Control of the triceps surae during the postural sway of quiet standing

Difference in the recruitment of intrinsic foot muscles in the elderly under static and dynamic postural conditions

Background

The effect of foot, especially intrinsic muscles, on postural control and its related mechanisms remain unclear due to the complex structure. Therefore, this study aims to investigate the activation of intrinsic foot muscles in the elderly under static and dynamic postural tasks.

Methods

Twenty-one elderly participants were included to perform different postural tests (sensory organization test (SOT), motor control test (MCT), limit of stability test (LOS), and unilateral stance test) by a NeuroCom Balance Manager System. The participants were instructed to maintain postural stability under conditions with combined different sensory inputs (vision, vestibular, and proprioception) in SOT as well as conditions with translation disturbance in MCT, and to perform an active weight-shifting tasks in LOS. During these tasks, muscle activation were simultaneously acquired from intrinsic foot muscles (abductor halluces (AbH) and flexor digitorum brevis (FDB)) and ankle muscles (anterior tibialis, medial head of gastrocnemius, lateral head of gastrocnemius, and peroneus longus). The root-mean-square amplitude of these muscles in postural tasks was calculated and normalized with the EMG activity in unilateral stance task.

Results

The activation of intrinsic foot muscles significantly differed among different SOT tasks (p < 0.001). Post-hoc tests showed that compared with that under normal condition 1 without sensory interference, EMGs increased significantly under sensory disturbance (conditions 2-6). By contrast, compared with that under the single-sensory disturbed conditions (conditions 2-4; 2 for disturbed vision, 3 for disturbed vestibular sensation, 4 for disturbed proprioception), activation was significantly greater under the dual-sensory disturbed postural tasks (conditions 5 and 6; 5 for disturbed vision and proprioception, 6 for disturbed vestibular sensation and proprioception). In MCT, EMGs of foot muscles increased significantly under different translation speeds (p < 0.001). In LOS, moderate and significant correlations were found between muscle activations and postural stability parameters (AbH, r = 0. 355-0.636, p < 0.05; FDB, r = 0.336-0.622, p < 0.05).

Conclusion

Intrinsic foot muscles play a complementary role to regulate postural stability when disturbances occur. In addition, the recruitment magnitude of intrinsic foot muscles is positively correlated with the limit of stability, indicating their contribution to increasing the limits of stability in the elderly.

Sex differences concerning the effects of ankle muscle fatigue on static postural control and spinal proprioceptive input at the ankle

Aims

The main aim of this study was to determine sex differences in postural control changes with ankle muscle fatigue during a standing forward leaning (FL) task under different vision conditions. The secondary aim was to examine sex differences in the effect of fatigue on soleus (SOL) H-reflex amplitude, a measure of motoneuron excitability with activation of Ia afferents.

Methods

Fifteen healthy young adult males (mean age: 28.0 years) and 16 healthy young adult females (mean age: 26.1 years) were asked to perform four consecutive FL tasks [30 s; two with eyes open (EO) and two with eyes closed (EC)] before, and immediately following a fatiguing exercise consisting of alternating ankle plantarflexion (6 s) and dorsiflexion (2 s) maximal isometric contractions, and at 5 and 10 min of recovery. Center of pressure (COP) sway variables (mean position, standard deviation, ellipse area, average velocity, and frequency), an ankle co-contraction index, and a ratio of SOL H-reflex to the maximum amplitude of the compound muscle action potential (M-max) were obtained during the FL tasks. A rating of perceived fatigue (RPF) was also documented at the different time points.

Results

Time to task failure (reduction of 50% in maximal voluntary isometric contraction torque of ankle plantar flexors) and the increase in RPF value were not significantly different between males and females. Both sex groups showed similar and significant increases (p < 0.05) in mean COP sway velocity with no significant changes in co-contraction indices. No significant effects of fatigue and related interactions were found for SOL H/M-max ratio.

Discussion

The absence of a significant sex difference in postural control change (sway and co-contraction) with fatigue could be explained by similar perceived (RPF) and performance fatigability (exercise duration) between males and females in the present study. Fatigue did not lead to significant changes in SOL spinal motoneuron excitability with activation of Ia afferents.

Systematic review: exercise training for equinus deformity in children with cerebral palsy

Background

Children with spastic cerebral palsy have motor deficits that can lead to joint contractures. Ankle equinus deformity is the most common foot deformity among children with CP. It is caused by spasticity and muscular imbalance in the gastrocnemius-soleus complex. Exercise enhances ankle function, improves gait in children with CP, and prevents permanent impairment. Therefore, there is a need to investigate the effectiveness of different types of exercise used in equine management. The aim of this review is to assess the evidence of the effectiveness of exercise training on equinus deformity in children with cerebral palsy.

Methodology

The American Academy for Cerebral Palsy and Developmental Medicine and Preferred Reporting Items for Systematic Reviews and Meta-Analyses methodology were used to conduct this systematic review. Four databases (PubMed, Cochrane Library, Physiotherapy Evidence Database (PEDro), and Google Scholar) were searched till January 2022 using predefined terms by two independent reviewers. Randomized controlled trials published in English were included. This review included seven studies with 203 participants ranging in age from 5 to 18 years. Methodological quality was assessed using AACPDM, PEDro scale; also, levels of evidence adopted from modified Sacket’s scale were used for each study. Primary outcomes were dorsiflexion angle, plantar flexion angle, and plantar flexors strength.

Results

The quality of studies ranged from good (six studies) to fair (one study). The level of evidence was level 1 (six studies) and level 2 (one study) on modified Sacket’s scale. There is a low risk of bias in the included studies. Meta-analysis revealed a non-significant difference in plantar flexor strength, plantar flexion angle, and dorsiflexion angle between the study and control group.

