Assessment of soleus motoneuronal excitability using the joint angle dependent H reflex in humans

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

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

M-wave and H-reflex recruitment curves in boys and men

The aim of the present study was to check whether the M-wave and H-reflex recruitment curves differ between prepubertal boys and men. Eleven boys (9-11 yr) and eleven men (18-35 yr) were magnetically stimulated at the tibial nerve in a prone position. M-wave and H-reflex maximal amplitudes (H_max; M_max ; H_max /M_max ), thresholds, regression slopes (H_slp ; M_slp ; H_slp /M_slp ) were extracted from M-wave and H-reflex recruitment curves and compared between the two age groups. Overall, no significant difference in M-wave and H-reflex recruitment curve parameters was found between the two populations. Nevertheless, the size of the M-wave associated with maximal H-reflex amplitude was lower in boys as compared to men when expressed relative to maximal M-wave amplitude (M_Hmax /M_max : 0.18 ± 0.06 vs. 0.31 ± 0.13; p < .05). This result suggests that the development of peripheral nerve was completed in 9 to 11-year-old boys and did not affect the M-wave and H-reflex recruitment curves parameters. In neuromuscular function studies, it implies that H_max /M_max and H_slp /M_slp could be used indifferently to compare spinal motoneuron excitability between 9-11-year-old boys and men. Conversely, evoking H-reflexes at a given percentage of M_max may bias the comparison between boys and men.

Posture influences on vestibulospinal tract excitability

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

The effects of forearm position and contraction intensity on cortical and spinal excitability during a submaximal force steadiness task of the elbow flexors

Elbow flexor force steadiness is less with the forearm pronated (PRO) compared with neutral (NEU) or supinated (SUP) and may relate to neural excitability. Although not tested in a force steadiness paradigm, lower spinal and cortical excitability was observed separately for biceps brachii in PRO, possibly dependent on contractile status at the time of assessment. This study aimed to investigate position-dependent changes in force steadiness as well as spinal and cortical excitability at a variety of contraction intensities. Thirteen males (26 ± 7 yr; means ± SD) performed three blocks (PRO, NEU, and SUP) of 24 brief (~6 s) isometric elbow flexor contractions (5, 10, 25 or 50% of maximal force). During each contraction, transcranial magnetic stimulation or transmastoid stimulation was delivered to elicit a motor-evoked potential (MEP) or cervicomedullary motor-evoked potential (CMEP), respectively. Force steadiness was lower in PRO compared with NEU and SUP (P ≤ 0.001), with no difference between NEU and SUP. Similarly, spinal excitability (CMEP/maximal M wave) was lower in PRO than NEU (25 and 50% maximal force; P ≤ 0.010) and SUP (all force levels; P ≤ 0.004), with no difference between NEU and SUP. Cortical excitability (MEP/CMEP) did not change with forearm position (P = 0.055); however, a priori post hoc testing for position showed excitability was 39.8 ± 38.3% lower for PRO than NEU at 25% maximal force (P = 0.006). The data suggest that contraction intensity influences the effect of forearm position on neural excitability and that reduced spinal and, to a lesser extent, cortical excitability could contribute to lower force steadiness in PRO compared with NEU and SUP.NEW & NOTEWORTHY To address conflicting reports about the effect of forearm position on spinal and cortical excitability of the elbow flexors, we examine the influence of contraction intensity. For the first time, excitability data are considered in a force steadiness context. Motoneuronal excitability is lowest in pronation and this disparity increases with contraction intensity. Cortical excitability exhibits a similar pattern from 5 to 25% of maximal force. Lower corticospinal excitability likely contributes to relatively poor force steadiness in pronation.

Effect of Ankle Angles on the Soleus H-Reflex Excitability During Standing

This study investigated effects of ankle joint angle on the Hoffman's reflex (H-reflex) excitability during loaded (weight borne with both legs) and unloaded (full body weight borne with the contralateral leg) standing in people without neurological injuries. Soleus H-reflex/M-wave recruitment curves were examined during upright standing on three different slopes that imposed plantar flexion (-15°), dorsiflexion (+15°), and neutral (0°) angles at the ankle, with the test leg loaded and unloaded. With the leg loaded and unloaded, maximum H-reflex/maximum M-wave ratio of -15° was significantly larger than those of 0° and +15° conditions. The maximum H-reflex/maximum M-wave ratios were 51%, 43%, and 41% with loaded and 56%, 46%, and 44% with unloaded for -15°, 0°, and +15° slope conditions, respectively. Thus, limb loading/unloading had limited impact on the extent of influence that ankle angles exert on the H-reflex excitability. This suggests that task-dependent central nervous system control of reflex excitability may regulate the influence of sensory input on the spinal reflex during standing.

