Inhibition Of Soleus Research Articles

Effects of Dry Needling on Spinal Reciprocal Inhibition

<h3>Research Objectives</h3> Spinal reciprocal inhibition is an important spinal mechanism of motor control. It is often impaired in persons after stroke, negatively impacting locomotion; at the same time, it changes in response to therapeutic interventions (e.g., FES). An increasing number of physical therapists worldwide have been administering trigger point deep dry needling (DDN) to treat post-stroke spasticity. Yet, currently, effects of DDN on spinal somatosensory processing are not well understood. Thus, to understand neurophysiological mechanisms of DDN, we are investigating the effects of DDN on spinal pathways in persons with and without stroke. <h3>Design</h3> Before and 0, 90 minutes, and 72 hours after DDN of the medial gastrocnemius (MG) at myofascial trigger points, reciprocal inhibition of the soleus, MG and lateral gastrocnemius (LG) was examined while the participant stood and maintained his/her standing posture and triceps muscle activity. Passive ankle range of motion (ROM) was also measured before and after DDN. <h3>Setting</h3> University. <h3>Participants</h3> 11 adults 22-57 years old (median 37.5) without known neurological conditions. <h3>Interventions</h3> Trigger point dry needling to medial gastrocnemius <h3>Main Outcome Measures</h3> Reciprocal inhibition of ongoing muscle activity elicited by common peroneal nerve (CPN) stimulation. ROM. <h3>Results</h3> In the MG and LG, there was no systematic changes in the amount of inhibition across participants or measurement times. Soleus inhibition elicited by suprathreshold CPN stimulation was increased significantly at 0 minute and 72 hours post DDN (p>0.05). Ankle dorsiflexion ROM was increased by 4 deg at 0 minutes and 72 hours post DDN. <h3>Conclusions</h3> DDN alters soleus inhibition, which may partially explain increased ankle ROM post DDN. In order to understand how DDN affects reciprocal inhibition in the stroke-impaired CNS, studies of individuals after stroke are currently underway. <h3>Author(s) Disclosures</h3> The authors have no conflicts of interest to declare.

The Influence of a Cognitive Task on the Postural Phase of Gait Initiation in Parkinson's Disease: An Electromyographic-Based Analysis.

The aim of this study was to compare postural control strategies during gait initiation in single- and dual-task conditions in individuals in early stages of Parkinson's Disease (PD). The activation timing of tibialis anterior occurred significantly later in the individuals with PD than in the controls (p = .05), and a significant interaction between the groups, conditions and limbs was found (p = .027). Differences between the single- and dual-task conditions were observed for the activation timing of the tibialis anterior (p = .042) and for the magnitude of soleus (p = .007), with lower values for the dual-task condition. Furthermore, not all the individuals followed the previously reported pattern of soleus inhibition followed by tibialis anterior activation. The duration of the mediolateral displacement of the center of pressure was longer in the individuals with PD than in the controls (p = .019). The anticipatory postural adjustments during gait initiation are impaired in PD and are expressed by an activation failure of tibialis anterior in both single- and dual-task conditions. Hence, it is important that during rehabilitation, intervention should concentrate on the tibialis anterior TA.

Task-dependent inhibition of slow-twitch soleus and excitation of fast-twitch gastrocnemius do not require high movement speed and velocity-dependent sensory feedback.

Although individual heads of triceps surae, soleus (SO) and medial gastrocnemius (MG) muscles, are often considered close functional synergists, previous studies have shown distinct activity patterns between them in some motor behaviors. The goal of this study was to test two hypotheses explaining inhibition of slow SO with respect to fast MG: (1) inhibition occurs at high movement velocities and mediated by velocity-dependent sensory feedback and (2) inhibition depends on the ankle-knee joint moment combination and does not require high movement velocities. The hypotheses were tested by comparing the SO EMG/MG EMG ratio during fast and slow motor behaviors (cat paw shake responses vs. back, straight leg load lifting in humans), which had the same ankle extension-knee flexion moment combination; and during fast and slow behaviors with the ankle extension-knee extension moment combination (human vertical jumping and stance phase of walking in cats and leg load lifting in humans). In addition, SO EMG/MG EMG ratio was determined during cat paw shake responses and walking before and after removal of stretch velocity-dependent sensory feedback by self-reinnervating SO and/or gastrocnemius. We found the ratio SO EMG/MG EMG below 1 (p < 0.05) during fast paw shake responses and slow back load lifting, requiring the ankle extension-knee flexion moment combination; whereas the ratio SO EMG/MG EMG was above 1 (p < 0.05) during fast vertical jumping and slow tasks of walking and leg load lifting, requiring ankle extension-knee extension moments. Removal of velocity-dependent sensory feedback did not affect the SO EMG/MG EMG ratio in cats. We concluded that the relative inhibition of SO does not require high muscle velocities, depends on ankle-knee moment combinations, and is mechanically advantageous for allowing a greater MG contribution to ankle extension and knee flexion moments.