Conclusions

There is a need for high-quality studies to draw a clear conclusion as the current level of evidence supporting the effectiveness of various types of exercises on equinus deformity in children with cerebral palsy is still weak.

Biosignal processing methods to explore the effects of side-dominance on patterns of bi- and unilateral standing stability in healthy young adults

We examined the effects of side-dominance on the laterality of standing stability using ground reaction force, motion capture (MoCap), and EMG data in healthy young adults. We recruited participants with strong right (n = 15) and left (n = 9) hand and leg dominance (side-dominance). They stood on one or two legs on a pair of synchronized force platforms for 50 s with 60 s rest between three randomized stance trials. In addition to 23 CoP-related variables, we also computed six MoCap variables representing each lower-limb joint motion time series. Moreover, 39 time- and frequency-domain features of EMG data from five muscles in three muscle groups were analyzed. Data from the multitude of biosignals converged and revealed concordant patterns: no differences occurred between left- and right-side dominant participants in kinetic, kinematic, or EMG outcomes during bipedal stance. Regarding single leg stance, larger knee but lower ankle joint kinematic values appeared in left vs right-sided participants during non-dominant stance. Left-vs right-sided participants also had lower medial gastrocnemius EMG activation during non-dominant stance. While right-side dominant participants always produced larger values for kinematic data of ankle joint and medial gastrocnemius EMG activation during non-dominant vs dominant unilateral stance, this pattern was the opposite for left-sided participants, showing larger values when standing on their dominant vs non-dominant leg, i.e., participants had a more stable balance when standing on their right leg. Our results suggest that side-dominance affects biomechanical and neuromuscular control strategies during unilateral standing.

Transcutaneous spinal direct current stimulation (tsDCS) does not affect postural sway of young and healthy subjects during quiet upright standing

Transcutaneous spinal direct current stimulation (tsDCS) is an effective non-invasive spinal cord electrical stimulation technique to induce neuromodulation of local and distal neural circuits of the central nervous system (CNS). Applied to the spinal cord lumbosacral region, tsDCS changes electrophysiological responses of the motor, proprioceptive and nociceptive pathways, alters the performance of some lower limb motor tasks and can even modulate the behavior of supramedullary neuronal networks. In this study an experimental protocol was conducted to verify if tsDCS (5 mA, 20 minutes) of two different polarizations, applied over the lumbosacral region (tenth thoracic vertebrae (T10)), can induce changes in postural sway oscillations of young healthy individuals during quiet standing. A novel initialization of the electrical stimulation was developed to improve subject blinding to the different stimulus conditions including the sham trials. Measures of postural sway, both global and structural, were computed before, during and following the DC stimulation period. The results indicated that, for the adopted conditions, tsDCS did not induce statistically significant changes in postural sway of young healthy individuals during quiet standing.

Balance Adaptation While Standing on a Compliant Base Depends on the Current Sensory Condition in Healthy Young Adults

Background

Several investigations have addressed the process of balance adaptation to external perturbations. The adaptation during unperturbed stance has received little attention. Further, whether the current sensory conditions affect the adaptation rate has not been established. We have addressed the role of vision and haptic feedback on adaptation while standing on foam.

Methods

In 22 young subjects, the analysis of geometric (path length and sway area) and spectral variables (median frequency and mean level of both total spectrum and selected frequency windows) of the oscillation of the centre of feet pressure (CoP) identified the effects of vision, light-touch (LT) or both in the anteroposterior (AP) and mediolateral (ML) direction over 8 consecutive 90 s standing trials.

Results

Adaptation was obvious without vision (eyes closed; EC) and tenuous with vision (eyes open; EO). With trial repetition, path length and median frequency diminished with EC (p < 0.001) while sway area and mean level of the spectrum increased (p < 0.001). The low- and high-frequency range of the spectrum increased and decreased in AP and ML directions, respectively. Touch compared to no-touch enhanced the rate of increase of the low-frequency power (p < 0.05). Spectral differences in distinct sensory conditions persisted after adaptation.

Conclusion

Balance adaptation occurs during standing on foam. Adaptation leads to a progressive increase in the amplitude of the lowest frequencies of the spectrum and a concurrent decrease in the high-frequency range. Within this common behaviour, touch adds to its stabilising action a modest effect on the adaptation rate. Stabilisation is improved by favouring slow oscillations at the expense of sway minimisation. These findings are preliminary to investigations of balance problems in persons with sensory deficits, ageing, and peripheral or central nervous lesion.

Soleus H-Reflex Change in Poststroke Spasticity: Modulation due to Body Position

Purpose

This study is aimed at exploring how soleus H-reflex change in poststroke patients with spasticity influenced by body position.

Materials and Methods

Twenty-four stroke patients with spastic hemiplegia and twelve age-matched healthy controls were investigated. Maximal Hoffmann-reflex (Hmax) and motor potential (Mmax) were elicited at the popliteal fossa in both prone and standing positions, respectively, and the Hmax/Mmax ratio at each body position was determined. Compare changes in reflex behavior in both spastic and contralateral muscles of stroke survivors in prone and standing positions, and match healthy subjects in the same position.

Results

In healthy subjects, Hmax and Hmax/Mmax ratios were significantly decreased in the standing position compared to the prone position (Hmax: p = 0.000, Hmax/Mmax: p = 0.016). However, Hmax/Mmax ratios were increased in standing position on both sides in poststroke patients with spasticity (unaffected side: p = 0.006, affected side: p = 0.095). The Hmax and Hmax/Mmax ratios were significantly more increased on the affected side than unaffected side irrespective of the position.

Conclusions

The motor neuron excitability of both sides was not suppressed but instead upregulated in the standing position in subjects with spasticity, which may suggest that there was abnormal regulation of the Ia pathway on both sides.

