Modulation of H reflex of pretibial muscles and reciprocal Ia inhibition of soleus muscle during voluntary teeth clenching in humans

A previous study has demonstrated that the soleus H reflex is facilitated in association with voluntary teeth clenching in proportion with biting force in humans. The present study tried to elucidate the functional significance of this facilitation of the soleus H reflex, by examining 1) whether the facilitation of the H reflex is reciprocal or nonreciprocal between the ankle extensors and flexors and 2) whether the reciprocal Ia inhibition of crural muscles is facilitated or depressed in association with voluntary teeth clenching. The H reflex of the pretibial muscles was evoked by stimulation of the common peroneal nerve in seven healthy subjects with no oral dysfunction. The pretibial H reflex was facilitated in association with voluntary teeth clenching in a force-dependent manner. The facilitation started preceding the onset of electromyographic activity of the masseter muscle. Stimulation of the common peroneal nerve at low intensities subthreshold for evoking the M wave of the pretibial muscles inhibited the soleus H reflex after a short latency corresponding with a disynaptic inhibition, indicating that the reciprocal Ia inhibition was depressed in association with voluntary teeth clenching. Thus, the present study has shown that voluntary teeth clenching evokes a nonreciprocal facilitation of ankle extensor and flexor muscles and attenuated reciprocal Ia inhibition from the pretibial muscles to the soleus muscle. It is concluded that voluntary teeth clenching contributes to improve stability of stance rather than smoothness of movements.     

Role of Ia afferents in the soleus motoneurones inhibition during a tibialis anterior voluntary contraction in man

Summary Variations of the soleus H-reflex were studied during voluntary isometric or anisometric contractions of the tibialis anterior in man. At the onset of isometric contractions there was a weak inhibition of the soleus H-reflex, which was not related to the force of the tibialis anterior contraction. 110 msec after the onset of the EMG activity, the inhibition became secondarily more marked and was then related to the force of the contraction. This secondary potentiation of the H-reflex inhibition is brought about by group I fibres activity, since it was markedly reduced during is-chemia of the leg. It persisted during local muscular fatigue, this indicating that Ib fibres from tibialis anterior are not involved and that, by the process of elimination, group Ia fibres must be responsible for the supplementary secondary inhibition. It is concluded that the early inhibition is only due to suprasegmental activity, whereas during the secondary part of the inhibition there is a supplementary inhibitory action brought about by Ia fibres from tibialis anterior. The secondary potentiation of the inhibition is therefore likely to be produced via the gamma loop.Attachée de Recherches à l'I.N.S.E.R.M.     

Changes in reciprocal ia inhibition during voluntary contraction in man

Summary Reciprocal Ia inhibition from ankle flexors to extensors was studied during voluntary tonic isometric dorsiflexion and plantar flexion in five normal subjects. The Ia inhibition was examined as the short-latency suppression of the soleus H-reflexes by stimulation of the low-threshold afferents in the common peroneal nerve (Mizuno et al. 1971). At rest, weak Ia inhibition was demonstrated in four subjects out of five, the maximal amount being 14.1 ± 5.0% suppression of the control H-reflex. The absolute amount of inhibition, which was calculated by subtracting the mean size of the conditioned H-reflex from that of the control H-reflex and expressed as a percentage of the maximal M-response, increased during ankle dorsiflexion, and decreased or disappeared during plantar flexion in parallel with the amount of contraction. The neural mechanisms for facilitation of the Ia inhibitory pathway during dorsiflexion were considered to support the hypothesis of --linkage in reciprocal inhibition, i.e. combined facilitatory effects on the Ia inhibitory interneurone from the supraspinal centers directly and indirectly via the motoneurone — Ia afferent route. The mechanism for inhibition of the pathway during plantar flexion was considered to be inhibition of the Ia interneurone of the flexor side by Ia interneurone of antagonist extensors. A quantitative aspect of activity in the reciprocal Ia inhibitory pathway on the performance of voluntary movement is revealed in this study.     

