Age-dependent variations in the directional sensitivity of balance corrections and compensatory arm movements in man

Selective engagement of long-latency reflexes in postural control through wobble board training

Long-latency reflexes (LLRs) are critical precursors to intricate postural coordination of muscular adaptations that sustain equilibrium following abrupt disturbances. Both disturbances and adaptive responses reflect excursions of postural control from quiescent Gaussian stability under a narrow bell curve, excursions beyond Gaussianity unfolding at many timescales. LLRs slow with age, accentuating the risk of falls and undermining dexterity, particularly in settings with concurrent additional tasks. We investigated whether the wobble board could cultivate the engagement of LLRs selectively in healthy young participants executing a suprapostural Trail Making Task (TMT). A concurrent additional-task demand constituted visual precision predominantly along the anteroposterior (AP) axis and mechanical instability mainly along the mediolateral (ML) axis. We scrutinized planar center-of-pressure (CoP) trajectories to quantify postural non-Gaussianity across various temporal scales. Wobble board increased engagement of LLRs and decreased engagement of compensatory postural adjustments (CPAs), indicated by the peak in non-Gaussianity of CoP planar displacements over LLR-specific timescales (50-100 ms) and non-Gaussianity of CoP planar displacements progressively diminishing over CPA-specific timescales ([Formula: see text] ms). Engagement with TMT did not show any noticeable influence on non-Gaussian postural sway patterns. Despite aligning the unstable axis of the wobble board with participants' ML axis, thus rendering posture more unstable along the ML axis, the wobble board increased engagement of LLRs significantly more along the AP axis and reduced engagement of CPAs significantly more along the ML axis. These findings offer initial mechanistic insights into how wobble boards may bolster balance and potentially reduce the occurrence of falls by catalyzing the engagement of LLRs selectively.

The effect of arm restriction on dynamic stability and upper-body responses to lateral loss of balance during walking: an observational study

When losing balance, upper-body movements serve as mechanical aids to regain stability. However, it remains unclear how these movements contribute to dynamic stability during recovery from a lateral loss of balance while walking with arm restriction. We aimed to (i) quantify the effect of arm restriction on gait stability and upper-body velocities and (ii) characterize upper-body kinematic strategies in response to lateral surface translations under different arm restriction conditions. Healthy adults were exposed to lateral surface translations while walking on a computerized treadmill under three conditions: 'free arms', '1-arm restricted' and '2-arms restricted'. Dynamic stability and upper-body velocities for the first step after perturbation onset were extracted. We found decreased dynamic stability in the sagittal plane and increased trunk velocity in the '2-arms restricted' condition compared with the 'free arms' condition. Head and trunk movements in the medio-lateral plane were in opposite directions in 44.31% of responses. Additionally, significant trunk velocities were observed in the opposite direction to the perturbation-induced loss of balance. Our results support the contribution of increased upper-body velocities to balance responses following arm-restricted walking perturbations and suggest that the '2-arms restricted' condition may be utilized as a perturbation-based balance training, focusing on head and trunk responses.

Single session of direction-specific training using auditory cues improves anticipatory postural adjustments to lateral perturbations

When exposed to a predictable external perturbation, humans typically generate anticipatory postural adjustments (APAs) to minimize potential body disturbance. After a single session of training, individuals demonstrated the ability to rely solely on an auditory cue to elicit appropriate APAs in response to an external postural perturbation. However, whether the generation of APAs requires directional specific training remains unclear. The aim of this study was to assess whether directional-specific training with auditory cues is necessary for the generation of appropriate APA responses. Ten young adults were exposed to external perturbations targeting either their left or right shoulders, with or without an auditory cue prior to the physical impact. Electromyography (EMG) activities of sixteen trunk and leg muscles and center-of-pressure (COP) displacements were recorded and analyzed during the anticipatory and compensatory phases of postural control. Outcome measures included the latencies and integrals of muscle activities, COP displacements, and indices of co-contraction and reciprocal activation of muscles. The results revealed that, after training with right-side perturbations accompanied by an auditory cue, young adults exhibited earlier and more efficient APA responses to right-side perturbations relying only on the auditory cue. Additionally, they displayed earlier APA responses in some muscles to left-side perturbations, although these responses were less efficient. Our findings suggest that young adults could generate effective APAs to external perturbations relying on an auditory cue after a single training session; however, these responses were directional specific.

Human foot force suggests different balance control between younger and older adults

Aging can cause the decline of balance ability, which can lead to increased falls and decreased mobility. This work aimed to discern differences in balance control between healthy older and younger adults. Foot force data of 38 older and 65 younger participants (older and younger than 60 yr, respectively) were analyzed. To first determine whether the two groups exhibited any differences, this study incorporated the orientation of the foot-ground interaction force in addition to its point of application. Specifically, the frequency dependence of the "intersection point" of the lines of actions of the foot-ground interaction forces was evaluated. Results demonstrated that, like the mean center-of-pressure speed, a traditionally employed measure, the intersection-point analysis could distinguish between the two participant groups. Then, to further explore age-specific control strategies, simulations of standing balance were conducted. An optimal controller stabilized a double-inverted-pendulum model with torque-actuated ankle and hip joints corrupted with white noise. The experimental data were compared with the simulation results to identify the controller parameters that best described the human data. Older participants showed significantly more use of the ankle than hip compared with younger participants. Best-fit controller gains suggested increased preference for asymmetric inter-joint neural feedback, possibly to compensate for the effects of aging such as sarcopenia. These results underscore the advantages of the intersection-point analysis to quantify possible shifts in inter-joint control with age, thus highlighting its potential to be used as a balance assessment tool in research and clinical settings.NEW & NOTEWORTHY Age groups were distinguished by analyzing foot-ground force data during quiet standing in older and younger adults to calculate the foot-force vector intersection point that emerges across frequency bands. Modeling balance and comparing the simulations' outcomes to experimental results suggested that older adults increased reliance on neural feedback, possibly compensating for muscle strength deficiency. This novel analysis also quantified the apparent balance controller for each participant, highlighting its potential as a balance assessment tool.

Balance in obese and normal weight pregnant women: A longitudinal study

INTRODUCTION

Pregnancy's physiological changes and weight gain can decrease balance control, increasing the risk of falls and impacting quality of life. The effect of obesity on balance control in pregnant women needs further investigation.

OBJECTIVE

Investigate the effect of obesity during pregnancy on center of pressure (COP) control under sensory disturbances at different gestational age.

METHODS

The COP (95% ellipse area, mean velocity and total length) of 23 pregnant women classified as normal weight (CG: n = 12) and obese (OG: n = 11) was evaluated at three gestational-age week times (T1 = 16th, T2 = 24th, T3 = 32nd week) on a force platform on standing position (SP) and under three disturbances: 1) visual disturbance (eyes closed), 2) proprioceptive disturbance (over an unstable surface) and, 3) vestibular disturbance (head movement). A mixed ANOVA (3 repeated measures vs. 2 groups) and LSD post-hoc was applied to examine main effects and interactions. Effect sizes were assessed using generalized eta squared (η2).

RESULTS

As main effect, in SP, COP area increased from T2 to T3 for both groups (P = 0.021, η2 = 0.193). One interaction was observed, under visual disturbance, the area increased from T2 to T3 only in the CG group (P = 0.020, η2 = 0.195).

CONCLUSION

The progression of pregnancy negatively impacts postural stability throughout its course. Visual disturbance only affected the balance of the CG. Hence, obesity combined with pregnancy does not appear to alter COP variables.

Center of mass states render multi-joint torques throughout standing balance recovery

Successful reactive balance control requires coordinated modulation of hip, knee, and ankle torques. Stabilizing joint torques arise from feedforward neural signals that modulate the musculoskeletal system's intrinsic mechanical properties, namely muscle short-range stiffness, and neural feedback pathways that activate muscles in response to sensory input. Although feedforward and feedback pathways are known to modulate the torque at each joint, the role of each pathway to the balance-correcting response across joints is poorly understood. Since the feedforward and feedback torque responses act at different delays following perturbations to balance, we modified the sensorimotor response model (SRM), previously used to analyze the muscle activation response to perturbations, to consist of parallel feedback loops with different delays. Each loop within the model is driven by the same information, center of mass (CoM) kinematics, but each loop has an independent delay. We evaluated if a parallel loop SRM could decompose the reactive torques into the feedforward and feedback contributions during balance-correcting responses to backward support surface translations at four magnitudes. The SRM accurately reconstructed reactive joint torques at the hip, knee, and ankle, across all perturbation magnitudes (R 2 >0.84 & VAF>0.83). Moreover, the hip and knee exhibited feedforward and feedback components, while the ankle only exhibited feedback components. The lack of a feedforward component at the ankle may occur because the compliance of the Achilles tendon attenuates muscle short-range stiffness. Our model may provide a framework for evaluating changes in the feedforward and feedback contributions to balance that occur due to aging, injury, or disease.