Effect of natural trunk inclination on variability in soleus inhibition and tibialis anterior activation during gait initiation in young adults

BackgroundAnticipatory postural adjustments (APAs) of gait initiation (GI) permit first step execution. APAs are characterized by a structured pattern of soleus (SOL) inhibition followed by tibialis anterior (TA) activation. This pattern shows variability among young adults where SOL is not always inhibited before TA activation. Initial posture preceding GI could explain a part of this variability. The aim of the study was to investigate the effect of natural trunk inclination on APAs during GI. MethodsTwo groups of twelve subjects divided by natural trunk inclination angle performed five gait initiation trials. A regression model was computed to predict SOL inhibition and TA activation. ResultsBackward leaners showed less SOL inhibition in stance leg (25.8% of trials) compared to forward leaners (55.6% of trials). Regression model revealed that high tonic EMG activity in SOL in the stance leg is the variable that best explains SOL inhibition variation within trials but not TA activation. ConclusionSlight variations in APAs are due to natural trunk inclination but more contribution in APAs is due to initial posture, future step speed and initial tonic soleus activity. Absence of SOL inhibition could be in part explained by natural trunk inclination, where the backward inclination leads to lower tonic SOL activity in quiet standing. These effects could be due to inherent and functional variability, which depend on postural variation, muscular coordination and limb roles.

Biomechanical reorganisation of stepping initiation during acute dorsiflexor fatigue

During voluntary step initiation (SI), propulsive forces are generated during anticipatory postural adjustments (APA) which displace the centre-of-gravity (CoG) in the desired direction. These propulsive forces are implemented by ankle synergy, bilateral soleus inhibition followed by activation of tibialis anterior (TA). The aim of this study was to investigate the effect of fatigue applied to ankle dorsiflexors on APA associated with SI and on related motor performance. Eight young healthy participants initiated stepping before and after a protocol designed to generate fatigue in ankle dorsiflexors. Fatigue was induced by series of high-level isometric contractions performed until exhaustion. Results showed that, with fatigue, the level of TA activation during APA, anticipatory postural dynamics (backward centre-of-pressure displacement and forward CoG velocity) and related motor performance (peak of CoG velocity) were attenuated, while APA duration and total SI duration increased. These changes were interpreted as reflecting a protective strategy aiming to preserve the integrity of the fatigued muscles, rather than an impairment associated with muscle weakness.

Postural responses explored through classical conditioning

The purpose of the study was to determine whether the central nervous system (CNS) requires the sensory feedback generated by balance perturbations in order to trigger postural responses (PRs). In Experiment 1, twenty-one participants experienced toes-up support-surface tilts in two blocks. Control blocks involved unexpected balance perturbations whereas an auditory tone cued the onset of balance perturbations in Conditioning blocks. A single Cue-Only trial followed each block (Cue-OnlyControl and Cue-OnlyConditioning trials) in the absence of balance perturbations. Cue-OnlyConditioning trials were used to determine whether postural perturbations were required in order to trigger PRs. Counter-balancing the order of Control and Conditioning blocks allowed Cue-OnlyControl trials to examine both the audio–spinal/acoustic startle effects of the auditory cue and the carryover effects of the initial conditioning procedure. In Experiment 2, six participants first experienced five consecutive Tone-Only trials that were followed by twenty-five conditioning trials. After conditioning, five Tone-Only trials were again presented consecutively to first elicit and then extinguish the conditioned PRs. Surface electromyography (EMG) recorded muscle activity in soleus (SOL), tibialis anterior (TA) and rectus femoris (RF). EMG onset latencies and amplitudes were calculated together with the onset latency, peak and time-to-peak of shank angular accelerations. Results indicated that an auditory cue could be conditioned to initiate PRs in multiple muscles without balance-relevant sensory triggers generated by balance perturbations. Postural synergies involving excitation of TA and RF and inhibition of SOL were observed following the Cue-OnlyConditioning trials that resulted in shank angular accelerations in the direction required to counter the expected toes-up tilt. Postural synergies were triggered in response to the auditory cue even 15 min post-conditioning. Furthermore, conditioned PRs were quickly extinguished as participants became unresponsive by the third trial in extinction. In conclusion, our results reveal that the CNS does not require sensory feedback from postural perturbations in order to trigger PRs.