Immediate Effects of Local Vibration and Whole-body Vibration on Postural Control in Patients with Ataxia: an Assessor-Blind, Cross-over randomized trial

Vibration interventions are used in neurorehabilitation to improve postural control in recent years. Little is known about the immediate effects of vibration interventions on postural control in patients with ataxia. The aim of this study is to investigate and compare the immediate effects of local vibration (LV) and whole-body vibration (WBV) on postural control in patients with ataxia. This study was designed as cross-over, single blind randomized clinical trial. Twenty-one patients with ataxia met the inclusion criteria. LV (frequency, 80 Hz; amplitude, 1 mm) and WBV (30 Hz, 2 mm) were applied to all patients. There was a 1-week washout time between interventions. Each patient was assessed 3 times: pre-intervention and 1 and 60 min post-intervention. The assessor was blinded to the interventions. Outcome measures were limits of stability (LoS), and postural sways (Bertec Balance Check Screener), gait parameters (GAITRite), and static balance (one-leg stance test). Twenty patients completed both interventions. The mean patient age was 39.43 ± 9.67 years. LV increased the left-LoS post-vibration (1 and 60 min post) more than WBV did (p ˂ 0.05). LV increased the LoS stability score and the base of support at 1 min post-vibration, while WBV decreased them (p ˂ 0.05). This study demonstrated different immediate effects of a single session of LV versus WBV and showed that LV has better effects on postural control in patients with ataxia. ClinicalTrials.gov. nr NCT04183647.

Acute effects of kinesiology tape tension on soleus muscle h-reflex modulations during lying and standing postures

Kinesiology tape (KT) has been widely used in the areas of sports and rehabilitation. However, there is no gold standard for the tape tension used during a KT application. The purpose of this study was to examine the effects of KT application with different tension intensities on soleus muscle Hoffmann-reflex (H-reflex) modulation during lying and standing postures. Fifteen healthy university students were tested with 3 tape tension intensities during separate visits with a randomized sequence: tape-on no tension (0KT), moderate (about 50% of the maximal tape tension: (ModKT), and maximal tape tension (MaxKT). During each experimental visit, the H-reflex measurements on the soleus muscle were taken before, during, and after the KT application for both lying and standing postures. The H-wave and M-wave recruitment curves were generated using surface electromyography (EMG). There was a main effect for posture (p = 0.001) for the maximal peak-to-peak amplitude of the H-wave and M-wave (H_max/M_max) ratio, showing the depressed H_max/M_max ratio during standing, when compared to the lying posture. Even though the tension factor had a large effect (η_p² = 0.165), different tape tensions showed no significant differential effects for the H_max/M_max ratio. The spinal motoneuron excitability was not altered, even during the maximal tension KT application on the soleus muscle. Thus, the tension used during a KT application should not be a concern in terms of modulating the sensorimotor activity ascribed to elastic taping during lying and standing postures.

Improvement of dynamic postural stability by an exercise program

BACKGROUND

Central processing of multi-sensory feedback and motor commands responsible for force production are critical for postural control. An exercise program was developed to realign spinal curvature, but its effect on postural control is unknown.

RESEARCH QUESTION

To what extent would the exercise program influence on center of pressure (CoP) sway on stable and unstable surfaces?

METHODS

Subjects (n = 30) were randomly assigned into one of three groups: exercise on a cylinder-shaped tube (98-cm length, 15-cm diameter, n = 10), exercise on a flat surface (n = 10), and a control group that laid supine on a flat surface (n = 10). Standing posture of each subject was quantified using anterior-, posterior-, and lateral-view photography. Each subject's CoP sway was measured while standing on a static and dynamic platform with eyes open and eyes closed. Subjects were instructed to stand still when the platform was held stationary (e.g., no tilt) during the static condition. During the dynamic condition the platform was allowed to tilt in response to changes of CoP and subjects were instructed to maintain the platform in a horizontal position.

RESULTS

Only when subjects performed the exercise program on the tube, the angles of neck flexion and pelvis tilt decreased, and CoP sway in the sagittal, but not frontal plane, decreased during the dynamic platform conditions with both eyes open and eyes closed (p < 0.05).

SIGNIFICANCE

It is speculated that performing the exercise program on the tube might enhance a) central processing of somatosensory and vestibular inputs, b) motor commands responsible for force production in postural control, and c) biomechanical advantage by the realigned posture. The exercise program can be used by a variety of populations as home-exercise to realign the neck and pelvic posture and improve dynamic postural stability.

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.

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.

Increasing mediolateral standing sway is associated with increasing corticospinal excitability, and decreasing M1 inhibition and facilitation

In standing, corticospinal excitability increases and primary motor cortex (M1) inhibition decreases in response to anterior posterior or direction unspecific manipulations that increase task difficulty. However, mediolateral (ML) sway control requires greater active neural involvement. Therefore, the primary purpose of this study was to determine the pattern of change in neural excitability when ML postural task difficulty is manipulated and to test whether the neural excitability is proportional to ML sway magnitude across conditions. Tibialis anterior corticospinal excitability was quantified using motor evoked potential (MEP) and postural sway was indexed using ML center of pressure (COP) velocity. Additionally, we examined inhibition and facilitation processes in the primary motor cortex using the paired pulse short interval intracortical inhibition (SICI) and intracortical facilitation (ICF) techniques respectively. Measurements were repeated in four conditions with quiet stance as a control. Differences between conditions were tested using one-way repeated measures ANOVAs, on log transformed data. Associations were quantified using Spearman's Rank Correlation Coefficient. There was a significant main effect of condition on all the neural excitability measures with MEP (p<0.001) being highest in the most difficult condition, and SICI (p=0.01), ICF (p<0.001) being lowest in the most difficult condition. Increasing ML COP velocity was significantly associated with increasing MEP amplitude (r=0.68, p<0.001), but decreasing SICI (r=0.24, p=0.03) and ICF (r=-0.54, p<0.001). Our results show that both corticospinal and M1 excitability in standing are scaled in proportion to ML task difficulty.