NEWS AND NOTEWORTHY

Reactive balance control requires coordination of neurally-mediated feedforward and feedback pathways to generate stabilizing joint torques at the hip, knee, and ankle. Using a sensorimotor response model, we decomposed reactive joint torques into feedforward and feedback contributions based on delays relative to center of mass kinematics. Responses across joints were driven by the same signals, but contributions from feedforward versus feedback pathways differed, likely due to differences in musculotendon properties between proximal and distal muscles.

Immediate Effect of Wearable Balance Training Device on Muscle Co-Contraction and Postural Control During Standing

Postural control is one of the primary body functions for fall prevention. Unexpected perturbation-based balance training is effective for improving postural control. However, the effect of perturbation-based training using assistive devices on muscle activity and co-contraction for standing balance is still unclear. This training is also difficult to perform easily because it requires large instruments or expert guidance. The purpose of this study is to demonstrate the effect of perturbation-based balance training using a wearable balance training device (WBTD) on postural control. In this study, fourteen healthy young adult males were assigned to either a WBTD group or a sham group. In the intervention session, participants in the WBTD group were perturbed either left or right direction at random timing by the WBTD during tandem stance balance training. Participants in the Sham group did not receive external perturbation during tandem stance balance training. Before and after the intervention session, participants of both groups underwent unexpected lateral perturbation postural control testing (pre- and post-test). The normalized integral of electromyography (IEMG), co-contraction index (CCI), and center of pressure (COP) parameters were measured in the pre- and post-test. Experimental results showed that the WBTD group in the post-test significantly decreased left Gluteus Medius IEMG, CCI of both Gluteus Medius, and peak [Formula: see text] velocity, compared to those of the pre-test ( [Formula: see text], p =0.024 , p =0.031 , respectively). We conclude that balance training using WBTD could improve flexible postural control adjustment via cooperative muscle activation.

Age-related impairments and influence of visual feedback when learning to stand with unexpected sensorimotor delays

Background

While standing upright, the brain must accurately accommodate for delays between sensory feedback and self-generated motor commands. Natural aging may limit adaptation to sensorimotor delays due to age-related decline in sensory acuity, neuromuscular capacity and cognitive function. This study examined balance learning in young and older adults as they stood with robot-induced sensorimotor delays.

Methods

A cohort of community dwelling young (mean = 23.6 years, N = 20) and older adults (mean = 70.1 years, N = 20) participated in this balance learning study. Participants stood on a robotic balance simulator which was used to artificially impose a 250 ms delay into their control of standing. Young and older adults practiced to balance with the imposed delay either with or without visual feedback (i.e., eyes open or closed), resulting in four training groups. We assessed their balance behavior and performance (i.e., variability in postural sway and ability to maintain upright posture) before, during and after training. We further evaluated whether training benefits gained in one visual condition transferred to the untrained condition.

Results

All participants, regardless of age or visual training condition, improved their balance performance through training to stand with the imposed delay. Compared to young adults, however, older adults had larger postural oscillations at all stages of the experiments, exhibited less relative learning to balance with the delay and had slower rates of balance improvement. Visual feedback was not required to learn to stand with the imposed delay, but it had a modest effect on the amount of time participants could remain upright. For all groups, balance improvements gained from training in one visual condition transferred to the untrained visual condition.

Conclusion

Our study reveals that while advanced age partially impairs balance learning, the older nervous system maintains the ability to recalibrate motor control to stand with initially destabilizing sensorimotor delays under differing visual feedback conditions.

Fear of heights shapes postural responses to vibration-induced balance perturbation at virtual height

Introduction

Standing upright at height is a challenging situation involving intense threat of balance loss and fall. The ability to maintain balance in such conditions requires properly resolving sensory conflicts and is influenced by fear. To get more insight on the role of fear in balance control at height, we explored the dynamics of postural behavior in the situation of enhanced threat of potential balance loss.

Methods

In 40 young individuals with varying fear of heights, we combined simulated exposure to height in a virtual reality environment with bilateral vibration of tibialis anterior muscles which evokes posture destabilization (the so-called vibration-induced falling).

Results

Under such condition of enhanced postural threat, individuals with intense fear of heights showed stronger stiffening of posture compared with individuals with low fear of heights who react more flexibly and adaptively to posture destabilization. This group difference was evident already at ground level but further increased during virtual height exposure.

Discussion

Our data show that fear of height significantly affects posture adaptation to balance-destabilizing events. Our findings demonstrate that the assessment of postural behavior during threatening situations in the virtual reality environment provides valuable insights into the mechanisms of balance control and may be used to develop novel strategies aimed at prevention of falls.

Peroneal muscle response to single-leg drop-jump and unexpected leg-drop in young and middle-aged adults before and after one session of neuromuscular training

BACKGROUND

Changes in neuromuscular ability in middle age (MA) may lead to deterioration of postural control. The aim of this study was to investigate the anticipatory response of the peroneus longus muscle (PL) to landing after a single-leg drop-jump (SLDJ), and its postural response after an unexpected leg-drop in MA and young adults. A second aim was to investigate the influence of neuromuscular training on PL postural responses in both age groups.

METHODS

Twenty-six healthy MA (55.3 ± 4 years) and 26 healthy young adults (26.3 ± 3.6 years) participated in the study. Assessments were performed before (T0) and after (T1) PL EMG biofeedback (BF) neuromuscular training. Subjects performed SLDJ, and PL EMG activity in preparation for landing (% of flight time) was calculated. To measure PL time to activation onset and time to peak activation in response to an unexpected leg-drop, subjects stood on a customized trapdoor device that produced a sudden 30° ankle inversion.

RESULTS

Before training, the MA group showed significantly shorter PL activity in preparation for landing compared to the young adults (25.0% vs. 30.0%, p = 0.016), while after training there was no difference between the groups (28.0% vs. 29.0%, p = 0.387). There were no differences between groups in peroneal activity after the unexpected leg-drop before and after training.

CONCLUSIONS

Our results suggest that automatic anticipatory peroneal postural responses are decreased at MA, whereas reflexive postural responses appear to be intact in this age group. A short PL EMG-BF neuromuscular training may have an immediate positive effect on PL muscle activity at MA. This should encourage the development of specific interventions to ensure better postural control in this group.

TRIAL REGISTRATION

ClinicalTrials.gov NCT05006547.

Differences between physical therapist ratings, self-ratings, and posturographic measures when assessing static balance exercise intensity

Introduction

In order for balance therapy to be successful, the training must occur at the appropriate dosage. However, physical therapist (PT) visual evaluation, the current standard of care for intensity assessment, is not always effective during telerehabilitation. Alternative balance exercise intensity assessment methods have not previously been compared to expert PT evaluations. The aim of this study was therefore to assess the relationship between PT participant ratings of standing balance exercise intensity and balance participant self-ratings or quantitative posturographic measures.

Methods

Ten balance participants with age or vestibular disorder-related balance concerns completed a total of 450 standing balance exercises (three trials each of 150 exercises) while wearing an inertial measurement unit on their lower back. They provided per-trial and per-exercise self-ratings of balance intensity on a scale from 1 (steady) to 5 (loss of balance). Eight PT participants reviewed video recordings and provided a total of 1,935 per-trial and 645 per-exercise balance intensity expert ratings.

Results

PT ratings were of good inter-rater reliability and significantly correlated with exercise difficulty, supporting the use of this intensity scale. Per-trial and per-exercise PT ratings were significantly correlated with both self-ratings (r = 0.77-0.79) and kinematic data (r = 0.35-0.74). However, the self-ratings were significantly lower than the PT ratings (difference of 0.314-0.385). Resulting predictions from self-ratings or kinematic data agreed with PT ratings approximately 43.0-52.4% of the time, and agreement was highest for ratings of a 5.

Discussion

These preliminary findings suggested that self-ratings best indicated two intensity levels (i.e., higher/lower) and sway kinematics were most reliable at intensity extremes.

Effects of arm movement strategies on emotional state and balance control during height-induced postural threat in young adults

BACKGROUND

It is firmly established that postural threat seems to lead to an increased. reliance on an ankle control ('stiffening') strategy. However, little is known about how. postural threat affects performance in challenging tasks that require the use of upper. body postural control strategies for stability. It is logical to assume that in such. conditions, being able to utilise an upper body strategy may reduce the reliance on. such ankle stiffening strategy. Research question The objective of this study was to determine how arm movement. influences balance control during a challenging balance task performed under. conditions of postural threat.