Transmission in Heteronymous Spinal Pathways Is Modified after Stroke and Related to Motor Incoordination

Changes in reflex spinal pathways after stroke have been shown to affect motor activity in agonist and antagonist muscles acting at the same joint. However, only a few studies have evaluated the heteronymous reflex pathways modulating motoneuronal activity at different joints. This study investigates whether there are changes in the spinal facilitatory and inhibitory pathways linking knee to ankle extensors and if such changes may be related to motor deficits after stroke. The early facilitation and later inhibition of soleus H reflex evoked by the stimulation of femoral nerve at 2 times the motor threshold of the quadriceps were assessed in 15 healthy participants and on the paretic and the non-paretic sides of 15 stroke participants. The relationships between this reflex modulation and the levels of motor recovery, coordination and spasticity were then studied. Results show a significant (Mann-Whitney U; P<0.05) increase in both the peak amplitude (mean±SEM: 80±22% enhancement of the control H reflex) and duration (4.2±0.5 ms) of the facilitation on the paretic side of the stroke individuals compared to their non-paretic side (36±6% and 2.9±0.4 ms) and to the values of the control subjects (33±4% and 2.8±0.4 ms, respectively). Moreover, the later strong inhibition observed in all control subjects was decreased in the stroke subjects. Both the peak amplitude and the duration of the increased facilitation were inversely correlated (Spearman r = −0.65; P = 0.009 and r = −0.67; P = 0.007, respectively) with the level of coordination (LEMOCOT) of the paretic leg. Duration of this facilitation was also correlated (r = −0.58, P = 0.024) with the level of motor recovery (CMSA). These results confirm changes in transmission in heteronymous spinal pathways that are related to motor deficits after stroke.

Seeking functions for spinal recurrent inhibition

In this issue of The Journal of Physiology, Lamy et al. (2008) describe a rhythmic modulation of recurrent inhibition during walking in man. This provides some insight into the function of recurrent inhibition, first described by Renshaw (1941) as an inhibition of cat motoneurones produced by antidromic stimulation of the axons of neighbouring motoneurones (homonymous action). Subsequent work, also in cats, showed that motor axon collaterals activate interneurones (the eponymous Renshaw cells) that in turn inhibit motoneurones. Actions extend to more distant muscle groups (heteronymous) and parallel the pattern of heteronymous monosynaptic excitation from muscle spindle Ia afferents. Renshaw cells receive peripheral sensory and descending inputs and have output connections to other spinal interneurones and to other Renshaw cells. So recurrent inhibition is more than just negative feedback from and to motoneurones and may have several functions depending upon motor context. Lamy et al. have focused on heteronymous actions (which are more widespread in man than in cats or monkeys) in the context of our unique bipedal form of walking. The knee extensor quadriceps is a source of recurrent inhibition to the ankle extensor and the flexor muscles soleus and tibialis anterior. But given that these ankle muscles are antagonists, how is differential activation achieved? The authors found that around heel strike and early in stance, when quadriceps is active and soleus activity is increasing, recurrent inhibition of soleus is weak. Conversely, later in stance, when quadriceps is active and soleus activity is declining, recurrent inhibition of soleus is strong. Recurrent inhibition of tibialis anterior motoneurones was neither task nor phase dependent. The reduction in soleus recurrent inhibition would assist the transition from swing to stance and enhanced inhibition later in stance would favour the transition from stance to swing by reducing reciprocal inhibition of tibialis anterior. The origin of these changes in recurrent inhibition remains uncertain but activity of peripheral sensory or descending motor pathways could be involved. Two interesting possibilities are the corticospinal pathway, known to depress homonymous recurrent inhibition of soleus in sitting subjects (Mazzocchio et al. 1994) and the vestibulospinal pathway that depresses recurrent inhibition of soleus in standing stance (Iles & Pisini, 1992). It might be possible to extend such studies to heteronymous actions in walking stance. More meticulous experiments built upon the framework of human research protocols pioneered by the Paris group will likely increase our understanding of recurrent inhibition even further.