Robot controlled, continuous passive movement of the ankle reduces spinal cord excitability in participants with spasticity: a pilot study

Spasticity of the ankle reduces quality of life by impeding walking and other activities of daily living. Robot-driven continuous passive movement (CPM) is a strategy for lower limb spasticity management but effects on spasticity, walking ability and spinal cord excitability (SCE) are unknown. The objectives of this experiment were to evaluate (1) acute changes in SCE induced by 30 min of CPM at the ankle joint, in individuals without neurological impairment and those with lower limb spasticity; and, (2) the effects of 6 weeks of CPM training on SCE, spasticity and walking ability in those with lower limb spasticity. SCE was assessed using soleus Hoffmann (H-) reflexes, collected prior to and immediately after CPM for acute assessments, whereas a multiple baseline repeated measures design assessed changes following 18 CPM sessions. Spasticity and walking ability were assessed using the Modified Ashworth Scale, the 10 m Walk test, and the Timed Up and Go test. Twenty-one neurologically intact and nine participants with spasticity (various neurological conditions) were recruited. In the neurologically intact group, CPM caused bi-directional modulation of H-reflexes creating 'facilitation' and 'suppression' groups. In contrast, amongst participants with spasticity, acute CPM facilitated H-reflexes. After CPM training, H-reflex excitability on both the more-affected and less-affected sides was reduced; on the more affected side H@Thres, H@50 and H@100 all significantly decreased following CPM training by 96.5 ± 7.7%, 90.9 ± 9.2%, and 62.9 ± 21.1%, respectively. After training there were modest improvements in walking and clinical measures of spasticity for some participants. We conclude that CPM of the ankle can significantly alter SCE. The use of CPM in those with spasticity can provide a temporary period of improved walking, but efficacy of treatment remains unknown.

The effect of imperceptible Gaussian tendon vibration on the Hoffmann reflex

Imperceptible vibratory Gaussian noise stimulation to the periphery is frequently being applied to humans to enhance motor performance. It is commonly theorized that this stimulation creates a Stochastic Resonance-like effect across both sensory and motor systems, but this idea has no empirical support. In contrast, there is substantial work showing that tendon vibration can be both excitatory and inhibitory on the lower motor neuron output. In this work, we demonstrate that delivery of imperceptible vibratory Gaussian noise stimulation to the wrist flexor tendons results in a 27% increase in excitability of the lower motor neuron pool in the median nerve, as evidenced by changes in the Hoffmann reflex. We argue that the well-documented tonic vibration reflex is a sufficient mechanistic explanation for the behavioral changes observed during the introduction of vibratory noise stimulation.

Soleus H-Reflex Inhibition Decreases During 30 s Static Stretching of Plantar Flexors, Showing Two Recovery Steps

During the period when the ankle joint is kept in a dorsiflexed position, the soleus (SOL) H-reflex is inhibited. The nature of this inhibition is not fully understood. One hypothesis is that the decrease in spinal excitability could be attributed to post-activation depression of muscle spindle afferents due to their higher firing rate during the stretch-and-hold procedure. As the static stretching position is maintained though, a partial restoration of the neurotransmitter is expected and should mirror a decrease in H-reflex inhibition. In the present study, we explored the time course of spinal excitability during a period of stretching. SOL H-reflex was elicited during a passive dorsiflexion movement, at 3, 6, 9, 12, 18, 21, and 25 s during maximal ankle dorsiflexion, during plantar flexion (PF) and after stretching, in 12 healthy young individuals. Measurements during passive dorsiflexion, PF and after stretching were all performed with the ankle at 100° angle; measurements during static stretching were performed at individual maximal dorsiflexion. H-reflex was strongly inhibited during the dorsiflexion movement and at maximal dorsiflexion (p < 0.0001) but recovered during PF and after stretching. During stretching H-reflex showed a recovery pattern (r = 0.836, P = 0.019) with two distinct recovery steps at 6 and 21 s into stretching. It is hypothesized that the H-reflex inhibition observed until 18 s into stretching is the result of post-activation depression of Ia afferent caused by the passive dorsiflexion movement needed to move the ankle into testing position. From 21 s into stretching, the lower inhibition could be caused by a weaker post-activation depression, inhibition from secondary afferents or post-synaptic inhibitions.

