The influence of ankle muscle activation on postural sway during quiet stance

The influence of false interoceptive feedback on emotional state and balance responses to height-induced postural threat

Postural threat elicits a robust emotional response (e.g., fear and anxiety about falling), with concomitant modifications in balance. Recent theoretical accounts propose that emotional responses to postural threats are manifested, in part, from the conscious monitoring and appraisal of bodily signals ('interoception'). Here, we empirically probe the role of interoception in shaping emotional responses to a postural threat by experimentally manipulating interoceptive cardiac feedback. Sixty young adults completed a single 60-s trial under the following conditions: Ground (no threat) without heart rate (HR) feedback, followed by Threat (standing on the edge of a raised surface), during which participants received either false heart rate feedback (either slow [n = 20] or fast [n = 20] HR feedback) or no feedback (n = 20). Participants provided with false fast HR feedback during postural threat felt more fearful, reported feeling less stable, and rated the task more difficult than participants who did not receive HR feedback, or those who received false slow HR feedback (Cohen's d effect size = 0.79 - 1.78). However, behavioural responses did not significantly differ across the three groups. When compared to the no HR feedback group, false slow HR feedback did not significantly affect emotional or behavioural responses to the postural threat. These observations provide the first experimental evidence for emerging theoretical accounts describing the role of interoception in the generation of emotional responses to postural threats.

Effects of arousal and valence on center of pressure and ankle muscle activity during quiet standing

Emotion affects postural control during quiet standing. Emotional states can be defined as two-dimensional models comprising valence (pleasant/unpleasant) and arousal (aroused/calm). Most previous studies have investigated the effects of valence on postural control without considering arousal. In addition, studies have focused on the center of pressure (COP) trajectory to examine emotional effects on the quiet standing control; however, the relationship between neuromuscular mechanisms and the emotionally affected quiet standing control is largely unknown. This study aimed to investigate the effects of arousal and valence on the COP trajectory and ankle muscle activity during quiet standing. Twenty-two participants were instructed to stand on a force platform and look at affective pictures for 72 seconds. The tasks were repeated six times, according to the picture conditions composed of arousal (High and Low) and valence (Pleasant, Neutral, and Unpleasant). During the task, the COP, electromyogram (EMG) of the tibialis anterior and soleus muscles, and electrocardiogram (ECG) were recorded. The heart rate calculated from the ECG was significantly affected by valence; the value was lower in Unpleasant than that in Neutral and Pleasant. The 95% confidence ellipse area and standard deviation of COP in the anterior-posterior direction were lower, and the mean power frequency of COP in the anterior-posterior direction was higher in Unpleasant than in Pleasant. Although the mean velocity of the COP in the medio-lateral direction was significantly lower in Unpleasant than in Pleasant, the effect was observed only when arousal was low. Although the EMG variables were not significantly affected by emotional conditions, some EMG variables were significantly correlated with the COP variables that were affected by emotional conditions. Therefore, ankle muscle activity may be partially associated with postural changes triggered by emotional intervention. In conclusion, both valence and arousal affect the COP variables, and ankle muscle activity may be partially associated with these COP changes.

Perceived cognitive fatigue has only marginal effects on static balance control in healthy young adults

We examined the influence of perceived cognitive fatigue on static balance control in healthy young adults to gain greater clarity about this issue than provided in previous research. Based on the prevailing assumption in pertinent literature, we hypothesized that the influence of cognitive fatigue on balance control depends on the attentional effort required by the balance tasks being performed. To test this hypothesis, 44 young adults (24 women and 20 men) were alternately assigned to either the experimental group that was cognitively fatigued (using the 16-min TloadDback-task with individualized settings) or the control group (who watched a documentary). Before and after the intervention, the participants performed six balance tasks that differed in (attentional) control requirements, while recording the center of pressure (COP). From these time series, sway variability, mean speed, and sample entropy were calculated and analyzed statistically. Additionally, perceived cognitive fatigue was assessed using VAS scales. Statistical analyses confirmed that the balance tasks differed in control characteristics and that cognitive fatigue was elevated in the experimental group, but not in the control group. Nevertheless, no significant main effects of cognitive fatigue were found on any of the COP measures of interest, except for some non-robust interaction effects related primarily to sample entropy. These results suggest that, in young adults, postural control in static balance tasks is largely automatic and unaffected by task-induced state fatigue.

Pain alleviation improves balance control and muscular coordination of lower limbs in patients with chronic ankle instability during sinusoidal perturbations

BACKGROUND

It is well established that individuals with chronic ankle instability manifest deficits in balance control and muscle activation. Given the prevalence of pain as a prominent symptom in this population, there is a need for in-depth investigation of its role in contributing to these impairments.

METHODS

A Stewart platform was used to generate translational sinusoidal perturbations in the antero-posterior direction. Eighteen individuals with chronic ankle instability and concurrent ankle pain were recruited. They were instructed to assume a central stance on the support surface with open eyes both before and 30 min after local analgesia. Data of center of pressure and electromyography of the tibialis anterior and medial gastrocnemius were recorded. Statistical analysis was performed to make comparisons pre- and post-analgesia using two-tailed paired t-test for the continuous variables.

FINDINGS

Pain intensity was significantly decreased after local anesthetic injections. Antero-posterior center of pressure parameters significantly decreased following the injection. Also, there was an increase in the regularity of the center of pressure pattern. The electromyographic pattern of the tibialis anterior and medial gastrocnemius exhibited various activation patterns. After pain alleviation, the characteristic electromyographic response of the tibialis anterior and medial gastrocnemius was reciprocal contraction and relaxation that corresponded with the sinusoidal pattern of the perturbations.

INTERPRETATION

Individuals who had chronic ankle instability and ankle pain demonstrated impaired balance control during sinusoidal perturbations. Mitigating pain improved their balance performance, evident in the center of pressure pattern and the coordination of lower limb muscles.

Effect of Moving Tactile Stimuli to Mimic Altered Weight Distribution During Gait on Quiet Stance Body Sway

Our purpose in the present study was to examine whether moving tactile stimuli to the sole to mimic moving weight distribution over the feet during gait would influence body sway in quiet stance. Fifteen healthy males maintained the quiet stance, and we delivered moving tactile stimuli to mimic the change in their weight distribution during gait. Moving tactile stimuli did not change the length of the center of pressure (COP) displacement and COP position. Vision decreased the length of the COP, but it did not interact with moving tactile stimuli for the COP length and position. The COP position rhythmically moved in the medial-lateral axis along with the cycle of moving tactile stimuli. The COP was at the lateral peak position at the period at which moving tactile stimuli mimicked the weight distribution in the transition between the swing and stance phases of the gait cycle. This finding may indicate that the body is positioned at the lateral peak position in quiet stance when people perceive the sensation of weight distribution over the feet at the most unstable phase of the gait cycle. We suggest that moving tactile stimuli to the sole may induce medial-lateral body sway before gait initiation for patients with Parkinson's disease to improve their freezing of gait initiation.

Effects of free versus restricted arm movements on postural control in normal and modified sensory conditions in young and older adults

The purpose of this study was to explore the effects of arm movements on postural control when standing under different sensory conditions in healthy young and older adults. Fifteen young (mean ± SD age; 21.3 ± 4.2 years) and 15 older (mean ± SD age; 73.3 ± 5.0 years) adults completed the modified Romberg test, which uses four task manipulations (i.e. eyes open and eyes closed on a firm and foam surface) to compromise the fidelity of sensory feedback mechanisms. Each participant completed the tasks under two arm movement conditions: restricted and free arm movements. Centre of pressure (COP) range and frequency were calculated to characterise postural performance and strategy, respectively. Older adults showed greater COP range with restricted compared to free arm movements during all modified sensory conditions, with these effects most prominent in the medio-lateral (ML) plane (all p < .05, Cohen's d = 0.69-1.61). Compared to the free arm movement condition, there was an increase in ML displacement and frequency when arm movements were restricted during only the most challenging (i.e. vestibular dominant) task in young adults (all p < .05, d = 0.645-0.83). Finally, main age effects for the arm restriction cost (p < .05) indicates a greater reliance on an upper body strategy in older compared to young adults, independent of sensory availability/accuracy. These findings indicate that older adults compensate for the loss of accuracy in sensory input by increasing reliance on upper body movement strategies.

