Postural modulation of the segmental reflex: effect of body tilt and postural sway

Virtual reality does not fool the brain only: spinal excitability changes during virtually simulated falling

Virtual reality (VR) is known to induce substantial activation of brain's motor regions. It remains unclear to what extent virtual reality can trigger the sensorimotor system, and more particularly, whether it can affect lower nervous levels. In this study, we aimed to assess whether VR simulation of challenging and stressful postural situations (Richie's plank experience) could interfere with spinal excitability of postural muscles in 15 healthy young participants. The H-reflex of the triceps surae muscles was elicited with electrical nerve stimulation while participants were standing and wearing a VR headset. Participants went through several conditions, during which stimulations were evoked: standing still (noVR), standing in VR on the ground (groundVR), standing on the edge of a building (plankVR), and falling from the building (fallingVR). Myoelectrical activity of the triceps surae muscles was measured throughout the experiment. Leg and head movements were also measured by means of accelerometers to account for body oscillations. First, no differences in head rotations and myoelectrical activity were to be noted between conditions. Second, triceps H-reflex (H_MAX/M_MAX) was not affected from noVR to groundVR and plankVR. The most significant finding was a drastic decrease in H-reflex during falling (-47 ± 26.9% between noVR and fallingVR, P = 0.015). It is suggested that experiencing a postural threat in VR efficiently modulates spinal excitability, despite remaining in a quiet standing posture. This study suggests that simulated falling mimics the neural adjustments observed during actual postural challenge tasks.NEW & NOTEWORTHY The present study showed a modulation of spinal excitability induced by virtual reality (VR). In the standing position, soleus H-reflex was downmodulated during a simulated falling, in the absence of apparent changes in body oscillations. Since the same behavior is usually observed during real falling, it was suggested that the visual cues provided by VR were sufficiently strong to lead the neuromuscular system to mimic the actual modulation.

Modulation of the Fibularis Longus Hoffmann Reflex and Postural Instability Associated With Chronic Ankle Instability

CONTEXT

Individuals with chronic ankle instability (CAI) present with decreased modulation of the Hoffmann reflex (H-reflex) from a simple to a more challenging task. The neural alteration is associated with impaired postural control, but the relationship has not been investigated in individuals with CAI.

OBJECTIVE

To determine differences in H-reflex modulation and postural control between individuals with or without CAI and to identify if they are correlated in individuals with CAI.

DESIGN

Descriptive laboratory study.

SETTING

Laboratory.

PATIENTS OR OTHER PARTICIPANTS

A total of 15 volunteers with CAI (9 males, 6 females; age = 22.6 ± 5.8 years, height = 174.7 ± 8.1 cm, mass = 74.9 ± 12.8 kg) and 15 healthy sex-matched volunteers serving as controls (9 males, 6 females; age = 23.8 ± 5.8 years, height = 171.9 ± 9.9 cm, mass = 68.9 ± 15.5 kg) participated.

INTERVENTION(S)

Maximum H-reflex (H_max) and motor wave (M_max) from the soleus and fibularis longus were recorded while participants lay prone and then stood in unipedal stance. We assessed postural tasks of unipedal stance with participants' eyes closed for 10 seconds using a forceplate.

MAIN OUTCOME MEASURE(S)

We normalized H_max to M_max to obtain H_max : M_max ratios for the 2 positions. For each muscle, H-reflex modulation was quantified using the percentage change scores in H_max : M_max ratios calculated from prone position to unipedal stance. Center-of-pressure data were used to compute 4 time-to-boundary variables. Separate independent-samples t tests were performed to determine group differences. Pearson product moment correlation coefficients were calculated between the modulation and balance measures in the CAI group.

RESULTS

The CAI group presented less H-reflex modulation in the soleus (t₂₆ = -3.77, P = .001) and fibularis longus (t₂₅ = -2.59, P = .02). The mean of the time-to-boundary minima in the anteroposterior direction was lower in the CAI group (t₂₈ = -2.06, P = .048). We observed a correlation (r = 0.578, P = .049) between the fibular longus modulation and mean of time-to-boundary minima in the anteroposterior direction.

CONCLUSIONS

The strong relationship indicated that, as H-reflex amplitude in unipedal stance was less down modulated, unipedal postural control was more impaired. Given the deficits in H-reflex modulation and postural control in the CAI group, the relationship may provide insights into the neurophysiologic mechanism of postural instability.