Transient Increase in Cortical Excitability Following Static Stretching of Plantar Flexor Muscles

Spinal excitability in humans is inhibited by both passively holding a static position with the muscle lengthened (static stretching) and by a single non-active lengthening movement. However, whilst immediately after a passive lengthening movement the inhibition persists for several seconds, there seem to be an immediate recovery following static stretching. This result is counter intuitive and could be attributed to methodological procedures. Indeed, differently to what has been done until now, in order to study whether static stretching has a transient effect on the neuromuscular pathway, the procedure should be repeated many times and measurements collected at different time points after stretching. In the present study we repeated 60 times 30 s static stretching of ankle plantar flexors and measured tap reflex (T-reflex), Hoffman reflex (H-reflex), and motor evoked potentials (MEPs) from the Soleus muscle at several time points, starting from immediately after until 30 s following the procedure. T-reflex was strongly inhibited (range 31–91%, p = 0.005) and the inhibition persisted for 30 s showing a slow recovery (r = 0.541, p = 0.037). H-reflex was not affected by the procedure. Stretching increased the size of the MEPs (p < 0.0001), differences at times 0 and 2 s after stretching (p = 0.015 and p = 0.047, respectively). These results confirm that static stretching reduces muscle spindle sensitivity. Moreover it is suggested that post-activation depression of Ia afferents, which is commonly considered the cause of H-reflex depression during both dorsiflexion and static stretching, vanished immediately following stretching or is counteracted by an increased corticospinal excitability.

Effect of knee angle on neuromuscular assessment of plantar flexor muscles: A reliability study

INTRODUCTION

This study aimed to determine the intra- and inter-session reliability of neuromuscular assessment of plantar flexor (PF) muscles at three knee angles.

METHODS

Twelve young adults were tested for three knee angles (90°, 30° and 0°) and at three time points separated by 1 hour (intra-session) and 7 days (inter-session). Electrical (H reflex, M wave) and mechanical (evoked and maximal voluntary torque, activation level) parameters were measured on the PF muscles. Intraclass correlation coefficients (ICC) and coefficients of variation were calculated to determine intra- and inter-session reliability.

RESULTS

The mechanical measurements presented excellent (ICC>0.75) intra- and inter-session reliabilities regardless of the knee angle considered. The reliability of electrical measurements was better for the 90° knee angle compared to the 0° and 30° angles.

CONCLUSIONS

Changes in the knee angle may influence the reliability of neuromuscular assessments, which indicates the importance of considering the knee angle to collect consistent outcomes on the PF muscles.

Assessing sensorimotor excitability after spinal cord injury: a reflex testing method based on cycling with afferent stimulation

Several studies have examined spinal reflex modulation during leg cycling in healthy and spinal cord injury (SCI) subjects. However, the effect of cutaneous plantar afferent input on spinal excitability during leg cycling after SCI has not been characterised. The aim of the study was to test the feasibility of using controlled leg cycling in combination with plantar cutaneous electrical stimulation (ES) cycling to assess lower limb spinal sensorimotor excitability in subjects with motor complete or incomplete SCI. Spinal sensorimotor excitability was estimated by measuring cutaneomuscular-conditioned soleus H-reflex activity. Reflex excitability was tested before and after a 10-min ES cycling session in 13 non-injured subjects, 6 subjects with motor incomplete SCI (iSCI) who had moderately impaired gait function, 4 subjects with motor iSCI who had severely impaired gait function, and 5 subjects with motor complete SCI (cSCI). No modulation of soleus H-reflex with plantar cutaneous stimuli was observed after either iSCI or cSCI when compared to non-injured subjects. However, after ES cycling, reflex excitability significantly increased in subjects with iSCI and moderately impaired gait function. ES cycling facilitated spinal sensorimotor excitability only in subjects with motor iSCI with residual gait function. Increased spinal excitability induced with a combination of exercise and afferent stimulation could be adopted with diagnostic and prognostic purposes to reveal the activity-based neurorehabilitation profile of individual subjects with motor iSCI.

TRIAL REGISTRATION

ISRCTN 26172500 ; retrospectively registered on 15 July 2016 Graphical abstract ᅟ.

