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

The underlying mechanisms of improved balance after one and ten sessions of balance training in older adults

Training improves balance control in older adults, but the time course and neural mechanisms underlying these improvements are unclear. We studied balance robustness and performance, H-reflex gains, paired reflex depression, and co-contraction duration in ankle muscles after one and ten training sessions in 22 older adults (+65 yrs). Mediolateral balance robustness, time to balance loss in unipedal standing on a platform with decreasing rotational stiffness, improved (33%) after one session, with no further improvement after ten sessions. Balance performance, absolute mediolateral center of mass velocity, improved (18.75%) after one session in perturbed unipedal standing and (18.18%) after ten sessions in unperturbed unipedal standing. Co-contraction duration of soleus/tibialis anterior increased (16%) after ten sessions. H-reflex gain and paired reflex depression excitability did not change. H-reflex gains were lower, and soleus/tibialis anterior co-contraction duration was higher in participants with more robust balance after ten sessions, and co-contraction duration was higher in participants with better balance performance at several time-points. Changes in robustness and performance were uncorrelated with changes in co-contraction duration, H-reflex gain, or paired reflex depression. In older adults, balance robustness improved over a single session, while performance improved gradually over multiple sessions. Changes in co-contraction and excitability of ankle muscles were not exclusive causes of improved balance.

Presynaptic inhibition in restless legs syndrome

Introduction: Restless legs syndrome (RLS) is a condition that particularly urges at night in resting and causes the need to move the legs. Although the pathophysiology has not yet been clarified, dopamine and iron metabolism and spinal cord pathologies are blamed for causing the condition. There are few studies on spinal reflex mechanisms on RLS. In the present study, we aimed to investigate the role of presynaptic inhibition (PreI) in the spinal cord in RLS.Methods: Fourteen patients with RLS and 14 controls with similar demographic characteristics were included in the study. Soleus muscle H-reflex (Ht) investigation was performed for subjects whose electrophysiologic investigation was normal. The Ht response was conditioned to the stimulation of the common peroneal nerve (CPN) (Hc). The test and conditioned stimulation intervals were kept between 10 ms, 20 ms, 30 ms, 40 ms, 50 ms, 75 ms, 100 ms, 150 ms and 200 ms. In each inter-stimulus interval, nonparametric repeat measurement evaluations were conducted with the percentage value of Hc/Ht. The Hc/Ht values of the study and control groups in the same intervals were compared separately.Results: A significant decrease was detected in Hc values in the control group in the repeat measurement values at 20 ms and 100 ms inter-stimulus intervals; however, there was not decrease in any intervals in the patient's group.Conclusion: The absence of any decrease in Hc reflexes for 20-100 ms intervals revealed that discernible PreI was vanished in RLS patients.

Motoneuron Function Does not Change Following Whole-Body Vibration in Individuals With Chronic Ankle Instability

CONTEXT

Following a lateral ankle sprain, ∼40% of individuals develop chronic ankle instability (CAI), characterized by recurrent injury and sensations of giving way. Deafferentation due to mechanoreceptor damage postinjury is suggested to contribute to arthrogenic muscle inhibition (AMI). Whole-body vibration (WBV) has the potential to address the neurophysiologic deficits accompanied by CAI and, therefore, possibly prevent reinjury.

OBJECTIVE

To determine if an acute bout of WBV can improve AMI and proprioception in individuals with CAI.

DESIGN AND PARTICIPANTS

The authors examined if an acute bout of WBV can improve AMI and proprioception in individuals with CAI with a repeated-measures design. A total of 10 young adults with CAI and 10 age-matched healthy controls underwent a control, sham, and WBV condition in randomized order.

SETTING

Biomechanics laboratory.

INTERVENTION

WBV.

MAIN OUTCOME MEASURES

Motoneuron pool recruitment was assessed via Hoffmann reflex (H-reflex) in the soleus. Proprioception was evaluated using ankle joint position sense at 15° and 20° of inversion. Both were assessed prior to, immediately following, and 30 minutes after the intervention (pretest, posttest, and 30mPost, respectively).

RESULTS

Soleus maximum H-reflex:M-response (H:M) ratios were 25% lower in the CAI group compared with the control group (P = .03). Joint position sense mean constant error did not differ between groups (P = .45). Error at 15° in the CAI (pretest 0.8 [1.6], posttest 2.0 [2.8], 30mPost 2.0 [1.9]) and control group (pretest 0.8 [2.0], posttest 0.6 [2.9], 30mPost 0.5 [2.1]) did not improve post-WBV. Error at 20° did not change post-WBV in the CAI (pretest 1.3 [1.7], posttest 1.0 [2.4], 30mPost 1.5 [2.2]) or control group (pretest -0.3 [3.0], posttest 0.8 [2.1], 30mPost 0.6 [1.8]).

CONCLUSION

AMI is present in the involved limb of individuals with CAI. The acute response following a single bout of WBV did not ameliorate the presence of AMI nor improve proprioception in those with CAI.