High lateral plantar pressure is related to an increased tibialis anterior/fibularis longus activity ratio in patients with recurrent lateral ankle sprain

Introduction

Center of pressure (COP) is a sudden displacement at the time of a lateral ankle sprain (LAS). It has been suggested that the distribution of plantar pressure and the quantity of COP displacement are important for assessing the risk of LAS. Therefore, we evaluated the plantar pressure during a single-leg balance test with eyes closed (SLB-C) to identify the factors and characteristics of plantar pressure in people with repeated cases of LAS.

Methods

We recruited 22 collegiate athletes and divided them into an instability group (IG; n=11) and a control group (CG; n=11). We measured the distribution of plantar pressure and lower extremity muscle activity during a SLB-C along with static alignment and isometric ankle strength.

Results

The fibularis longus (FL) activity was significantly lower in the IG than in the CG. The lateral plantar pressure (LPP)/medial plantar pressure (MPP) ratio was also higher in the IG than in the CG. In addition, the LPP/MPP ratio was correlated with the tibialis anterior (TA)/FL ratio.

Conclusion

These results suggest that increased lateral plantar pressure is related to decreased FL activity and increased TA/FL ratio.

Frequency characteristics of human muscle and cortical responses evoked by noisy Achilles tendon vibration

Noisy stimuli, along with linear systems analysis, have proven to be effective for mapping functional neural connections. We explored the use of noisy (10-115 Hz) Achilles tendon vibration to examine somatosensory reflexes in the triceps surae muscles in standing healthy young adults (n = 8). We also examined the association between noisy vibration and electrical activity recorded over the sensorimotor cortex using electroencephalography. We applied 2 min of vibration and recorded ongoing muscle activity of the soleus and gastrocnemii using surface electromyography (EMG). Vibration amplitude was varied to characterize reflex scaling and to examine how different stimulus levels affected postural sway. Muscle activity from the soleus and gastrocnemii was significantly correlated with the tendon vibration across a broad frequency range (~10-80 Hz), with a peak located at ~40 Hz. Vibration-EMG coherence positively scaled with stimulus amplitude in all three muscles, with soleus displaying the strongest coupling and steepest scaling. EMG responses lagged the vibration by ~38 ms, a delay that paralleled observed response latencies to tendon taps. Vibration-evoked cortical oscillations were observed at frequencies ~40-70 Hz (peak ~54 Hz) in most subjects, a finding in line with previous reports of sensory-evoked γ-band oscillations. Further examination of the method revealed 1) accurate reflex estimates could be obtained with <60 s of low-level (root mean square = 10 m/s²) vibration; 2) responses did not habituate over 2 min of exposure; and importantly, 3) noisy vibration had a minimal influence on standing balance. Our findings suggest noisy tendon vibration is an effective novel approach to characterize somatosensory reflexes during standing.NEW & NOTEWORTHY We applied noisy (10-115 Hz) vibration to the Achilles tendon to examine the frequency characteristics of lower limb somatosensory reflexes during standing. Ongoing muscle activity was coherent with the noisy vibration (peak coherence ~40 Hz), and coherence positively scaled with increases in stimulus amplitude. Our findings suggest that noisy tendon vibration, along with linear systems analysis, is an effective novel approach to study somatosensory reflex actions in active muscles.

Comparison of temporal and kinetic walking parameters among young people and falling and non-falling elderly persons

Objective : Comparison of the biomechanical parameters (spatiotemporal and kinetic) during walking of young people, falling, and non-falling elderly persons. Methods : A cross-sectional study was performed of 29 individuals divided into three groups: young persons (n=10); falling elderly individuals (n=7) and non-falling individuals (n=12). Gait analysis was performed based on the recording of three walking gait cycles along an 8 meter platform, which was attached to a force plate with a recording frequency of 200 Hz. Gait cycles were also recorded by three video cameras positioned perpendicular to the force plate with a recording frequency of 60 Hz. The data analyzed was: average step velocity, stance time, Froude number and anteroposterior ground reaction force. Results : The average step velocity was higher among young persons and there was no difference in the Froude number among the three groups. During the stance and impulse phase, anterior and posterior force was higher among young persons than in the non-falling elderly group. The foot stance time of young individuals was also lower than the non-falling elderly group (p=0.000) and the foot stance time of the falling elderly group was lower than that of the non-falling elderly group (p=0.004). Conclusion : Falling and non-falling elderly persons have different gait biomechanical characteristics than young women, other than with respect to the Froude number. Furthermore, falling elderly persons spend more time in the gait swing phase than non-falling elderly persons.

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.

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.

Behavior of medial gastrocnemius motor units during postural reactions to external perturbations after stroke

OBJECTIVE

This study investigated the behavior of medial gastrocnemius (GM) motor units (MU) during external perturbations in standing in people with chronic stroke.

METHODS

GM MUs were recorded in standing while anteriorly-directed perturbations were introduced by applying loads of 1% body mass (BM) at the pelvis every 25-40s until 5% BM was maintained. Joint kinematics, surface electromyography (EMG), and force platform measurements were assessed.

RESULTS

Although external loads caused a forward progression of the anterior-posterior centre of pressure (APCOP), people with stroke decreased APCOP velocity and centre of mass (COM) velocity immediately following the highest perturbations, thereby limiting movement velocity in response to perturbations. MU firing rate did not increase with loading but the GM EMG magnitude increased, reflecting MU recruitment. MU inter spike interval (ISI) during the dynamic response was negatively correlated with COM velocity and hip angular velocity.

CONCLUSIONS

The GM utilized primarily MU recruitment to maintain standing during external perturbations. The lack of MU firing rate modulation occurred with a change in postural central set. However, the relationship of MU firing rate with kinematic variables suggests underlying long-loop responses may be somewhat intact after stroke.

