Between‐day reliability of the trapezius muscle H‐reflex and M‐wave

Introduction: The aim of this study was to investigate the between‐day reliability of the trapezius muscle H‐reflex and M‐wave. Methods: Sixteen healthy subjects were studied on 2 consecutive days. Trapezius muscle H‐reflexes were evoked by electrical stimulation of the C3/4 cervical nerves; M‐waves were evoked by electrical stimulation of the accessory nerve. Relative reliability was estimated by intraclass correlation coefficients (ICC_2,1). Absolute reliability was estimated by computing the standard error of measurement (SEM) and the smallest real difference (SRD). Bland–Altman plots were constructed to detect any systematic bias. Results: Variables showed substantial to excellent relative reliability (ICC = 0.70–0.99). The relative SEM ranged from 1.4% to 34.8%; relative SRD ranged from 3.8% to 96.5%. No systematic bias was present in the data. Conclusions: The amplitude and latency of the trapezius muscle H‐reflex and M‐wave in healthy young subjects can be measured reliably across days. Muscle Nerve 52 : 1066–1071, 2015     

Crossed spinal soleus muscle communication demonstrated by H-reflex conditioning

Introduction: A conditioning volley to the ipsilateral tibial nerve (iTN) inhibits contralateral soleus (cSOL) electromyographic activity at latencies of 37–41 ms. This is evidence for spinal muscular communication in opposing limbs. The aim of our study was to determine whether the cSOL H‐reflex would be inhibited in a similar manner. Methods: Thirteen subjects participated in two experiments: (1) stimuli delivered to the iTN at 85% of the maximal peak‐to‐peak M‐wave (85% M‐max) with a pre‐contracted cSOL; (2) 510 stimuli delivered at 85% M‐max to the iTN with a test volley delivered to the contralateral tibial nerve at interstimulus intervals of ?6 to 100 ms. Results: Significant inhibition was observed in the cSOL H‐reflex when conditioning stimuli were delivered 3–33 ms before the test H‐reflex. Conclusions: The activity of this spinal pathway can be quantified using H‐reflex conditioning to provide a controlled model for further studies of this response. Muscle Nerve, 2011     

Soleus and lateral gastrocnemius H‐reflexes during standing with unstable footwear

Introduction: Unstable footwear has been shown to increase lower extremity muscle activity, but the reflex response to perturbations induced by this intervention is unknown. Methods: Twenty healthy subjects stood in stable and unstable footwear conditions (presented randomly) while H‐reflex amplitude and background muscle activity were measured in the soleus and lateral gastrocnemius (LG) muscles. Results: Wearing unstable footwear resulted in larger H‐reflexes (normalized to the maximal M‐wave) for the LG (+12%; P = 0.025), but not for the soleus (+4%; P > 0.05). Background activity of both muscles was significantly higher in the unstable condition. Conclusions: The H‐reflex facilitation observed with unstable footwear was unexpected, as challenging postural conditions usually result in reflex depression. Increased muscle activity, decreased presynaptic inhibition, and/or more forward postural position may have (over‐)compensated the expected reflex depression. Differences between LG and soleus H‐reflex modulation may be due to diverging motor unit recruitment thresholds. Muscle Nerve 51 :764–766, 2015     

H‐reflexes reduce fatigue of evoked contractions after spinal cord injury

Introduction: Neuromuscular electrical stimulation (NMES) over a muscle belly (mNMES) generates contractions predominantly through M‐waves, while NMES over a nerve trunk (nNMES) can generate contractions through H‐reflexes in people who are neurologically intact. We tested whether the differences between mNMES and nNMES are present in people with chronic motor‐complete spinal cord injury and, if so, whether they influence contraction fatigue. Methods: Plantar‐flexion torque and soleus electromyography were recorded from 8 participants. Fatigue protocols were delivered using mNMES and nNMES on separate days. Results: nNMES generated contractions that fatigued less than mNMES. Torque decreased the least when nNMES generated contractions, at least partly through H‐reflexes (n = 4 participants; 39% decrease), and torque decreased the most when contractions were generated through M‐waves, regardless of NMES site (nNMES 71% decrease, n = 4; mNMES, 73% decrease, n = 8). Conclusions: nNMES generates contractions that fatigue less than mNMES, but only when H‐reflexes contribute to the evoked contractions. Muscle Nerve 50:224–234, 2014     

Soleus H‐reflex modulation during balance recovery after forward falling

Introduction: Our purpose was to examine the Hoffmann reflex (H‐reflex) during balance recovery after a simulated forward fall from 2 different inclination angles. Methods: The soleus H‐reflex of 15 healthy adults was measured in 2 different leaning positions (exerting a horizontal force at 15% and 30% of body weight, respectively), with no release (Int0) and at 2 different intervals (Int1, Int2) after the release (～45 and ～65 ms, respectively). Results: During Int2, the H‐reflex, which was evoked before the onset of the soleus electromyography, was significantly higher than the H‐reflex induced 20 ms earlier (Int1). No significant difference was observed between Int0 and Int1 and between the 2 leaning positions. Conclusions: These findings indicate that Ia afferent input is facilitated before muscle activation during forward falling. This could be important for the timely activation and increased rate of force development required during this task. Muscle Nerve 54 : 952–958, 2016     