Operant conditioning of reciprocal inhibition in rat soleus muscle

Operant conditioning of the H-reflex, the electrical analog of the spinal stretch reflex (SSR), induces activity-dependent plasticity in the spinal cord and might be used to improve locomotion after spinal cord injury. To further assess the potential clinical significance of spinal reflex conditioning, this study asks whether another well-defined spinal reflex pathway, the disynaptic pathway underlying reciprocal inhibition (RI), can also be operantly conditioned. Sprague-Dawley rats were implanted with electromyographic (EMG) electrodes in right soleus (SOL) and tibialis anterior (TA) muscles and a stimulating cuff on the common peroneal (CP) nerve. When background EMG in both muscles remained in defined ranges, CP stimulation elicited the TA H-reflex and SOL RI. After collection of control data for 20 days, each rat was exposed for 50 days to up-conditioning (RIup mode) or down-conditioning (RIdown mode) in which food reward occurred if SOL RI evoked by CP stimulation was more (RIup mode) or less (RIdown mode) than a criterion. TA and SOL background EMG and TA M response remained stable. In every rat, RI conditioning was successful (i.e., change > or =20% in the correct direction). In the RIup rats, final SOL RI averaged 171+/- 28% (mean +/- SE) of control, and final TA H-reflex averaged 114 +/- 14%. In the RIdown rats, final SOL RI averaged 37 +/- 13% of control, and final TA H-reflex averaged 60 +/- 18%. Final SOL RI and TA H-reflex sizes were significantly correlated. Thus like the SSR and the H-reflex, RI can be operantly conditioned; and conditioning one reflex can affect another reflex as well.     

Localized muscle pain causes prolonged recovery after fatiguing isometric contractions

The purpose of this study was to investigate the force and electromyographic (EMG) signal recorded from the muscles immediately after a sustained fatiguing contraction with or without muscle pain. Ten subjects performed sustained dorsi- and plantarflexions at two contraction levels (50 and 80% of maximum voluntary contraction) until exhaustion with or without muscle pain induced by injection of 6% hypertonic saline in one of the torque producing muscles. The muscle pain intensity was scored on a visual analogue scale (VAS, 0–10 cm). The root mean square (RMS) of the surface EMG signal from plantarflexors and dorsiflexors were estimated during maximum voluntary contractions (MVC) and ramp contractions before and after the fatiguing task at 0, 5, 10 and 15 min during the recovery phase. VAS scores immediately after the contractions with hypertonic saline (on average 3.2 ± 1.1 cm) progressively decreased during recovery and no pain was experienced 15 min after the contraction. After the painful contraction the RMS-EMG during MVC was on average decreased (23.4 ± 7.4%) compared to the non-painful condition both in muscles where pain was previously induced and in non-painful synergists. During recovery, the slope of the torque–EMG curve during ramp contraction was significantly decreased (28.4 ± 8.1%) after the painful contraction compared to the control contraction both for the muscle previously exposed to pain and also the other active synergists. The decreased EMG during recovery after painful contractions compared with control was not accompanied by significant reductions in force during MVC indicating a change in the strategy for motor unit recruitment. This study shows that localized muscle pain inhibits muscle activation and increases the effects of fatigue on EMG recovery curves both for painful and non-painful synergists probably by a central effect. These effects can modify the normal patterns of synergistic activation and can also generate overload problems in muscle pain patients if compensatory motor control strategies are applied.     

Heat production and chemical change during isometric contraction of rat soleus muscle.

1. Methods are described whereby the soleus muscle of the rat may be used for the investigation of initial processes in the absence of oxidative recovery. 2. The anaerobic conditions employed had no effect on the concentration of phosphocreatine in resting muscle or the mechanical response during contraction. 3. Muscles were stimulated tetanically for 10 s at 17-18 degrees C. Measurements were made of the heat production and metabolic changes that occurred during a 13 s period following the first stimulus. 4. There was no detectable change in the concentration of ATP. Neither was there detectable activity of adenylate kinase or adenylate deaminase. The changes in the concentration of glycolytic intermediaries were undetectable or very small. 5. The change in the concentration of phosphocreatine was large and amounted to -127 +/- 11-4 mumol/mmol Ct (mean and S.E. of the mean, negative sign indicates break-down, Ct = free creatine + phosphocreatine) which is equivalent to about -2-13 mumol/g wet weight of muscle. The heat production was 6549 +/- 408 mJ/mmol Ct (mean and S.E. of mean) which is equivalent to about 110 mJ/g. 6. About 30% of the observed energy output is unaccounted for by measured metabolic changes. 7. The ratio of heat production (corrected for small amounts of glycolytic activity) to phosphocreatine hydrolysis was -49-7 +/- 5-6 kJ/mol (mean and S.E. of mean), in agreement with previous results using comparable contractions of frog muscle, but different from the enthalpy change associated with phosphocreatine hydrolysis under in vivo conditions (-34 kJ/mol). 8. The results support the notion that the discrepancy between energy output and metabolism is an indication of an unidentified process of substantial energetic significance that is common to a number of species.     

