Modulation of soleus H reflex due to stance pattern and haptic stabilization of posture

Influence of oral motor tasks on postural muscle activity during dynamic reactive balance

BACKGROUND

Jaw clenching improves dynamic reactive balance on an oscillating platform during forward acceleration and is associated with decreased mean sway speed of different body regions.

OBJECTIVE

It is suggested that jaw clenching as a concurrent muscle activity facilitates human motor excitability, increasing the neural drive to distal muscles. The underlying mechanism behind this phenomenon was studied based on leg and trunk muscle activity (iEMG) and co-contraction ratio (CCR).

METHODS

Forty-eight physically active and healthy adults were assigned to three groups, performing three oral motor tasks (jaw clenching, tongue pressing against the palate or habitual lower jaw position) during a dynamic one-legged stance reactive balance task on an oscillating platform. The iEMG and CCR of posture-relevant muscles and muscle pairs were analysed during platform forward acceleration.

RESULTS

Tongue pressing caused an adjustment of co-contraction patterns of distal muscle groups based on changes in biomechanical coupling between the head and trunk during static balancing at the beginning of the experiment. Neither iEMG nor CCR measurement helped detect a general neuromuscular effect of jaw clenching on the dynamic reactive balance.

CONCLUSION

The findings might indicate the existence of robust fixed patterns of rapid postural responses during the important initial phases of balance recovery.

Comparison of acute responses in spinal excitability between older and young people after neuromuscular electrical stimulation

PURPOSE

This study aims at comparing acute responses in spinal excitability, as measured by H-reflex, between older and young individuals, following a single session of NMES superimposed onto voluntary isometric contractions of the ankle plantar-flexor muscles (NMES+), with respect to passive NMES (pNMES) and voluntary isometric contractions only (ISO).

METHODS

Thirty-two volunteers, 16 older (OLDER) and 16 young (YOUNG), were asked to sustain a constant force at 20% of maximal voluntary isometric contraction (MVIC) of the ankle plantar-flexor muscles in the dominant limb during each of the 3 conditions (NMES+ , pNMES and ISO). Fifteen repetitions of 6 s were performed, with a resting interval of 6 s between repetitions. Before and after each condition, soleus H-reflexes were elicited by percutaneous electrical stimulation of the posterior tibial nerve and H-reflex amplitudes recorded by surface EMG.

RESULTS

In OLDER, H-reflex amplitude did not change following any experimental condition (ISO: p = 0.203; pNMES: p = 0.542; NMES+: p = 0.431) compared to baseline. On the contrary, in YOUNG, H-reflex amplitudes significantly increased (p < 0.000) and decreased (p = 0.001) following NMES+ and pNMES, respectively, while there was no significant change in reflex responses following ISO (p = 0.772).

CONCLUSION

The lack of change in H-reflex responses following either NMES+ or pNMES might reflect a reduced ability of older people in modulating spinal excitability after the conditions. Specifically, an age-related alteration in controlling mechanisms at presynaptic level was suggested.

Soleus H-reflex modulation in cerebral palsy and its relationship with neural control complexity: a pilot study

Individuals with cerebral palsy (CP) display motor control patterns that suggest decreased supraspinal input, but it remains unknown if they are able to modulate lower-limb reflexes in response to more complex tasks, or whether global motor control patterns relate to reflex modulation capacity in this population. Eight ambulatory individuals with CP (12-18 years old) were recruited to complete a task complexity protocol, where soleus H-reflex excitability was compared between bilateral (baseline) and unilateral (complex) standing. We also investigated the relationship between each participant's ability to modulate soleus H-reflex excitability and the complexity of their walking neural control pattern determined from muscle synergy analysis. Finally, six of the eight participants completed an exoskeleton walking protocol, where soleus H-reflexes were collected during the stance phase of walking with and without stance-phase plantar flexor resistance. Participants displayed a significant reduction in soleus H-reflex excitability (- 26 ± 25%, p = 0.04) with unilateral standing, and a strong positive relationship was observed between more refined neural control during walking and an increased ability to modulate reflex excitability (R = 0.79, p = 0.04). There was no difference in neuromuscular outcome measures with and without the ankle exoskeleton (p values all > 0.05), with variable reflex responses to walking with ankle exoskeleton resistance. These findings provide evidence that ambulatory individuals with CP retain some capacity to modulate lower-limb reflexes in response to increased task complexity, and that less refined neural control during walking appears to be related to deficits in reflex modulation.