Discriminative Changes in Sitting and Standing Postural Steadiness in Patients With Chronic Low Back Pain

Postural control is reduced in patients with low back pain (LBP), which is considered an important factor attributing to the chronicity of LBP and a target for treatment. It is proposed that the changes in postural steadiness in sitting reflect the trunk control better than those in standing, but the previous study results are inconsistent. Thus, this study aimed to compare trunk sway parameters during standing and sitting between patients with chronic LPB (CLBP) and controls using a tracker-based posturography to record the trunk displacement trajectories at the lumbar level (TD [Formula: see text]. A total of 64 participants (32 patients with CLBP and 32 pain-free controls) were included in this study. The postural sway was measured under four conditions, sitting or standing on unstable surface, with eyes open or closed. The TDL parameters were compared between the two groups to explore their discriminative ability. The CLBP group had more body sway than the control group, characterized by several TDL parameters in sitting with eyes closed and standing with eyes open. The TDL parameters with the highest area under the curve according to the receiver operating characteristic curve analysis were the root mean square distance and mean frequency in the medial-lateral direction obtained in the sitting with eyes closed. In conclusion, we confirmed the advantage of using sitting posturographic parameters as a sensitive measure to detect impaired trunk control in patients with CLBP. The results would help choose sensitive outcome measures to reflect the postural control of patients with LBP.

History-dependent muscle resistance to stretch remains high after small, posturally relevant pre-movements

The contributions of intrinsic muscle fiber resistance during mechanical perturbations to standing and other postural behaviors are unclear. Muscle short-range stiffness is known to vary depending on the current level and history of the muscle's activation, as well as the muscle's recent movement history; this property has been referred to as history dependence or muscle thixotropy. However, we currently lack sufficient data about the degree to which muscle stiffness is modulated across posturally relevant characteristics of muscle stretch and activation. We characterized the history dependence of muscle's resistance to stretch in single, permeabilized, activated, muscle fibers in posturally relevant stretch conditions and activation levels. We used a classic paired muscle stretch paradigm, varying the amplitude of a 'conditioning' triangular stretch-shorten cycle followed by a 'test' ramp-and-hold imposed after a variable inter-stretch interval. We tested low (<15%), intermediate (15-50%) and high (>50%) muscle fiber activation levels, evaluating short-range stiffness and total impulse in the test stretch. Muscle fiber resistance to stretch remained high at conditioning amplitudes of <1% optimal fiber length, L0, and inter-stretch intervals of >1 s, characteristic of healthy standing postural sway. An ∼70% attenuation of muscle resistance to stretch was reached at conditioning amplitudes of >3% L0 and inter-stretch intervals of <0.1 s, characteristic of larger, faster postural sway in balance-impaired individuals. The thixotropic changes cannot be predicted solely on muscle force at the time of stretch. Consistent with the disruption of muscle cross-bridges, muscle resistance to stretch during behavior can be substantially attenuated if the prior motion is large enough and/or frequent enough.

The 'Postural Rhythm' of the Ground Reaction Force during Upright Stance and Its Conversion to Body Sway-The Effect of Vision, Support Surface and Adaptation to Repeated Trials

The ground reaction force (GRF) recorded by a platform when a person stands upright lies at the interface between the neural networks controlling stance and the body sway deduced from centre of pressure (CoP) displacement. It can be decomposed into vertical (VGRF) and horizontal (HGRF) vectors. Few studies have addressed the modulation of the GRFs by the sensory conditions and their relationship with body sway. We reconsidered the features of the GRFs oscillations in healthy young subjects (n = 24) standing for 90 s, with the aim of characterising the possible effects of vision, support surface and adaptation to repeated trials, and the correspondence between HGRF and CoP time-series. We compared the frequency spectra of these variables with eyes open or closed on solid support surface (EOS, ECS) and on foam (EOF, ECF). All stance trials were repeated in a sequence of eight. Conditions were randomised across different days. The oscillations of the VGRF, HGRF and CoP differed between each other, as per the dominant frequency of their spectra (around 4 Hz, 0.8 Hz and <0.4 Hz, respectively) featuring a low-pass filter effect from VGRF to HGRF to CoP. GRF frequencies hardly changed as a function of the experimental conditions, including adaptation. CoP frequencies diminished to <0.2 Hz when vision was available on hard support surface. Amplitudes of both GRFs and CoP oscillations decreased in the order ECF > EOF > ECS ≈ EOS. Adaptation had no effect except in ECF condition. Specific rhythms of the GRFs do not transfer to the CoP frequency, whereas the magnitude of the forces acting on the ground ultimately determines body sway. The discrepancies in the time-series of the HGRF and CoP oscillations confirm that the body's oscillation mode cannot be dictated by the inverted pendulum model in any experimental conditions. The findings emphasise the robustness of the VGRF "postural rhythm" and its correspondence with the cortical theta rhythm, shed new insight on current principles of balance control and on understanding of upright stance in healthy and elderly people as well as on injury prevention and rehabilitation.

Association of Piriformis Thickness, Hip Muscle Strength, and Low Back Pain Patients with and without Piriformis Syndrome in Malaysia

Low back pain is a serious threat to human health and the illness jeopardizes the human workforce and pressurizes the health system in the community. Low back pain might be related to piriformis syndrome (PS), which is a disorder presented as muscular spasm and hypertrophy that is strongly associated with piriformis thickness. Nevertheless, the relationship between piriformis thickness and morphological and functional changes of the gluteal muscles in PS remains unclear. This study aimed to investigate the association between the thickness, strength, and activation of piriformis and gluteus muscles (maximus and medius) among low back pain (LBP) patients with and without PS. This is a case-control study conducted at HSNZ and UiTM from 2019-2020. A total number of 91 participants (LBP + PS (n = 36), LBP - PS (n = 24), and healthy (n = 31)) were recruited in this study. Negative radiography, specific symptoms, and a positive PS test were applied for PS diagnoses. The thickness, strength, and activation of piriformis and gluteus muscles were measured using ultrasonography (USG) and a surface electromyogram, respectively. Resultantly, the one-way ANOVA test demonstrated no significant difference in piriformis thickness between LBP + PS and LBP - PS (p > 0.01). Piriformis thickness was inversely correlated with gluteus maximus strength (r = -0.4, p < 0.05) and positively correlated with gluteus medius activation (r = 0.48, p < 0.01) in LBP + PS. Stepwise linear regression for LBP + PS revealed a significant association between piriformis thickness and gluteus maximus strength (R = -0.34, accounted for 11% of the variance) and gluteus medius activation in prone lying with the hip in an externally rotated, abducted, and extended (ERABEX) position (R = 0.43, accounted for 23% of the variance). With the adjustment of age and gender, piriformis thickness, gluteus maximus strength, and gluteus medius activation in prone lying with hip ERABEX demonstrated a significant association, but no independent effect of age and gender was detected within the range. Meanwhile, a significant association between piriformis thickness and gluteus maximus thickness was observed (R = 0.44, accounted for 19% of the variance) in the LBP - PS group. These findings may assist to elucidate the actions and functions of piriformis and gluteus muscle in LBP with and without PS.

Introduction to a Twin Dual-Axis Robotic Platform for Studies of Lower Limb Biomechanics

This paper presents a twin dual-axis robotic platform system which is designed for the characterization of postural balance under various environmental conditions and quantification of bilateral ankle mechanics in 2 degrees-of-freedom (DOF) during standing and walking. Methods: Validation experiments were conducted to evaluate performance of the system: 1) to apply accurate position perturbations under different loading conditions; 2) to simulate a range of stiffness-defined mechanical environments; and 3) to reliably quantify the joint impedance of mechanical systems. In addition, several human experiments were performed to demonstrate the system's applicability for various lower limb biomechanics studies. The first two experiments quantified postural balance on a compliance-controlled surface (passive perturbations) and under oscillatory perturbations with various frequencies and amplitudes (active perturbations). The second two experiments quantified bilateral ankle mechanics, specifically, ankle impedance in 2-DOF during standing and walking. The validation experiments showed high accuracy of the platform system to apply position perturbations, simulate a range of mechanical environments, and quantify the joint impedance. Results of the human experiments further demonstrated that the platform system is sensitive enough to detect differences in postural balance control under challenging environmental conditions as well as bilateral differences in 2-DOF ankle mechanics. This robotic platform system will allow us to better understand lower limb biomechanics during functional tasks, while also providing invaluable knowledge for the design and control of many robotic systems including robotic exoskeletons, prostheses and robot-assisted balance training programs. Clinical and Translational Impact Statement- Our robotic platform system serves as a tool to better understand the biomechanics of both healthy and neurologically impaired individuals and to develop assistive robotics and rehabilitation training programs using this information.