Changes in H-reflex amplitude to muscle stretch and lengthening in humans

Spinal reflex excitability is traditionally assessed to investigate neural adjustments that occur during human movement. Different experimental procedures are known to condition spinal reflex excitability. Among these, lengthening movements and static stretching the human triceps have been investigated over the last 50 years. The purpose of this review is to shed light on several apparent incongruities in terms of magnitude and duration of the reported results. In the present review dissimilarities in neuro-spinal changes are examined in relation to the methodologies applied to condition and measure them. Literature that investigated three different conditioning procedures was reviewed: passive dorsiflexion, active dorsiflexion through antagonists shortening and eccentric plantar-flexors contractions. Measurements were obtained before, during and after lengthening or stretching. Stimulation intensities and time delays between conditioning procedures and stimuli varied considerably. H-reflex decreases immediately as static stretching is applied and in proportion to the stretch degree. During dorsiflexions the inhibition is stronger with greater dorsiflexion angular velocity and at lower nerve stimulation intensities, while it is weaker if any concomitant muscle contraction is performed. Within 2 s after a single passive dorsiflexion movement, H-reflex is strongly inhibited, and this effect disappears within 15 s. Dorsiflexions repeated over 1 h and prolonged static stretching training induce long-lasting inhibition. This review highlights that the apparent disagreement between studies is ascribable to small methodological differences. Lengthening movements and stretching can strongly influence spinal neural pathways. Results interpretation, however, needs careful consideration of the methodology applied.

Physiological tremor increases when skeletal muscle is shortened: implications for fusimotor control

KEY POINTS

In tonic, isometric, plantarflexion contractions, physiological tremor increases as the ankle joint becomes plantarflexed. Modulation of physiological tremor as a function of muscle stretch differs from that of the stretch reflex amplitude. Amplitude of physiological tremor may be altered as a function of reflex pathway gains. Healthy humans likely increase their γ-static fusimotor drive when muscles shorten. Quantification of physiological tremor by manipulation of joint angle may be a useful experimental probe of afferent gains and/or the integrity of automatic fusimotor control.

ABSTRACT

The involuntary force fluctuations associated with physiological (as distinct from pathological) tremor are an unavoidable component of human motor control. While the origins of physiological tremor are known to depend on muscle afferentation, it is possible that the mechanical properties of muscle-tendon systems also affect its generation, amplification and maintenance. In this paper, we investigated the dependence of physiological tremor on muscle length in healthy individuals. We measured physiological tremor during tonic, isometric plantarflexion torque at 30% of maximum at three ankle angles. The amplitude of physiological tremor increased as calf muscles shortened in contrast to the stretch reflex whose amplitude decreases as muscle shortens. We used a published closed-loop simulation model of afferented muscle to explore the mechanisms responsible for this behaviour. We demonstrate that changing muscle lengths does not suffice to explain our experimental findings. Rather, the model consistently required the modulation of  γ-static fusimotor drive to produce increases in physiological tremor with muscle shortening - while successfully replicating the concomitant reduction in stretch reflex amplitude. This need to control γ-static fusimotor drive explicitly as a function of muscle length has important implications. First, it permits the amplitudes of physiological tremor and stretch reflex to be decoupled. Second, it postulates neuromechanical interactions that require length-dependent γ drive modulation to be independent from α drive to the parent muscle. Lastly, it suggests that physiological tremor can be used as a simple, non-invasive measure of the afferent mechanisms underlying healthy motor function, and their disruption in neurological conditions.

Local vibration inhibits H-reflex but does not compromise manual dexterity and does not increase tremor

The present work aimed at investigating the effects of local vibration on upper limb postural and kinetic tremor, on manual dexterity and on spinal reflex excitability. Previous studies have demonstrated a decrease in spinal reflex excitability and in force fluctuations in the lower limb but an increase in force fluctuation in the upper limbs. As hand steadiness is of vital importance in many daily-based tasks, and local vibration may also be applied in movement disorders, we decided to further explore this phenomenon. Ten healthy volunteers (26±3years) were tested for H reflex, postural and kinetic tremor and manual dexterity through a Purdue test. EMG was recorded from flexor carpi radialis (FCR) and extensor digitorum communis (EDC). Measurements were repeated at baseline, after a control period during which no vibration was delivered and after vibration. Intervention consisted in holding for two minutes a vibrating handle (frequency 75Hz, displacement∼7mm), control consisted in holding for two minutes the same handle powered off. Reflex excitability decreased after vibration whilst postural tremor and manual dexterity were not affected. Peak kinetic tremor frequency increased from baseline to control measurements (P=0.002). Co-activation EDC/FCR increased from control to vibration (P=0.021). These results show that two minutes local vibration lead to a decrease in spinal excitability, did not compromise manual dexterity and did not increase tremor; however, in contrast with expectations, tremor did not decrease. It is suggested that vibration activated several mechanisms with opposite effects, which resulted in a neutral outcome on postural and kinetic tremor.