Specific Posture-Stabilising Effects of Vision and Touch Are Revealed by Distinct Changes of Body Oscillation Frequencies

We addressed postural instability during stance with eyes closed (EC) on a compliant surface in healthy young people. Spectral analysis of the centre of foot pressure oscillations was used to identify the effects of haptic information (light-touch, EC-LT), or vision (eyes open, EO), or both (EO-LT). Spectral median frequency was strongly reduced by EO and EO-LT, while spectral amplitude was reduced by all "stabilising" sensory conditions. Reduction in spectrum level by EO mainly appeared in the high-frequency range. Reduction by LT was much larger than that induced by the vision in the low-frequency range, less so in the high-frequency range. Touch and vision together produced a fall in spectral amplitude across all windows, more so in anteroposterior (AP) direction. Lowermost frequencies contributed poorly to geometric measures (sway path and area) for all sensory conditions. The same subjects participated in control experiments on a solid base of support. Median frequency and amplitude of the spectrum and geometric measures were largely smaller when standing on solid than on foam base but poorly affected by the sensory conditions. Frequency analysis but not geometric measures allowed to disclose unique tuning of the postural control mode by haptic and visual information. During standing on foam, the vision did not reduce low-frequency oscillations, while touch diminished the entire spectrum, except for the medium-high frequencies, as if sway reduction by touch would rely on rapid balance corrections. The combination of frequency analysis with sensory conditions is a promising approach to explore altered postural mechanisms and prospective interventions in subjects with central or peripheral nervous system disorders.

Modulation of spinal cord excitability following remote limb ischemic preconditioning in healthy young men

Remote limb ischemic preconditioning (RIPC) has shown to improve dynamic postural control in humans. However, studies on the underlying adaptations of spinal cord networks have never been performed. The present work addresses this issue by investigating parameters from the soleus H-reflex recruitment curve (RC), presynaptic mechanisms of reflex modulation (presynaptic inhibition-PSI, and post activation depression-PAD), and the excursion of the center of pressure (CP) recorded during 1 min in upright stance over a compliant surface. A sham ischemic protocol (partial obstruction of blood flow) was applied to the contralateral thigh along four consecutive days. The same procedure was repeated with full obstruction (RIPC) three days after ending the sham protocol. Data were collected before and after both sham and RIPC protocols. The follow-up data were collected five days after the last ischemic intervention. Significant reduction was detected for both the fast oscillations of the CP (higher frequency components) and the parameter estimated from the RC corresponding to the high amplitude H-reflexes (p < 0.05). Even though the magnitude of effects was similar, it was washed out within three days after sham, but persisted for at least five days after RIPC. No significant differences were found for PSI and PAD levels across conditions. These findings indicate that RIPC leads to enduring changes in spinal cord excitability for the latest reflexively recruited motoneurons, along with improvement in balance control. However, these adaptations were not mediated by the presynaptic mechanisms currently assessed.

Modulation of the Hoffmann reflex in the tibialis anterior with a change in posture

Hoffmann (H-) reflex amplitudes in plantar flexor soleus muscle are modulated by posture, yet dorsiflexor tibialis anterior (TA) H-reflex parameters have sparingly been studied. The purpose was to investigate modulation of the TA H-reflex when postural demands are increased from sitting to standing. In this study, data from 18 participants (Age: 25 ± 4 years, Height: 170.9 ± 9.5 cm, Weight: 75.9 ± 17.2 kg) allowed comparison of two experimental conditions involving different postures (i.e. sitting and standing). Maximal amplitude of the TA H-reflex (Hmax ) as a percent of the maximal M-wave amplitude (Mmax ) (Hmax (% Mmax )) during sitting and standing was compared using ANOVA. Modulation of TA H-reflex amplitude was found: Eleven participants showed facilitation and seven showed no change of reflex amplitudes. Only participants in the facilitation group showed modulation related to changes in posture (sitting: 8.7 ± 2.9%; standing: 14.8 ± 6.7%, P = 0.005). These data provide evidence of the sensitivity to posture of TA H-reflexes. As with task-dependent changes in soleus H-reflexes, presynaptic regulation of Ia afferent transmission is a possible mechanism. Further investigations into causes of modulation are warranted.