Glycolytic intermediates in human muscle after isometric contraction

Isometric contraction of the quadriceps muscle sustained to fatigue with a force of 66% of the maximum voluntary contraction force resulted in a mean glycogen utilization of 80.4 (S.D. 58.4) mmol glucosyl units/kg dry muscle (d.m.) and an accumulation of glycolytic intermediates and glucose corresponding to 82.9 (S.D. 17.5) mmol glucosyl units/kg d.m. Accumulation of hexose phosphates (principally glucose 6-phosphate) accounted for 35.4% (S.D. 4.1) of the total increase and lactate for 59.3% (S.D. 2.8). During a 4 min recovery period glucose 6-[hosphate content showed a linear decrease with a half time of 2.0 min and lactate decreased exponentially with a half time of 2.5 min. The rate of lactate disappearance from the muscle was approximately 4 times as fast as that observed previously after maximal bicycle exercise. This was probably due to a lower lactate concentration in blood after isometric contraction resulting in a larger muscle-blood gradient for lactate. Muscle content of free glucose was increased after contraction and increased further during recovery. It is concluded that the glucose increase is confined to the intracellular pool and is an effect of hexokinase inhibition by accumulated glucose 6-phosphate. Occlusion, of the local circulation after the contraction inhibited the recovery processes for lactate and glucose 6-phosphate.     

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

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

Evoked H-reflex and V-wave responses during maximal isometric, concentric, and eccentric muscle contraction

This study was designed to investigate the modulations of H-reflex and V-wave responses during passive and maximal active dynamic actions. Experiments were performed on 16 healthy males [age: 24 +/- 4 (SD) yr]. Maximal H-reflexes (Hmax) and M-waves (MmaxR) were evoked at the same muscle length during passive isometric, shortening and lengthening actions and during maximal voluntary isometric, concentric, and eccentric plantar-flexion. In all contraction types, supra-maximal stimulus intensity was used to evoke the superimposed maximal M wave (MmaxA) and V wave (V) of the soleus muscle. At rest, the Hmax/MmaxR ratio was significantly reduced during lengthening with respect to isometric and shortening actions (P < 0.05). For each action type, the ratio between H reflex superimposed to the contraction (Hsup) and MmaxA was not different from Hmax/MmaxR ratio. When plantar flexors were maximally voluntary activated, the Hsup/MmaxA ratio was still lower during eccentric contraction as compared with isometric and concentric efforts (0.33 +/- 0.03 vs. 0.47 +/- 0.02 and 0.50 +/- 0.03, P < 0.001), whereas V/MmaxA ratios were similar for all contraction types (isometric 0.26 +/- 0.02; concentric 0.23 +/- 0.03, and eccentric 0.24 +/- 0.02; P > 0.05). The V/MmaxA ratio was significantly lower than Hsup/MmaxA during isometric and concentric MVC (P < 0.001). No difference was observed between V/MmaxA and Hsup/MmaxA ratios during eccentric efforts. The H-reflex modulations, present during lengthening actions, were mainly attributed to presynaptic inhibition of Ia afferents and to homosynaptic postactivation depression. Results on V wave and H reflex suggest that during eccentric MVC, the spinal loop is specifically modulated by the supra-spinal centers and/or neural mechanisms at spinal level.     