METHODS

Thirty young adults (mean ± SD age; 22.0 ± 4.0 years) balanced in tandem. stance whilst standing at both ground-level (no threat) and 0.8 m above ground. (threat). In both conditions, participants performed the task under two different arm.

POSITIONS

restricted arm movements and free arm movements. Postural sway. amplitude and frequency were calculated to infer postural stiffening response. Selfreported. emotional responses were quantified by assessing balance confidence, fear. of falling, perceived stability, and conscious balance processing.

RESULTS

Independent of arm movements, postural threat evoked an increase in fear of. falling and conscious balance processing, and reductions in balance confidence and. perceived stability. These threat-related changes in emotional state were further. amplified when arm movements were restricted. Whilst significant increases in sway. frequency during threat were observed in both arm conditions, reductions in sway. amplitude were only observed during the restricted arm movement condition.

SIGNIFICANCE

We propose that these responses likely reflect a fear-related cautious. strategy intended to reduce the postural destabilisation associated with individuals. being unable to use their arms to counter any destabilisation, as would normally be the. case in daily life.

Systematic review of candidate prognostic factors for falling in older adults identified from motion analysis of challenging walking tasks

The objective of this systematic review is to identify motion analysis parameters measured during challenging walking tasks which can predict fall risk in the older population. Numerous studies have attempted to predict fall risk from the motion analysis of standing balance or steady walking. However, most falls do not occur during steady gait but occur due to challenging centre of mass displacements or environmental hazards resulting in slipping, tripping or falls on stairs. We conducted a systematic review of motion analysis parameters during stair climbing, perturbed walking and obstacle crossing, predictive of fall risk in healthy older adults. We searched the databases of Pubmed, Scopus and IEEEexplore.A total of 78 articles were included, of which 62 simply compared a group of younger to a group of older adults. Importantly, the differences found between younger and older adults did not match those found between older adults at higher and lower risk of falls. Two prospective and six retrospective fall history studies were included. The other eight studies compared two groups of older adults with higher or lower risk based on mental or physical performance, functional decline, unsteadiness complaints or task performance. A wide range of parameters were reported, including outcomes related to success, timing, foot and step, centre of mass, force plates, dynamic stability, joints and segments. Due to the large variety in parameter assessment methods, a meta-analysis was not possible. Despite the range of parameters assessed, only a few candidate prognostic factors could be identified: older adults with a retrospective fall history demonstrated a significant larger step length variability, larger step time variability, and prolonged anticipatory postural adjustments in obstacle crossing compared to older adults without a fall history. Older adults who fell during a tripping perturbation had a larger angular momentum than those who did not fall. Lastly, in an obstacle course, reduced gait flexibility (i.e., change in stepping pattern relative to unobstructed walking) was a prognostic factor for falling in daily life. We provided recommendations for future fall risk assessment in terms of study design.In conclusion, studies comparing older to younger adults cannot be used to explore relationships between fall risk and motion analysis parameters. Even when comparing two older adult populations, it is necessary to measure fall history to identify fall risk prognostic factors.

Neural Plasticity in Spinal and Corticospinal Pathways Induced by Balance Training in Neurologically Intact Adults: A Systematic Review

Balance training, defined here as training of postural equilibrium, improves postural control and reduces the rate of falls especially in older adults. This systematic review aimed to determine the neuroplasticity induced by such training in younger (18–30 years old) and older adults (≥65 years old). We focused on spinal and corticospinal pathways, as studied with electrophysiology, in people without neurological or other systemic disorders. We were specifically interested in the change in the excitability of these pathways before and after training. Searches were conducted in four databases: MEDLINE, CINAHL, Scopus, and Embase. A total of 1,172 abstracts were screened, and 14 articles were included. Quality of the studies was evaluated with the Downs and Black checklist. Twelve of the studies measured spinal reflexes, with ten measuring the soleus H-reflex. The H-reflex amplitude was consistently reduced in younger adults after balance training, while mixed results were found in older adults, with many showing an increase in the H-reflex after training. The differences in results between studies of younger vs. older adults may be related to the differences in their H-reflexes at baseline, with older adults showing much smaller H-reflexes than younger adults. Five studies measured corticospinal and intracortical excitability using transcranial magnetic stimulation. Younger adults showed reduced corticospinal excitability and enhanced intracortical inhibition after balance training. Two studies on older adults reported mixed results after training. No conclusions could be drawn for corticospinal and intracortical plasticity given the small number of studies. Overall, balance training induced measurable change in spinal excitability, with different changes seen in younger compared to older adults.

Systematic Review of the Importance of Hip Muscle Strength, Activation, and Structure in Balance and Mobility Tasks

OBJECTIVE

The aim of this systematic review was to identify the associations of the hip abductor muscle strength, structure, and neuromuscular activation on balance and mobility in younger, middle-aged, and older adults.

DATA SOURCES

We followed PRISMA guidelines and performed searches in PubMed, Embase, CINAHL, and Physiotherapy Evidence Database.

STUDY SELECTION

Study selection included: (1) studies with patients aged 18 years or older and (2) studies that measured hip abduction torque, surface electromyography, and/or muscle structure and compared these measures with balance or mobility outcomes.

DATA EXTRACTION

The extracted data included the study population, setting, sample size, sex, and measurement evaluated.

DATA SYNTHESIS

The present systematic review is composed of 59 research articles including a total of 2144 young, middle-aged, and older adults (1337 women). We found that hip abductor strength is critical for balance and mobility function, independent of age. Hip abductor neuromuscular activation is also important for balance and mobility, although it may differ across ages depending on the task. Finally, the amount of fat inside the muscle appears to be one of the important factors of muscle structure influencing balance.

CONCLUSIONS

In conclusion, a change in all investigated variables (hip abduction torque, neuromuscular activation, and intramuscular fat) appears to have an effect during balance or mobility tasks across age ranges and may elicit better performance. Future studies are necessary to confirm the effect of these variables across age ranges and the effects of interventions.

Perturbation-Induced Protective Arm Responses: Effect of Age, Perturbation-Intensity, and Relationship with Stepping Stability: A Pilot Study

During balance recovery from slip perturbations, forward flexion (elevation) of the arms serves to counterbalance the posteriorly displaced center of mass (CoM). We aimed to investigate whether aging affects modulation of arm responses to various intensities of unpredictable slip perturbations and whether arm responses are related to compensatory stepping stability. Ten healthy young adults and ten healthy older adults participated. Participants were asked to react naturally to three randomly administered levels of slip-like surface perturbations (intensity 1 (7.75 m/s2), intensity 2 (12.00 m/s2) and intensity 3 (16.75 m/s2), which occurred by means of forward acceleration of the treadmill belt while standing. Kinematic data were collected using a motion capture system. Outcomes included arm elevation displacement, velocity, and margin of stability (MoS) of compensatory stepping. The results reveal no modulation of arm elevation velocity in older adults from perturbation intensity 1 to 2, whereas younger adults demonstrated progressive increases from intensity 1 to 2 to 3. At intensity 3, older adults demonstrated reduced maximal arm elevation velocity compared to younger adults (p = 0.02). The results in both groups combined reveal a positive correlation between maximal arm elevation velocity and first compensatory step MoS at intensity 3 (p = 0.01). Together, these findings indicate age-related decreases in arm response modulation and the association of arm elevation response with protective stepping stability, suggesting that fall prevention interventions may benefit from an emphasis on arm elevation velocity control in response to greater perturbation intensities.

The Reduced Adaptability of H-Reflex Parameters to Postural Change With Deficiency of Foot Plantar Sensitivity

Purpose: The project was to examine the influence of peripheral neuropathy (PN) severity on the relationship between Hoffmann-reflex (H-reflex) and postures.

Methods: A total of 34 participants were recruited. H-reflex (H/M ratio and H-index) during prone, standing, and the heel-contact phase of walking was tested, along with foot sole sensitivity.

Results: The participants were divided into three groups based on the severity of the foot sole sensitivity deficit: control, less (LA), and more (MA) affected with both feet 5.07 monofilament test scores ranging 10, 0–5, and 6–9, respectively. A significant group by the posture interaction was observed in the H/M ratio (F_{3.0, 41.9} = 2.904, p = 0.046, η_p² = 0.172). In the control group, the H/M ratio of prone (22 ± 7%) was greater than that of the standing (13 ± 3%, p = 0.013) and heel-contact phase (10 ± 2%, p = 0.004). In the MA group, the H/M ratio of standing (13 ± 3%) was greater than that of the heel-contact phase (8 ± 2%, p = 0.011). The H-index was significantly different among groups (F_2,28 = 5.711, p = 0.008, and η_p²= 0.290). Post hoc analysis showed that the H-index of the control group (80.6 ± 11.3) was greater than that of the LA (69.8 ± 12.1, p = 0.021) and MA groups (62.0 ± 10.6, p = 0.003).

