat

1. Experiments were carried out to test the effect of changes in spindle resting discharge on the size of monosynaptic reflexes in the cat and on the H reflex in humans. Resting discharge was altered by contracting the triceps surae muscle at longer (hold-long) or shorter (hold-short) lengths than that at which the reflex was tested. 2. The reflex in the cat was larger after hold-long than after hold-short conditioning, and the difference, after an initial decline, was well maintained. For the human H reflex a similar pattern was observed except that 15 s after muscle conditioning the difference in reflex size had disappeared. 3. Monosynaptic reflex depression immediately after hold-long conditioning, when most of the muscle spindles are silent, was attributed to the high level of spindle discharge during the immediately preceding hold-long period. The time course of this inhibition was too long to be accounted for by presynaptic inhibition. 4. In the cat heteronymous muscle conditioning was used to test whether presynaptic inhibition could be responsible for reflex depression using the synergist muscle pair lateral gastrocnemius-soleus and medial gastrocnemius. Conditioning one of the pair did not affect the reflex in the other, the opposite result to that expected with presynaptic inhibition. A similar experiment in which the triceps H reflex in human subjects was facilitated by a quadriceps volley gave the same result. 5. Thus this study presents evidence that monosynaptic reflexes are depressed by the on-going discharge of muscle spindles in the homonymous muscle, but that this depression does not appear to involve "classical' presynaptic inhibition.

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.

Increase in reciprocal Ia inhibition during antagonist contraction in the human leg: a study of motor units and the H reflex

1. The change in reciprocal Ia inhibition of soleus motoneurones produced by stimulation of the common peroneal nerve was investigated by the use of twenty-three soleus motor units as well as the soleus H reflex in six normal subjects during tonic pretibial contraction. 2. In the motor unit experiments, motoneuronal excitability was measured as the 'critical firing stimulus' (CFS), which is the difference between the test stimulus intensity needed to reach the threshold for the lowest threshold Ia fibres and the intensity which evokes firing of a motor unit with the probability of 50%. The conditioning effect, assessed from the change in the CFS, was expressed as a percentage of the unconditioned CFS. 3. At a conditioning intensity of 0.95 times the motor threshold value, there was Ia inhibition in sixteen of the twenty-three motor units (69.6%) at rest. Of these sixteen motor units, twelve showed increases in inhibition at intervals below 2.0 ms during pretibial contraction. In four of the remaining seven units, inhibition first appeared during contraction. There was no significant decrease in inhibition at any time during contraction. 4. Based on the conventional H reflex, reciprocal Ia inhibition increased during very weak (below 2% of the maximum) voluntary dorsiflexion and continued to increase at a slightly stronger (3-8% of the maximum) contraction, then decreased continuously when contraction was strengthened further. Maximal inhibition occurred at a relatively strong contraction when a weak conditioning stimulus was used, and vice versa. 5. We conclude that the activity of reciprocal Ia inhibitory interneurones increases during tonic antagonist contraction. The previous controversy about this inhibition is the result of occlusion at the Ia interneuronal level.

Increased resting discharge of human spindle afferents following voluntary contractions

1. The aim of this study was to assess the incidence of lasting alterations in discharge rate of muscle spindle afferents innervating human ankle and toe dorsiflexor muscles following isometric contractions. 2. The subjects performed controlled isometric ankle dorsiflexions maintained for approximately 5 s. During the contraction the discharge of all but one spindle afferent increased above the precontraction level. After complete relaxation, there was prolonged enhancement of the discharge rate of nineteen of fifty-five muscle spindle afferents and none of three Golgi tendon organ afferents. Ten of the nineteen spindle afferents had been silent prior to the contraction. For the population of fifty-five spindle afferents, the mean 'postcontraction' discharge rate was 65% higher than the mean precontraction discharge rate, with the mean rate increasing from 2.3 to 3.9 Hz (P < 0.001). The mean duration of the enhanced postcontraction discharge was 52 s (range, 8-240 s). 3. Stretch applied to the tendon of the receptor-bearing muscle in twelve of fourteen spindle afferents with an enhanced postcontraction discharge rate eliminated or reduced the enhanced discharge rate. 4. The high incidence of an enhanced spindle discharge after voluntary contraction (35% of spindle afferents) suggests that muscle 'history' should be taken into account when interpreting changes in spindle discharge rates. The enhanced discharge rates following contraction probably reflect a long-lasting effect of the contraction-associated increase in fusimotor drive on intrafusal stiffness, rather than the persistence of fusimotor drive following relaxation.