Tetanus toxin reduces local and descending regulation of the H‐reflex

Introduction: Skeletal muscles that are under the influence of tetanus toxin show an exaggerated reflex response to stretch. We examined which changes in the stretch reflex may underlie the exaggerated response. Methods: H‐reflexes were obtained from the tibialis anterior (TA) and flexor digitorum brevis (FDB) muscles in rats 7 days after intramuscular injection of tetanus toxin into the TA. Results: We found effects of the toxin on the threshold, amplitude, and duration of H‐waves from the TA. The toxin inhibited rate‐dependent depression in the FDB between the stimulation frequencies of 0.5–50 HZ and when a conditioning magnetic stimulus applied to the brain preceded a test electrical stimulus delivered to the plantar nerve. Conclusions: Tetanus toxin increased the amplitude of the H‐wave and reduced the normal depression of H‐wave amplitude that is associated with closely timed stimuli, two phenomena that could contribute to hyperactivity of the stretch reflex. Muscle Nerve 49:495–501, 2014     

Modulation of the soleus H‐reflex following galvanic vestibular stimulation and cutaneous stimulation in prone human subjects

There is evidence to suggest that vestibular and somatosensory inputs may interact when they are processed by the central nervous system, although the nature of the individual sensory contributions to this interaction is unknown. We examined the effects of a combined vestibular and cutaneous conditioning stimulus on the motoneuron pool that supplies the soleus muscle via the Hoffman reflex (H‐reflex). We applied galvanic vestibular stimulation (GVS; bipolar, binaural, 500 ms, 2.5‐mA square‐wave pulse) and cutaneous stimulation (medial plantar nerve; 11 ms, three‐pulse train, 200 HZ ) to prone human subjects and examined changes in the amplitude of the H‐reflex. GVS alone caused facilitation (approximately 20%) of the H‐reflex, whereas ipsilateral cutaneous stimulation alone caused a 26% inhibition. Paired GVS and cutaneous stimulation resulted in a linear summation of the individual conditioning effects. H‐reflex amplitudes observed after paired conditioning with GVS and cutaneous stimulation could be predicted from the amplitudes observed with individual conditioning. These results suggest that in the prone position, when the muscles are not posturally engaged, vestibular and somatosensory information appear to sum in a linear fashion to influence the reflex response of lower limb motoneurons. Muscle Nerve 40: 213–220, 2009     

Stiffness regulation provided by short-latency reflexes in human triceps surae muscles

The triceps surae muscles of normal human subjects were rapidly stretched and released by rotating the foot about the ankle joint with a torque motor. Following the initial intrinsic resistance, the yielding observed in incremental force records was more rapid for stretch than for release responses. Short-latency EMG responses elicited by stretch recruit force, to compensate for the yielding and to maintain the total (intrinsic plus reflex) resistance constant as the prior force level changes.     

Changes in soleus H‐reflex during walking in middle‐aged,healthy subjects

Introduction: To assess the effect of aging on stretch reflex modulation during walking, soleus H‐reflexes obtained in 15 middle‐aged (mean age 56.4 ± 6.9 years) and 15 young (mean age 23.7 ± 3.9 years) subjects were compared. Methods: The H‐reflex amplitude, muscle activity (EMG) of the soleus and tibialis anterior muscles, and EMG/H‐reflex gain were measured during 4‐km/h treadmill walking. Results: The normalized H‐reflex amplitude was lower in the swing phase for the middle‐aged group, and there was no difference in muscle activity. EMG/H‐reflex gain did not differ between groups. Conclusions: H‐reflex amplitude during walking was affected by aging, and changes during the swing phase could be seen in the middle‐aged subjects. Subdividing the 2 age groups into groups of facilitated or suppressed swing‐phase H‐reflex revealed that the H‐reflex amplitude modulation pattern in the group with facilitated swing‐phase H‐reflex may be influenced by aging. Muscle Nerve 51: 419–425, 2015     

Increased H‐reflex excitability is not accompanied by changes in neural drive following 24 days of unilateral lower limb suspension

The purpose of this study was to determine whether the gain in soleus H‐reflex excitability induced by unilateral lower limb suspension (ULLS) is associated with changes in neural drive to the plantar flexor muscles. Six male subjects (23 ± 2 years, 187 ± 7 cm, 79 ± 9 kg) underwent 24 days of ULLS of the dominant limb. Plantar flexor maximal voluntary contraction (MVC) torque, activation capacity (twitch interpolation), soleus maximal electromyographic (EMG) activity, Hoffman (H)‐reflex, and the first volitional (V) wave normalized to the compound muscle action potential (M‐wave) were quantified before and after ULLS. Following ULLS, MVC torque decreased by 15% (P < 0.05). However, neither activation capacity nor EMG activity was significantly altered after the suspension. The V‐wave remained unchanged consistently after ULLS, whereas the H‐reflex increased significantly (+20%). Furthermore, there was no significant relationship between changes in H‐reflex and V‐wave over the ULLS period. These findings indicate that 24 days of ULLS can result in a substantial reduction of muscle strength without any apparent change in voluntary activation capacity. H‐reflex and V‐wave findings suggest that the spinal adaptations that underlie the unloading‐induced increase in resting soleus H‐reflex excitability did not significantly affect the efferent motor output to the plantar flexor muscles. Muscle Nerve, 2009     