Differential changes in corticospinal and Ia input to tibialis anterior and soleus motor neurones during voluntary contraction in man

Motor-evoked potentials (MEPs) were recorded in the tibialis anterior and soleus muscles following transcranial magnetic stimulation (TMS) of the motor cortex. In the soleus, the H-reflex amplitude increased with the contraction level to the same extent as that of MEPs, whereas in the tibialis anterior, the H-reflex amplitude increased significantly less than that of MEPs. The latency of the MEPs decreased with contraction, whereas this was not the case of the H-reflexes. In the tibialis anterior, the response probability of single-motor units (SMU) to TMS increased more substantially during voluntary contraction than following stimulation of the peroneal nerve. In the tibialis anterior, the response probability of SMU increased more substantially during voluntary contraction than following stimulation of the peroneal nerve. The short-latency facilitation, presumably monosynaptic of origin, of the soleus H-reflex evoked by subthreshold TMS increased as a function of the plantarflexion force. This was not the case for the heteronymous Ia facilitation of the soleus H-reflex following stimulation of the femoral nerve. It is concluded that the corticospinal input to lower limb motor neurones generated by TMS increases with the level of voluntary contraction, whereas this is true only to a limited extent for the synaptic input from Ia afferents. It is suggested that this reflects changes in the susceptibility of corticospinal cells to TMS during voluntary contraction.     

M wave and H-reflex of soleus muscle before and after electrical muscle stimulation in healthy subjects

PURPOSE: This study was the preliminary research for applying the evaluation of muscle fatigue using the evoked electromyography in the field of physical therapy. In this study, we speculated that muscle fatigue was induced by electrical muscle stimulation (EMS). And we studied the M wave and amplitude ratio of H/M before and after EMS because the M wave and amplitude ratio of H/M have been often used as the parameter in the study of muscle fatigue. METHODS: Subjects were five healthy males. In this study, the intermittent EMS (30 Hz) was administered to the soleus muscle of dominant leg for 10 minutes and we analyzed the amplitude of maximal M wave and the amplitude ratio of H/M in this study. RESULTS: The amplitude of maximal M wave after EMS significantly decreased compared with that before the EMS (p < 0.05). And the results of amplitude ratio of H/M varied as follows; decrease in two subjects, increase in one subject and unchanged results in two subjects. DISCUSSION: M wave reflects the excitability of muscle membrane related to the change in force during muscle fatigue and the amplitude ratio of H/M has been considered as the index of a relative excitability of alpha motoneuron pool. From the results of this study, we considered that muscle fatigue was induced by EMS as predictability because the amplitude of maximal M wave significantly decreased after EMS. And it was considered that the excitability of spinal neural function corresponding to fatigued soleus muscle by EMS was not consist change in this study. Therefore we thought that results of amplitude ratio of H/M might be influenced by excitability of spinal neural function in subject's ordinary state. CONCLUSION: It was suggested that the muscle fatigue was induced because the amplitude of maximal M wave significantly decreased after EMS in this study. And also it was suggested the excitability of spinal neural function corresponding with fatigued soleus muscle by EMS was not consist change in this study.     

Surface EMG and motor unit activity of partially denervated human muscle during fatiguing submaximal isometric contraction

The extent to which the normal fatigue compensatory mechanisms are disturbed in partially denervated muscles was investigated in human patients. Surface EMG, as well as motor unit electrical and mechanical activity, were analyzed from the partially denervated first interosseous muscle, during fatiguing isometric submaximal contraction. The EMG power and frequency changes which reveal the local fatigue process of healthy muscle have not been systematically found. Motor unit firing rate changes were rather normal and twitch contraction time did not increase during the fatiguing exercise. Differences between normal and partially denervated muscles could be explain by the occurrence of a central fatigue process more or less important in neurogenic lesions.     