Relationships between presynaptic inhibition and static postural sway in subjects with and without diabetic neuropathy

[Purpose] Diabetic peripheral neuropathy can often lead to balance impairment. The spinal reflex is a mechanism that is reportedly important for balance, but it has not been investigated in diabetic peripheral neuropathy patients. Moreover, inhibitory or facilitatory behavior of the spinal reflex—known as presynaptic inhibition—is essential for controlling postural sway. The purpose of this study was to compare the differences in as presynaptic inhibition and balance in subjects with and without diabetic peripheral neuropathy to determine the influence of presynaptic inhibition on balance in diabetic peripheral neuropathy patients. [Subjects and Methods] Presynaptic inhibition and postural sway were tested in eight patients (mean age, 58±6 years) and eight normal subjects (mean age, 59±7 years). The mean percent difference in conditioned reflex amplitude relative to the unconditioned reflex amplitude was assessed to calculate as presynaptic inhibition. The single-leg balance index was measured using a computerized balance-measuring device. [Results] The diabetic peripheral neuropathy group showed lower presynaptic inhibition (47±30% vs. 75±22%) and decreased balance (0.65±0.24 vs. 0.38±0.06) as compared with the normal group. No significant correlation was found between as presynaptic inhibition and balance score (R=0.37). [Conclusion] Although the decreased as presynaptic inhibition observed in diabetic peripheral neuropathy patients may suggest central nervous system involvement, further research is necessary to explore the role of presynaptic inhibition in decreased balance in diabetic peripheral neuropathy patients.

Acute exposure to microgravity does not influence the H-reflex with or without whole body vibration and does not cause vibration-specific changes in muscular activity

PURPOSE

Many potential countermeasures for muscle and bone loss caused by exposure to microgravity require an uncompromised stretch reflex system. This is especially true for whole body vibration (WBV), as the main source of the neuromuscular activity during WBV has been attributed to stretch reflexes. A priori, it cannot be assumed that reflexes and Ia afferent transmission in particular have the same characteristics in microgravity as in normal gravity (NG). Therefore, the purpose of the study was to compare Ia afferent transmission in microgravity and NG and to assess how microgravity affects muscle activity during WBV.

METHODS

In 14 participants, electromyographic activity of four leg muscles as well as Hoffmann-reflexes were recorded during NG and microgravity induced by parabolic flights.

RESULTS

The size of the Hoffmann-reflex was reduced during WBV, but did not differ during acute exposure to microgravity compared to NG. The influence of the gravity conditions on the electromyographic activity did not change depending on the vibration condition.

CONCLUSIONS

As far as the electromyographic activity of the recorded leg muscles is concerned, the effect of WBV is the same in microgravity as in NG. Moreover, Ia afferent transmission does not seem to be affected by acute exposure to microgravity when subjects are loaded with body weight and postural sway is minimized.

Soleus H-reflex and its relation to static postural control

The Hoffmann reflex (H-reflex) test has been extensively used to investigate the responsiveness of Ia afferent spinal loop in animal and human studies. The H-reflex response is influenced by multiple neural pathways and the assessment of H-reflex variation is a useful tool in understanding the neural mechanisms in control of movement. Recently, several studies have examined the relationship between the H-reflex modulation and postural stability. For example, it has been reported that the amplitude of soleus (SOL) H-reflex is depressed in relation to increased body sway during upright standing on a soft surface compared to that on a solid surface. It has been suggested that the SOL H-reflex modulation under such condition is predominately affected by the presynaptic inhibitory mechanisms for avoiding oversaturation of the spinal motoneurons. It has also been reported that after balance training, the SOL H-reflex amplitude is down-modulated in parallel with improvement in balance control, suggesting a functional adaptation at the supraspinal levels. The aim of this review is to examine the current literature on the relationship between H-reflex modulation and postural control for a better understanding of the physiological mechanisms involved in control of posture in humans.

Spinal and supraspinal adaptations associated with balance training and their functional relevance

Traditionally, balance training has been used to rehabilitate ankle injuries and postural deficits. Prospective studies have shown preventive effects with respect to ankle and knee joint injuries. Presently, balance training is not only applied for rehabilitation and prevention but also for improving motor performance, especially muscle power. The recent application of noninvasive electrophysiological and brain imaging techniques revealed insights into the central control of posture and the adaptations induced by balance training. This information is important for our understanding of the basic control and adaptation mechanisms and to conceptualize appropriate training programmes for athletes, elderly people and patients. The present review presents neurophysiological adaptations induced by balance training and their influence on motor behaviour. It emphasizes the plasticity of the sensorimotor system, particularly the spinal and supraspinal structures. The relevance of balance training is highlighted with respect to athletic performance, postural control within elderly people as well as injury prevention and rehabilitation.