Spinal Reflexes in Ankle Flexor and Extensor Muscles After Chronic Central Nervous System Lesions and Functional Electrical Stimulation

Objective. Spinal reciprocal inhibitory and excitatory reflexes of ankle extensor and flexor muscles were investigated in ambulatory participants with chronic central nervous system (CNS) lesions causing foot drop as a function of time after lesion and stimulator use. Methods. Thirty-nine participants with progressive (eg, secondary progressive MS) and 36 with generally nonprogressive (eg, stroke) conditions were studied. The tibialis anterior (TA) and soleus maximum H-reflex/M-wave (Hmax/Mmax) ratios and maximum voluntary contractions (MVC) were measured and compared with those in age-matched control participants. Reciprocal inhibition was measured as a depression of the ongoing electromyographic (EMG) activity produced by antagonist muscle—nerve stimulation. Results. Participants with CNS lesions had significantly higher soleus Hmax/Mmax ratios than control participants, and reduced voluntary modulation of the reflexes occurred in both muscles. Reciprocal inhibition of soleus from common peroneal (CP) nerve stimulation was not significantly different from controls in either group. Inhibition of the TA by tibial nerve stimulation decreased and was eventually replaced by excitation in participants with nonprogressive disorders. No significant change occurred in progressive disorders. Use of a foot drop stimulator increased the TA, but not the soleus MVC overall. H-reflexes only showed small changes. Reciprocal inhibition of the TA increased considerably, while that of the soleus muscle decreased toward control values. Conclusions. Disorders that produce foot drop also produce reflex changes, some of which only develop over a period of years or even decades. Regular use of a foot drop stimulator strengthens voluntary pathways and changes some reflexes toward control values. Thus, stimulators may provide multiple benefits to people with foot drop.

Immobilization induces changes in presynaptic control of group Ia afferents in healthy humans

Neural plasticity occurs throughout adult life in response to maturation, use and disuse. Recent studies have documented that H-reflex amplitudes increase following a period of immobilization. To elucidate the mechanisms contributing to the increase in H-reflex size following immobilization we immobilized the left foot and ankle joint for 2 weeks in 12 able-bodied subjects. Disynaptic reciprocal inhibition of soleus (SOL) motoneurons and presynaptic control of SOL group Ia afferents was measured before and after the immobilization as well as following 2 weeks of recovery. Following immobilization, maximal voluntary plantar- and dorsiflexion torque (MVC) was significantly reduced and the maximal SOL H-reflex amplitude increased with no changes in the maximal compound motor response (M(max)). Decreased presynaptic inhibition of the Ia afferents probably contributed to the increase of the H-reflex size, since we observed a significant decrease in the long-latency depression of the SOL H-reflex evoked by peroneal nerve stimulation (D2 inhibition) and an increase in the size of the monosynaptic Ia facilitation of the SOL H-reflex evoked by femoral nerve stimulation. These two measures provide independent evidence of changes in presynaptic inhibition of SOL Ia afferents and taken together suggest that GABAergic presynaptic inhibition of the SOL Ia afferents is decreased following 2 weeks of immobilization. The depression of the SOL H-reflex when evoked at intervals shorter than 10 s (homosynaptic post-activation depression) also decreased following immobilization, suggesting that the activity-dependent regulation of transmitter release from the afferents was also affected by immobilization. We observed no significant changes in disynaptic reciprocal Ia inhibition. Two weeks after cast removal measurements returned to pre-immobilization levels. Together, these observations suggest that disuse causes plastic changes in spinal interneuronal circuitries responsible for presynaptic control of sensory input to the spinal cord. This may be of significance for the motor disabilities seen following immobilization as well as the development of spasticity following central motor lesions.