Changes in presynaptic inhibition of Ia fibres to soleus motoneurones during voluntary dorsiflexion of the foot

Summary Variations of presynaptic inhibition in heteronymous Ia fibres projecting to soleus motoneurones were studied during the first 250 ms of phasic voluntary isometric contractions of the antagonist tibialis anterior muscle in human subjects. During the first 60–80 ms of TA e.m.g activity, presynaptic inhibition was often more marked than at rest, but not in all experimental sessions. After 60–80 ms, presynaptic inhibition was always increased compared to rest and to the onset of TA e.m.g. activity. A rebound in femoral nerve induced Ia facilitation was often observed between 90–150 ms. The early increase in presynaptic inhibition was widespread and non specific since it was observed at the onset of extensor carpi radialis contractions of maximal strength. The rebound in heteronymous Ia facilitation was interpreted as a relative decrease in presynaptic inhibition to which nonspecific suprasegmental and cutaneous effects contributed. The late increase in presynaptic inhibition in Ia fibers to soleus motoneurones was considered as reciprocally inhibiting the Sol H-reflex, thus counteracting the phasic stretch of the antagonist muscle during TA contraction.     

Changes in reciprocal Ia inhibition from wrist extensors to wrist flexors during voluntary movement in man

Summary Transmission in the Ia inhibitory pathway from wrist extensor muscles onto flexor MNs was studied at various times after the onset of voluntary wrist extension or flexion. At the very onset of wrist movements Ia inhibition was not changed, as compared to at rest, whereas later it progressively increased during wrist extension and decreased during wrist flexion. These results are discussed in relation to the different inputs converging onto Ia interneurones and it is suggested that their inhibition by Renshaw cells might be responsible for the results found at the onset of contraction     

Sonomyographic responses during voluntary isometric ramp contraction of the human rectus femoris muscle

This paper aims to investigate the relationship between torque and muscle morphological change, which is derived from ultrasound image sequence and termed as sonomyography (SMG), during isometric ramp contraction of the rectus femoris (RF) muscle, and to further compare SMG with the electromyography (EMG) and mechanomyography (MMG), which represent the electrical and mechanical activities of the muscle. Nine subjects performed isometric ramp contraction of knee up to 90% of the maximal voluntary contraction (MVC) at speeds of 45, 22.5 and 15% MVC/s, and EMG, MMG and ultrasonography were simultaneously recorded from the RF muscle. Cross-sectional area, which was referred to as SMG, was automatically extracted from continuously captured ultrasound images using a newly developed image tracking algorithm. Polynomial regression analyses were applied to fit the EMG/MMG/SMG-to-torque relationships, and the regression coefficients of EMG, MMG, and SMG were compared. Moreover, the effect of contraction speed on SMG/EMG/MMG-to-torque relationships was tested by pair-wise comparisons of the mean relationship curves at different speeds for EMG, MMG and SMG. The results show that continuous SMG could provide important morphological parameters of continuous muscle contraction. Compared with EMG and MMG, SMG exhibits different changing patterns with the increase of torque during voluntary isometric ramp contraction, and it is less influenced by the contraction speed.     

The role of the menstrual cycle phase in pain perception before and after an isometric fatiguing contraction

The purpose of this study was to compare exercise-induced analgesia in young women after a fatiguing isometric contraction during different phases of the menstrual cycle. Twenty female subjects performed a submaximal (25% maximal voluntary contraction) isometric contraction until task failure during both the mid-follicular and mid-luteal phases of their menstrual cycle. Pain perception (i.e., pain threshold and pain ratings) was measured before and after the isometric fatiguing contraction. Other measures included mean arterial pressure, heart rate, and anxiety levels. Time to task failure of the fatiguing contraction was similar for the two phases of the menstrual cycle. Following the performance of the isometric contraction: (1) pain thresholds increased and pain ratings decreased; (2) anxiety levels increased; and (3) mean arterial pressure and heart rate increased. These changes were not dependent on the phase of the menstrual cycle. Thus, the menstrual cycle phase does not influence the magnitude of exercise-induced analgesia.     

Theoretical analysis of surface EMG in voluntary isometric contraction

Summary A new stochastic model of the surface EMG is suggested and the spectral density of the surface EMG is studied theoretically and experimentally to confirm the validity of this model.Theoretical results show that while the contraction level is not so high, the shape of the spectral density (distribution) does not change and its amplitude is directly proportional to the motor unit firing frequency and recruitment.To illustrate the theoretical results, experiments were carried out for rectus femoris and biceps brachii. The surface EMG was lead off by bipolar surface electrodes. And the spectral density of the surface EMG was calculated using FFT algorithm.From these experimental results, it was confirmed that our theoretical results were almost valid.     

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

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