Exploring how arm movement moderates the effect of task difficulty on balance performance in young and older adults

Emerging evidence highlights that arm movements exert a substantial and functionally relevant contribution on quiet standing balance control in young adults. Ageing is associated with "non-functional" compensatory postural control strategies (i.e., lower limb co-contraction), which in turn, may increase the reliance on an upper body strategy to control upright stance. Thus, the primary purpose of this study was to compare the effects of free versus restricted arm movements on balance performance in young and older adults, during tasks of different difficulty. Fifteen young (mean ± SD age; 21.3 ± 4.2 years) and fifteen older (mean ± SD age; 73.3 ± 5.0 years) adults performed bipedal, semi-tandem and tandem balance tasks under two arm position conditions: restricted arm movements and free arm movements. Centre of pressure (COP) amplitude and frequency were calculated as indices of postural performance and strategy, respectively. Especially in older adults, restriction of arm movement resulted in increased sway amplitude and frequency, which was primarily observed for the mediolateral direction. Further, increasing balance task difficulty raised the arm restriction cost (ARC; a new measure to quantify free vs. restricted arm movement differences in postural control) that was more prominent in older adults. These findings indicate the ARC provides a measure of reliance on the upper body for balance control and that arm movement is important for postural control in older adults, especially during tasks of greater difficulty.

A real-time and convex model for the estimation of muscle force from surface electromyographic signals in the upper and lower limbs

Surface electromyography (sEMG) is a signal consisting of different motor unit action potential trains and records from the surface of the muscles. One of the applications of sEMG is the estimation of muscle force. We proposed a new real-time convex and interpretable model for solving the sEMG-force estimation. We validated it on the upper limb during isometric voluntary flexions-extensions at 30%, 50%, and 70% Maximum Voluntary Contraction in five subjects, and lower limbs during standing tasks in thirty-three volunteers, without a history of neuromuscular disorders. Moreover, the performance of the proposed method was statistically compared with that of the state-of-the-art (13 methods, including linear-in-the-parameter models, Artificial Neural Networks and Supported Vector Machines, and non-linear models). The envelope of the sEMG signals was estimated, and the representative envelope of each muscle was used in our analysis. The convex form of an exponential EMG-force model was derived, and each muscle's coefficient was estimated using the Least Square method. The goodness-of-fit indices, the residual signal analysis (bias and Bland-Altman plot), and the running time analysis were provided. For the entire model, 30% of the data was used for estimation, while the remaining 20% and 50% were used for validation and testing, respectively. The average R-square (%) of the proposed method was 96.77 ± 1.67 [94.38, 98.06] for the test sets of the upper limb and 91.08 ± 6.84 [62.22, 96.62] for the lower-limb dataset (MEAN ± SD [min, max]). The proposed method was not significantly different from the recorded force signal (p-value = 0.610); that was not the case for the other tested models. The proposed method significantly outperformed the other methods (adj. p-value < 0.05). The average running time of each 250 ms signal of the training and testing of the proposed method was 25.7 ± 4.0 [22.3, 40.8] and 11.0 ± 2.9 [4.7, 17.8] in microseconds for the entire dataset. The proposed convex model is thus a promising method for estimating the force from the joints of the upper and lower limbs, with applications in load sharing, robotics, rehabilitation, and prosthesis control for the upper and lower limbs.

Advanced cueing of auditory stimulus to the head induces body sway in the direction opposite to the stimulus site during quiet stance in male participants

Under certain conditions, a tactile stimulus to the head induces the movement of the head away from the stimulus, and this is thought to be caused by a defense mechanism. In this study, we tested our hypothesis that predicting the stimulus site of the head in a quiet stance activates the defense mechanism, causing a body to sway to keep the head away from the stimulus. Fourteen healthy male participants aged 31.2 ± 6.8 years participated in this study. A visual cue predicting the forthcoming stimulus site (forehead, left side of the head, right side of the head, or back of the head) was given. Four seconds after this cue, an auditory or electrical tactile stimulus was given at the site predicted by the cue. The cue predicting the tactile stimulus site of the head did not induce a body sway. The cue predicting the auditory stimulus to the back of the head induced a forward body sway, and the cue predicting the stimulus to the forehead induced a backward body sway. The cue predicting the auditory stimulus to the left side of the head induced a rightward body sway, and the cue predicting the stimulus to the right side of the head induced a leftward body sway. These findings support our hypothesis that predicting the auditory stimulus site of the head induces a body sway in a quiet stance to keep the head away from the stimulus. The right gastrocnemius muscle contributes to the control of the body sway in the anterior-posterior axis related to this defense mechanism.

Vertical ground reaction force oscillation during standing on hard and compliant surfaces: The “postural rhythm”

When a person stands upright quietly, the position of the Centre of Mass (CoM), the vertical force acting on the ground and the geometrical configuration of body segments is accurately controlled around to the direction of gravity by multiple feedback mechanisms and by integrative brain centres that coordinate multi-joint movements. This is not always easy and the postural muscles continuously produce appropriate torques, recorded as ground reaction force by a force platform. We studied 23 young adults during a 90 s period, standing at ease on a hard (Solid) and on a compliant support (Foam) with eyes open (EO) and with eyes closed (EC), focusing on the vertical component of the ground reaction force (VGRF). Analysis of VGRF time series gave the amplitude of their rhythmic oscillations (the root mean square, RMS) and of their frequency spectrum. Sway Area and Path Length of the Centre of Pressure (CoP) were also calculated. VGRF RMS (as well as CoP sway measures) increased in the order EO Solid ≈ EC Solid < EO Foam < EC Foam. The VGRF frequency spectra featured prevailing frequencies around 4–5 Hz under all tested conditions, slightly higher on Solid than Foam support. Around that value, the VGRF frequencies varied in a larger range on hard than on compliant support. Sway Area and Path Length were inversely related to the prevailing VGRF frequency. Vision compared to no-vision decreased Sway Area and Path Length and VGRF RMS on Foam support. However, no significant effect of vision was found on VGRF mean frequency for either base of support condition. A description of the VGRF, at the interface between balance control mechanisms and sway of the CoP, can contribute information on how upright balance is maintained. Analysis of the frequency pattern of VGRF oscillations and its role in the maintenance of upright stance should complement the traditional measures of CoP excursions in the horizontal plane.

Postural Sway and Muscle Activity Dynamics of Upright Standing on Sloped Surfaces

During upright standing, individuals often use co-contraction muscle activity at the ankle joint when encountering increased postural difficulty; however, this strategy has been shown to be maladaptive. The purpose of the current investigation was to examine the effect of sloped standing on postural sway and muscle co-contraction at the ankle joint as a function of postural difficulty. Twelve young (21.67 ± 1.11 years) adults performed upright standing on flat, declined, and inclined support surfaces. Center of pressure displacements indexed postural sway while electromyography data were collected for the tibialis anterior and gastrocnemius medialis muscles. A co-contraction index and a nonlinear coupling metric (cross-approximate entropy) were computed between ankle dorsiflexor and plantar flexor muscles (tibialis anterior/gastrocnemius medialis) activity. The results showed that higher degrees of postural difficulty led to increased amounts of sway as well as increased sway regularity. Lower co-contraction index was observed for higher degrees of postural difficulty; however, increased dynamic coupling occurred with deviations from the flat standing condition. Overall, increased postural difficulty as manipulated by sloped standing (in either inclined or declined conditions) resulted in individuals adopting a more regular sway trajectory that may be due, in part, to a stronger dynamic coupling strategy occurring at the neuromuscular level.

The Walls Are Closing In: Postural Responses to a Virtual Reality Claustrophobic Simulation

Background: Changes in the visual environment and thereby, the spatial orientation, can induce postural instability leading to falls. Virtual reality (VR) has been used to expose individuals to virtual environments (VE) that increase postural threats. Claustrophobia is an anxiety disorder categorized under situational phobias and can induce such postural threats in a VE. Purpose: The purpose of the study was to investigate if VR-generated claustrophobic simulation has any impact on postural threats that might lead to postural instability. Methods: Thirty healthy men and women (age: 20.7 ± 1.2 years; height: 166.5 ± 7.3 cm; mass: 71.7 ± 16.2 kg) were tested for postural stability while standing on a force platform, upon exposure to five different testing trials, including a normal stance (NoVR), in stationary VE (VR), and three consecutive, randomly initiated, unexpected claustrophobia trials (VR CP1, VR CP2, VR CP3). The claustrophobia trials involved all four walls closing in towards the center of the room. Center of pressure (COP)-derived postural sway variables were analyzed with a one-way repeated measures analysis of variance at an alpha level of 0.05. Results: Significant main effect differences existed in all but one dependent COP-derived postural sway variables, at p < 0.05. Post-hoc pairwise comparisons with a Bonferroni correction revealed that, predominantly, postural sway excursions were significantly lower in claustrophobia trials compared to NoVR and VR, but only accomplished with significantly increased sway velocity. Conclusion: The VR CP trials induced lower postural sway magnitude, but with increased velocity, suggesting a bracing and co-contraction strategy when exposed to virtual claustrophobic postural threats. Additionally, postural sway decreased with subsequent claustrophobia trials, suggesting potential motor learning effects. Findings from the study offer insights to postural control behavior under virtual claustrophobic simulations and can aid in VR exposure therapy for claustrophobia.