Conclusion: In a non-PN population, the plantar sensory input plays an important role in maintaining standing postural control, while as for the PN population with foot sole sensitivity deficiency, type Ⅰ afferent fibers reflex loop (H-reflex) contributes more to the standing postural control. The H-index parameter is an excellent method to recognize the people with and without PN but not to distinguish the severity of PN with impaired foot sole sensitivity.

Strength and Power-Related Measures in Assessing Core Muscle Performance in Sport and Rehabilitation

While force-velocity-power characteristics of resistance exercises, such as bench presses and squats, have been well documented, little attention has been paid to load, force, and power-velocity relationships in exercises engaging core muscles. Given that power produced during lifting tasks or trunk rotations plays an important role in most sport-specific and daily life activities, its measurement should represent an important part of the test battery in both athletes and the general population. The aim of this scoping review was 1) to map the literature related to testing methods assessing core muscle strength and stability in sport and rehabilitation, chiefly studies with particular focus on force-velocity-power characteristics of exercises involving the use of core muscles, 2) and to identify gaps in existing studies and suggest further research in this field. The literature search was conducted on Cochrane Library databases, Scopus, Web of Science, PubMed and MEDLINE, which was completed by SpringerLink, Google Scholar and Elsevier. The inclusion criteria were met in 37 articles. Results revealed that among a variety of studies investigating the core stability and core strength in sport and rehabilitation, only few of them analyzed force-velocity-power characteristics of exercises involving the use of core muscles. Most of them evaluated maximal isometric strength of the core and its endurance. However, there are some studies that assessed muscle power during lifting tasks at different loads performed either with free weights or using the Smith machine. Similarly, power and velocity were assessed during trunk rotations performed with different weights when standing or sitting. Nevertheless, there is still scant research investigating the power-velocity and force-velocity relationship during exercises engaging core muscles in able-bodied and para athletes with different demands on stability and strength of the core. Therefore, more research is needed to address this gap in the literature and aim research at assessing strength and power-related measures within cross-sectional and intervention studies. A better understanding of the power-force-velocity profiles during exercises with high demands on the core musculature has implications for designing sport training and rehabilitation programs for enhancement of athletes' performance and/or decrease their risk of back pain.

A Lifespan Approach to Balance in Static and Dynamic Conditions: The Effect of Age on Balance Abilities

Postural control is a complex sensorimotor skill that is fundamental to our daily life. The abilities to maintain and recover balance degrade with age. However, the time decay of balance performance with age is not well understood. In this study, we aim at quantifying the age-dependent changes in standing balance under static and dynamic conditions. We tested 272 healthy subjects with ages ranging from 20 to 90. Subjects maintained the upright posture while standing on the robotic platform hunova®. In the evaluation of static balance, subjects stood on the fixed platform both with eyes open (EO) and eyes closed (EC). In the dynamic condition, subjects stood with eyes open on the moving foot platform that provided three different perturbations: (i) an inclination proportional to the center of pressure displacements, (ii) a pre-defined predictable motion, and (iii) an unpredictable and unexpected tilt. During all these tests, hunova® measured the inclination of the platform and the displacement of the center of pressure, while the trunk movements were recorded with an accelerometer placed on the sternum. To quantify balance performance, we computed spatio-temporal parameters typically used in clinical environments from the acceleration measures: mean velocity, variability of trunk motion, and trunk sway area. All subjects successfully completed all the proposed exercises. Their motor performance in the dynamic balance tasks quadratically changed with age. Also, we found that the reliance on visual feedback is not age-dependent in static conditions. All subjects well-tolerated the proposed protocol independently of their age without experiencing fatigue as we chose the timing of the evaluations based on clinical needs and routines. Thus, this study is a starting point for the definition of robot-based assessment protocols aiming at detecting the onset of age-related standing balance deficits and allowing the planning of tailored rehabilitation protocols to prevent falls in older adults.

The effect of increased cognitive processing on reactive balance control following perturbations to the upper limb

Reactive balance control following hand perturbations is important for everyday living as humans constantly encounter perturbations to the upper limb while performing functional tasks while standing. When multiple tasks are performed simultaneously, cognitive processing is increased, and performance on at least one of the tasks is often disrupted, owing to attentional resources being divided. The purpose here was to assess the effects of increased cognitive processing on whole-body balance responses to perturbations of the hand during continuous voluntary reaching. Sixteen participants (8 females; 22.9 ± 4.5 years) stood and grasped the handle of a KINARM - a robotic-controlled manipulandum paired with an augmented visual display. Participants completed 10 total trials of 100 mediolateral arm movements at a consistent speed of one reach per second, and an auditory n-back task (cognitive task). Twenty anteroposterior hand perturbations were interspersed randomly throughout the reaching trials. The arm movements with random arm perturbations were either performed simultaneously with the cognitive task (combined task) or in isolation (arm perturbation task). Peak centre of pressure (COP) displacement and velocity, time to COP displacement onset and peak, as well as hand displacement and velocity following the hand perturbation were evaluated. N-back response times were 8% slower and 11% less accurate for the combined than the cognitive task. Peak COP displacement following posterior perturbations increased by 8% during the combined compared to the arm perturbation task alone, with no other differences detected. Hand peak displacement decreased by 5% during the combined compared to the arm perturbation task. The main findings indicate that with increased cognitive processing, attentional resources were allocated from the cognitive task towards upper limb movements, while attentional resources for balance seemed unaltered.

Effects of age and surface instability on the control of the center of mass

During standing, posture can be controlled by accelerating the Center of Mass (CoM) through shifting the center of pressure (CoP) within the base of support by applying ankle moments ("CoP mechanism"), or through the "counter-rotation mechanism", i.e., changing the angular momentum of segments around the CoM to change the direction of the ground reaction force. Postural control develops over the lifespan; at both the beginning and the end of the lifespan adequate postural control appears more challenging. In this study, we aimed to assess mediolateral balance performance and the related use of the postural control mechanisms in children, older adults and younger adults when standing on different (unstable) surfaces. Sixteen pre-pubertal children (6-9y), 17 younger adults (18-24y) and eight older adults (65-80y) performed bipedal upright standing trials of 16 s on a rigid surface and on three balance boards that could freely move in the frontal plane, varying in height (15-19 cm) of the surface of the board above the point of contact with the floor. Full body kinematics (16 segments, 48 markers, using SIMI 3D-motion analysis system (GmbH) and DeepLabCut and Anipose) were retrieved. Performance related outcome measures, i.e., the number of trials with balance loss and the Root Mean Square (RMS) of the time series of the CoM acceleration, the contributions of the CoP mechanism and the counter-rotation mechanism to the CoM acceleration in the frontal plane and selected kinematic measures, i.e. the orientation of the board and the head and the Mean Power Frequency (MPF) of the balance board orientation and the CoM acceleration were determined. Balance loss only occurred when standing on the highest balance board, twice in one older adult once in one younger adult. In children and older adults, the RMS of the CoM accelerations were larger, corresponding to poorer balance performance. Across age groups and conditions, the contribution of the CoP mechanism to the total CoM acceleration was much larger than that of the counter-rotation mechanisms, ranging from 94% to 113% vs 23% to 38% (with totals higher than 100% indicating opposite effects of both mechanisms). Deviations in head orientation were small compared to deviations in balance board orientation. We suggest that the CoP mechanism is dominant, since the counter-rotation mechanism would conflict with stabilizing the orientation of the head in space.

Stabilization after postural transitions in the elderly: Experimental study on community-dwelling subjects and nursing home residents

BACKGROUND

Postural transitions have been identified as presenting challenging situations for the elderly.

RESEARCH QUESTION

This study hypothesizes a relationship between age-related factors and postural stabilization performance after a transition movement. In particular, the controlled factors in the experiment are: 1) assistance in living (independent living for community-dwelling subjects vs. assisted living for institutionalized subjects in nursing homes); 2) age of institutionalized individuals, by comparing groups with different age ranges.

METHODS

Sixty-three institutionalized individuals in nursing homes were recruited (17 in the age range 64-79; 46, including 6 drop-outs, in the age range 80-95). Moreover, seventeen (one drop out) community dwelling subjects (64-79 years) were enlisted. The study focuses on the postural stabilization phase after a "step forward" task.