Long-lasting conditioning of the human soleus H reflex following quadriceps tendon tap

Percussion of the quadriceps tendon was used to test the hypothesis that knee extensor muscle spindle discharge initiates down-regulation of the gain of the soleus H reflex. Seven subjects participated. Soleus H reflex magnitude was observed for up to 15 s, following conditioning tendon taps of 60 N or 80 N force and 10 ms duration, with the knee at 60 degrees or 90 degrees of flexion. The tap elicited quadriceps stretch reflexes in four subjects, with a mean latency of 42 ms. The major component of the conditioning of the soleus H reflex was significant attenuation of magnitude by 30-90% of controls, starting as early as 36 ms post-percussion and lasting as long as 3-8 s. The attenuation of reflex magnitude was evident, whichever combination of duration and force of tap was used. Preceding and/or following this inhibition, there was mild facilitation. Static stretch of quadriceps also significantly reduced soleus H reflex magnitude. These results support the spindle receptor origin for the gain attenuation seen during movement. The time course of the gain attenuation suggests a spinal route, by which the spindle discharge of the heteronymous extensor muscles initiates presynaptic inhibition of transmission through the reflex pathway.

Influence of physiotherapeutic facilitation techniques on motor evoked potentials in centrally paretic hand extensor muscles

In the rehabilitation of stroke patients, various facilitation techniques are applied to reduce weakness in centrally paretic muscles and to improve functional motor capacity. The present investigation compared the facilitatory effect of 5 different physiotherapeutic approaches onto the centrally paretic extensor carpi radialis muscle in 30 stroke patients classified into 3 groups according to the individual degree of paresis. In order to quantify the influence of the respective facilitation manoeuvre, single transcranial magnetic stimuli were applied before and during the application of cutaneous/proprioceptive stimuli, a weight bearing task, contraction of the affected and the non-affected extensor carpi radialis muscle and during proximal preinnervation on the affected side. All procedures, indeed, enhanced the frequency of occurrence of muscular response potentials and their amplitudes while diminishing their response latencies. The most prominent effects were observed when the muscle itself was voluntarily activated. A similarly strong facilitation was obtained in the most severely affected patients with cutaneous and proprioceptive stimuli, but such stimuli had inhibitory effects in the healthy control group. The present study illustrates the interaction of cortically evoked motor potentials with peripherally or centrally generated inputs, contributes to the understanding of the neurophysiological mechanisms underlying physiotherapeutic facilitation techniques and helps in providing rational criteria to decide about the most appropriate facilitation method.

Central control of disynaptic reciprocal inhibition in humans

The disynaptic pathway from muscle spindle Ia afferents to motoneurones of the antagonist muscle is one of the best studied pathways in the spinal cord. Early animal studies--mainly in the cat--have provided a detailed knowledge of the pathway itself and of the integration of segmental and supraspinal convergence at the interneuronal level. Although this knowledge was used to formulate hypotheses on the function of the pathway during natural movements, the reduced animal preparation limited the possibilities of testing these ideas. However, such information has more recently been obtained from human subjects by using indirect electrophysiological techniques. In most of these experiments the disynaptic Ia inhibition was demonstrated as a short-latency depression of a monosynaptic test reflex (H-reflex) following a conditioning stimulation of the antagonist nerve. Changes in the size of this depression during voluntary tasks were then taken as evidence of a central regulation of the pathway. It has for example been demonstrated in this way that the brain regulates the Ia inhibitory interneurones in parallel with their corresponding motoneurones during extension-flexion movements, but not during co-contraction of antagonistic muscles. The importance of the central control of the pathway has also been emphasized by the finding of a disordered regulation of its activity in patients with lesions of the brain. This may possibly contribute to the inappropriate co-contraction of antagonistic muscles observed in some of these patients. It seems reasonable to expect that this kind of experiment in the future may contribute significantly to the knowledge of the central control of spinal motor mechanisms.