Soleus H-reflex modulation and paired reflex depression from prone to standing and from standing to walking

Patterns of soleus H-reflex modulation as a function of posture, task, and reflex activation history were assessed with three experimental paradigms: lying prone compared with standing unsupported; standing compared with the initiation of walking; and standing compared with the mid stance phase of walking. Paired H-reflexes, 80 ms apart, were evoked under each condition. The paired reflex depression (PRD), the percentage depression of the second H-reflex relative to the first H-reflex, was modulated independently of the first H-reflex across the postures and tasks. These results reveal divergent patterns of segmental reflex modulation and support the idea that segmental reflexes are controlled by multiple mechanisms.

Prolonged quadriceps activity following imposed hip extension: a neurophysiological mechanism for stiff-knee gait?

The biomechanical characteristics of stiff knee gait following neurological injury include decreased knee flexion velocity at toe-off, which may be due to exaggerated quadriceps activity. The neuromuscular mechanism underlying this abnormal activity is unclear, although hyperexcitable heteronymous reflexes may be a source of impaired coordination. The present study examines the contribution of reflex activity from hip flexors on knee extensors following stroke and its association with reduced swing-phase knee flexion during walking. Twelve individuals poststroke and six control subjects were positioned in supine on a Biodex dynamometer with the ankle and knee held in a static position. Isolated hip extension movements were imposed at 60, 90, and 120 degrees /s through a 50 degrees excursion to end-range hip extension. Reflexive responses of the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) were quantified during and after the imposed hip rotation. Gait analysis was also performed for all subjects in the stroke group. In subjects with stroke, imposed hip extension evoked a brief reflexive response in the quadriceps, followed by a heightened level of sustained activity. The initial response was velocity dependent and was larger in the stroke group than in the control group. In contrast, the prolonged response was not velocity dependent, was significantly greater in the VL and RF in subjects with stroke, and, importantly, was correlated to decreased swing-phase knee flexion. Hyperexcitable heteronymous connections from hip flexors to knee extensors appear to elicit prolonged quadriceps activity and may contribute to altered swing-phase knee kinematics following stroke.

Hip joint position modulates volitional knee extensor muscle activity after stroke

Evidence from animal and human models has demonstrated the importance of hip proprioceptors and vestibular inputs in modulating lower-extremity muscle activity through reflex pathways. Comprehension of the role of these sensory inputs following stroke may be important in understanding pathological muscle activity during functional activities. We therefore examined the influence of both hip and head/trunk position on volitional quadriceps activity in chronic stroke and control subjects. With the knee held at 60 degrees, maximal voluntary isometric quadriceps contractions were elicited with trunk orientation (head position) and hip angle systematically positioned at 0 degrees, 45 degrees, and 90 degrees. Integrated electromyographic activity from the quadriceps was compared between groups and conditions. Vasti activity in the stroke group was greater in a seated upright posture (hip flexed) than supine (hip neutral). Controlling for vestibular input, the stroke group demonstrated greater quadriceps activity (VL and RF) with a neutral hip compared to flexion. Such findings may have implications for understanding inappropriate muscle activity during walking after stroke, as hip extension occurs immediately prior to toe off, when inappropriate quadriceps activity is commonly observed.

Static and dynamic changes in body orientation modulate spinal reflex excitability in humans

In the present study, we investigated the modulation pattern of the soleus H reflex in healthy subjects in response to imposed static and dynamic changes in body angle, referenced to the vertical plane. Soleus H reflexes were recorded using conventional methods with subjects either supine or while they were erect. Changes in body angle were initiated with subjects lying supine on a tilt table. Table position was controlled via a motor and could move from the horizontal to the upright position and beyond. Elastic bands around the trunk (upper and lower part) and around the thigh and shank secured subjects' position. In the vertical position, the soleus H reflex exhibited a strong depression in all subjects tested, reaching amplitudes as low as 40+/-8.1% of the control reflex (Ho). With subjects supine, positioning the body at 10 degrees, 20 degrees, 40 degrees, 60 degrees, 90 degrees, -50 degrees and -20 degrees all resulted in a significant facilitation of the soleus H reflex. The reflex magnitude at these angles ranged from 140+/-17.2% to 180+/-10.9% of the Ho. Reflex facilitation was also observed following dynamic tilt of the body in the sagittal plane (at 1.8 degrees /s) with the H reflex reaching amplitudes as high as 300+/-18.3% of Ho. Our findings indicate that changes in body orientation induced a significant facilitation of the H reflex magnitude in soleus motoneurones that were essentially independent of angular change in body orientation or of movement direction. In addition, they highlight the potent modulatory effects that natural stimulation of the vestibular system can have on reflex excitability. The implications of our findings are discussed in relation to the maintenance of body posture.