RESULTS

When comparing age-matched subjects from the two groups, the residents in nursing homes were characterised by a worse stabilization performance: the stabilization time more than doubled, Instability increased by 39 %, and Promptness decreased by 77 %, although there was no significant difference in the quiet erect posture between the groups. No difference was observed when comparing the two age groups of residents in the nursing homes, however a potential confounding effect has been identified in the unequal mortality rates between the two groups.

SIGNIFICANCE

It is hypothesized that an individual identification of abnormal values of Instability and/or Promptness may inform different rehabilitation approaches.

Center of pressure plausibility for the double-link human stance model under the intermittent control paradigm

Despite human balance maintenance in quiet conditions could seem a trivial motor task, it is not. Recently, the human stance was described through a double link inverted pendulum (DIP) actively controlled at the ankle with an intermittent proportional (P) and derivative (D) control actions based on the sway of a virtual inverted pendulum (VIP) that links the ankle joint with the DIP center of mass. Such description, encompassing both the mechanical model and the intermittent control policy, was referred as the DIP/VIP human stance model, and it showed physiologically plausible kinematic patterns. In this study a mathematical formalization of the Center of pressure (COP) for a DIP structure was developed. Then, it was used in conjunction with an intermittently controlled DIP/VIP model to assess its kinetic plausibility. Three descriptors commonly employed in posturography were selected among six based on their capability to discriminate between young (Y) and elderly (O) adults groups. Then, they were applied to assess whether variations of the P-D parameters affect the synthetic COP. The results showed that DIP/VIP model can reproduce COP trajectories, showing characteristics similar to the Y and O groups. Moreover, it was observed that both P and D parameters increased passing from Y to O, indicating that the COP obtained from the DIP/VIP model is able to highlight differences in balance control between groups. The study hence promote the use of DIP/VIP in posturography, where inferential techniques can be applied to characterize neural control.

Characteristics of upper-extremity reactions to sudden lateral loss of balance in persons with stroke

BACKGROUND

Upper-extremity reactions are part of a whole-body response to counterweight the falling center of mass after unexpected balance loss. Impairments in upper-extremity reactions due to unilateral hemiparesis may contribute to stroke survivors propensity for falling. We aimed to characterize upper-extremity (paretic and non-paretic sides) reactive movements in response to lateral balance perturbations in Persons with Stroke vs. healthy controls.

METHODS

Twenty-six subacute persons with stroke and 15 healthy controls were exposed to multidirectional sudden unannounced surface translations in stance. Spatiotemporal parameters of upper- and lower-extremity balance responses to lateral perturbations were analyzed.

FINDINGS

In both groups reactive upper-extremity movement initiation preceded reactive step initiation. In response to a loss of balance toward the paretic side, persons with stroke demonstrated delayed movement initiation of both upper- and lower-extremity compared with healthy controls (In persons with stroke: 234.7 ± 60.0 msec and 227.1 ± 39.6 msec for upper extremities vs. 272.1 ± 59.1 msec for lower-extremity; and in controls: 180.1 ± 39.9 msec and 197.8 ± 61.3 msec for upper-extremities vs. 219.3 ± 40.8 msec for lower-extremity; p = 0.001, Cohen's d's: 0.59-1.03) and a greater abduction excursion in the ipsilateral upper-extremity compared with the contralateral upper-extremity (In persons with stroke: 39.3 ± 23.6 cm vs. 24.9 ± 10.1 cm, respectively; In Controls: 42.6 ± 21.8 cm vs. 29.3 ± 17.3 cm, respectively).

INTERPRETATION

The faster upper-extremity reactive movement reactions compared to reactive step initiation in both persons with stroke and healthy controls suggests that balance recovery is an automatic "reflex-like" response. Delayed upper-extremity reactive reactions in conditions of surface translation toward the non-paretic side in persons with stroke may increase the risk of falls in the direction of the paretic side.

Selective preservation of changes to standing balance control despite psychological and autonomic habituation to a postural threat

Humans exhibit changes in postural control when confronted with threats to stability. This study used a prolonged threat exposure protocol to manipulate emotional state within a threatening context to determine if any threat-induced standing behaviours are employed independent of emotional state. Retention of balance adaptations was also explored. Thirty-seven adults completed a series of 90-s standing trials at two surface heights (LOW: 0.8 m above ground, away from edge; HIGH: 3.2 m above ground, at edge) on two visits 2-4 weeks apart. Psychological and autonomic state was assessed using self-report and electrodermal measures. Balance control was assessed using centre of pressure (COP) and lower limb electromyographic recordings. Upon initial threat exposure, individuals leaned backward, reduced low-frequency centre of pressure (COP) power, and increased high-frequency COP power and plantar/dorsiflexor coactivation. Following repeated exposure, the psychological and autonomic response to threat was substantially reduced, yet only high-frequency COP power and plantar/dorsiflexor coactivation habituated. Upon re-exposure after 2-4 weeks, there was partial recovery of the emotional response to threat and few standing balance adaptations were retained. This study suggests that some threat-induced standing behaviours are coupled with the psychological and autonomic state changes induced by threat, while others may reflect context-appropriate adaptations resistant to habituation.

Interactions between initial posture and task-level goal explain experimental variability in postural responses to perturbations of standing balance

Postural responses to similar perturbations of standing balance vary widely within and across subjects. Here, we identified two sources of variability and their interactions by combining experimental observations with computational modeling: differences in posture at perturbation onset across trials and differences in task-level goals across subjects. We first collected postural responses to unpredictable backward support-surface translations during standing in 10 young adults. We found that maximal trunk lean in postural responses to backward translations were highly variable both within subjects (mean of ranges = 28.3°) and across subjects (range of means = 39.9°). Initial center of mass (COM) position was correlated with maximal trunk lean during the response, but this relation was subject specific (R² = 0.29-0.82). We then used predictive simulations to assess causal relations and interactions with task-level goal. Our simulations showed that initial posture explains the experimentally observed intrasubject variability with a more anterior initial COM position increasing the use of the hip strategy. Differences in task-level goal explain observed intersubject variability with prioritizing effort minimization leading to ankle strategies and prioritizing stability leading to hip strategies. Interactions between initial posture and task-level goal explain observed differences in intrasubject variability across subjects. Our findings suggest that variability in initial posture due to increased sway as observed in older adults might increase the occurrence of less stable postural responses to perturbations. Insight in factors causing movement variability will advance our ability to study the origin of differences between groups and conditions.NEW & NOTEWORTHY Responses to perturbations of standing balance vary both within and between individuals. By combining experimental observations with computational modeling, we identified causes of observed kinematic variability in healthy young adults. First, we found that trial-by-trial differences in posture at perturbation onset explain most of the kinematic variability observed within subjects. Second, we found that differences in prioritizing effort versus stability explained differences in the postural response as well as differences in trial-by-trial variability across subjects.

A systematic review of upper extremity responses during reactive balance perturbations in aging

BACKGROUND

Balance responses to perturbations often involve the arms in an attempt to either restore balance or protect against impact. Although a majority of research has been dedicated to understanding age-related changes in lower limb balance responses, there is a growing body of evidence supporting age-related changes in arm responses. This systematic review aimed to summarize differences in arm responses between older and younger adults under conditions requiring counterbalancing, reaching to grasping, and protection against impact.

METHODS

Following a systematic review and critical appraisal of the literature, data regarding the arm response in studies comparing young and older adults was extracted. The resulting articles were also assessed for quality to determine risk of bias.

RESULTS

Fifteen high quality studies were identified. The majority of these studies reported delayed onsets in muscle activation, differences in arm movement strategies, delayed movement timing, increased impact forces, and greater grasp errors in older compared to young adults. These differences were also identified under varied visual and cognitive conditions.

CONCLUSIONS

The studies included in this review demonstrate age-related differences in arm responses regardless of the direction and nature of the perturbation. These differences could provide insight into developing more targeted rehabilitation and fall prevention strategies. More research is needed to assess whether the identified age-related differences are a necessary compensation or a contributory factor to balance impairments and fall risk in older adults.

Preserving Mobility in Older Adults with Physical Frailty and Sarcopenia: Opportunities, Challenges, and Recommendations for Physical Activity Interventions

One of the most widely conserved hallmarks of aging is a decline in functional capabilities. Mobility loss is particularly burdensome due to its association with negative health outcomes, loss of independence and disability, and the heavy impact on quality of life. Recently, a new condition, physical frailty and sarcopenia, has been proposed to define a critical stage in the disabling cascade. Physical frailty and sarcopenia are characterized by weakness, slowness, and reduced muscle mass, yet with preserved ability to move independently. One of the strategies that have shown some benefits in combatting mobility loss and its consequences for older adults is physical activity. Here, we describe the opportunities and challenges for the development of physical activity interventions in people with physical frailty and sarcopenia. The aim of this article is to review age-related physio(patho)logical changes that impact mobility in old age and to provide recommendations and procedures in accordance with the available literature.