Association between muscle architecture and quadriceps femoris H-reflex

The purpose of the present study was to establish the relationship between muscle architecture and H-reflex recordings in quadriceps femoris muscle. H-reflexes were elicited in human quadriceps femoris muscle over a broad area of skin to document the shape and amplitude of the H-potentials. This, in combination with recording monopolar and bipolar H-potentials, was performed to determine the location and method for measuring maximum-amplitude H-reflexes. The influence of neural and peripheral factors on the H-potential during passive length changes was studied by comparing the amplitude of H-potentials to motor unit action potentials. Monopolar recordings of the H-potential were found to be preferable to bipolar recordings because of the reproducibility of shape and easier distinction between the M- and H-potentials. The location for recording maximum H-potentials was in the distal one third of the quadriceps femoris muscle, over the border between vastus lateralis and rectus femoris. The inferred relationship between H-potential amplitude and reflex excitability must be made with caution in quadriceps femoris muscle because the amplitude of both the motor unit potential and H-potential change as a function of muscle length.

H-reflexes are less depressed following muscle stretch in spastic spinal cord injured patients than in healthy subjects

The size of the soleus H-reflex was measured after a slow (17 deg/s) passive stretch of ankle plantarflexors and compared to its control size without muscle stretch in ten neurologically healthy subjects and in six spastic spinal-cord-injured patients. Two seconds after the end of the stretch, the size of the H-reflex was reduced to about 30% of its pre-stretch size in the healthy subjects. The depression remained for 10-15 s. In the spastic, spinal-cord-injured patients, stretch caused significantly less reduction in the size of the H-reflex. The H-reflex also regained its pre-stretch size much faster than in healthy subjects. We suggest that the smaller depression of the H-reflex observed in spastic patients may be involved in the pathophysiology of spasticity.

Differential projection of the sural nerve to early and late recruited human tibialis anterior motor units: change of recruitment gain

The effect of a stimulation of the cutaneous sural nerve [three shocks, 2.5 x perception threshold (PT)] was studied on the tibialis anterior (TA) H-reflex and single voluntarily activated TA motor units using post-stimulus time histograms (PSTH). In both cases, when studying only the first recruited motor units, an inhibition with a delay of 10 ms, in relation to the monosynaptic latency of Ia afferents in the common peroneal nerve, was observed. This inhibition had a duration of 10-20 ms. The inhibition was evoked by low-threshold cutaneous fibres, since it could be seen at a stimulation strength close to the perception threshold. The central delay of the inhibition was calculated in two subjects to be 1.8 ms and 1.2 ms respectively. The TA motor units were characterized by their recruitment threshold and minimal firing frequency and the effect of the sural nerve stimulation was subsequently investigated. Early recruited low frequency motor units were found to be inhibited, whereas later recruited motor units with a higher minimal firing frequency were facilitated. Similarly small TA H-reflexes were inhibited, whereas large reflexes were facilitated. This difference in the effect of the sural nerve stimulation was not caused by a difference in the descending command, since the same early recruited motor unit was still inhibited when firing at a high frequency and at a high torque level. Stimulation of the femoral nerve was found to produce a monosynaptic facilitation of the TA H-reflex and a heteronymous monosynaptic peak in the PSTH of single motor units. A stimulation of the sural nerve increased the size of the reflex facilitation, but had no effect on the size of the monosynaptic peak in the PSTH of the single motor units. It is concluded that the effect of the sural nerve stimulation on human TA motor units is similar to observations in the cat and that a similar interneuronal system may be responsible. It is furthermore suggested that the sural nerve stimulation increases the recruitment gain of the TA motoneuronal pool.