Neural Substrates of Cognitive Motor Interference During Walking; Peripheral and Central Mechanisms

Current gait control models suggest that independent locomotion depends on central and peripheral mechanisms. However, less information is available on the integration of these mechanisms for adaptive walking. In this cross-sectional study, we investigated gait control mechanisms in people with Parkinson’s disease (PD) and healthy older (HO) adults: at self-selected walking speed (SSWS) and at fast walking speed (FWS). We measured effect of additional cognitive task (DT) and increased speed on prefrontal (PFC) and motor cortex (M1) activation, and Soleus H-reflex gain. Under DT-conditions we observed increased activation in PFC and M1. Whilst H-reflex gain decreased with additional cognitive load for both groups and speeds, H-reflex gain was lower in PD compared to HO while walking under ST condition at SSWS. Attentional load in PFC excites M1, which in turn increases inhibition on H-reflex activity during walking and reduces activity and sensitivity of peripheral reflex during the stance phase of gait. Importantly this effect on sensitivity was greater in HO. We have previously observed that the PFC copes with increased attentional load in young adults with no impact on peripheral reflexes and we suggest that gait instability in PD may in part be due to altered sensorimotor functioning reducing the sensitivity of peripheral reflexes.

Modulation of H-Reflex Depression with Paired-Pulse Stimulation in Healthy Active Humans

Depression of the Hoffman reflex (H-reflex) is used to examine spinal control mechanisms during exercise, fatigue, and vibration and in response to training. H-reflex depression protocols frequently use trains of stimuli; this is time-consuming and prevents instantaneous assessment of motor neuronal excitability. The purpose of this study was to determine if paired-pulse H-reflex depression is reproducible and whether paired-pulse stimulation adequately estimates the depression induced by the more traditional ten-pulse train. H-reflexes were elicited via ten-pulse trains at 0.1, 0.2, 1, 2, and 5 Hz in ten neurologically intact individuals on two separate days. We measured the depression elicited by the second pulse (H2) and the mean depression elicited by pulses 2–10 (Hmean). H2 was consistent at all frequencies on both days (r² = 0.97, p < 0.05, and ICC_(3,1) = 0.81). H2 did not differ from Hmean (p > 0.05). The results indicate that paired-pulse H-reflex depression has high between-day reliability and yields depression estimates that are comparable to those obtained via ten-pulse trains. Paired-pulse H-reflex depression may be especially useful for studies that require rapid assessment of motor neuronal excitability, such as during exercise, fatigue, and vibration, or to establish recovery curves following inhibition.

Impaired H-Reflex Gain during Postural Loaded Locomotion in Individuals Post-Stroke

Objective

Successful execution of upright locomotion requires coordinated interaction between controllers for locomotion and posture. Our earlier research supported this model in the non-impaired and found impaired interaction in the post-stroke nervous system during locomotion. In this study, we sought to examine the role of the Ia afferent spinal loop, via the H-reflex response, under postural influence during a locomotor task. We tested the hypothesis that the ability to increase stretch reflex gain in response to postural loads during locomotion would be reduced post-stroke.

Methods

Fifteen individuals with chronic post-stroke hemiparesis and 13 non-impaired controls pedaled on a motorized cycle ergometer with specialized backboard support system under (1) seated supported, and (2) non-seated postural-loaded conditions, generating matched pedal force outputs of two levels. H-reflexes were elicited at 90°crank angle.

Results

We observed increased H-reflex gain with postural influence in non-impaired individuals, but a lack of increase in individuals post-stroke. Furthermore, we observed decreased H-reflex gain at higher postural loads in the stroke-impaired group.

Conclusion

These findings suggest an impaired Ia afferent pathway potentially underlies the defects in the interaction between postural and locomotor control post-stroke and may explain reduced ability of paretic limb support during locomotor weight-bearing in individuals post-stroke.

Significance

These results support the judicious use of bodyweight support training when first helping individuals post-stroke to regain locomotor pattern generation and weight-bearing capability.

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.

Associations between prefrontal cortex activation and H-reflex modulation during dual task gait

Walking, although a largely automatic process, is controlled by the cortex and the spinal cord with corrective reflexes modulated through integration of neural signals from central and peripheral inputs at supraspinal level throughout the gait cycle. In this study we used an additional cognitive task to interfere with the automatic processing during walking in order to explore the neural mechanisms involved in healthy young adults. Participants were asked to walk on a treadmill at two speeds, both with and without additional cognitive load. We evaluated the impact of speed and cognitive load by analyzing activity of the prefrontal cortex (PFC) using functional Near-Infrared Spectroscopy (fNIRS) alongside spinal cord reflex activity measured by soleus H-reflex amplitude and gait changes obtained by using an inertial measuring unit. Repeated measures ANOVA revealed that fNIRS Oxy-Hb concentrations significantly increased in the PFC with dual task (walking while performing a cognitive task) compared to a single task (walking only; p < 0.05). PFC activity was unaffected by increases of walking speed. H-reflex amplitude and gait variables did not change in response to either dual task or increases in walking speed. When walking under additional cognitive load participants adapted by using greater activity in the PFC, but this adaptation did not detrimentally affect H-reflex amplitude or gait variables. Our findings suggest that in a healthy young population central mechanisms (PFC) are activated in response to cognitive loads but that H-reflex activity and gait performance can successfully be maintained. This study provides insights into the mechanisms behind healthy individuals safely performing dual task walking.