Changes in Corticospinal and Spinal Excitability to the Biceps Brachii with a Neutral vs. Pronated Handgrip Position Differ between Arm Cycling and Tonic Elbow Flexion

The purpose of this study was to examine the influence of neutral and pronated handgrip positions on corticospinal excitability to the biceps brachii during arm cycling. Corticospinal and spinal excitability were assessed using motor evoked potentials (MEPs) elicited via transcranial magnetic stimulation (TMS) and cervicomedullary-evoked potentials (CMEPs) elicited via transmastoid electrical stimulation (TMES), respectively. Participants were seated upright in front on arm cycle ergometer. Responses were recorded from the biceps brachii at two different crank positions (6 and 12 o'clock positions relative to a clock face) while arm cycling with neutral and pronated handgrip positions. Responses were also elicited during tonic elbow flexion to compare/contrast the results to a non-rhythmic motor output. MEP and CMEP amplitudes were significantly larger at the 6 o'clock position while arm cycling with a neutral handgrip position compared to pronated (45.6 and 29.9%, respectively). There were no differences in MEP and CMEP amplitudes at the 12 o'clock position for either handgrip position. For the tonic contractions, MEPs were significantly larger with a neutral vs. pronated handgrip position (32.6% greater) while there were no difference in CMEPs. Corticospinal excitability was higher with a neutral handgrip position for both arm cycling and tonic elbow flexion. While spinal excitability was also higher with a neutral handgrip position during arm cycling, no difference was observed during tonic elbow flexion. These findings suggest that not only is corticospinal excitability to the biceps brachii modulated at both the supraspinal and spinal level, but that it is influenced differently between rhythmic arm cycling and tonic elbow flexion.

Maintenance of cutaneomuscular neuronal excitability after leg-cycling predicts lower limb muscle strength after incomplete spinal cord injury

OBJECTIVE

Controlled leg-cycling modulates H-reflex activity after spinal cord injury (SCI). Preserved cutaneomuscular reflex activity is also essential for recovery of residual motor function after SCI. Here the effect of a single leg-cycling session was assessed on cutaneomuscular-conditioned H-reflex excitability in relation to residual lower limb muscle function after incomplete SCI (iSCI).

METHODS

Modulation of Soleus H-reflex activity was evaluated following ipsilateral plantar electrical stimulation applied at 25-100ms inter-stimulus intervals (ISI's), before and after leg-cycling in ten healthy individuals and nine subjects with iSCI.

RESULTS

Leg-cycling in healthy subjects increased cutaneomuscular-conditioned H-reflex excitability between 25 and 75ms ISI (p<0.001), compared to a small loss of excitability at 75ms ISI after iSCI (p<0.05). In addition, change in cutaneomuscular-conditioned H-reflex excitability at 50ms and 75ms ISI in subjects with iSCI after leg-cycling predicted lower ankle joint hypertonia and higher Triceps Surae muscle strength, respectively.

CONCLUSION

Leg-cycling modulates cutaneomuscular-conditioned spinal neuronal excitability in healthy subjects and individuals with iSCI, and is related to residual lower limb muscle function.

SIGNIFICANCE

Cutaneomuscular-conditioned H reflex modulation could be used as a surrogate biomarker of both central neuroplasticity and lower limb muscle function, and could benchmark lower-limb rehabilitation programs in subjects with iSCI.

Arm posture-dependent changes in corticospinal excitability are largely spinal in origin

Biceps brachii motor evoked potentials (MEPs) from cortical stimulation are influenced by arm posture. We used subcortical stimulation of corticospinal axons to determine whether this postural effect is spinal in origin. While seated at rest, 12 subjects assumed several static arm postures, which varied in upper-arm (shoulder flexed, shoulder abducted, arm hanging to side) and forearm orientation (pronated, neutral, supinated). Transcranial magnetic stimulation over the contralateral motor cortex elicited MEPs in resting biceps and triceps brachii, and electrical stimulation of corticospinal tract axons at the cervicomedullary junction elicited cervicomedullary motor evoked potentials (CMEPs). MEPs and CMEPs were normalized to the maximal compound muscle action potential (Mmax). Responses in biceps were influenced by upper-arm and forearm orientation. For upper-arm orientation, biceps CMEPs were 68% smaller (P= 0.001), and biceps MEPs 31% smaller (P= 0.012), with the arm hanging to the side compared with when the shoulder was flexed. For forearm orientation, both biceps CMEPs and MEPs were 34% smaller (both P< 0.046) in pronation compared with supination. Responses in triceps were influenced by upper-arm, but not forearm, orientation. Triceps CMEPs were 46% smaller (P= 0.007) with the arm hanging to the side compared with when the shoulder was flexed. Triceps MEPs and biceps and triceps MEP/CMEP ratios were unaffected by arm posture. The novel finding is that arm posture-dependent changes in corticospinal excitability in humans are largely spinal in origin. An interplay of multiple reflex inputs to motoneurons likely explains the results.