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.

Age-Related Differences in Arm and Trunk Responses to First and Repeated Exposure to Laterally Induced Imbalances

The objective of this study was to examine age-related differences in arm and trunk responses during first and repeated step induced balance perturbations. Young and older adults received 10 trials of unpredictable lateral platform translations. Outcomes included maximum arm and trunk displacement within 1 s of perturbation and at first foot lift off (FFLO), arm and neck muscle activity as recorded using electromyography (EMG), initial step type, balance confidence, and percentage of harness-assisted trials. Compared to young adults, older adults demonstrated greater arm and trunk angular displacements during the first trial, which were present at FFLO and negatively associated with balance confidence. Unlike young adults, recovery steps in older adults were directed towards the fall with a narrowed base of support. Over repeated trials, rapid habituation of first-trial responses of bilateral arm and trunk displacement and EMG amplitude was demonstrated in young adults, but was absent or limited in older adults. Older adults also relied more on harness assistance during balance recovery. Exaggerated arm and trunk responses to sudden lateral balance perturbations in older adults appear to influence step type and balance recovery. Associations of these persistently amplified movements with an increased reliance on harness assistance suggest that training to reduce these deficits could have positive effects in older adults with and without neurological disorders.

Interactions among obesity and age-related effects on the gait pattern and muscle activity across the ankle joint

OBJECTIVE

The purposes of this study were to investigate the combined effects of age and obesity on gait and to analyze the relationship between age and obesity on ankle muscle activities during walking.

MATERIALS AND METHODS

4 groups; the young non-obese control group (CG, n = 50, age = 31.8 ± 4.5 years; BMI = 21.4 ± 2.2 kg/m²), the young obese group (OB, n = 30, age = 35.4 ± 4.1 years; BMI = 38.6 ± 3.5 kg/m²), the non-obese older adults group (OA, n = 20, age = 76.1 ± 3.5 years; BMI = 24.4 ± 1.1 kg/m²) and the obese older adults group (OBOA, n = 20, age = 79.6 ± 5.7 years; BMI = 35.5 ± 2.7 kg/m²) walked on an instrumented gait analysis treadmill at their preferred walking speed. Spatiotemporal parameters, walking cycle phases, Vertical ground reaction force (GRFv) and center of pressure (CoP) velocity were sampled from the treadmill software. Electromyography (EMG) activity of the gastrocnemius medialis (GM), the soleus (SOL) and tibialis anterior (TA) were also collected during the walking test. A forward stepwise multiple regression analysis was performed to determine if body weight or age could predict ankle muscle activities during the different walking cycle phases.

RESULTS

Compared to OB, OBOA walked with higher CoP velocity, shorter stride, spending more time in support phase (p < .05). These manifestations were associated with higher TA and SOL activities during the 1st double support (1st DS) and higher TA activity during the single support (SS) (p < .05). Compared to OA, OBOA walked with lower GRFv, shorter and wider stride and spend more time in SU (p < .05). Moreover, SOL, TA and GM activities of OBOA were higher compared to OAG during 1st DS, SS and 2nd Double support (2nd DS), respectively (p < .05). During the 1ST DS, the stepwise multiple regression revealed that age accounted for 87% of the variance of TA activity. The addition of age contributed a further 16% to explain the variance TA activity. During the SS, age accounted for 64% and 46% of the variance of SOL and TA activity respectively. The addition of the body weight added further 15% and 66% of the variation of SOL and TA activity respectively. During the 2nd DS, body weight accounted for 86% of the variance and the addition of the body weight added a further 17% to explain the high level of GM.

CONCLUSION

Age in obese adults and obesity in older adults should be considered separately to evaluate neuromuscular responses during walking and, subsequently, optimize the modality of treatment and rehabilitation processes in obese individuals in order to reduce and/or prevent the risk of falls.

Can Momentum-Based Control Predict Human Balance Recovery Strategies?

Human-like balance controllers are desired for wearable exoskeletons in order to enhance human-robot interaction. Momentum-based controllers (MBC) have been successfully applied in bipeds, however, it is unknown to what degree they are able to mimic human balance responses. In this paper, we investigated the ability of an MBC to generate human-like balance recovery strategies during stance, and compared the results to those obtained with a linear full-state feedback (FSF) law. We used experimental data consisting of balance recovery responses of nine healthy subjects to anteroposterior platform translations of three different amplitudes. The MBC was not able to mimic the combination of trunk, thigh and shank angle trajectories that humans generated to recover from a perturbation. Compared to the FSF, the MBC was better at tracking thigh angles and worse at tracking trunk angles, whereas both controllers performed similarly in tracking shank angles. Although the MBC predicted stable balance responses, the human-likeness of the simulated responses generally decreased with an increased perturbation magnitude. Specifically, the shifts from ankle to hip strategy generated by the MBC were not similar to the ones observed in the human data. Although the MBC was not superior to the FSF in predicting human-like balance, we consider the MBC to be more suitable for implementation in exoskeletons, because of its ability to handle constraints (e.g. ankle torque limits). Additionally, more research into the control of angular momentum and the implementation of constraints could eventually result in the generation of more human-like balance recovery strategies by the MBC.

Effects of somatosensory information provision to fingertips for mitigation of postural sway and promotion of muscle coactivation in an upright posture

This study was conducted to investigate the effects of somatosensory information provision to the fingertips using a device proposed by the authors for mitigation of postural sway and muscle co-contraction in an upright posture, which both increase with aging and result in inefficient postural control. In the research, center of pressure (CoP) fluctuation index values and muscle co-contraction for ankle joint movement were monitored with healthy young adults in a standing position. The results showed that the proposed device helped to reduce the root mean square (RMS) of the CoP and muscle co-contraction in the right ankle joint, thereby suggesting its potential for contribution to the assistance of efficient postural control.

Spike-induced ordering: Stochastic neural spikes provide immediate adaptability to the sensorimotor system

The functional advantages of using a stochastically spiking neural network (sSNN) instead of a nonspiking neural network (NS-NN) have remained largely unknown. We developed an architecture which enabled the parametric adjustment of the spikiness (i.e., impulsive dynamics and stochasticity) of the sSNN output and observed that stochastic spikes instantaneously induced the ordered motion of a dynamical system. We demonstrated the benefits of sSNNs using a musculoskeletal bipedal walker and, moreover, showed that the decrease in the spikiness of motor neuron output leads to a reduction in adaptability. Stochastic spikes may aid the adaptation of a biological system to sudden perturbations or environmental changes. Our architecture can easily be connected to the conventional NS-NN and may superimpose the on-site adaptability.

Most biological neurons exhibit stochastic and spiking action potentials. However, the benefits of stochastic spikes versus continuous signals other than noise tolerance and energy efficiency remain largely unknown. In this study, we provide an insight into the potential roles of stochastic spikes, which may be beneficial for producing on-site adaptability in biological sensorimotor agents. We developed a platform that enables parametric modulation of the stochastic and discontinuous output of a stochastically spiking neural network (sSNN) to the rate-coded smooth output. This platform was applied to a complex musculoskeletal–neural system of a bipedal walker, and we demonstrated how stochastic spikes may help improve on-site adaptability of a bipedal walker to slippery surfaces or perturbation of random external forces. We further applied our sSNN platform to more general and simple sensorimotor agents and demonstrated four basic functions provided by an sSNN: 1) synchronization to a natural frequency, 2) amplification of the resonant motion in a natural frequency, 3) basin enlargement of the behavioral goal state, and 4) rapid complexity reduction and regular motion pattern formation. We propose that the benefits of sSNNs are not limited to musculoskeletal dynamics. Indeed, a wide range of the stability and adaptability of biological systems may arise from stochastic spiking dynamics.

Reliability and validity of the mCTSIB dynamic platform test to assess balance in a population of older women living in the community

OBJECTIVES

Test the reliability and validity of the modified Clinical test of Sensory Interaction in Balance (mCTSIB) of the Balance Platform Biodex Balance System (BBS) in a female community dwelling population.

METHOD

100 women over 65 years community dwellers mean age 71.8 (SD±6, ranging from 65 to 91) years, were examined using the posturography modified Clinical test of Sensory Interaction on Balance (mCTSIB) protocol of the Biodex Balance system SD and the Greek Mini-Best Test (miniBESTest-GR) to assess concurrent validity, with 24 undergoing a second measurement after one week to test the reliability of the method.