Postural control in adolescent boys and girls before the age of peak height velocity: Effects of task difficulty

BACKGROUND

Adolescent children experience a critical developmental period marked by rapid biological changes. Research question To describe the longitudinal changes in postural control that occur in adolescent boys and girls before the age of peak height velocity (PHV).

METHODS

Here, to address the gap of knowledge, we compared the postural control and activation strategies of the muscles that control the ankle joint in twenty-three boys (age 12.5 ± 0.29) and twenty-one girls (age 10.5 ± 0.32). They performed easy (two legs) and difficult (two legs-eyes closed; one leg) postural balance tasks at 18 and 9 months before PHV and at PHV. We quantified the center of pressure (COP) displacements in the anterior-posterior (AP) and mediolateral (ML) directions and electromyographic (EMG) activity of tibialis anterior (TA) and medial gastrocnemius (MG) muscles.

RESULTS

Boys exhibited greater AP and ML COP displacement than girls only for the one leg task (difficult task). Although boys and girls had similar postural control 18 months prior to PHV, girls exhibited lesser COP displacement at 9 months before PHV, which related to greater TA-MG coactivation (R² = 0.26; p < 0.01). In contrast, postural control was not different between boys and girls with an easy balance task (two legs) performed with eyes open and closed. Rather, we found that all children improved their COP displacement in the ML direction with maturity and both AP and ML COP was significantly lower with eyes open.

CONCLUSION

These findings provide novel evidence that postural control is superior in early adolescent girls than boys 9 months prior to PHV, likely associated with an earlier maturation of muscle coordination.

Correlation between ankle plantar flexion strength and degree of body sway

[Purpose] The purpose of this study is to consider the correlation between ankle plantar flexion strength and degree of body sway. [Participants and Methods] Twenty-one healthy adult males were targeted. A handheld dynamometer was used to measure ankle plantar flexion strength. The Body Pressure Measurement System was used to measure the degree of body sway. Lastly, correlation between ankle plantar flexion strength and degree of body sway was considered. [Results] A negative correlation was observed between ankle plantar flexion strength and degree of body sway. [Conclusion] Regarding ankle plantar flexion strength and degree of body sway, results of a consideration using a handheld dynamometer and Body Pressure Measurement System were the same as that of previous studies using large equipment.

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.

Effects of Concurrent and Terminal Visual Feedback on Ankle Co-Contraction in Older Adults during Standing Balance

This preliminary investigation studied the effects of concurrent and terminal visual feedback during a standing balance task on ankle co-contraction, which was accomplished via surface electromyography of an agonist–antagonist muscle pair (medial gastrocnemius and tibialis anterior muscles). Two complementary mathematical definitions of co-contraction indices captured changes in ankle muscle recruitment and modulation strategies. Nineteen healthy older adults received both feedback types in a randomized order. Following an analysis of co-contraction index reliability as a function of surface electromyography normalization technique, linear mixed-effects regression analyses revealed participants learned or utilized different ankle co-contraction recruitment (i.e., relative muscle pair activity magnitudes) and modulation (i.e., absolute muscle pair activity magnitudes) strategies depending on feedback type and following the cessation of feedback use. Ankle co-contraction modulation increased when concurrent feedback was used and significantly decreased when concurrent feedback was removed. Ankle co-contraction recruitment and modulation did not significantly change when terminal feedback was used or when it was removed. Neither ankle co-contraction recruitment nor modulation was significantly different when concurrent feedback was used compared to when terminal feedback was used. The changes in ankle co-contraction recruitment and modulation were significantly different when concurrent feedback was removed as compared to when terminal feedback was removed. Finally, this study found a significant interaction between feedback type, removal of feedback, and order of use of feedback type. These results have implications for the design of balance training technologies using visual feedback.

Immediate Effects of Vibrotactile Biofeedback Instructions on Human Postural Control

Vibrotactile biofeedback can improve balance and consequently be helpful in fall prevention. However, it remains unclear how different types of stimulus presentations affect not only trunk tilt, but also Center of Pressure (CoP) displacements, and whether an instruction on how to move contributes to a better understanding of vibrotactile feedback.Based on lower back tilt angles (L5), we applied individualized multi-directional vibrotactile feedback to the upper torso by a haptic vest in 30 healthy young adults. Subjects were equally distributed to three instruction groups (attractive - move in the direction of feedback, repulsive - move in the opposite direction of feedback & no instruction - with attractive stimuli). We conducted four conditions with eyes closed (feedback on/off, Narrow Stance with head extended, Semi-Tandem stance), with seven trials of 45s each. For CoP and L5, we computed Root Mean Square (RMS) of position/angle and standard deviation (SD) of velocity, and for L5 additionally, the percentage in time above threshold. The analysis consisted of mixed model ANOVAs and t-tests (α-level: 0.05).In the attractive and repulsive groups feedback significantly decreased the percentage above threshold (p<0.05). Feedback decreased RMS of L5, whereas RMS of CoP and SD of velocity in L5 and COP increased (p<0.05). Finally, an instruction on how to move contributed to a better understanding of the vibrotactile biofeedback.

Co-contraction of ankle muscle activity during quiet standing in individuals with incomplete spinal cord injury is associated with postural instability

Previous findings indicate that co-contractions of plantarflexors and dorsiflexors during quiet standing increase the ankle mechanical joint stiffness, resulting in increased postural sway. Balance impairments in individuals with incomplete spinal cord injury (iSCI) may be due to co-contractions like in other individuals with reduced balance ability. Here we investigated the effect of co-contraction between plantar- and dorsiflexors on postural balance in individuals with iSCI (iSCI-group) and able-bodied individuals (AB-group). Thirteen able-bodied individuals and 13 individuals with iSCI were asked to perform quiet standing with their eyes open (EO) and eyes closed (EC). Kinetics and electromyograms from the tibialis anterior (TA), soleus and medial gastrocnemius were collected bilaterally. The iSCI-group exhibited more co-contractions than the AB-group (EO: 0.208% vs. 75.163%, p = 0.004; EC: 1.767% vs. 92.373%, p = 0.016). Furthermore, postural sway was larger during co-contractions than during no co-contraction in the iSCI-group (EO: 1.405 cm/s² vs. 0.867 cm/s², p = 0.023; EC: 1.831 cm/s² vs. 1.179 cm/s², p = 0.030), but no differences were found for the AB-group (EO: 0.393 cm/s² vs. 0.499 cm/s², p = 1.00; EC: 0.686 cm/s² vs. 0.654 cm/s², p = 1.00). To investigate the mechanism, we performed a computational simulation study using an inverted pendulum model and linear controllers. An increase of mechanical stiffness in the simulated iSCI-group resulted in increased postural sway (EO: 2.520 cm/s² vs. 1.174 cm/s², p < 0.001; EC: 4.226 cm/s² vs. 1.836 cm/s², p < 0.001), but not for the simulated AB-group (EO: 0.658 cm/s² vs. 0.658 cm/s², p = 1.00; EC: 0.943 cm/s² vs. 0.926 cm/s², p = 0.190). Thus, we demonstrated that co-contractions may be a compensatory strategy for individuals with iSCI to accommodate for decreased motor function, but co-contractions may result in increased ankle mechanical joint stiffness and consequently postural sway.

Effects of initial foot position on postural responses to lateral standing surface perturbations in younger and older adults

BACKGROUND

An age-related decline in standing balance control in the medio-lateral direction is associated with increased risk of falls. A potential approach to improve postural stability is to change initial foot position (IFP).

RESEARCH QUESTIONS

In response to a lateral surface perturbation, how are lower extremity muscle activation levels different and what are the effects of different IFPs on muscle activation patterns and postural stability in younger versus older adults?

METHODS

Ten younger and ten older healthy adults participated in this study. Three IFPs were tested [Reference (REF): feet were placed parallel, shoulder-width apart; Toes-out with heels together (TOHT): heels together with toes pointing outward; Modified Semi-Tandem (M-ST): the heel of the anterior foot was placed by the big toe of the posterior foot]. Unexpected lateral translations of the standing surface were applied. Electromyographic (EMG) activity of the lower extremity muscles, standard deviation (SD) of the body's CoM acceleration (SD of CoMAccel), and center of pressure (CoP) sway area were compared across IFPs and age.

RESULTS

Activation levels of the muscles serving the ankle and gluteus medius were greater than for the knee joint muscles and gluteus maximus in the loaded leg across all IFPs in both groups. TOHT showed greater EMG peak amplitude of the soleus and fibularis longus compared to REF, and had smaller SD of CoMAccel and CoP sway area than M-ST. Compared to younger adults, older adults demonstrated lower EMG peak amplitude and delayed peak timing of the fibularis longus and greater SD of CoMAccel and CoP sway area in all IFPs during balance recovery.

SIGNIFICANCE

During standing balance recovery, ankle muscles and gluteus medius are important active responders to unexpected lateral surface perturbations and a toes-out IFP could be a viable option to enhance ankle muscle activation that diminishes with age to improve postural stability.