Biomechanical analysis of movement strategies in human forward trunk bending. II. Experimental study

The large mass of the human upper trunk, its elevated position during erect stance, and the small area limited by the size of the feet, stress the importance of equilibrium control during trunk movements. The objective of the present study was to perform a biomechanical analysis of fast forward trunk movements in order to understand the coordination between movement and posture. The analysis is based on a comparison between experimentally observed bending and hypothetical "optimal bending" performed on an infinitely narrow support, as presented in a companion paper. The experimental data were obtained from 16 subjects who performed fast forward bending while standing on a wide platform or on a narrow beam. The analysis is performed by decomposition of the movement into three dynamically independent components, each representing a movement along one of the three eigenvectors of the motion equation. The eigenmovements are termed "hip", "ankle", and "knee" eigenmovements, according to the dominant joint. The experimentally observed movement is characterized mainly by the hip and ankle eigenmovements, whereas the knee eigenmovement is negligible. Similarly to the "optimal bending" the ankle eigenmovement starts earlier and lasts longer than the hip eigenmovement. An early forward acceleration of the center of gravity in the ankle eigenmovement is caused by anticipatory changes in the ankle joint torque. This clarifies the role of the early tibialis anterior burst and/or soleus inhibition usually observed in electromyographic recordings during forward bending. The results suggest that the hip and the ankle eigenmovements can be treated as independently controlled motion units aimed at functionally different behavioral goals: the bending per se and postural adjustment. It is proposed that the central nervous system has to control these motion units sequentially in order to perform the movement and maintain equilibrium. It is also suggested that the hip and ankle eigenmovements can be regarded as a biomechanical background for the hip and ankle strategies introduced by Horak and Nashner (1986) on the basis of electromyographic recordings and kinematic patterns in response to postural perturbations.

Recruitment of triceps surae motor units in the decerebrate cat. I. Independence of type S units in soleus and medial gastrocnemius muscles.

1. We tested the hypothesis that reflex inhibition of soleus motor units reflects selective inhibition of slow-twitch (type S) motor units throughout the triceps surae. Physiological properties including type, together with firing behavior, were measured from single motor units in the medial gastrocnemius (MG) muscle of decerebrate cats with the use of intra-axonal recording and stimulation. MG unit firing was contrasted during net inhibition or excitation of the slow-twitch soleus muscle produced by ramp-hold-release stretches of MG. 2. Stretch of the MG muscle increased the firing of type S motor units in the MG regardless of the reflex response of the soleus muscle. When stretch inhibited soleus, each of the 14 type S units sampled from MG either was newly recruited or exhibited increases in the rate of ongoing firing. Increased firing was observed in 320 of 321 stretch trials. For 8 of these 14 units, a total of 155 stretch trials evoked reflex excitation of soleus, and unit firing increased in all trials. 3. For the eight MG type S motor units studied during both reflex inhibition and excitation of soleus, firing rate tended to be higher during inhibition. The higher rates were also associated with the higher MG forces required to elicit soleus inhibition. For one MG type S unit it was possible to compare firing rates during soleus inhibition and excitation for trials of overlapping levels of MG force. For this unit, firing rate was similar, but still appreciably higher, during inhibition. 4. Soleus inhibition was also produced by stretch of the plantaris (PL) or lateral gastrocnemius (LG) muscles. Type S units in PL (n = 2) or in LG (n = 1) were recruited or increased firing rate even when stretch of these muscles produced soleus inhibition. 5. The firing behavior of 12 fast-twitch (type F) units was studied (11 from MG, 1 from PL). All type F units either were recruited or accelerated the rate of firing during soleus inhibition, as well as during soleus excitation. 6. These findings give evidence that reflex inhibition of type S motor units in the soleus muscle does not necessarily reflect an organizational scheme in which there is inactivation of type S units in other active muscles. In the DISCUSSION we point out the absence of direct evidence for selective inactivation of units on the basis of their type classification.

Recruitment of triceps surae motor units in the decerebrate cat. II. Heterogeneity among soleus motor units.