RESULTS

The m-CTSIB-"Composite Score" test was significantly and positively correlated with the mini-BESTest-GR (r= -0.652, p<0.001) indicating good validity properties. The test-retest reliability was measured using the intra-class correlation coefficient (ICC) using a two-way mixed-effects absolute-agreement single-measurement model, among the two measurements of mCTSIB test (test-retest). No statistical difference was found between the two samples (N1=100, N2=24, t= -1.755, df=122, p=0.08). ICC estimates as 0.628 with 95% confident interval=0.31-0.82.

CONCLUSION

The mCTSIB test from the BBS has a moderate validity and reliability to evaluate balance in elderly women living in the community and can be used as a screening tool.

Effects of Whole-Body Vibration on Motor Impairments in Patients With Neurological Disorders: A Systematic Review

OBJECTIVE

This systematic review was conducted to examine the effects of whole-body vibration training on motor impairments among patients with neurological disorders and to investigate which the whole-body vibration training parameters induced improvement in motor impairments.

DESIGN

PubMed, SCOPUS, PEDro, REHABDATA, and Web of Science were searched for randomized controlled trials and pseudo-randomized controlled trials investigated the effect of whole-body vibration on motor impairments in patients with neurological disorders. The methodological quality was rated using the Cochrane Collaboration's tool.

RESULTS

Twenty studies were included in this systematic review. Four studies included patients with multiple sclerosis, cerebral palsy (n = 2), stroke (n = 9), Parkinson disease (n = 3), spinal cord injuries (n = 1), and spinocerebellar ataxia (n = 1). The results showed different evidence of benefits and nonbenefits for whole-body vibration training in motor impairments outcomes.

CONCLUSIONS

There is weak evidence for a positive effect of short-term whole-body vibration training on spasticity of lower limbs, mobility, balance, and postural control. Besides, positive effect of the long-term effect of whole-body vibration training on mobility in patients with neurological disorders. The optimal whole-body vibration training parameters in treating patients with neurological disorders remain unclear.

A new approach to characterize postural deficits in chemotherapy-induced peripheral neuropathy and to analyze postural adaptions after an exercise intervention

Background

Postural instability presents a common and disabling consequence of chemotherapy-induced peripheral neuropathy (CIPN). However, knowledge about postural behavior of CIPN patients is sparse. With this pilot study, we used a new approach to i) characterize postural impairments as compared to healthy subjects, ii) allocate possible abnormalities to a set of parameters describing sensorimotor function, and iii) evaluate the effects of a balance-based exercise intervention.

Methods

We analyzed spontaneous and externally perturbed postural control in eight CIPN patients before and after a balance-based exercise intervention by using a modification of an established postural control model. These findings were compared to 15 matched healthy subjects.

Results

Spontaneous sway amplitude and velocity were larger in CIPN patients compared to healthy subjects. CIPN patients’ reactions to external perturbations were smaller compared to healthy subjects, indicating that patients favor vestibular over proprioceptive sensory information. The balance-based exercise intervention up-weighted proprioceptive information in patients.

Conclusions

CIPN patients’ major postural deficit may relate to underuse of proprioceptive information that results in a less accurate posture control as spontaneous sway results indicate. The balance-based exercise intervention is able to partially correct for this abnormality. Our study contributes to a better understanding of postural impairments in CIPN patients and suggests an effective treatment strategy.

Trial registration

German Clinical Trials Register: DRKS00004340, retrospectively registered 04 January 2013.

Multi-sensory training and wrist fractures: a randomized, controlled trial

Background

Asymmetric vestibular function, decreased plantar sensation, postural control and functional ability have been associated with fall-related wrist fractures.

Objective

To investigate whether multi-sensory training (MST) improves postural control, vestibular function, foot sensation and functional ability among people with fall-related wrist fractures compared to wrist stabilization training (WT).

Methods

This was an assessor-blinded, randomized controlled trial. Ninety-eight participants, age 50–75 years, were randomized to MST or WT. Pre- and post-training measurements: Head Shake Test (HST), Video-Head Impulse Test (vHIT), Semmes–Weinstein Monofilaments (SWF), Biothesiometer (BT), Sensory Organization Test (SOT), 10-m Walk Test (10MWT), Five Times Sit to Stand Test (FTSTS), Activities-Specific Balance Confidence (ABC) and Dizziness Handicap Inventory Scales (DHI). The training period was 12 weeks, with six supervised sessions by a physical therapist and daily home exercises for both groups.

Results

There were significant endpoint differences in SOT (p = 0.01) between the two groups, in favor of the MST group, but no changes were seen in other outcome variables. Subgroup analysis with participants below normal baseline SOT composite scores indicated that the MST was more effective in improving 10MWT fast (p = 0.04), FTSTS (p = 0.04), SWF (p = 0.04) and SOT scores (p = 0.04) than the WT.

Conclusions

MST improves postural control among people with a fall-related wrist fracture. The results further suggest that the program is more effective for those with SOT balance scores below age-related norms.

Reactive Postural Responses to Continuous Yaw Perturbations in Healthy Humans: The Effect of Aging

Maintaining balance stability while turning in a quasi-static stance and/or in dynamic motion requires proper recovery mechanisms to manage sudden center-of-mass displacement. Furthermore, falls during turning are among the main concerns of community-dwelling elderly population. This study investigates the effect of aging on reactive postural responses to continuous yaw perturbations on a cohort of 10 young adults (mean age 28 ± 3 years old) and 10 older adults (mean age 61 ± 4 years old). Subjects underwent external continuous yaw perturbations provided by the RotoBit^1D platform. Different conditions of visual feedback (eyes opened and eyes closed) and perturbation intensity, i.e., sinusoidal rotations on the horizontal plane at different frequencies (0.2 Hz and 0.3 Hz), were applied. Kinematics of axial body segments was gathered using three inertial measurement units. In order to measure reactive postural responses, we measured body-absolute and joint absolute rotations, center-of-mass displacement, body sway, and inter-joint coordination. Older adults showed significant reduction in horizontal rotations of body segments and joints, as well as in center-of-mass displacement. Furthermore, older adults manifested a greater variability in reactive postural responses than younger adults. The abnormal reactive postural responses observed in older adults might contribute to the well-known age-related difficulty in dealing with balance control during turning.

Anticipatory coadaptation of ankle stiffness and sensorimotor gain for standing balance

External perturbation forces may compromise standing balance. The nervous system can intervene only after a delay greater than 100 ms, during which the body falls freely. With ageing, sensorimotor delays are prolonged, posing a critical threat to balance. We study a generic model of stabilisation with neural delays to understand how the organism should adapt to challenging balance conditions. The model suggests that ankle stiffness should be increased in anticipation of perturbations, for example by muscle co-contraction, so as to slow down body fall during the neural response delay. Increased ankle muscle co-contraction is indeed observed in young adults when standing in challenging balance conditions, and in older relative to young adults during normal stance. In parallel, the analysis of the model shows that increases in either stiffness or neural delay must be coordinated with decreases in spinal sensorimotor gains, otherwise the feedback itself becomes destabilizing. Accordingly, a decrease in spinal feedback is observed in challenging conditions, and with age-related increases in neural delay. These observations have been previously interpreted as indicating an increased reliance on cortical rather than spinal control of balance, despite the fact that cortical responses have a longer latency. Our analysis challenges this interpretation by showing that these observations are consistent with a functional coadaptation of spinal feedback gains to functional changes in stiffness and neural delay.

Being able to stand still can be difficult when faced with an unexpected push. It takes the nervous system more than a tenth of a second to respond to such a perturbation, and during this delay the body falls under the influence of its own weight. By co-contracting their ankle muscles in anticipation of a perturbation, subjects can increase their ankle stiffness, which slows down their fall during the neural delay. Young subjects indeed adopt this strategy when they need to remain particularly still (for example when they stand in front of a cliff). Older subjects adopt this strategy even during normal standing. We present a model of standing balance that shows that this postural strategy provides partial compensation for the increase in neural delays with ageing. According to our model, increasing ankle stiffness only improves balance if it is accompanied by a decrease in sensorimotor gain. This provides a novel and functional interpretation for the decrease in spinal feedback observed during ageing, and observed in young subjects when they stand in challenging balance conditions.

Effect of handrail height and age on the timing and speed of reach-to-grasp balance reactions during slope descent

We investigated the effect of handrail height on the timing and speed of reach-to-grasp balance reactions during slope descent, in fourteen younger and thirteen older adults. Participants walked along an 8° slope mounted to a robotic platform. Platform perturbations evoked reach-to-grasp reactions. Handrail height did not significantly affect handrail contact time (i.e., time from perturbation onset to handrail contact) or movement time (i.e., time from EMG latency to handrail contact). Participants appeared to compensate for the increased hand-handrail distance with higher rails via increased peak upward hand speed, and decreased vertical handrail overshoot. Aging was associated with slower EMG latency, reduced hand acceleration time, and increased hand deceleration time. Our findings suggest that participants were not disadvantaged by higher handrails from reach-to-grasp timing or speed perspectives, and that other metrics (e.g., center-of-mass control after grasping) may be more important when evaluating handrail designs for balance recovery.