Influence of joint position on synergistic muscle activity after fatigue of a single muscle head

INTRODUCTION

We investigated synergistic muscle activity after fatigue of a single muscle in different joint positions.

METHODS

Two experimental groups (n = 12 each) performed maximal voluntary contractions (MVCs) before and after fatiguing the gastrocnemius lateralis (GL), using neuromuscular electrical stimulation (NMES). Neuromuscular tests, including muscle activity during MVC, H-reflex, and twitch interpolation, were performed. One group completed the experiment in a knee-extended position with the second group in a knee-flexed position.

RESULTS

In the knee-flexed position, the muscle activity increased in non-stimulated synergistic muscles. In contrast, in the knee-extended position, muscle activity of the synergistic muscles remained unaltered. The MVC force remained unaltered in the flexed position and decreased in the extended position.

CONCLUSIONS

Synergistic muscles compensate for the fatigued muscle in the flexed position but not in the extended position. Compensation mechanisms seem to depend on joint position.

Phase dependent modulation of soleus H-reflex in healthy, non-injured individuals while walking with an ankle foot orthosis

OBJECTIVE

To examine the dynamic modulation of the soleus H-reflex while walking with a posterior leaf spring ankle foot orthosis (PAFO).

METHODS

Soleus H-reflexes were evoked on randomly chosen lower limb of fourteen healthy individuals (age range of 22-36 years, 7 women) while walking on a treadmill with and without a PAFO. In order to capture excitability across the duration of the gait cycle, H-reflexes were evoked at heel strike (HS), HS+100ms, HS+200ms, HS+300ms, HS+400ms in the stance phase and at toe-off (TO), TO+100ms, TO+200ms, TO+300ms, TO+400ms in the swing phase respectively.

RESULTS

H-reflex excitability was significantly higher in the form of greater slope of the rise in H-reflex amplitude across the swing phase (p=0.024) and greater mean H-reflex amplitude (p=0.014) in the swing phase of walking with a PAFO. There was no change in the slope (p=0.25) or the mean amplitude of H-reflexes (p=0.22) in the stance phase of walking with a PAFO. Mean background EMG activity between the two walking conditions was not significantly different for both the tibialis anterior (p=0.69) and soleus muscles (p=0.59).

CONCLUSION

PAFO increased reflex excitability in the swing phase of walking in healthy individuals. Altered sensory input originating from joint, muscle and cutaneous receptors may be the underlying mechanism for greater reflex excitability. The neurophysiological effect of PAFOs on reflex modulation during walking needs to be tested in persons with neurological injury. The relationship between the sensory input and the reflex output during walking may assist in determining if there exists a neurological disadvantage of using a compensatory device such as a PAFO.

Repeatability of corticospinal and spinal measures during lengthening and shortening contractions in the human tibialis anterior muscle

Elements of the human central nervous system (CNS) constantly oscillate. In addition, there are also methodological factors and changes in muscle mechanics during dynamic muscle contractions that threaten the stability and consistency of transcranial magnetic stimulation (TMS) and perpherial nerve stimulation (PNS) measures.

Plantarflexor stretch training increases reciprocal inhibition measured during voluntary dorsiflexion