Lateralized α-motoneuron excitabilities during lying and standing of healthy individuals in relation to parkinsonian rigidity

OBJECTIVES

To elucidate mechanisms of Parkinsonian rigidity by assessing excitability of alpha-motoneurons innervating right and left soleus muscles in healthy controls and Parkinson's disease (PD) patients with rigidities in the right, left and both legs.

METHODS

One group of 45 controls was recruited and 60 PD patients in three groups: rigidities, predominantly in the right, left and both legs. H-reflex (H) and muscle response (M) were recorded from right and left soleus muscles during stimulations of the posterior tibial nerve at the popliteal fossa while lying and standing. The H/M ratio was taken as an index for motoneuron excitability.

RESULTS

Mean H/M ratios were significantly different on the right and left sides, modified by postural changes in controls and PD patients. Analysis of variance showed that in healthy subjects the H/M ratio was: standing>lying (right), lying>standing (left). In right leg rigidity patients, the H/M ratio was greatest during standing, and smallest during lying. In left leg rigidity patients, the H/M ratios on the right and left sides were equally independent of posture. In controls, left H/M>right while lying, <right while standing. In right leg rigidity patients, right H/M>left, but <right in left leg rigidity patients, independent of posture. There was no side difference in patients with rigidity in both legs.

CONCLUSIONS

(i) motoneuron excitability may show side and postural differences in healthy individuals and PD patients; (ii) posture may be associated with lateralized motoneuron excitability in these subjects; and (iii) Parkinsonian rigidity may have spinal motor origins.

Postactivation depression and recovery of reflex transmission during repetitive electrical stimulation of the human tibial nerve

H-reflexes are progressively depressed, relative to the first response, at stimulation frequencies above 0.1 Hz (postactivation depression; PAD). Presently, we investigated whether H-reflexes "recover" from this depression throughout 10-s trains of stimulation delivered at physiologically relevant frequencies (5-20 Hz) during functionally relevant tasks (sitting and standing) and contraction amplitudes [relaxed to 20% maximum voluntary contraction (MVC)]. When participants held a 10% MVC, reflex amplitudes did not change during 5-Hz stimulation. During stimulation at 10 Hz, reflexes were initially depressed by 43% but recovered completely by the end of the stimulation period. During 20-Hz stimulation, reflexes were depressed to 10% and recovered to 36% of the first response, respectively. This "postactivation depression and recovery" (PAD&R) of reflex amplitude was not different between sitting and standing. In contrast, PAD&R were strongly influenced by contraction amplitude. Reflexes were depressed to 10% of the first response during the relaxed condition (10-Hz stimulation) and showed no depression during a 20% MVC contraction. A partial recovery of reflex amplitude occurred when participants were relaxed and during contractions of 1-5% MVC. Surprisingly, reflexes could recover completely by the third pulse within a stimulation train when participants held a contraction between 5 and 10% MVC during stimulation at 10 Hz, a finding that challenges classical ideas regarding PAD mechanisms. Our results support the idea that there is an ongoing interplay between depression and facilitation when motoneurons receive trains of afferent input. This interplay depends strongly on the frequency of the afferent input and the magnitude of the background contraction but is relatively insensitive to changes in task.

Soleus H-reflex modulation after motor incomplete spinal cord injury: effects of body position and walking speed

OBJECTIVE

To examine position-dependent (semireclined to standing) and walking speed-dependent soleus H-reflex modulation after motor incomplete spinal cord injury (SCI).

PARTICIPANTS

Twenty-six patients with motor incomplete SCI (mean: 45 +/- 15 years) and 16 noninjured people (mean: 38 +/- 14 years).

METHODS

Soleus H-reflexes were evoked by tibial nerve stimulation. Patients were tested in semireclined and standing positions (experiment 1) and in midstance and midswing positions (experiment 2).

RESULTS

H-reflexes were significantly greater after SCI in all positions compared with noninjured people (P < 0.05). Position-dependent modulation from semireclined to standing (normally observed in noninjured people) was absent after SCI. In SCI patients, H-reflex modulation was not significantly different at 1.2 m/s compared with 0.6 m/s treadmill walking speed; in noninjured people, H-reflex modulation was significantly greater at 1.2 m/s compared with 0.6 m/s treadmill walking speed. There was a significant positive correlation between modified Ashworth scores, a clinical measure of spasticity and soleus H-reflex amplitudes tested in all positions. A significant negative correlation was also found between H-reflexes in standing and midstance positions and the amount of assistance patients required to walk.

CONCLUSIONS

An improvement in position-dependent and walking speed-dependent reflex modulation after SCI may indicate functional recovery. Future studies will use H-reflex testing to track changes as a result of therapeutic interventions.