H-reflex in abductor hallucis and postural performance between flexible flatfoot and normal foot

OBJECTIVE

Morphological changes of the abductor hallucis muscle (AbH) in flexible flatfoot (FF) individuals influence regulations of the medial longitudinal arch (MLA). Prolonged and repeated stretching of AbH in flexible flatfoot may cause changes in muscle reflex properties and further influence postural performance. However, AbH muscle reflex under different postural conditions have never been examined. The purpose of this study was to investigate differences in AbH H-reflex and postural performance between individuals with normal foot (NF) alignment and FF under prone, double-leg stance (DLS), and single-leg stance (SLS) conditions.

DESIGN

Cross-sectional study.

SETTING

University laboratory.

PARTICIPANTS

Individuals with FF (n = 12) and NF (n = 12).

MAIN OUTCOME MEASURES

AbH H-reflex, AbH EMG and center of pressure (CoP) displacement.

RESULTS

Under all postural conditions, AbH H-reflex was significantly lower in the FF group (P < .05). Under the SLS condition, AbH EMG was significantly higher in the FF group (P < .05), and CoP displacement for the medial-lateral and anterior-posterior directions were significantly higher in the FF group (P < .05).

CONCLUSIONS

With increased postural demand, FF individuals maintained their postural stability by recruiting greater AbH activities than through automatic stretch reflex, but FF individuals still showed inferior posture stability.

Haptic Cues for Balance: Use of a Cane Provides Immediate Body Stabilization

Haptic cues are important for balance. Knowledge of the temporal features of their effect may be crucial for the design of neural prostheses. Touching a stable surface with a fingertip reduces body sway in standing subjects eyes closed (EC), and removal of haptic cue reinstates a large sway pattern. Changes in sway occur rapidly on changing haptic conditions. Here, we describe the effects and time-course of stabilization produced by a haptic cue derived from a walking cane. We intended to confirm that cane use reduces body sway, to evaluate the effect of vision on stabilization by a cane, and to estimate the delay of the changes in body sway after addition and withdrawal of haptic input. Seventeen healthy young subjects stood in tandem position on a force platform, with eyes closed or open (EO). They gently lowered the cane onto and lifted it from a second force platform. Sixty trials per direction of haptic shift (Touch → NoTouch, T-NT; NoTouch → Touch, NT-T) and visual condition (EC-EO) were acquired. Traces of Center of foot Pressure (CoP) and the force exerted by cane were filtered, rectified, and averaged. The position in space of a reflective marker positioned on the cane tip was also acquired by an optoelectronic device. Cross-correlation (CC) analysis was performed between traces of cane tip and CoP displacement. Latencies of changes in CoP oscillation in the frontal plane EC following the T-NT and NT-T haptic shift were statistically estimated. The CoP oscillations were larger in EC than EO under both T and NT (p < 0.001) and larger during NT than T conditions (p < 0.001). Haptic-induced effect under EC (Romberg quotient NT/T ~ 1.2) was less effective than that of vision under NT condition (EC/EO ~ 1.5) (p < 0.001). With EO cane had little effect. Cane displacement lagged CoP displacement under both EC and EO. Latencies to changes in CoP oscillations were longer after addition (NT-T, about 1.6 s) than withdrawal (T-NT, about 0.9 s) of haptic input (p < 0.001). These latencies were similar to those occurring on fingertip touch, as previously shown. Overall, data speak in favor of substantial equivalence of the haptic information derived from both “direct” fingertip contact and “indirect” contact with the floor mediated by the cane. Cane, finger and visual inputs would be similarly integrated in the same neural centers for balance control. Haptic input from a walking aid and its processing time should be considered when designing prostheses for locomotion.

Limb Segment Load Inhibits the Recovery of Soleus H-Reflex After Segmental Vibration in Humans

We investigated the effects of vertical vibration and compressive load on soleus H-reflex amplitude and postactivation depression. We hypothesized that, in the presence of a compressive load, limb vibration induces a longer suppression of soleus H-reflex. Eleven healthy adults received vibratory stimulation at a fixed frequency (30 Hz) over two loading conditions (0% and 50% of individual's body weight). H-reflex amplitude was depressed ∼88% in both conditions during vibration. Cyclic application of compression after cessation of the vibration caused a persistent reduction in H-reflex excitability and postactivation depression for > 2.5 min. A combination of limb segment vibration and compression may offer a nonpharmacologic method to modulate spinal reflex excitability in people after CNS injury.