SIGNIFICANCE

People with stroke demonstrate alterations in postural control strategies which may explain MU behavior with external perturbations.

Diagnostic accuracy of common clinical tests for assessing abdominal muscle function after motor-complete spinal cord injury above T6

STUDY DESIGN

Diagnostic study.

OBJECTIVES

The objective of this study was to compare patterns of electromyography (EMG) recordings of abdominal muscle function in persons with motor-complete spinal cord injury (SCI) above T6 and in able-bodied controls, and to determine whether manual examination or ultrasound measures of muscle activation can be accurate alternatives to EMG.

SETTING

Research center focused on SCI and University laboratory, Vancouver, Canada.

METHODS

Thirteen people with SCI (11 with American Spinal Injury Association Impairment Scale (AIS) A and 2 AIS B; C4-T5), and 13 matched able-bodied participants volunteered for the study. Participants completed trunk tasks during manual examination of the abdominal muscles and then performed maximal voluntary isometric contractions, while EMG activity and muscle thickness changes were recorded. The frequency of muscle responses detected by manual examination and ultrasound were compared with detection by EMG (sensitivity and specificity).

RESULTS

All individuals with SCI were able to elicit EMG activity above resting levels in at least one abdominal muscle during one task. In general, the activation pattern was task specific, confirming voluntary control of the muscles. Ultrasound, when compared with EMG, showed low sensitivity but was highly specific in its ability to detect preserved abdominal muscle function in persons with SCI. Conversely, manual examination was more sensitive than ultrasound but showed lower specificity.

CONCLUSION

The results from this study confirm preserved voluntary abdominal muscle function in individuals classified with motor-complete SCI above T6 and highlight the need for further research in developing more accurate clinical measures to diagnose the level of trunk muscle preservation in individuals with SCI.

Time-interval for integration of stabilizing haptic and visual information in subjects balancing under static and dynamic conditions

Maintaining equilibrium is basically a sensorimotor integration task. The central nervous system (CNS) continually and selectively weights and rapidly integrates sensory inputs from multiple sources, and coordinates multiple outputs. The weighting process is based on the availability and accuracy of afferent signals at a given instant, on the time-period required to process each input, and possibly on the plasticity of the relevant pathways. The likelihood that sensory inflow changes while balancing under static or dynamic conditions is high, because subjects can pass from a dark to a well-lit environment or from a tactile-guided stabilization to loss of haptic inflow. This review article presents recent data on the temporal events accompanying sensory transition, on which basic information is fragmentary. The processing time from sensory shift to reaching a new steady state includes the time to (a) subtract or integrate sensory inputs; (b) move from allocentric to egocentric reference or vice versa; and (c) adjust the calibration of motor activity in time and amplitude to the new sensory set. We present examples of processes of integration of posture-stabilizing information, and of the respective sensorimotor time-intervals while allowing or occluding vision or adding or subtracting tactile information. These intervals are short, in the order of 1–2 s for different postural conditions, modalities and deliberate or passive shift. They are just longer for haptic than visual shift, just shorter on withdrawal than on addition of stabilizing input, and on deliberate than unexpected mode. The delays are the shortest (for haptic shift) in blind subjects. Since automatic balance stabilization may be vulnerable to sensory-integration delays and to interference from concurrent cognitive tasks in patients with sensorimotor problems, insight into the processing time for balance control represents a critical step in the design of new balance- and locomotion training devices.

The efficacy of a multidisciplinary group program for patients with refractory chronic pain

Comprehensive programs for chronic pain management provided at multidisciplinary clinics have been shown to be successful in Western countries. However, similar results have not yet been reported in Japan, and it is unclear whether these results are applicable to the Japanese culture. Accordingly, the authors report the results of the ‘Chronic Pain Class’, a program initiated at a multidisciplinary pain centre in Nagakute, Japan.

BACKGROUND:

Chronic pain is a major problem because it can result in not only a reduction in activities of daily living and quality of life but also requires initiation of social assistance. Seeking only to eliminate pain itself would appear to be too narrow an objective, in addition to often being unachievable; therefore, a multifaceted, comprehensive approach with multiple objectives is needed.

OBJECTIVE:

To describe the effects of a program (the ‘Chronic Pain Class’) offering cognitive behavioural therapy to small groups of individuals with refractory chronic pain in Japan. Exercise was an important feature of the program.

METHODS:

A total of 46 patients who were experiencing treatment difficulties and decreased activity participated in the program. The programs were conducted in groups of five to seven patients who met weekly for nine weeks. Weekly sessions, which were approximately 2 h in duration, combined lectures with exercise. Several measures related to pain and physical function were administered at the beginning and the conclusion of the program.

RESULTS:

Nine patients dropped out during the program. A number of measures (eg, pain intensity, disability, catastrophizing thoughts) showed significant improvements after intervention (P<0.002 after Bonferroni correction). Furthermore, most measures of physical function showed substantial improvement, especially seated forward bends, zig-zag walking, self-care and 6 min walk test (P<0.001).

CONCLUSION:

The results of the present study provide evidence that a combination of cognitive behavioural therapy and exercise should be recommended to patients with refractory chronic pain.

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.