Differential effects of plantar cutaneous afferent excitation on soleus stretch and H‐reflex

Previous studies have demonstrated that plantar cutaneous afferents can adjust motoneuron excitability, which may contribute significantly to the control of human posture and locomotion. However, the role of plantar cutaneous afferents in modulating the excitability of stretch and H‐reflex with respect to the location of their excitation remains unclear. In the present study, it was hypothesized that electrical stimulation delivered to the sole of the foot might be followed by modulation of spinal excitability that depends on: (1) the stimulation location and (2) the reflex studied. In these experiments, conditioned and unconditioned stretch and H‐reflexes were evoked in 16 healthy subjects in a seated position. Both reflexes were conditioned by non‐noxious electrical plantar cutaneous afferent stimulation at two different sites, the heel and metatarsal regions, at four different conditioning–test (CT) intervals. The conditioning stimulation delivered to the heel caused a significant facilitation of the soleus stretch reflex for all CT intervals, whereas the soleus H‐reflex had significant facilitation only at CT interval of 50 ms and significant inhibition at longer CT intervals. Stimulation delivered to the metatarsal region, however, resulted mainly in reduced stretch and H‐reflex sizes. This study extends the reported findings on the contribution of plantar cutaneous afferents within spinal interneuron reflex circuits as a function of their location and the reflex studied. Muscle Nerve, 2008     

Posterior root-muscle reflexes elicited by transcutaneous stimulation of the human lumbosacral cord

Continuous epidural stimulation of lumbar posterior root afferents can modify the activity of lumbar cord networks and motoneurons, resulting in suppression of spasticity or elicitation of locomotor-like movements in spinal cord-injured people. The aim of the present study was to demonstrate that posterior root afferents can also be depolarized by transcutaneous stimulation with moderate stimulus intensities. In healthy subjects, single stimuli applied through surface electrodes placed over the T11-T12 vertebrae with a mean intensity of 28.6 V elicited simultaneous, bilateral monosynaptic reflexes in quadriceps, hamstrings, tibialis anterior, and triceps surae by depolarization of lumbosacral posterior root fibers. The nature of these posterior root-muscle reflexes was demonstrated by the duration of the refractory period, and by modifying the responses with vibration and active and passive movements. Stimulation over the L4-L5 vertebrae selectively depolarized posterior root fibers or additionally activated anterior root fibers within the cauda equina depending on stimulus intensity. Transcutaneous posterior root stimulation with single pulses allows neurophysiological studies of state- and task-dependent modulations of monosynaptic reflexes at multiple segmental levels. Continuous transcutaneous posterior root stimulation represents a novel, non-invasive, neuromodulative approach for individuals with different neurological disorders.     

Recovery of long-term denervated human muscles induced by electrical stimulation

We investigated the restorative potential of intensive electrical stimulation in a patient with long-standing quadriceps denervation. Stimulation started 18 months after injury. After 26 months, the thighs were visibly less wasted. Muscle cross-sectional areas, measured by computerized tomography, increased from 36.0 cm(2) to 57.9 cm(2) (right) and from 36.1 cm(2) to 52.4 cm(2) (left). Knee torque had become sufficient to maintain standing without upper extremity support. Biopsies revealed evidence of both growth and regeneration of myofibers. The results suggest that electrical stimulation may offer a route to the future development of mobility aids in patients with lower motor neuron lesions.     

Effect of Electrical Stimulation of Hamstrings and L3/4 Dermatome on Gait in Spinal Cord Injury

Objective. To determine the effect of electrical stimulation of hamstrings and L3/4 dermatome on the swing phase of gait. Materials and Methods. Five subjects with incomplete spinal cord injury (SCI) with spasticity were included. Two electrical stimulation methods were investigated, i.e., hamstrings and L3/4 dermatome stimulation. Both interventions were applied during the swing phase of gait. The main outcome measures were step length, maximum hip, and knee flexion during the swing phase of gait. In three subjects changes of spinal inhibition during gait were evaluated using the Hoffman reflex/m (motor)–wave (H/M) ratio at mid swing. Results. The hip flexion decreased 4.6° (p < 0.05) when the hamstrings were stimulated during the swing phase, whereas the knee flexion was not changed. The step length did not change significantly. One subject showed a decrease of the H/M ratio to a nonpathologic level during hamstrings stimulation. Conclusion. It was concluded that hamstrings stimulation during the swing phase results in a reduction of the hip flexion in all five SCI subjects. The H/M ratio of the vastus lateralis was normalized using hamstrings stimulation in one of three subjects. Stimulation of the L3/4 dermatome provides no significant changes in gait performance, but in one subject the H/M ratio increased.