EEG frequency analysis of cortical brain activities induced by effect of light touch

In human postural control, touching a fingertip to a stable object with a slight force (<1 N) reduces postural sway independent of mechanical support, which is referred to as the effect of light touch (LT effect). The LT effect is achieved by the spatial orientation according to haptic feedback acquired from an external spatial reference. However, the neural mechanism of the LT effect is incompletely understood. Therefore, the purpose of this study was to employ EEG frequency analysis to investigate the cortical brain activity associated with the LT effect when attentional focus was strictly controlled with the eyes closed during standing (i.e., control, fixed-point touch, sway-referenced touch, and only fingertip attention). We used EEG to measure low-alpha (about 8-10 Hz) and high-alpha rhythm (about 10-12 Hz) task-related power decrease/increase (TRPD/TRPI). The LT effect was apparent only when the subject acquired the stable external spatial reference (i.e., fixed-point touch). Furthermore, the LT-specific effect increased the high-alpha TRPD of two electrodes (C3, P3), which were mainly projected from cortical brain activities of the left primary sensorimotor cortex area and left posterior parietal cortex area. Furthermore, there was a negative correlation between the LT effect and increased TRPD of C3. In contrast, the LT effect correlated positively with increased TRPD of P3. These results suggest that central and parietal high-alpha TRPD of the contralateral hemisphere reflects the sensorimotor information processing and sensory integration for the LT effect. These novel findings reveal a partial contribution of a cortical neural mechanism for the LT effect.

The functional role of the triceps surae muscle during human locomotion

Aim

Despite numerous studies addressing the issue, it remains unclear whether the triceps surae muscle group generates forward propulsive force during gait, commonly identified as ‘push-off’. In order to challenge the push-off postulate, one must probe the effect of varying the propulsive force while annulling the effect of the progression velocity. This can be obtained by adding a load to the subject while maintaining the same progression velocity.

Methods

Ten healthy subjects initiated gait in both unloaded and loaded conditions (about 30% of body weight attached at abdominal level), for two walking velocities, spontaneous and fast. Ground reaction force and EMG activity of soleus and gastrocnemius medialis and lateralis muscles of the stance leg were recorded. Centre of mass velocity and position, centre of pressure position, and disequilibrium torque were calculated.

Results

At spontaneous velocity, adding the load increased disequilibrium torque and propulsive force. However, load had no effect on the vertical braking force or amplitude of triceps activity. At fast progression velocity, disequilibrium torque, vertical braking force and triceps EMG increased with respect to spontaneous velocity. Still, adding the load did not further increase braking force or EMG.

Conclusions

Triceps surae is not responsible for the generation of propulsive force but is merely supporting the body during walking and restraining it from falling. By controlling the disequilibrium torque, however, triceps can affect the propulsive force through the exchange of potential into kinetic energy.

Limb compressive load does not inhibit post activation depression of soleus H-reflex in indiviudals with chronic spinal cord injury

OBJECTIVE

We investigated the effect of various doses of limb compressive load on soleus H-reflex amplitude and post activation depression in individuals with/without chronic SCI. We hypothesized that SCI reorganization changes the typical reflex response to an external load.

METHODS

Ten healthy adults and 10 individuals with SCI received three doses of compressive load to the top of their knee (10%, 25%, and 50% of the body weight, BW). Soleus H-reflexes were measured before (baseline) and during the loading phase.

RESULTS

With persistent background muscle activity across all testing sessions, segment compressive load significantly decreased post activation depression in the control group, but did not change the post activation ratio in the SCI group. Normalized H2 amplitude significantly increased according to load (50%> 25%> 10%) in the control group whereas was minimally modulated to load in those with SCI.

CONCLUSIONS

Segment compressive load inhibits post activation depression in humans without SCI, but minimally modulates the reflex circuitry in people with chronic SCI. These findings suggest that spinal cord reorganization mitigates the typical response to load in people with chronic SCI.

SIGNIFICANCE

Early limb load training may impact the reorganization of the spinal cord in humans with acute SCI.