By counteracting gravity, triceps surae sets both kinematics and kinetics of gait

In the single-stance phase of gait, gravity acting on the center of mass (CoM) causes a disequilibrium torque, which generates propulsive force. Triceps surae activity resists gravity by restraining forward tibial rotation thereby tuning CoM momentum. We hypothesized that time and amplitude modulation of triceps surae activity determines the kinematics (step length and cadence) and kinetics of gait. Nineteen young subjects participated in two experiments. In the gait initiation (GI) protocol, subjects deliberately initiated walking at different velocities for the same step length. In the balance-recovery (BR) protocol, subjects executed steps of different length after being unexpectedly released from an inclined posture. Ground reaction force was recorded by a large force platform and electromyography of soleus, gastrocnemius medialis and lateralis, and tibialis anterior muscles was collected by wireless surface electrodes. In both protocols, the duration of triceps activity was highly correlated with single-stance duration (GI, R (2) = 0.68; BR, R (2) = 0.91). In turn, step length was highly correlated with single-stance duration (BR, R (2) = 0.70). Control of CoM momentum was obtained by decelerating the CoM fall via modulation of amplitude of triceps activity. By modulation of triceps activity, the central nervous system (CNS) varied the position of CoM with respect to the center of pressure (CoP). The CoM-CoP gap in the sagittal plane was determinant for setting the disequilibrium torque and thus walking velocity. Thus, by controlling the gap, CNS-modified walking velocity (GI, R (2) = 0.86; BR, R (2) = 0.92). This study is the first to highlight that by merely counteracting gravity, triceps activity sets the kinematics and kinetics of gait. It also provides evidence that the surge in triceps activity during fast walking is due to the increased requirement of braking the fall of CoM in late stance in order to perform a smoother step-to-step transition.

By counteracting gravity, triceps surae sets both kinematics and kinetics of gait

In the single-stance phase of gait, gravity acting on the center of mass (CoM) causes a disequilibrium torque, which generates propulsive force. Triceps surae activity resists gravity by restraining forward tibial rotation thereby tuning CoM momentum. We hypothesized that time and amplitude modulation of triceps surae activity determines the kinematics (step length and cadence) and kinetics of gait. Nineteen young subjects participated in two experiments. In the gait initiation (GI) protocol, subjects deliberately initiated walking at different velocities for the same step length. In the balance-recovery (BR) protocol, subjects executed steps of different length after being unexpectedly released from an inclined posture. Ground reaction force was recorded by a large force platform and electromyography of soleus, gastrocnemius medialis and lateralis, and tibialis anterior muscles was collected by wireless surface electrodes. In both protocols, the duration of triceps activity was highly correlated with single-stance duration (GI, R (2) = 0.68; BR, R (2) = 0.91). In turn, step length was highly correlated with single-stance duration (BR, R (2) = 0.70). Control of CoM momentum was obtained by decelerating the CoM fall via modulation of amplitude of triceps activity. By modulation of triceps activity, the central nervous system (CNS) varied the position of CoM with respect to the center of pressure (CoP). The CoM-CoP gap in the sagittal plane was determinant for setting the disequilibrium torque and thus walking velocity. Thus, by controlling the gap, CNS-modified walking velocity (GI, R (2) = 0.86; BR, R (2) = 0.92). This study is the first to highlight that by merely counteracting gravity, triceps activity sets the kinematics and kinetics of gait. It also provides evidence that the surge in triceps activity during fast walking is due to the increased requirement of braking the fall of CoM in late stance in order to perform a smoother step-to-step transition.

Intrinsic foot muscles have the capacity to control deformation of the longitudinal arch

The human foot is characterized by a pronounced longitudinal arch (LA) that compresses and recoils in response to external load during locomotion, allowing for storage and return of elastic energy within the passive structures of the arch and contributing to metabolic energy savings. Here, we examine the potential for active muscular contribution to the biomechanics of arch deformation and recoil. We test the hypotheses that activation of the three largest plantar intrinsic foot muscles, abductor hallucis, flexor digitorum and quadratus plantae is associated with muscle stretch in response to external load on the foot and that activation of these muscles (via electrical stimulation) will generate sufficient force to counter the deformation of LA caused by the external load. We found that recruitment of the intrinsic foot muscles increased with increasing load, beyond specific load thresholds. Interestingly, LA deformation and muscle stretch plateaued towards the maximum load of 150% body weight, when muscle activity was greatest. Electrical stimulation of the plantar intrinsic muscles countered the deformation that occurred owing to the application of external load by reducing the length and increasing the height of the LA. These findings demonstrate that these muscles have the capacity to control foot posture and LA stiffness and may provide a buttressing effect during foot loading. This active arch stiffening mechanism may have important implications for how forces are transmitted during locomotion and postural activities as well as consequences for metabolic energy saving.

Tibialis anterior muscle fascicle dynamics adequately represent postural sway during standing balance

To maintain a stable, upright posture, the central nervous system (CNS) must integrate sensory information from multiple sources and subsequently generate corrective torque about the ankle joint. Although proprioceptive information from the muscles that cross this joint has been shown to be vital in this process, the specific source of this information remains questionable. Recent research has been focused on the potential role of tibialis anterior (TA) muscle during standing, largely due to the lack of modulation of its activity throughout the sway cycle. Ten young, healthy subjects were asked to stand normally under varying conditions, for periods of 60 s. During these trials, intramuscular electromyographic (EMG) activity and the fascicle length of three distinct anatomical regions of TA were sampled synchronously with kinematic data regarding sway position. In the quiet standing conditions, TA muscle activity was unmodulated and fascicle length changes in each region were tightly coupled with changes in sway position. In the active sway condition, more EMG activity was observed in TA and the fascicle length changes were decoupled from sway position. No regional specific differences in correlation values were observed, contrasting previous observations. The ability of the fascicles to follow sway position builds upon the suggestion that TA is well placed to provide accurate, straightforward sensory information to the CNS. As previously suggested, through reciprocal inhibition, afferent information from TA could help to regulate plantar flexor torque at relevant phases of the sway cycle. The proprioceptive role of TA appears to become complicated during more challenging conditions.

Afferent control of walking: are there distinct deficits associated to loss of fibres of different diameter?

OBJECTIVES

To compare the gait pattern in patients affected by different types of neuropathy.

METHODS

We recruited healthy subjects (HS, n=38), patients with Charcot-Marie-Tooth disease type 1A (CMT1A) (n=10) and patients with diabetic neuropathy (DNP) (n=12). Neuropathy impairment score and neuropathy score were assessed. Body sway during quiet stance, and spatio-temporal gait parameters were recorded.