Agonist-mediated reciprocal inhibition (RI) in distal skeletal muscles is an important neurophysiological phenomenon leading to improved movement coordination and efficiency. It has been shown to be reduced in aged and clinical populations, so the development of interventions augmenting RI is an important research goal. We examined the efficacy of using chronic passive muscle stretching to augment RI. The influence of 3 wk of plantarflexor stretching (4 × 30 s, two times/day) on RI of soleus and gastrocnemius initiated by tonic, voluntary dorsiflexion contractions [20% of maximum voluntary contraction (MVC)] was examined in 11 healthy men who performed stretch training and in nine nontraining controls. Hoffmann's reflexes (H-reflexes) were elicited by tibial nerve stimulation during both weak isometric (2% MVC) plantarflexions and dorsiflexion contractions at 20% MVC. Changes were examined at three joint angles, normalized to each subject's range of motion (ROM; plantarflexed = 10 ± 0°, neutral = −3.3 ± 2.9°, dorsiflexed = −16.5 ± 5.6°). No changes were detected in controls. A 20% increase in ROM in the stretch subjects was associated with a significant decrease in maximum H-reflex (Hmax): maximum evoked potential (Mmax), measured during 2% plantarflexion at the plantarflexed and neutral angles in soleus and at the plantarflexed angle in gastrocnemius ( P < 0.05–0.01). By contrast, decreases in Hmax:Mmax during 20% dorsiflexion contract were also seen at each angle in soleus and at the dorsiflexed angle in gastrocnemius. However, a greater decrease in Hmax:Mmax measured during voluntary dorsiflexion rather than during plantarflexion, which indicates a specific change in RI, was detected only at the dorsiflexed angle (−30.7 ± 9.4% and −35.8 ± 6.8% for soleus and gastrocnemius, respectively). These results demonstrate the efficacy of soleus-gastrocnemius stretch training in increasing agonist-mediated RI from tibialis anterior onto soleus-gastrocnemius in young, healthy individuals at dorsiflexed, but not plantarflexed, joint angles.

Improved postural control after slackline training is accompanied by reduced H-reflexes

"Slacklining" represents a modern sports activity where people have to keep balance on a tightened ribbon. The first trials on the slackline result in uncontrollable lateral swing of the supporting leg. Training decreases those oscillations and therefore improves postural control. However, the underlying neural mechanisms are not known. Therefore, the present study aimed to highlight spinal adaptations going along with slackline training. Twenty-four subjects were either assigned to a training or a control group and postural control was assessed before and after the 10 training sessions. Additionally, soleus Hoffmann (H)-reflexes were elicited to evaluate changes in the excitability of the spinal reflex circuitry. Trained subjects were able to maintain balance on the slackline for at least 20 s (P<0.001) and reduced platform movements on the balance board (P<0.05). The H-reflexes were significantly diminished (P<0.05) while no changes occurred in the background electromyography (bEMG). The control group showed no significant changes. From a functional point of view the reflex reduction may serve to suppress uncontrollable reflex mediated joint oscillations. As the bEMG remained unchanged, presynaptic rather than post-synaptic mechanisms are speculated to be responsible for the changes in the Ia-afferent transmission.

The effect of tone-reducing orthotic devices on soleus muscle reflex excitability while standing in patients with spasticity following stroke

Orthoses are commonly prescribed for the management of spasticity but their neurophysiologic effect on spasticity remains unsubstantiated. The purpose of this study was to investigate the effect of three tone-reducing devices (dynamic foot orthosis, muscle stretch, and orthokinetic compression garment) on soleus muscle reflex excitability while standing in patients with spasticity following stroke. A repeated measures intervention study was conducted on 13 patients with stroke selected from a sample of convenience. A custom-made dynamic foot orthosis, a range of motion walker to stretch the soleus muscle and class 1 and class 2 orthokinetic compression garments were assessed using the ratio of maximum Hoffmann reflex amplitude to maximum M-response amplitude (Hmax:Mmax) to determine their effect on soleus muscle reflex excitability. Only 10 subjects were able to complete the testing. There were no significant treatment effects for the interventions (F=1.208, df=3.232, p=0.328); however, when analyzed subject-by-subject, two subjects responded to the dynamic foot orthosis and one of those two subjects also responded to the class 1 orthokinetic compression garment. Overall, the results demonstrated that the tone-reducing devices had no significant effect on soleus reflex excitability suggesting that these tone-reducing orthotic devices have no significant neurophysiologic effect on spasticity.

Monopolar surface electromyography: a better tool to assess motoneuron excitability upon passive muscle stretching

Bipolar and monopolar surface electromyography (sEMG) are known procedures to measure the H-reflex. However, signal cancellation is a potential experimental problem of bipolar sEMG. The results of our study show that monopolar sEMG was the more sensitive procedure to differentiate motoneuron excitability at different passive muscle stretching speeds as it overcame signal cancellation.