The tightening parameters of the vibratory devices modify their disturbing postural effects

The purpose was to specify the impact of two different forces exerted by vibratory devices on the Achilles tendon on postural balance. The postural balance of 13 participants was evaluated on a force platform in two 40 s bipedal stance conditions with closed eyes. Tendon vibrations (80 Hz) were triggered 10 s after the beginning of the postural evaluation and applied during 20 s. Two levels of the force exerted by the vibrators were calibrated using load cells to control the tightening parameters of the vibrators: a strong tightening (ST) condition at 45 N and a light tightening (LT) condition at 5 N. The soleus electromyographic (EMG) activity and the spatio-temporal parameters of displacement of the centre of foot pressure (COP) were analysed. To analyse the effects of the introduction, the adaptation and the end of the stimulation, non-parametric tests were used. The results indicated that the soleus EMG activity increased only in the ST condition. However, during the vibration the anteroposterior COP position was significantly more in a backward position in the LT condition. At the end of the vibration, COP parameters increased more in the LT condition than the ST condition. This study demonstrated that the effects of the vibration depended on the force exerted by the devices on the tendons. The ST increased the vibration effects on EMG activity through greater stimulating effects compared to the LT. However, the ST could also increase the ankle joint stiffness and/or somaesthetic sensory information, which attenuated the COP backward shift.

State-Space Characterization of Balance Capabilities in Biped Systems with Segmented Feet

The human ability of keeping balance during various locomotion tasks is attributed to our capability of withstanding complex interactions with the environment and coordinating whole-body movements. Despite this, several stability analysis methods are limited by the use of overly simplified biped and foot structures and corresponding contact models. As a result, existing stability criteria tend to be overly restrictive and do not represent the full balance capabilities of complex biped systems. The proposed methodology allows for the characterization of the balance capabilities of general biped models (ranging from reduced-order to whole-body) with segmented feet. Limits of dynamic balance are evaluated by the Boundary of Balance (BoB) and the associated novel balance indicators, both formulated in the Center of Mass (COM) state space. Intermittent heel, flat, and toe contacts are enabled by a contact model that maps discrete contact modes into corresponding center of pressure constraints. For demonstration purposes, the BoB and balance indicators are evaluated for a whole-body biped model with segmented feet representative of the human-like standing posture in the sagittal plane. The BoB is numerically constructed as the set of maximum allowable COM perturbations that the biped can sustain along a prescribed direction. For each point of the BoB, a constrained trajectory optimization algorithm generates the biped's whole-body trajectory as it recovers from extreme COM velocity perturbations in the anterior-posterior direction. Balance capabilities for the cases of flat and segmented feet are compared, demonstrating the functional role the foot model plays in the limits of postural balance. The state-space evaluation of the BoB and balance indicators allows for a direct comparison between the proposed balance benchmark and existing stability criteria based on reduced-order models [e.g., Linear Inverted Pendulum (LIP)] and their associated stability metrics [e.g., Margin of Stability (MOS)]. The proposed characterization of balance capabilities provides an important benchmarking framework for the stability of general biped/foot systems.

Effects of two passive back-support exoskeletons on postural balance during quiet stance and functional limits of stability

While occupational back-support exoskeletons (BSEs) are considered as potential workplace interventions, BSE use may compromise postural control. Thus, we investigated the effects of passive BSEs on postural balance during quiet upright stance and functional limits of stability. Twenty healthy adults completed trials of quiet upright stance with differing levels of difficulty (bipedal and unipedal stance; each with eyes open and closed), and executed maximal voluntary leans. Trials were done while wearing two different BSEs (SuitX™, Laevo™) and in a control (no-BSE) condition. BSE use significantly increased center-of-pressure (COP) median frequency and mean velocity during bipedal stance. In unipedal stance, using the Laevo™ was associated with a significant improvement in postural balance, especially among males, as indicated by smaller COP displacement and sway area, and a longer time to contact the stability boundary. BSE use may affect postural balance, through translation of the human + BSE center-of-mass, restricted motion, and added supportive torques. Furthermore, larger effects of BSEs on postural balance were evident among males. Future work should further investigate the gender-specificity of BSE effects on postural balance and consider the effects of BSEs on dynamic stability.

Influence of age on the frequency characteristics of the soleus muscle response to Achilles tendon vibration during standing

KEY POINTS

Proprioceptive sensory information from the ankle joint is critical for the control of upright posture and balance. We examined the influence of age (n = 54 healthy adults, 20-82 years old) on lower limb muscle responses to proprioceptive perturbations evoked by Achilles tendon vibration during standing. The frequency bandwidth of the muscle response became narrower, and the gain (the muscle response relative to the stimulus) and scaling (increases in response amplitude with increases in stimulus amplitude) decreased with age. Mechanics of the muscle-tendon unit (mechanical admittance) did not differ with age during standing, and thus probably did not mediate the age-related changes observed in soleus muscle responses to vibration. These findings add to our understanding of how altered proprioceptive responses may contribute to impaired mobility and falls with ageing.

ABSTRACT

Proprioceptive information from the ankle joint plays an important role in the control of upright posture and balance. Ageing influences many components of the sensorimotor system, which leads to poor mobility and falls. However, little is known about the influence of age on the characteristics of short latency muscle responses to proprioceptive stimuli during standing across frequencies that are encoded by muscle spindles. We examined the frequency characteristics of the soleus muscle response to noisy (10-115 Hz) Achilles tendon vibration during standing in 54 healthy adults across a broad age range (20-82 years). The results showed the frequency bandwidth of the soleus response (vibration-electromyography coherence) became progressively narrower with ageing. Coherence was significantly lower in middle-aged relative to young adults between ∼7-11 and 28-62 Hz, lower in older relative to middle-aged adults between ∼30-50 Hz and lower in older relative to young adults between ∼7-64 Hz. Muscle response gain was similar between age groups at low frequencies, although gain was lower in older relative to young adults between ∼28-54 Hz. Across the age range, the response amplitude (peak-to-peak cross-covariance) and the scaling of the response with stimulus amplitude were both negatively correlated with age. Muscle-tendon mechanics (admittance) did not differ with age, suggesting this did not mediate differences in soleus responses. Our findings suggest there is a progressive change in the soleus response to proprioceptive stimuli with ageing during standing, which could contribute to poorer mobility and falls.

Contribution of the dorsolateral prefrontal cortex activation, ankle muscle activities, and coactivation during dual-tasks to postural steadiness: a pilot study

[Purpose] To examine the influence of dorsolateral prefrontal cortex (DLPFC) activation, ankle muscle activities, and coactivation on postural steadiness during dual-tasks. [Participants and Methods] A total of 14 participants (8 males, 6 females) were included. The participants stood straight on the force plate, and performed 3 different tasks: 1) a quiet standing (single-task), 2) a repetition of a number (dual-task 1: DT1), and 3) a serial subtraction (dual-task 2: DT2). We divided the participants into 2 groups (S and L group) according to whether their center of pressure paths in the dual-tasks were shorter or longer than those in the single-task. The EMG activity of the gastrocnemius lateralis and tibialis anterior were measured; the oxygenated hemoglobin (oxy-Hb) level in the DLPFC were measured using fNIRS. [Results] The results revealed that oxy-Hb in the left DLPFC increased significantly in all participants during DT2 compared to a single-task. Further, we found that the S group exhibited a higher rate of tibialis anterior activity and ankle muscle coactivation than the L group during DT2. [Conclusion] We concluded that the increase of the DLPFC activation varied with the dual-tasks; moreover, younger individuals modulate their standing posture using different strategies for posture steadiness during posture-calculating task.

Impairments of Postural Balance in Surgically Treated Lumbar Disc Herniation Patients

Reports on body sway control following microdiscectomy lack reports on side-specific balance deficits as well as the effects of trunk balance control deficits on body sway during upright stances. About 3 weeks post microdiscectomy, the body sway of 27 patients and 25 controls was measured while standing in an upright quiet stance with feet positioned parallel on an unstable support surface, a tandem stance with the involved leg positioned in front or at the back, a single-leg stance with both legs, and sitting on an unstable surface. Velocity, average amplitude, and frequency-direction-specific parameters were analyzed from the center of pressure movement, measured by the force plate. Statistically significant differences between the 2 groups were observed for the medial-lateral body sway frequency in parallel stance on a stable and unstable support surface and for the sitting balance task in medial-lateral body sway parameters. Medium to high correlations were observed between body sway during sitting and the parallel stance, as well as between the tandem and single-legged stances. Following microdiscectomy, deficits in postural balance were side specific, as expected by the nature of the pathology. In addition, the results of this study confirmed the connection between proximal balance control deficits and balance during upright quiet balance tasks.