RESULTS

Most patients had reduced or absent tendon-tap reflexes. Strength of foot dorsiflexor muscles (p<0.05) and conduction velocity (CV) of leg nerves (p<0.0001) were more impaired in CMT1A than DNP, whereas joint-position sense was more affected (p<0.05) in DNP. Body sway while standing was larger in DNP compared to CMT1A and HS (p<0.01 and p<0.0001 respectively). During gait, the distribution of foot sole contact pressure was abnormal in CMT1A (p<0.05) but not in DNP. Velocity and step length were decreased, and foot yaw angle at foot flat increased, in DNP with respect to CMT1A and HS (both variables, p<0.001). Gait velocity and step length were decreased (p<0.005) also in CMT1A, but to a smaller extent than in DNP, so that the difference between patient groups was significant (p<0.0005). Duration of the double support was protracted in DNP compared to CMT1A and HS (p<0.0005). For DNP only, velocity of gait and duration of single support were correlated (p<0.05) both to sway path and lower limb muscle strength.

CONCLUSIONS

Changes in both body sway and stance phase of gait were larger in DNP than CMT1A, indicating more impaired static and dynamic control of balance when neuropathy affects the small in addition to the large afferent fibres. Diminished somatosensory input from the smaller fibres rather than muscle weakness or foot deformity plays a critical role in the modulation of the support phase of gait.

SIGNIFICANCE

The analysis of balance and gait in patients with neuropathy can offer a tool for understanding the nature and functional impact of the neuropathy and should be included in their functional evaluation.

Effects of postural threat on spinal stretch reflexes: evidence for increased muscle spindle sensitivity?

Standing balance is often threatened in everyday life. These threats typically involve scenarios in which either the likelihood or the consequence of falling is higher than normal. When cats are placed in these scenarios they respond by increasing the sensitivity of muscle spindles imbedded in the leg muscles, presumably to increase balance-relevant afferent information available to the nervous system. At present, it is unknown whether humans also respond to such postural threats by altering muscle spindle sensitivity. Here we present two studies that probed the effects of postural threat on spinal stretch reflexes. In study 1 we manipulated the threat associated with an increased consequence of a fall by having subjects stand at the edge of an elevated surface (3.2 m). In study 2 we manipulated the threat by increasing the likelihood of a fall by occasionally tilting the support surface on which subjects stood. In both scenarios we used Hoffmann (H) and tendon stretch (T) reflexes to probe the spinal stretch reflex circuit of the soleus muscle. We observed increased T-reflex amplitudes and unchanged H-reflex amplitudes in both threat scenarios. These results suggest that the synaptic state of the spinal stretch reflex is unaffected by postural threat and that therefore the muscle spindles activated in the T-reflexes must be more sensitive in the threatening conditions. We propose that this increase in sensitivity may function to satisfy the conflicting needs to restrict movement with threat, while maintaining a certain amount of sensory information related to postural control.

The inflow of sensory information for the control of standing is graded and bidirectional

The control of upright standing is accomplished through the integration of different sources of sensory information and by providing an appropriate motor program to control both expected and unexpected perturbations imposed on the system. However, the dynamic characteristics of postural sway and its interplay with the regulation of Ia sensory information within the spinal cord are largely unknown. Here, using a stochastic technique for analyzing the dynamics of upright standing, we demonstrate that the changes in the dynamics of postural sway were accompanied by modulation of the soleus H-reflex during quiet standing. While the causality of this relation was not established, the results showed that these changes were independent of the sway of the center of pressure and were bidirectional and purposeful. With this novel perspective, the appropriate reflex gain, which is important for balance control, can be predicted from the dynamic characteristics of postural sway. Our current findings provide the first human behavioral evidence to suggest the contribution of the spinal cord in fulfilling the desired motor programming of a complex task. This contribution is, by conventional guess, carried out through interneuronal adjustments, which are under the control of different brain areas.

Changes in spinal excitability during dual task performance

The authors investigated how the nervous system responds to dual task performance. Because dual tasking is associated with greater postural challenges, it was hypothesized that spinal excitability would be reduced when simultaneously performing 2 tasks. For this experiment, participants maintained a lying or standing posture with or without performing a concurrent cognitive task (i.e., reacting to an auditory tone). Spinal excitability was assessed by eliciting the soleus Hoffmann reflex (H-reflex). Results indicated that the H-reflex was 6.4 ± 2.3% smaller (p = .011) when dual compared to single tasking. The reduced H-reflex amplitude, indicating a depressed spinal excitability, when dual tasking is suggested to reflect a neural strategy that individuals adopt to maintain postural stability when cognitive resources are divided between 2 concurrent tasks.

Association of Force Steadiness of Plantar Flexor Muscles and Postural Sway during Quiet Standing by Young Adults

This study was conducted to assess the relations of force fluctuations during isometric plantar-flexion and postural sway during quiet standing. Twelve healthy men ( M age = 21 yr., SD = 1) performed unilateral plantar flexion measured by a strain gauge force transducer. Participants performed force-matching tasks; sustained plantar flexion for 20 sec. at levels corresponding to 10% and 20% of maximum voluntary contraction with the visual feedback. Also, participants were asked to stand quietly with their eyes open, and then the center of mass displacement and velocity in the anteroposterior were measured. In analysis, postural sway was associated with force fluctuation at only 10% of maximum voluntary contraction. The statistically significant correlation between variables was found only at corresponding contraction intensities for plantar-flexor muscles. From this one may infer neural strategies in plantar-flexor muscles during quiet standing may be characteristics similar to those controlling the plantar-flexion force in young adults.