Soleus H-reflex gain in healthy elderly and young adults when lying, standing, and balancing

Soleus Hoffman-reflex (H-reflex) gain was compared at the same background level of electromyographic activity across lying, natural standing, and tandem stance postures, in 12 young and 16 elderly adults. When compared to a lying posture, young adults significantly depressed soleus H-reflex gain when in a natural standing (19% decrease) and a tandem stance position (30% decrease; p <.0125 for both positions). For elderly adults, there was no significant decrease in H-reflex gain while standing naturally, but there was a significant 28% decrease when performing tandem stance (p <.0125). The data indicate that, although the mild motor control challenge of natural standing does not induce a decrease in soleus H-reflex gain in the elderly adults, as it does in young adults, in the more difficult task of tandem stance, soleus H-reflex gain is significantly decreased in both young and elderly adults.

Modulations of soleus H-reflex excitability during gait initiation: central versus peripheral influences

Soleus and tibialis anterior electromyogram (EMG) and soleus H-reflexes were recorded from the stance limb of an individual who suffered a traumatic peroneal nerve injury and of four nonimpaired individuals during gait initiation. The control subjects also initiated walking after swaying forward (sway-gait initiation), which eliminated the initial tibialis anterior activation. During the initial period of gait initiation, H-reflexes were depressed to 43% of standing values during normal-gait initiation and 86% during sway-gait initiation in the nonimpaired subjects. H-reflexes of the nerve-injured subject were depressed to 37%, even though no tibialis anterior EMG was observed. The findings support the view that reciprocal inhibition of the soleus during a task, which normally involves tibialis anterior activation, is due to a centrally mediated process.

Effect of a reduced base of support in standing and balance training on the soleus H-reflex

The present study provides evidence that a reflex at the segmental level can adapt over a two hour experiment in a functionally appropriate manner in response to a balance training task. Subjects (N=9) received soleus (S) H-reflexes in blocks of seven trials while free standing on a normal base-of-support (NBOS) and while standing on a plafform with a reduced base-of-support (RBOS) in the sagittal plane. During the RBOS condition, the H-reflex served as a postural perturbation. Subjects were instructed to suppress the H-reflex when it was evoked, as an attempt to maintain a balanced state. Background EMG from the S and tibialis anterior muscles, the S M-wave, and stimulus current were maintained at a constant level during the experiment. Subjects initially received a block of NBOS trials, followed by 4 RBOS blocks (training), a second NBOS block, four additional RBOS blocks, and a third NBOS block with the protocol repeated on three different days (D1, D2 and D3) within the same week. The S H/M ratio was depressed 9% upon standing on the RBOS. With training the S H/M ratio decreased by 22% on Dl, 18% on D2 and 6% on D3. The ratio between the H-reflex and background S EMG (H-reflex gain) decreased 10% on D1, 40% on D2 and 23% on D3 when the first NBOS and first RBOS blocks were compared. Due to a slight increase in the S EMG across blocks, the H-reflex gain decreased considerably more across blocks than the H/M ratio. Although the S H/M ratio underwent an 7% decrease from D1 to D3, the differences were not significant. Individually, however, six of the nine subjects decreased their H/M ratios from 12-42% across days. The results may reflect the inception of longer-term adaptations of the segmental stretch reflex system.

Modulation of triceps surae H-reflexes as a function of the reflex activation history during standing and stepping

The facilitatory effectiveness of spindle afferent feedback is controlled by modulation of segmental reflex excitability such that the level of muscle activation is appropriate for the task. Phase-dependent modes of reflex modulation have been well-characterized. We hypothesized that segmental reflex excitability of the triceps surae was also modulated in a manner associated with the activation history of the spindle afferents and the segmental reflex pathway during isometric contractions, standing and stepping. In the first experiment. pairs of soleus (S) H-reflexes were evoked 80 ms apart with equal strength stimuli at rest and while subjects isometrically contracted their S against loads of 10%. 20%. and 50% of their maximum voluntary efforts. The percent depression of the second H-reflex relative to the first was used as a measure of the effect of reflex activation history. At rest, the second H-reflexes were depressed an average of 73% relative to the first. The degree of depression was progressively reduced as the plantarflexion torque increased. In the second experiment, paired H-reflexes were obtained from the S and medial (MG) and lateral gastrocnemii (LG) muscles while subjects were standing and during the stance phase of step initiation. The degree of depression of the second H-reflex during standing ( > 78%) was similar in magnitude to that produced at rest in Experiment I. At the end of the stance phase of stepping. depression of the second H-reflex of all three muscles was reduced to less than 25%. We conclude that the segmental reflex excitability is modulated as a function of the reflex activation history during these tasks.