Out of balance - Postural control in cancer patients before and after neurotoxic chemotherapy

BACKGROUND

Chemotherapy-induced peripheral neuropathy (CIPN) is a serious side effect deriving from neurotoxic chemotherapeutic agents. The underlying nerve injury can affect proprioception causing impaired postural control, gait difficulties and a higher risk of falling. Overall, the symptoms and functional limitations negatively affect patients' independence and quality of life.

RESEARCH QUESTION

Our objective was to analyze postural control in cancer patients before and after neurotoxic chemotherapy and to compare these data to healthy controls.

METHODS

Participants were 35 cancer patients (PAT) and 35 healthy, one-to-one gender, age, height, and weight matched controls (HMC). Postural control of HMC was tested once, whereas PAT were tested prior to (PAT_pre) and three weeks after completion of neurotoxic chemotherapy (PAT_post). Temporal, spatial and frequency domain measures of the center of pressure (COP) were calculated using a force plate. The following balance conditions were analyzed: bipedal stance with open (BP_EO) and closed eyes (BP_EC), semi-tandem (ST_EO, ST_EC) and monopedal stance (MP_EO). CIPN was assessed clinically (Total Neuropathy Score) and via questionnaire. Time and group differences were determined by using Wilcoxon-signed-rank tests. Spearman correlation was applied to analyze associations between severity of CIPN and postural control.

RESULTS

PAT_post showed significantly increased temporal and spatial measures of the COP (p < .05) - both after neurotoxic chemotherapy (PAT_pre-PAT_post) and in comparison to HMC. Withdrawal of visual control resulted in greater temporal and spatial COP displacements in PAT_post than in the comparative groups (PAT_pre, HMC). Correlation analyzes revealed moderate associations of COP measures with clinical CIPN measures and low to none for the questionnaires.

SIGNIFICANCE

Three weeks after completion of neurotoxic chemotherapy, PAT_post showed significant balance deficits compared to PAT_pre and HMC. Especially the deficits in the standing conditions with closed eyes may indicate an impaired proprioception. This hypothesis is supported by the finding that stronger CIPN symptoms were associated with poorer postural control. However, future studies need to take further influencing factors on postural control into account (e.g. strength) in order to generate efficacious rehabilitation measures.

Could Ankle Muscle Activation be Used as a Simple Measure of Balance Exercise Intensity?

Few, if any, studies have reported the effects of intensity of balance exercise for balance training and rehabilitation. The aim of the present study was to find a relative measure of intensity of balance exercise. On this basis, we analysed ankle muscle activation in the sagittal plane with increasing difficulty for a one leg stance on a T-board. Ten adults (7 men, 24.1 ± 3.5 years; 3 women, 30.6 ± 5.8 years) performed 3 trials on a T-board within 6 randomly assigned stability levels. T-board swaying velocities in the sagittal plane were manipulated to attain different stability levels (conditions). Concurrently, angular distance of the T-board and active balance time (i.e., percentage of a total time balancing) under each condition were measured. Surface electromyography from the tibialis anterior, gastrocnemius and soleus were monitored during one leg stance. The surface electromyography amplitude in the time domain was quantified using the root-mean-square values. Significant effect of stability levels on angular distance (F_5,45 = 3.4; p = 0.01) and velocity of the T-board (F_5,45 = 4.6; p = 0.002) were obtained. Active balance time decreased by ∼15% (p = 0.001) from the maximal to the minimal stability conditions. The graded level of balance board stability conditions did not generate significantly higher root-mean-square values in any muscles and hence could not be used as a relative measure of intensity of balance exercise. These findings imply that there could be a plateau in difficulty of balance exercise for enhancement of ankle muscle activity.

Virtual-Reality-Induced Visual Perturbations Impact Postural Control System Behavior

Background: Virtual reality (VR) is becoming a widespread tool in rehabilitation, especially for postural stability. However, the impact of using VR in a “moving wall paradigm” (visual perturbation), specifically without and with anticipation of the perturbation, is unknown. Methods: Nineteen healthy subjects performed three trials of static balance testing on a force plate under three different conditions: baseline (no perturbation), unexpected VR perturbation, and expected VR perturbation. The statistical analysis consisted of a 1 × 3 repeated-measures ANOVA to test for differences in the center of pressure (COP) displacement, 95% ellipsoid area, and COP sway velocity. Results: The expected perturbation rendered significantly lower (p < 0.05) COP displacements and 95% ellipsoid area compared to the unexpected condition. A significantly higher (p < 0.05) sway velocity was also observed in the expected condition compared to the unexpected condition. Conclusions: Postural stability was lowered during unexpected visual perturbations compared to both during baseline and during expected visual perturbations, suggesting that conflicting visual feedback induced postural instability due to compensatory postural responses. However, during expected visual perturbations, significantly lowered postural sway displacement and area were achieved by increasing the sway velocity, suggesting the occurrence of postural behavior due to anticipatory postural responses. Finally, the study also concluded that VR could be used to induce different postural responses by providing visual perturbations to the postural control system, which can subsequently be used as an effective and low-cost tool for postural stability training and rehabilitation.

Does a Passive Unilateral Lower Limb Exoskeleton Affect Human Static and Dynamic Balance Control?

Exoskeletons are wearable devices closely coupled to the human, which can interact with the musculoskeletal system, e. g., to augment physical and functional capabilities. A main prerequisite for the development and application of exoskeletons is to investigate the human-exoskeleton interaction, particularly in terms of potential inferences with human motor control. Therefore, the purpose of the present study was to investigate whether a passive unilateral lower limb exoskeleton has an impact on static and dynamic reactive balance control. Eleven healthy subjects (22.9 ± 2.5 years, five females) volunteered for this study and performed three different balance tasks: bipedal standing, single-leg standing, and platform perturbations in single-leg standing. All the balance tasks were conducted with and without a passive unilateral lower limb exoskeleton, while force plates and a motion capture system were used to capture the center of pressure mean sway velocity and the time to stabilization, respectively. Dependent t-tests were separately run for both static balance tests, and a repeated-measure analysis of variance with factors exoskeleton and direction of perturbation was calculated for the dynamic reactive balance task. The exoskeleton did not significantly influence postural sway in bipedal stance. However, in single-leg stance, the mediolateral mean sway velocity of the center of pressure was significantly shorter for the exoskeleton condition. For the dynamic reactive balance task, the participants tended to regain stability less quickly with the exoskeleton, as indicated by a large effect size and longer time to stabilization for all directions of perturbation. In summary, the study showed that the exoskeleton provided some assistive support under static conditions, which however may disappear when sufficient stability is available (bipedal stance). Besides, the exoskeleton tended to impair dynamic reactive balance, potentially by impeding adequate compensatory adjustments. These are important findings with strong implications for the future design and application of exoskeletons, emphasizing the significance of taking into account the mechanisms of human motor control.

Adaptation of emotional state and standing balance parameters following repeated exposure to height-induced postural threat

Height-induced postural threat influences standing balance control. However, it is unknown if minimizing individuals’ emotional response to threat moderates this relationship. This study repeatedly exposed individuals to height-induced postural threat to determine if reducing the emotional response to threat influences standing balance control. Sixty-eight young adults completed a series of standing trials at LOW (0.8 m above ground, away from edge) and HIGH (3.2 m above ground, at edge) postural threat conditions. Emotional state was assessed using self-report and electrodermal measures. Standing balance was assessed through analysis of centre of pressure (COP) movement and lower leg electromyographic activity. Individuals’ emotional response to threat was attenuated following repeated threat exposure. However, threat-induced changes in standing balance were largely preserved. When initially threatened, individuals leaned backward and demonstrated smaller amplitude and higher frequency of COP adjustments; these balance outcomes did not change following repeated threat exposure. Only high frequency COP oscillations (>1.8 Hz) and ankle muscle co-contraction showed any adaptation; regression analyses showed that these behavioural adaptations were accounted for by a combination of emotional and cognitive state changes. This suggests that some threat-induced standing balance changes are more closely linked with the emotional response to threat than others, and are therefore amendable to intervention.

Absence of Ankle Stiffening While Standing in Focus and Cognitive Task Conditions in Older Adults

Research suggests that an external focus or cognitive task may improve postural control. Removing attention from movement production may promote automaticity, or the tasks may promote ankle stiffening. To investigate these two theories, twenty older adults stood while performing baseline standing, internal focus, external focus, and two cognitive tasks. Changes in postural control occurred in external focus and cognitive task conditions compared to baseline and internal focus, while no change occurred in cocontraction indices. This suggests that an external focus and cognitive task can improve postural control in older adults. Since no change occurred in cocontraction indices across conditions, this suggests that stiffening cannot explain these changes. Instead, changes could be due to automaticity of sway.