Modulation of the soleus H-reflex during knee rotations is not consistent with muscle fascicle length changes

The purpose of this study was to examine whether passively rotating the knee would result in parallel or differential changes to the medial gastrocnemius (MG) and soleus (SOL) H-reflex amplitudes. Since passive knee rotation alters the muscle length of the MG, but not the SOL, it was hypothesized that the MG H-reflex would reflect the lengthening or shortening actions that occur during knee rotation, whereas the SOL H-reflex would remain unaltered. MG and SOL Hoffman reflexes (H-reflexes) were evoked with the knee joint held static at 10° or as the joint was passively flexed or extended past 10°. Ultrasound recordings were used to confirm whether the knee rotations altered MG but not SOL muscle fascicle lengths. In contrast to our hypothesis, results indicated that the MG and SOL H-reflexes were similarly affected during knee rotations, with both MG and SOL H(max):M(max) smaller during the knee extension than the knee flexion (33-43% reduction) and static (22-28% reduction) conditions. Parallel changes to the MG and SOL H-reflexes occurred despite a differential effect of knee rotation on muscle fascicle lengths. Whereas, MG muscle fascicles lengthened and shortened during knee extension and flexion, respectively, SOL fascicles length remained unchanged. Given the strong neural coupling between the MG and SOL motoneuron pools, the results highlight the difficulty in isolating specific variables (e.g., muscle length) when determining the modulatory influences on the triceps surae H-reflex amplitude.

Recruitment of the plantar intrinsic foot muscles with increasing postural demand

BACKGROUND

The aim of this study was to determine the difference in activation patterns of the plantar intrinsic foot muscles during two quiet standing tasks with increasing postural difficulty. We hypothesised that activation of these muscles would increase with increasing postural demand and be correlated with postural sway.

METHODS

Intra-muscular electromyographic (EMG) activity was recorded from abductor hallucis, flexor digitorum brevis and quadratus plantae in 10 healthy participants while performing two balance tasks of graded difficulty (double leg stance and single leg stance). These two standing postures were used to appraise any relationship between postural sway and intrinsic foot muscle activity.

FINDINGS

Single leg stance compared to double leg stance resulted in greater mean centre of pressure speed (0.24 m s(-1) versus 0.06 m s(-1), respectively, P ≤ 0.05) and greater mean EMG amplitude for abductor hallucis (P ≥ 0.001, ES=0.83), flexor digitorum brevis (P ≤ 0.001, ES=0.79) and quadratus plantae (P ≤ 0.05, ES=0.4). EMG amplitude waveforms for all muscles were moderate to strongly correlated to centre of pressure (CoP) medio-lateral waveforms (all r ≥ 0.4), with muscle activity amplitude increasing with medial deviations of the CoP. Intra-muscular EMG waveforms were all strongly correlated with each other (all r ≥ 0.85).

INTERPRETATIONS

Activation of the plantar intrinsic foot muscles increases with increasing postural demand. These muscles are clearly important in postural control and are recruited in a highly co-ordinated manner to stabilise the foot and maintain balance in the medio-lateral direction, particularly during single leg stance.

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.

An enhanced level of motor cortical excitability during the control of human standing

AIM

The study examined the role of the motor cortex in the control of human standing.

METHODS

Subjects (n = 15) stood quietly with or without body support. The supported standing condition enabled subjects to stand with a reduced amount of postural sway. Peripheral electrical stimulation, transcranial magnetic stimulation (TMS) or transcranial electrical stimulation (TES) was applied to elicit a soleus (SOL) H-reflex, or motor-evoked potentials (MEPs) in the SOL and the tibialis anterior (TA). Trials were grouped based on the standing condition (i.e. supported vs. normal) as well as sway direction (i.e. forward and backward) while subjects were standing normally.

RESULTS

During normal when compared to supported standing, the SOL H-reflex was depressed (-11 +/- 4%), while the TMS-evoked MEPs from the SOL and TA were facilitated (35 +/- 11% for the SOL, 51 +/- 15% for the TA). TES-evoked SOL and TA MEPs were, however, not different between the normal and supported standing conditions. The data based on sway direction indicated that the SOL H-reflex, as well as the SOL TMS- and TES-evoked MEPs were all greater during forward when compared to backward sway. In contrast, the TMS- and TES-evoked MEPs from the TA were smaller when swaying forward as compared to backward.

CONCLUSIONS

The results indicated the presence of an enhanced cortical excitability because of the need to control for postural sway during normal standing. The increased cortical excitability was, however, unlikely to be involved in an on-going control of postural sway, suggesting that postural sway is controlled at the spinal and/or subcortical levels.

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.

Sway-dependent modulation of the triceps surae H-reflex during standing

Previous research has shown that changes in spinal excitability occur during the postural sway of quiet standing. In the present study, it was of interest to examine the independent effects of sway position and sway direction on the efficacy of the triceps surae Ia pathway, as reflected by the Hoffman (H)-reflex amplitude, during standing. Eighteen participants, tested under two different experimental protocols, stood quietly on a force platform. Percutaneous electrical stimulation was applied to the posterior tibial nerve when the position and direction of anteroposterior (A-P) center of pressure (COP) signal satisfied the criteria for the various experimental conditions. It was found that, regardless of sway position, a larger amplitude of the triceps surae H-reflex (difference of 9–14%; P = 0.005) occurred when subjects were swaying in the forward compared with the backward direction. The effects of sway position, independent of the sway direction, on spinal excitability exhibited a trend ( P = 0.075), with an 8.9 ± 3.7% increase in the H-reflex amplitude occurring when subjects were in a more forward position. The observed changes to the efficacy of the Ia pathway cannot be attributed to changes in stimulus intensity, as indicated by a constant M-wave amplitude, or to the small changes in the level of background electromyographic activity. One explanation for the changes in reflex excitability with respect to the postural sway of standing is that the neural modulation may be related to the small lengthening and shortening contractions occurring in the muscles of the triceps surae.