Limb segment load inhibits post activation depression of soleus H-reflex in humans

OBJECTIVE

We investigated the effect of various doses of limb segment load on soleus H-reflex amplitude and post activation depression in healthy individuals. We also explored the influence of limb segment load on spinal circuitry in one individual with chronic SCI.

METHODS

Twenty-eight healthy adults and one SCI subject received compressive loads applied to the top of their knee at varied doses of load (10%, 25%, and 50% of the body weight). Soleus H-reflexes were measured before (baseline) and during the loading phase.

RESULTS

There were no significant differences in H-reflex amplitudes during the 50% BW load-on phase as compared to either baseline session or the load-off phase. However, the post activation depression was decreased over 9% (p<0.05) during the load-on phase compared to the load-off phase and scaled according to load (50%>25%>10%). The post activation depression ratio also appears less responsive to varying loads after chronic SCI.

CONCLUSIONS

Limb segment load decreases post-activation depression in humans. These findings suggest that the mechanism associated with post activation depression is modulated by limb segment load, and may be influenced by spinal reorganization after SCI.

SIGNIFICANCE

Future studies will determine if various levels of spasticity modulate the response of limb segment load on post activation depression in those with acute and chronic SCI.

Behavioral data and neural correlates for postural prioritization and flexible resource allocation in concurrent postural and motor tasks

This study was undertaken to investigate the reciprocity effect between postural and suprapostural performances and its underlying neural mechanisms wherein subjects executed a perceptual-motor suprapostural task and maintained steady upright postures. Fourteen healthy individuals conducted force-matching maneuvers (static vs. dynamic) under two stance conditions (bipedal stance vs. unipedal stance); meanwhile, force-matching error, center of pressure dynamics, event-related potentials (ERPs), and the movement-related potential (MRP) were monitored. The behavioral results showed that force-matching error and postural sway were differently modulated by variations in stance pattern and force-matching version. Increase in postural challenge undermined the precision of static force-matching but facilitated a dynamic force-matching task. Both static and dynamic force-matching tasks improved postural control of unipedal stance but not of bipedal stance, in reference to the control conditions. ERP results revealed a stance-dependent N1 response, which was greater around the parietal cortex in the unipedal stance conditions. Instead, P2 was modulated by the effect of the suprapostural motor task, with a smaller P2 in the right parietal cortex for dynamic force-matching. Spatiotemporal evolution of the MRP commenced at the left frontal-central area and spread bilaterally over the frontal-central and parietal cortex. MRP onset was subject to an analogous interaction effect on force-matching performance. Our findings suggest postural prioritization and a structural alternation effect of stance pattern on postural performance, relevant to implicit expansion and selective allocation of central resources for relative task-loads of a postural-suprapostural task.

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.

Role of vision and task complexity on soleus H-reflex gain

There exists extensive evidence supporting the presence of reflex modulation in humans during a variety of motor tasks. The soleus H-reflex has been shown to be modulated during static and dynamic balance conditions as well as during various motor tasks. The purpose of this study was to examine the effects of two different stance positions and visual conditions on soleus H-reflex gain in 15 apparently healthy adults (mean age=30.27+/-6.92 yrs). The soleus H-reflexes were examined in two experimental stance conditions: two-legged (stable) and one-leg (unstable), and two visual conditions: eyes open and eyes closed. To assess the reflex gain, subjects performed ten trials under each of the four conditions and a soleus H-reflex was elicited during the performance of each trial. For each condition the peak-to-peak amplitude of the H-reflex and the EMG activity 50 ms prior to the stimulus was recorded. Differences in the peak-to-peak amplitudes of the soleus H-reflex for the experimental conditions were compared with a 2x2 (Stance x Vision) repeated measures ANOVA. The level of significance was p<0.05. Results demonstrated significant differences in reflex gain for both the vision (F(l,15)=4.87, p<0.05) and the stance condition (F(l,15)=14.86, p<0.05). Although both the stance condition and vision significantly affected the H-reflex gain, there was no interaction between these two variables (F(l,15)=0.17). From these results, we conclude that H-reflex gain was decreased both as stance complexity increased and as visual inputs were removed. Consistent with previous reports, it may be speculated that changes in presynaptic inhibition to the soleus Ia fibers regulate these gain changes. We propose that vision and stability of stance affect soleus H-reflex gain, but do so without any interactive effects.