1. On the basis of the orderly activation of motoneurons in a pool, one would predict that motor unit activity and whole muscle force will change at least roughly in parallel: active motor units should continue to fire as net muscle force increases and quiescent motor units should remain inactive as muscle force decreases. We have consistently observed this relationship in our studies of the medial gastrocnemius (MG) muscle, but here we report an uncoupling of the soleus muscle and some of its motor units. 2. Physiological properties and firing behaviors of 20 soleus motor units were characterized in five decerebrate cats with the use of intra-axonal stimulation and recording. Motor unit firing was elicited in reflexes initiated by muscle stretch, nerve stimulation, and mechanical stimulation of the heel. Particular emphasis was placed on the heterogenic reflexes produced in soleus by ramphold-release stretches of the MG muscle. In agreement with previous reports, either net heterogenic excitation or inhibition of the soleus muscle was produced in separate trials of MG stretch. 3. During excitation of soleus in autogenic stretch reflexes and in crossed-extension reflexes, all 20 units were recruited or increased firing, i.e., unit firing was coupled with soleus force. In the other reflexes, however, unit firing and muscle force were uncoupled for 10 of these units. Six tonically active motor units were inhibited during an increase in soleus force produced by MG stretch or by mechanical stimulation of the heel. Four motor units were activated during a decrease in soleus force produced by the same stimuli. 4. Six motor units were studied during both soleus inhibition and excitation evoked by MG stretches. One motor unit was consistently coupled to the soleus muscle response; firing increased during soleus excitation and decreased during inhibition. However, four soleus motor units were inhibited under both conditions, and one unit was excited under both conditions. Thus the firing behavior of five of these six motor units was the same in response to MG stretch, irrespective of the soleus response. 5. The uncoupling was most clearly recognized when tonically active units ceased firing during net excitation of the soleus muscle and when silent units began firing during net inhibition of the soleus muscle. Unit responses were not as striking in all trials of MG stretch (spike number increased or decreased relative to prestretch values by 1-4 spikes), but the responses were consistent across trials; in multiple stretches, spike number commonly either increased or decreased. Intertrial regularity was also observed in units for which firing was coupled with the net reflex response of the soleus muscle. 6. Divergence in the firing of soleus motor units was also observed in three cases in which records were taken simultaneously from two motor units. In one pair, one unit increased and the other decreased firing during MG stretch-evoked inhibition of soleus. In the other two pairs, one unit increased and the other decreased firing when soleus was excited by heel stimulation. In all pairs, the unit that decreased firing under these conditions had the lowest recruitment threshold in response to the soleus stretch. 7. Although all soleus motor units were classified as slow-twitch (type S), variation in their physiological properties bore some relation to firing behavior. Those units recruited during periods of soleus inhibition exhibited among the fastest conduction velocities and contraction times in our sample. In all three unit pairs sampled, the unit expressing decreases in firing had the slower conduction velocity and contraction time. 8. These findings demonstrate that soleus motor units are differentially activated and deactivated by peripheral afferents. (ABSTRACT TRUNCATED)

Central motor conduction in Hirayama disease

The pathogenesis of Hirayama disease is usually attributed to microcirculatory disturbances in the anterior spinal artery territory, leading to segmental anterior horn cell loss and occasional lower limb hyperreflexia. In 7 patients with Hirayama disease, central motor conduction to upper (CMCT-ADM) and lower limbs (CMCT-TA) was evaluated. CMCT-TA was normal in all, but CMCT-ADM was marginally prolonged (8.4 msec, amplitude 0.8 mV) on one side only. Peripheral delay in the upper limbs was found in 2 patients (1 side each) which might be due to fall-out of anterior horn cells. In 2 patients with lower limb hyperreflexia, HM ratio, vibratory inhibition and reciprocal inhibition of soleus H reflex were also normal, suggesting lack of pyramidal dysfunction. Our results do not suggest any pyramidal dysfunction as a cause of lower limb hyperreflexia in Hirayama disease.

Depression of Renshaw recurrent inhibition by activation of corticospinal fibres in human upper and lower limb.