Slowed sensory reweighting and postural illusions in older adults: the moving platform illusion

We investigated whether postural aftereffects witnessed during transitions from a moving to a stable support are accompanied by a delayed perception of platform stabilization in older adults, in two experiments. In experiment 1, postural sway and muscle cocontraction were assessed in 11 healthy young, 11 healthy older, and 11 fall-prone older adults during blindfolded stance on a fixed platform, followed by a sway-referenced platform and then by a fixed platform again. The sway-referenced platform was more compliant for young adults, to induce similar levels of postural sway in both age groups. Participants were asked to press a button whenever they perceived that the platform had stopped moving. Both older groups showed significantly larger and longer postural sway aftereffects during platform stabilization compared with young adults, which were pronounced in fall-prone older adults. In both older groups elevated muscle cocontraction aftereffect was also witnessed. Importantly, these aftereffects were accompanied by an illusory perception of prolonged platform movement. After this, experiment 2 examined whether this illusory perception was a robust age effect or an experimental confound due to greater surface compliance in young adults, which could create a larger perceptual discrepancy between moving and stable conditions. Despite exposure to the same surface compliance levels during sway-reference, the perceptual illusion was maintained in experiment 2 in a new group of 14 healthy older adults compared with 11 young adults. In both studies, older adults took five times longer than young adults to perceive platform stabilization. This supports that sensory reweighting is inefficient in older adults. NEW & NOTEWORTHY This is the first paper to show that postural sway aftereffects witnessed in older adults after platform stabilization may be due to a perceptual illusion of platform movement. Surprisingly, in both experiments presented it took older adults five times longer than young adults to perceive platform stabilization. This supports a hypothesis of less efficient sensory reintegration in this age group, which may delay the formation of an accurate postural percept.

Antagonist muscle activity during reactive balance responses is elevated in Parkinson's disease and in balance impairment

BACKGROUND

Abnormal antagonist leg muscle activity could indicate increased muscle co-contraction and clarify mechanisms of balance impairments in Parkinson's disease (PD). Prior studies in carefully selected patients showed PD patients demonstrate earlier, longer, and larger antagonist muscle activation during reactive balance responses to perturbations.

RESEARCH QUESTION

Here, we tested whether antagonist leg muscle activity was abnormal in a group of PD patients who were not selected for phenotype and most of whom had volunteered for exercise-based rehabilitation.

METHODS

We compared antagonist activation during reactive balance responses to multidirectional support-surface translation perturbations in 31 patients with mild-moderate PD (age 68±9; H&Y 1-3; UPDRS-III 32±10) and 13 matched individuals (age 65±9). We quantified modulation of muscle activity (i.e., the ability to activate and inhibit muscles appropriately according to the perturbation direction) using modulation indices (MI) derived from minimum and maximum EMG activation levels observed across perturbation directions.

RESULTS

Antagonist leg muscle activity was abnormal in unselected PD patients compared to controls. Linear mixed models identified significant associations between impaired modulation and PD (P<0.05) and PD severity (P<0.01); models assessing the entire sample without referencing PD status identified associations with balance ability (P<0.05), but not age (P = 0.10).

SIGNIFICANCE

Antagonist activity is increased during reactive balance responses in PD patients who are not selected on phenotype and are candidates for exercise-based rehabilitation. This activity may be a mechanism of balance impairment in PD and a potential rehabilitation target or outcome measure.

Standing Versus Stepping-Exploring the Relationships Between Postural Steadiness and Dynamic Reactive Balance Control

While the literature has characterized balance control during quasi-static and/or dynamic tasks, comparatively few studies have examined relationships across paradigms. This study investigated whether quiet-stance postural steadiness metrics were associated with reactive control parameters (during both stepping and restabilization phases) following a lean-and-release perturbation. A total of 40 older adults participated. Postural steadiness (center of the pressure range, root mean square, velocity, and frequency) was evaluated in "feet together" and "tandem stance" positions. During the reactive control trials, the step length, step width, movement time, and reaction time were measured, in addition to the postural steadiness variables measured during the restabilization phase following the stepping response. Out of 64 comparisons, only 10 moderate correlations were observed between postural steadiness and reactive spatio-temporal stepping parameters (P ≤ .05, r = -.312 to -.534). However, postural steadiness metrics were associated with the center of pressure velocity and frequency during the restabilization phase of the reactive control trials (P ≤ .02, r = .383 to .775 for velocity and P ≤ .01, r = .386 to .550 for frequency). Although some elements of quasi-static center of pressure control demonstrated moderate associations with dynamic stepping responses, relationships were stronger for restabilization phase dynamics after foot-contact. Future work should examine the potential association between restabilization phase control and older adult fall-risk.

The center of pressure and ankle muscle co-contraction in response to anterior-posterior perturbations

Though both contraction of agonist muscles and co-contraction of antagonistic muscle pairs across the ankle joint are essential to postural stability, they are perceived to operate independently of each other, In an antagonistic setup, agonist muscles contract generating moment about the joint, while antagonist muscles contract generating stiffness across the joint. While both work together in maintaining robustness in the face of external perturbations, contractions of agonist muscles and co-contractions of antagonistic muscle pairs across the ankle joint play different roles in responding to and adapting to external perturbations. To determine their respective roles, we exposed participants to repeated perturbations in both large and small magnitudes. The center of pressure (COP) and a co-contraction index (CCI) were used to quantify the activation of agonist muscles and antagonistic muscle pairs across the ankle joint. Our results found that participants generated moment of a large magnitude across the ankle joint—a large deviation in the COP curve—in response to perturbations of a large magnitude (p <0.05), whereas the same participants generated higher stiffness about the ankle—a larger value in CCI—in response to perturbations of a small magnitude (p <0.05). These results indicate that participants use different postural strategies pertaining to circumstances. Further, the moment across the ankle decreased with repetitions of the same perturbation (p <0.05), and CCI tended to remain unchanged even in response to a different perturbation following repetition of the same perturbation (p <0.05). These findings suggest that ankle muscle contraction and co-contraction play different roles in regaining and maintaining postural stability. This study demonstrates that ankle moment and stiffness are not correlated in response to external perturbations.

Graded forward and backward walking at a matched intensity on cardiorespiratory responses and postural control

BACKGROUND

While several studies compare backward walking (BW) and forward walking (FW) in terms of heart rate (HR) and rating of perceived exertion (RPE), workload (VO₂) was not matched to control for intensity levels (Hooper et al. [1]). Moreover, acute effects of inclined BW on postural control and ankle musculature has not been investigated. This study was designed to compare cardiovascular, metabolic and perceptual responses, changes in center of pressure (COP) motion, and muscle activation of tibialis anterior (TA) and gastrocnemius (GM) to control quiet stance posture immediately following inclined BW and FW at a matched intensity.

METHODS

Seventeen healthy young adults completed three lab sessions 7-14 days apart. Session one, maximal oxygen consumption (VO_2max) was measured using open-circuit spirometry for each participant. Session two, participants performed BW for 15-min. Session three, participants performed FW for 15-min at matched intensity of BW. Surface electromyography (SEMG) measured the muscular activity of the TA and GM during bilateral stance on a force plate for 30 s prior to and immediately following BW and FW under both eyes open (EO), and eyes closed (EC) conditions.

RESULTS

HR, VCO₂, RER and RPE were significantly greater during BW compared to FW. Increased muscle activation and COP motion was elicited immediately following BW compared to FW under EO and EC.

CONCLUSION

Results of this study indicate BW requires greater cardiovascular, metabolic, perceptual and neuromuscular demands than FW, which may cause postural instability particularly to those with compromised balance. While there are benefits to BW in rehabilitation settings, these factors should be considered when prescribing BW for training and/or rehabilitation exercise program (Duffell et al. [2], Warnica et al. [3]).

Influence of foot position and vision on dynamic postural strategies during the "grand plié" ballet movement (squatting) in young and adult ballet dancers

PURPOSE

To analyse dynamic postural strategies during the "grand plié" in two different foot positions (parallel or turned out), with and without vision, and as a function of age in ballet dancers.

METHOD

Twenty young dancers (YD) aged from 8 to 16 years, and 20 adult dancers (AD) aged from 17 to 30 years were recruited. Center of pressure (CoP) and ground reaction forces (GRF) were recorded (500 Hz) during the grand plié (lowering, squatting and rising). This movement was tested with the feet parallel and with both lower limbs turned out (foot angle >140°), with eyes open (EO) and eyes closed (EC). Groups were compared using Student t-tests. Repeated analysis of variance was used to examine the effects of eyes and foot conditions, with a significance level of p < 0.05.

RESULTS

The results of this study showed that dynamic postural strategies during the "grand plié" ballet movement are influenced by age, foot position and visual condition. CoP displacement length (p < 0.003) and CoP speed (p < 0.003) were higher in YD compared with AD. CoP surface (p < 0.05), mediolateral CoP speed (p < 0.048) and GRF parameters, particularly the mediolateral (p < 0.049), were higher than in the parallel than the turned out position. In both groups all CoP (p < 0.042) and GRF parameters (p < 0.049), except the vertical component, were higher with EC than EO.