Effect of inversion and ankle bracing on peroneus longus Hoffmann reflex

This study examined peroneus longus (PL) Hoffmann reflex (H-reflex) during sudden inversion perturbation of the ankle/foot complex under an ankle brace and non-brace condition. Ten healthy subjects volunteered. H-reflexes were tested on the up-sloping portion of the recruitment curve, utilizing a control trial M-wave above motor threshold to maintain consistency between subjects and conditions. The PL H/maximum M-wave (M(max)) ratio was established using the PL H-reflex and PL M(max) peak-to-peak measures. The mean ratio across five trials for each subject under each ankle brace (brace, no brace) and surface (flat, inversion) conditions was utilized for analysis. The 1 x 4 repeated measures ANOVA revealed a significant main effect for treatment condition (P<0.0001). The PL H/M(max) ratio significantly increased during sudden inversion-no ankle brace condition compared with the flat surface no-ankle brace condition (P=0.04). Application of an ankle brace had no effect on PL H/M(max) ratio during inversion (P=0.78). During this study PL H/M(max) ratios increased during an inversion perturbation in healthy ankles. This is believed to occur due to heightened sensorimotor demand placed on the nervous system during this motion. Moreover, application of an ankle brace during inversion does not appear to affect PL H/M(max) ratio.

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

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

A review of the H-reflex and M-wave in the human triceps surae

The soleus is the most commonly used muscle for H-reflex studies in humans, while limited comparable data have been produced from the gastrocnemii muscles. This article reviews the fundamental differences between the structure and function of the human soleus and gastrocnemii muscles, including recent data published about their complex innervation zones. Protocols for eliciting, recording, and assessing the H-reflex and M-wave magnitude in the human triceps surae are also discussed.

Responsiveness of the H reflex to loading and posture in patients following stroke

The objective of the research was to examine the effects of loading and posture on motoneuronal excitability of the triceps surae (TS) for patients with hemiplegia. Twelve healthy subjects and 12 patient subjects with post-stroke hemiparesis (onset period: 3-60 months) were enrolled in this study. The subjects were instructed to remain in quiet sitting with the test knee straight and three standing conditions of different superincumbent loads by shifting body weight to the test leg (10%, 50%, and 90% of body weight), while the H reflexes and M waves of the TS were measured. The results clearly indicated that H reflex amplitudes were not affected by different loading conditions in standing for both healthy subjects and patients who had a previous stroke. In addition, the H reflex amplitude in quiet standing for healthy subjects was significantly downward modulated relative to that in relaxed sitting with the test knee straight, but this posturally driven modulation was impaired in patients following stroke. Current electrophysiological findings imply that body weight as a means for rehabilitation facilitation had little immediate effect on paretic TS, and absence in postural gating of reflex excitability appeared to be an incentive for postural instability resulting from post-stroke hemiparesis.

Reliability of a new measure of H-reflex excitability

OBJECTIVE

This study examined the intraclass reliability of different measures extracted from Hoffmann reflex (H-reflex) stimulus-response curve that are used to assess neuromuscular excitability. The following measures were compared: (1) the peak-to-peak amplitude of the H-reflex at a stimulus intensity associated with 5% of the maximum M-wave; (2) the slope of the regression line of the H-reflex stimulus-response curve; and (3) the peak of the first derivative of the H-reflex stimulus-response curve, a new measure introduced in this paper.

METHODS

The H-reflex was elicited in the soleus for 24 subjects (12 males and 12 females) on 5 separate days. Vibration was applied to the Achilles tendon prior to stimulation to test the sensitivity of the measures on test day 4. The stimulus intensity was gradually increased from below the threshold for an H-reflex response to above the maximum M-wave (Mmax) response. The means of 5 evoked potentials at each intensity level were used to create both the H-reflex and M-wave stimulus-response curves for each subject across test days. Determination of reliability involves the consideration of both the stability and consistency of the measures. A repeated measures analysis of variance evaluated the stability of the group means across test sessions. The consistency of scores within individuals was determined by calculating the intraclass correlation coefficient (ICC). Calculation of the 95% confidence interval of estimation was used to assess significant differences between ICCs.

RESULTS

The H-reflex measures were both stable and consistent across the first 3 test days. Achilles tendon vibration resulted in a profound reduction (59-70%) on test day 4, and then there was a return to baseline levels on test day 5. The ICC for H-reflex at a stimulus intensity associated with 5% of the maximum M-wave was 0.85. The ICC for the slope of the regression line of the H-reflex stimulus-response curve was 0.79, while it was 0.89 for the peak of the first derivative of the H-reflex stimulus-response curve. However, there was no statistical significance (P>0.05) between the 3 EMG measures of the H-reflex arc. Maximum M-wave amplitude had an ICC of 0.96 attesting to careful methodological controls.

CONCLUSIONS

The peak of the first derivative of the H-reflex stimulus-response curve was shown to have comparable sensitivity and reliability as other more established measures.

SIGNIFICANCE

The first derivative of the H-reflex stimulus-response curve provides the rate of change, rather than amplitude, making it a robust measure of reflex arc excitability. The higher ICC for the first derivative offers greater statistical power, which is of practical significance.