1. This study tested whether the recurrent inhibition of soleus and wrist flexor motoneurones could be modified by transcranial magnetic stimulation in human subjects. 2. Magnetic stimulation was given through a circular coil centred at the vertex. The intensity of the magnetic stimulus was subthreshold for evoking a motor response in the active soleus and wrist flexor muscles. The recurrent inhibition brought about by a conditioning H1 reflex discharge was estimated by a test H' reflex. The modifications of the recurrent inhibition after cortical stimulation were distinguished from the motoneuronal changes by comparing H' to a reference H reflex. 3. In the soleus motoneurones, the reference H reflex was inhibited at a minimum conditioning-test interval of -2 ms (H reflex stimulus before magnetic stimulation). In contrast, the H' reflex was facilitated at minimum conditioning-test intervals of +1 ms. In the wrist flexor motoneurones, both H' and reference H reflexes were facilitated. However, at lower cortical stimulus intensities, only the H' reflex was facilitated at minimum conditioning-test intervals of +1 ms. 4. In both motoneurone pools, H' facilitation started 3-4 ms later than the earliest changes in the reference H reflex. Also, the threshold of H' facilitation was lower than that of reference H reflex. 5. It is concluded that facilitation of the H' reflex is produced by corticospinal inhibition of Renshaw cells via a short interneuronal chain in both the upper and lower limb.

The regulation of presynaptic inhibition during co-contraction of antagonistic muscles in man.

1. The size of the soleus, tibialis anterior and medial gastrocnemius H reflexes were measured at different levels of EMG activity in ankle plantar- and dorsiflexors. At similar levels of EMG activity in the agonist muscle, the soleus and tibials anterior H reflexes were found to be smaller during co-contraction than during an isolated agonist contraction. In contrast to this, the medial gastrocnemius H reflex had the same size during co-contraction as during an isolated plantarflexion at comparable EMG levels. 2. Peaks occurred at monosynaptic reflex latency in the post-stimulus time histogram (PSTH) of firing of soleus and tibialis anterior motor units following stimulation of their respective motor nerves. These homonymous Ia peaks were found to be smaller during co-contraction than during isolated agonist contraction, whereas that of medial gastrocnemius motor units was unchanged. As the monosynaptic peak in the PSTH may give an indirect measure of the size of the monosynaptic EPSP in the motoneurone, it is suggested that these findings reflect a differential control of presynaptic inhibition of soleus and tibialis anterior motor units with respect to medial gastrocnemius motor units. 3. The monosynaptic Ia facilitation of the soleus H reflex evoked by a stimulation of the femoral nerve, was also found to be smaller during co-contraction of tibials anterior and soleus than during isolated plantarflexion. This suggests that presynaptic inhibition of heteronymous Ia fibres from the quadriceps muscle projecting to soleus motoneurones is regulated in the same way as presynaptic inhibition of homonymous Ia fibres during co-contraction. 4. The femoral nerve-induced monosynaptic facilitation of the soleus H reflex was found to decrease with increasing tonic levels of both co-contraction and isolated dorsi- and plantarflexion. The amount of facilitation decreased to the same degree with increasing levels of dorsiflexion and co-contraction. For the same level of EMG in the dorsiflexors there was thus no difference in the amount of facilitation whether the dorsiflexors were activated separately or in a co-contraction. 5. Whereas the soleus H reflex increased in size 50 ms prior to the onset of a dynamic plantarflexion, it did not change prior to the onset of a co-contraction. The femoral nerve-induced facilitation increased prior to the plantarflexion, but decreased prior to the co-contraction.(ABSTRACT TRUNCATED AT 400 WORDS)

Contralateral inhibition of soleus H reflexes with different velocities of passive movement of the opposite leg

The research question was, do events arising from rhythmic passive movement of the human leg lead to inhibition of the H reflex pathway in the stationary leg contralateral to that movement? Further, as the angular velocity of the passive movement increases, does the contralateral reflex inhibition also increase? Stable stimulation of the tibial nerve elicited H reflexes in the EMG of soleus. Trials involved the stimulated or the contralateral leg being rotated passively in a pedalling motion, at various velocities. The controls were made with the subjects seated and relaxed. The results showed that reflex magnitudes were significantly depressed when the test limb was passively rotated at 60 rpm, in comparison to the seated control trials. Rotation of the opposite limb depressed reflex magnitudes in the test limb, which was stationary. This contralateral inhibition increased, (mean reflex magnitudes of 62.68%, 41.04%, 16.65% and 9.58% of peak-to-peak M max), as the velocity of rotation of the opposite limb increased (10, 30, 60, 90 rpm, respectively) ( P < 0.01). The effect of movement velocity was interpreted as the result of altered sensory receptor discharge arising from the passive movement. It is concluded that contralateral sensory activity contributes to the movement-elicited afferent discharge which tunes the spinal somatosensory-motor mechanisms for human locomotion.