CONCLUSION

The effect of foot position was greatest with EO. The parallel position was less stable. The YD were more unstable in the parallel position, particularly with EC. For both groups, the lack of vision increased instability. These results show the importance of integrating balance training in a variety of foot positions and visual conditions, particularly during the initial stages of training to prevent injury.

The influence of wearing ski-boots with different rigidity characteristics on postural control

External supports that reduce ankle joint mobility such as ski-boots can impair postural control of healthy participants. Although this disruptive effect has been attributed to the rigidity of the external supports, the results remained controversial and no study has been conducted in order to evaluate the influence of ski-boots rigidity. Hence, the question about the influence of ankle support rigidity on postural control remains open. This study was therefore undertaken in order to investigate the effect of ski-boots rigidity on postural control. Ten healthy active participants were recruited. The wearing of soft and rigid ski-boots was compared to barefoot while standing on a seesaw generating mediolateral and anteroposterior instability. Centre of pressure displacements were sampled with a force platform. The surface electromyographic activity of the main muscles from the leg, thigh and trunk was recorded. A motion analysis system was also used to calculate the ankle, knee and hip angles. The results did not reveal any negative influence of ski-boot rigidity on postural control but rather suggest a less active postural control with the rigid ski-boots which offered a higher mechanical contribution.

Assessment of ankle muscle activation by muscle synergies in healthy and post-stroke gait

OBJECTIVE

Impaired ankle dorsal and plantar flexor function is a frequent sequela of stroke. A better assessment of ankle muscle activation would be highly significant for stroke rehabilitation. The challenge in implementing current electromyography (EMG)-based assessments is due to problems with the variability and individuality of ankle muscle EMG profiles during walking. We have been studying a new technique using the muscle synergy method to quantify the characteristics that underlie ankle muscle activation to address this issue.

APPROACH

We processed surface EMG signals from ankle muscles and gait parameters collected from 20 healthy and 22 post-stroke subjects during walking. A non-negative matrix factorization algorithm was used to extract muscle synergies.

MAIN RESULTS

Our results suggest a featured muscle synergy structure (R  =  0.84, 95% CI: 0.83-0.85) underlying ankle muscle activation in both healthy and post-stroke subjects. The structure of the featured muscle synergy was robust in the same subjects across different conditions in the healthy group (R  =  0.97, 95% CI: 0.96-0.98) and the post-stroke group (R  =  0.95, 95% CI: 0.88-0.97). Compared to the stroke group, the synergy patterns of healthy subjects showed better regularity and higher inter-subject similarity (P  =  0.001). In addition, the results of muscle synergies were indicative of locomotor performance.

SIGNIFICANCE

The innovative quantitative results of this study can help us to better understand ankle muscle activation and will be a reference for clinical assessments and intervention studies.

Effects of the availability of accurate proprioceptive information on older adults' postural sway and muscle co-contraction

During conditions of increased postural instability, older adults exhibit greater lower limb muscle co-contraction. This response has been interpreted as a compensatory postural strategy, which may be used to increase proprioceptive information from muscle spindles or to stiffen the lower limb as a general response to minimise postural sway. The current study aimed to test these two hypotheses by investigating use of muscle co-contraction during sensory transitions that manipulated proprioceptive input. Surface EMG was recorded from the bilateral tibialis anterior and gastrocnemius medialis muscles, in young (aged 18-30) and older adults (aged 68-80) during blind-folded postural assessment. This commenced on a fixed platform (baseline: 2 min), followed by 3 min on a sway-referenced platform (adaptation) and a final 3 min on a fixed platform again (reintegration). Sensory reweighting was slower in older adults, as shown by a significantly larger and longer postural sway after-effect once a stable platform was restored. Muscle co-contraction showed similar after-effects, whereby older adults showed a larger increase in co-contraction once the stable platform had been restored, compared to young adults. This co-contraction after-effect did not return to baseline until after 1 min. Our evidence for high muscle co-contraction during the reintroduction of veridical proprioceptive input suggests that increased co-contraction in older adults is not dependent on contemporaneous proprioceptive input. Rather, it is more likely that co-contraction is a general postural strategy used to minimise postural sway, which is increased during this sensory transition. Future research should examine whether muscle co-contraction is typically a reactive or anticipatory response.

Ankle muscle co-contractions during quiet standing are associated with decreased postural steadiness in the elderly

It has been reported that the elderly use co-contraction of the tibialis anterior (TA) and plantarflexor muscles for longer duration during quiet standing than the young. However, the particular role of ankle muscle co-contractions in the elderly during quiet standing remains unclear. Therefore, the objective of this study was to investigate the association between ankle muscle co-contractions and postural steadiness during standing in the elderly. Twenty-seven young (27.2±4.5yrs) and twenty-three elderly (66.2±5.0yrs) subjects were asked to stand quietly on a force plate for five trials. The center of pressure (COP) trajectory and its velocity (COPv) as well as the center of mass (COM) trajectory and its velocity (COMv) and acceleration (ACC) were calculated using the force plate outputs. Electromyograms were obtained from the right TA, soleus (SOL), and medial gastrocnemius (MG) muscles. Periods of TA activity (TAon) and inactivity (TAoff) were determined using an EMG threshold based on TA resting level. Our results indicate that, in the elderly, the COPv, COMv, and ACC variability were significantly larger during TAon periods compared to TAoff periods. However, in the young, no significant association between respective variability and TA activity was found. We conclude that ankle muscle co-contractions in the elderly are not associated with an increase, but a decrease in postural steadiness. Future studies are needed to clarify the causal relationship between (1) ankle muscle co-contractions and (2) joint stiffness and multi-segmental actions during standing as well as their changes with aging.

The effects of attentional focus and cognitive tasks on postural sway may be the result of automaticity

Research reveals improvements in postural control when focus is placed on movement effects rather than movement production, and further improvements during the performance of a concurrent cognitive task. It has yet to be determined if these changes are due to the use of an ankle stiffening strategy or to the use of more automatic postural control processes. The objectives of the present study were to replicate the effect of attentional focus and cognitive tasks on postural control and to test that no change occurs in lower leg muscle activity in these conditions. Twenty five healthy young adults (20.7±2.76years, 10 male) were asked to stand still while performing various tasks: baseline standing, internally focusing on minimizing movement of the ankles, externally focusing on minimizing movement of an apparatus placed on their ankle joint, and two cognitive tasks consisting of counting and simultaneously summing one or two single digits in a series of three-digit numbers. Compared to baseline and internal focus, sway decreased in external focus conditions and decreased further in cognitive task conditions. Furthermore, sway velocity increased in cognitive task conditions and sway frequency increased in the medial-lateral direction in the more difficult cognitive task. Finally, no effect of condition was found on muscle activity around the ankle joint. Collectively, the findings lend support to the hypothesis that changes in postural control were the result of an automatic type of postural control rather than due to stiffening occurring at the ankle joint.

The Spatial Distribution of Ankle Muscles Activity Discriminates Aged from Young Subjects during Standing

During standing, age-related differences in the activation of ankle muscles have been reported from surface electromyograms (EMGs) sampled locally. Given though activity seems to distribute unevenly within ankle muscles, the local sampling of surface EMGs may provide a biased view on how often and how much elderly and young individuals activate these muscles during standing. This study aimed therefore at sampling EMGs from multiple regions of individual ankle muscles to evaluate whether the distribution of muscle activity differs between aged and young subjects during standing. Thirteen young and eleven aged, healthy subjects were tested. Surface EMGs were sampled at multiple skin locations from tibialis anterior, soleus and medial and lateral gastrocnemius muscles while subjects stood at ease. The root mean square amplitude of EMGs was considered to estimate the duration, the degree of activity and the size of the region where muscle activity was detected. Our main findings revealed the medial gastrocnemius was active for longer periods in aged (interquartile interval; 74.1-98.2%) than young (44.9-81.9%) individuals (P = 0.02). Similarly, while tibialis anterior was rarely active in young (0.7-4.4%), in elderly subjects (2.6-82.5%) it was often recruited (P = 0.01). Moreover, EMGs with relatively higher amplitude were detected over a significantly wider proximo-distal region of medial gastrocnemius in aged (29.4-45.6%) than young (20.1-31.3%) subjects (P = 0.04). These results indicate the duration and the size of active muscle volume, as quantified from the spatial distribution of surface EMGs, may discriminate aged from young individuals during standing; elderlies seem to rely more heavily on the active loading of ankle muscles to control their standing posture than young individuals. Most importantly, current results suggest different conclusions on the active control of standing posture may be drawn depending on the skin location from where EMGs are collected, in particular for the medial gastrocnemius.