Evaluation of balance recovery stability from unpredictable perturbations through the compensatory arm and leg movements (CALM) scale

Following unpredictable large-magnitude stance perturbations diverse patterns of arm and leg movements are performed to recover balance stability. Stability of these compensatory movements could be properly estimated through qualitative evaluation. In the present study, we present a scale for evaluation of compensatory arm and leg movements (CALM) in response to unpredictable displacements of the support base in the mediolateral direction. We tested the CALM scale for intra- and inter-rater reliability, correlation with kinematics of arm and leg movement amplitudes, and sensitivity to mode (rotation, translation and combined) and magnitude (velocity) of support base displacements, and also to perturbation-based balance training. Results showed significant intra- and inter-rater coefficients of agreement, ranging from moderate (0.46–0.53) for inter-rater reliability in the arm and global scores, to very high (0.87–0.99) for inter-rater leg scores and all intra-rater scores. Analysis showed significant correlation values between scale scores and the respective movement amplitudes both for arm and leg movements. Assessment of sensitivity revealed that the scale discriminated the responses between perturbation modes, platform velocities, in addition to higher balance recovery stability as a result of perturbation-based balance training. As a conclusion, the CALM scale was shown to provide adequate integrative evaluation of compensatory arm and leg movements for balance recovery stability after challenging stance perturbations, with potential application in fall risk prediction.

Body configuration as a predictor of centre of mass displacement in a forward reactive step

In balance perturbations that elicit backwards reactive steps, body configuration at stepping contact is related to likelihood of balance recovery. However, less is known about the relationship between body configuration (at stepping contact) and underlying centre of mass (COM) dynamics during dynamic perturbations requiring a forward reactive step. Accordingly, the primary objective of this study was to characterize the potential relationships between body configuration and COM displacement during simulated trips. Towards determining the robustness of these relationships, trips were simulated in both baseline and increased passive joint stiffness conditions. Sixteen healthy adults participated in this study. Trips were simulated using a tether release paradigm where participants were suddenly released, necessitating a forward step (onto a force plate) to recover their balance. Trials were performed in a baseline unconstrained condition, and in a 'corset' condition to increase passive stiffness of the trunk and hips. In all trials, whole body kinematics and kinetics were collected. Multiple linear regression models were run to assess the relationship of body angles to COM displacement in both the anteroposterior (AP) and mediolateral (ML) planes. Regression models showed a significant association of sagittal plane body configuration to both COM displacement at stepping contact and maximum COM displacement in the AP plane. Across models, the strongest predictor was the trail leg angle. Associations were stronger in the increased passive stiffness condition (average R² = 0.366) compared to the baseline condition (average R² = 0.266). Poor association of body configuration to COM displacement was found in the ML plane. The significant associations observed between body configuration and COM dynamics in simulated trips supports the potential downstream application of these models in identifying individuals with impaired balance control and increased fall risk.

Aging changes in protective balance and startle responses to sudden drop perturbations

This study investigated aging changes in protective balance and startle responses to sudden drop perturbations and their effect on landing impact forces (vertical ground reaction forces, vGRF) and balance stability. Twelve healthy older (6 men; mean age = 72.5 ± 2.32 yr, mean ± SE) and 12 younger adults (7 men; mean age = 28.09 ± 1.03 yr) stood atop a moveable platform and received externally triggered drop perturbations of the support surface. Electromyographic activity was recorded bilaterally over the sternocleidomastoid (SCM), middle deltoid, biceps brachii, vastus lateralis (VL), biceps femoris (BF), medial gastrocnemius (MG), and tibialis anterior (TA). Whole body kinematics were recorded with motion analysis. Stability in the anteroposterior direction was quantified using the margin of stability (MoS). Incidence of early onset of bilateral SCM activation within 120 ms after drop onset was present during the first-trial response (FTR) for all participants. Co-contraction indexes during FTRs between VL and BF as well as TA and MG were significantly greater in the older group (VL/BF by 26%, P < 0.05; TA/MG by 37%, P < 0.05). Reduced shoulder abduction between FTR and last-trial responses, indicative of habituation, was present across both groups. Significant age-related differences in landing strategy were present between groups, because older adults had greater trunk flexion (P < 0.05) and less knee flexion (P < 0.05) that resulted in greater peak vGRFs and decreased MoS compared with younger adults. These findings suggest age-associated abnormalities of delayed, exaggerated, and poorly habituated startle/postural FTRs are linked with greater landing impact force and diminished balance stabilization. NEW & NOTEWORTHY This study investigated the role of startle as a pathophysiological mechanism contributing to balance impairment in aging. We measured neuromotor responses as younger and older adults stood on a platform that dropped unexpectedly. Group differences in landing strategies indicated age-associated abnormalities of delayed, exaggerated, and poorly habituated startle/postural responses linked with a higher magnitude of impact force and decreased balance stabilization. The findings have implications for determining mechanisms contributing to falls and related injuries.

The influence of increased passive stiffness of the trunk and hips on balance control during reactive stepping

BACKGROUND

Age-related changes, which include increased trunk and hip stiffness, negatively influence postural balance. While previous studies suggest no net-effect of trunk and hip stiffness on initial trip-recovery responses, no study to date has examined potential effects during the dynamic restabilisation phase following foot contact.

RESEARCH QUESTION

Does increased trunk and hip stiffness, in isolation from other ageing effects, negatively influence balance during the restabilisation phase of reactive stepping.

METHODS

Balance perturbations were applied using a tether-release paradigm, which required participants to react with a single-forward step. Sixteen young adults completed two blocks of testing: a baseline and an increased stiffness (corset) condition. Whole-body kinematics were utilized to estimate spatial step parameters, center of mass (COM), COM incongruity (peak - final position) and time to restabilisation, in anteroposterior (AP) and mediolateral (ML) directions.

RESULTS

In the corset condition, peak COM displacement was increased in both directions (p < 0.024), which drove reductions in minimum margins of stability (p < 0.032) as step width and length were unchanged (p > 0.233). Increased passive stiffness also increased the magnitude and variability of peak shear ground reaction force, COM incongruity, and time to restabilisation in the ML (but not AP) direction (p < 0.027).

SIGNIFICANCE

In contrast to previous literature, increased stiffness resulted in greater peak COM displacement in both directions. Our results suggest increased trunk and hip stiffness have detrimental effects on dynamic stability following a reactive step, particularly in the ML direction. Observed increases in magnitude and variability of COM incongruity suggest the likelihood of a sufficiently large loss of ML stability - requiring additional steps - was increased by stiffening of the hips and trunk. The current findings suggest interventions aiming to mobilize the trunk and hips, in conjunction with strengthening, could improve balance and reduce the risk of falls.

Aging effects of motor prediction on protective balance and startle responses to sudden drop perturbations

This pilot study investigated the effect of age on the ability of motor prediction during self-triggered drop perturbations (SLF) to modulate startle-like first trial response (FTR) magnitude during externally-triggered (EXT) drop perturbations. Ten healthy older (71.4 ± 1.44 years) and younger adults (26.2 ± 1.63 years) stood atop a moveable platform and received blocks of twelve consecutive EXT and SLF drop perturbations. Following the last SLF trial, participants received an additional EXT trial spaced 20 min apart to assess retention (EXT RTN) of any modulation effects. Electromyographic (EMG) activity was recorded bilaterally over the sternocleidomastoid (SCM), vastus lateralis (VL), biceps femoris (BF), medial gastrocnemius (MG), and tibialis anterior (TA). Whole-body kinematics and kinetic data were recorded. Stability in the antero-posterior direction was quantified using the margin of stability (MoS). Compared with EXT trials, both groups reduced SCM peak amplitude responses during SLF and EXT RTN trials. VL/BF and TA/MG coactivation were reduced during SLF FTR compared to EXT FTR (p < 0.05) with reduced peak vertical ground reaction forces (vGRF) in both younger and older adults (p < 0.05). Older adults increased their MoS during SLF FTR compared to EXT FTR (p < 0.05). Both groups performed more eccentric work during SLF trials compared to EXT (p < 0.05). These findings indicate that abnormal startle effects with aging may interfere with balance recovery and increase risk of injury with external balance perturbations. Motor prediction may be used to acutely mitigate abnormal startle/postural responses with aging.