A motor programme for the initiation of forward‐oriented movements in humans.

1. The EMG sequence activated before the initiation of a number of fast forward-oriented voluntary movements was analysed quantitatively in normal subjects. 2. The sequence consisted of an initial inhibitory component directed to the soleus motor nucleus, followed by a second excitatory one directed to the tibialis anterior (TA). 3. The spectrum of functional utilization included motor tasks in which the prime movers are leg and thigh muscles (initiation of gait, rising on tip-toes), thigh and trunk muscles (fast-forward bending of the trunk, standing up) and upper-limb muscles (forward throw or catch). 4. In a same motor task and across the different motor tasks, performed at various speeds, the latency of soleus inhibition and TA activation with respect to the onset of movement co-varied according to a linear function, indicating a close temporal correlation between the two components. 5. In all the movements investigated, the earliest mechanical effect was a backward displacement of the centre of foot pressure in the sagittal plane. 6. Soleus inhibition alone and TA burst alone were each able to produce a backward displacement of the centre of foot pressure, but the effect was significantly slower after soleus inhibition. 7. The spatio-temporal parameters of the sequence were modulated according to the pre-existing postural conditions. For the gait initiation protocol, increasing initial forward leaning led to a decrease in the amplitude of soleus inhibition and the TA burst, and to a change in their relative time delays. Modulation was different on the two sides. We could define a postural boundary as the degree of forward leaning beyond which the full sequence is no longer called into action. 8. The spatio-temporal parameters of the sequence were pre-set according to the requirements of the forthcoming movement. In the gait initiation protocol, the amplitude and synchronization of the TA burst were directly correlated with velocity of movement, while the relative delay between soleus inhibition and TA activation was inversely correlated. Modulation on the two sides differed. We could define a velocity boundary as the velocity of movement below which the full sequence is no longer called into action. 9. We suggest that the EMG sequence described can be considered a motor programme that, through direct action on the position of the centre of foot pressure (the variable primarily controlled), will precisely adjust the configuration of forces external to the body, allowing the contraction of the prime mover(s) to interact appropriately with them for the production of a specific, forward-oriented movement.

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 < 0.01)), and with isometric contractions in the same subjects seated (1.03 ( r = 0.69, P < 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.

Biphasic inhibitory responses and their IPSPs evoked by tibial nerve stimulation in human soleus motor neurones.

A novel stimulating protocol was used to demonstrate the existence of a biphasic inhibitory reflex in single soleus motor neurones in response to weak stimulation of the tibial nerve in human subjects. The stimulus intensity was sufficient to evoke a small M-response but was subthreshold for the H-reflex. Stronger stimulation caused the 2 phases of inhibition to fuse. The experimental protocol allowed excitatory and inhibitory responses to be recognized unambiguously. The criterion for excitation was stimulus-induced advance in the timing of the next action potential in certain trials: for inhibition, the criterion was stimulus-induced delay in the timing of the next action potential. The shorter-latency inhibitory response began at about 45-50 ms and lasted for about 20 ms. With the stimulus intensity used, only inhibitory responses were evoked at short latency in tibialis motor neurones, which suggests that the soleus inhibition resulted from recurrent inhibition via the Renshaw pathway rather than by activation of Ib afferents in the tibial nerve. A novel approach to the determination of the waveform of the IPSP underlying the shorter-latency (presumably Renshaw-mediated) inhibition revealed only part of the waveform owing to the sharp fall in the membrane potential at the onset of (presumably-Renshaw) IPSP. This approach was more successful when applied to the longer-latency, slower IPSP. The longer-latency inhibitory response in soleus motor neurones began at about 80-120 ms. Overall, the latency of this inhibition was less when it followed an M-response than when it followed an H-reflex.(ABSTRACT TRUNCATED AT 250 WORDS)