Cortical modulation of transmission in spinal reflex pathways of man

1. The motor actions in the lower limb of transcranial electrical stimulation of the motor cortex have been studied in sitting human subjects. 2. Cortical stimulation induced a short latency inhibition of H reflexes evoked in soleus motoneurones both at rest and during small voluntary contractions of soleus. 3. Spatial interaction between cortical inhibition of soleus motoneurons and inhibition evoked through identified spinal reflex machinery was investigated. 4. Interactions were found between cortically evoked inhibition and spinal Ia reciprocal inhibition, group I non-reciprocal inhibition and higher threshold components of longer latency reciprocal inhibition (D1 and D2 inhibitions). 5. Interactions were facilitatory when cortical and spinal inhibitory actions were weak and reversed to occlusion when both actions were strong. 6. It is concluded that the corticospinal pathway converges on the interneurones which subserve Ia reciprocal, group I non-reciprocal, D1 and D2 inhibition of soleus motoneurones. 7. No significant interaction was found under the present experimental conditions between cortical stimulation and group Ia-Ia presynaptic inhibition of soleus afferents. 8. The statistical significance of spatial interactions observed with H reflex conditioning was investigated using a control experiment.

Central facilitation of Ia inhibition during tonic ankle dorsiflexion revealed after blockade of peripheral feedback

Recent studies have reported that no increase of the disynaptic reciprocal inhibition can be observed during tonic voluntary dorsiflexion of the foot as compared to rest, when the size of the control H-reflex is kept constant. Other studies have, however, shown that a voluntary contraction evokes a strong and long-lasting depression of the synaptic transmission from Ia afferents to motoneurones, most likely secondary to activation of these afferents during the contraction (post-activation depression). It was thought that this effect could also interfere with the demonstration of a central facilitation of the reciprocal inhibition during movement. The amount of disynaptic Ia reciprocal inhibition from the pretibial flexors to the soleus H-reflex was therefore estimated in normal human subjects at rest and during voluntary tonic dorsiflexion before, during and after blocking the peripheral feedback from the investigated muscles. It was observed that the reciprocal inhibition measured during dorsiflexion increased during occlusion of the blood supply to the leg, reaching a maximum of inhibition after 30 min of ischaemia. After release of the ischaemia the inhibition gradually decreased to its pre-ischaemic level. It is therefore suggested that the brain facilitates transmission in the Ia disynaptic reciprocal pathway during tonic voluntary dorsiflexion of the foot, but that this facilitation is normally not observed due to a post-activation depression following the peripheral feedback activation during the movement.

Dynamic control of muscle stiffness and H reflex modulation during hopping and jumping in man

1. The objective of the study was to evaluate the functional effects of reflexes on muscle mechanics during natural voluntary movements. The excitability of the H (Hoffmann) reflex was used as a measure of the excitability of the central component of the stretch reflex. 2. We recorded EMG, ground reaction forces and the H reflex in the soleus muscle in humans while landing from a downward jump, during drop jumping and during hopping. The movements were also recorded by high-speed cinematography. 3. The EMG pattern was adapted to the motor task. When landing the EMG in the soleus muscle and in the anterior tibial muscle showed preinnervation and alternating activity after touch down. When hopping there was little preinnervation in the soleus muscle, and the activity was initiated about 45 ms after touch down by a peak and continued unbroken until lift off. In the drop jumps the EMG pattern depended on the jumping style used by the subject. 4. The H reflex in the soleus muscle was strongly modulated in a manner appropriate to the requirements of the motor task. During landing from a downward jump the H reflex was low at touch down whereas while hopping it was high at touch down. During drop jumping it was variable and influenced by the jumping technique. 5. Muscle stiffness in the ankle joint was negative after touch down when landing, but always positive when hopping. 6. It is suggested that during landing the alternating EMG pattern after touch down was programmed and little influenced by reflexes. During hopping reflexes could contribute to the initial peak and the EMG during lift off. 7. The programmed EMG activity and the suppression of the H reflex while landing probably contribute to the development of the negative stiffness and change the muscles from a spring to a damping unit.