Cerebellar Transcranial Direct Current Stimulation Modulates Anticipatory Postural Adjustments in Healthy Adults

During forward swinging of the arm, the central nervous system must anticipate the effect of upraising upon the body. Little is known about the cerebellar network that coordinates these anticipatory postural adjustments (APAs). Stimulating different cerebellar regions with transcranial direct current stimulation (tDCS) and with different polarities modulated the APAs. We used surface electromyography (sEMG) to measure muscle activities in a bilateral rapid shoulder flexion task. The onset of APAs was altered after tDCS over the vermis, while the postural stability and the kinematics of arm raising were not affected. To our knowledge, this is the first human cerebellar-tDCS (c-tDCS) study to separate cerebellar involvement in core muscle APAs in bilateral rapid shoulder flexion. These data contribute to our understanding of the cerebellar network supporting APAs in healthy adults. Modulated APAs of the erector spinae by tDCS on the vermis may be related to altered cerebellar brain inhibition (CBI), suggesting the importance of the vermal-cerebral connections in APAs regulation.

Predicting slight freezing of gait in Parkinson's disease with anticipatory postural adjustments and limits of stability

INTRODUCTION

Gait initiation (GI) includes automatic and voluntary movements. However, research on their impact on the first step in patients with Parkinson's disease (PD) and their relationship to freezing of gait (FOG) is lacking. We examined the effects of automatic movements (anticipatory postural adjustments [APAs]) and voluntary movements (limits of stability [LOS]) on the first step (first-step duration and first-step range of motion), along with their early recognition and prediction of slight FOG.

METHODS

Twenty-three patients with PD and slight freezing (PD + FOG) and 25 non-freezing patients with PD (PD-FOG) were tested while off medications and compared with 24 healthy controls (HC). All participants completed a 7-m Stand and Walk Test (7 m SAW) and wore inertial sensors to quantify the APAs and first step. LOS was quantified by dynamic posturography in different directions using a pressure platform. We compared differences among all three groups, analysed correlations, and evaluated their predictive value for slight FOG.

RESULTS

In PD + FOG, APAs and LOS were worse than those in the PD-FOG and HC groups (p < 0.001), and the first step was worse than that in HC (p < 0.001). APAs were correlated mainly with the first-step duration. APAs and LOS were correlated with the first-step range of motion. APAs have been recognized as independent predictors of FOG, and their combination with LOS enhances predictive sensitivity.

CONCLUSION

APAs and LOS in patients with PD directly affect the first step during GI. In addition, the combination of APAs and LOS helped predict slight FOG.

Gait initiation in Parkinson's disease: comparison of timing and displacement during anticipatory postural adjustments as a function of motor severity and apathy in a large cohort

Introduction

Gait initiation is preceded by three anticipatory postural adjustment (APA) phases. In Parkinson's disease (PD) generated force, displacement and timing during APA differ from healthy controls. APA might be influenced by disease status, weight or emotion. It is unknown how motor severity, disease duration or presence of apathy influences APA timing and displacement.

Methods

We included 99 people with PD and 50 healthy controls (HC) to perform five gait initiation trials following an auditory cue. Force plates measured timing and center of pressure (CoP) displacement during APA phases.

Results

Time to gait initiation (tGI) was higher in the PD group (p < 0.001, t = 2.74, 95%CI (0.008, 0.066)). The first two APA phases (APA1 and APA2a) lasted longer in PD (respectively p < 0.001, t = 3.87, 95%CI (0.091, 0.28) and p < 0.001, t = 4.1, 95%CI (0.031, 0.091)). Mean CoP displacement, variability in timing and displacement did not differ. A multiple regression model was used to determine if clinical variables were related to gait initiation parameters. tGI was predicted by age (p < 0.001) and weight (p = 0.005). The duration of APA1 was predicted by weight (p = 0.006) and APA2a by age (p < 0.001). Variability in duration of the locomotor phase (LOC) was predicted by age (p < 0.001).

Conclusion

tGI and initial APA phases are longer in PD than in HC. There are no significant differences in variability of timing or displacement between the two groups. Gait initiation parameters are independent of disease duration, motor severity, medication usage or apathy in PD. Our findings suggest that cueing does not speed up gait initiation but reduces variability.

Gait initiation in multiple sclerosis patients with and without functional loss

BACKGROUND

Gait initiation (GI) is an important functional task related to balance and gait performance. In addition, it has predictive importance for falls and postural instability in patient with multiple sclerosis (MS). However, it is uncertain how GI is affected in patients in the early stage of MS (Expanded Disability Status Scale (EDSS) ≤3). In this study, it was aimed to investigate the anticipatory postural adjustments (APAs), posterior center of pressure (COPap) displacement, and spatiotemporal variability during GI in patients with and without functional loss in the early stage of MS.

METHODS

Forty-four participants (31 MS patients and 13 healthy subjects) involved in this prospective cross-sectional study were divided into three groups: Group-I: Patients without functional loss (EDSS 0 to 1.5) (n = 14), Group-II: Patients with functional loss (EDSS 2 to 3) (n = 17) and Group-III: Healthy subjects (n = 13). Electromyographic activity of the bilateral tibialis anterior (TA) and gastrocnemius medialis (GM) and COPap displacement were recorded during the postural phase of GI. Additionally, spatiotemporal parameters were recorded within the first three steps, and the coefficient of variation was calculated with 40 walks for variability.

RESULTS

There were significant differences in the Kruskal-Wallis tests of variables (p<0.05). Group-I demonstrated smaller APAs magnitudes in TA [stance (p = 0.01), swing (p = 0.01)], GM of swing limb (p<0.0001), and smaller COPap displacement (p<0.0001) compared to group-III. Group-II demonstrated smaller APAs magnitudes in all muscles (p<0.0001) compared to group-III and the smallest COPap displacement (p<0.0001). Group-I showed a significant increase in stride width variability compared to group-III (p = 0.01). Group-II showed a significant increase in several variabilities [first stride length (p<0.0001), second stride time (p<0.0001), first double support time (p<0.0001), stride width (p<0.0001)] compared to group-III.

CONCLUSION

Patients in the early stage of MS had impairment in both the postural and locomotor phases of GI with more obvious in the patients with functional loss. The results indicate that MS patients without functional loss have difficulty initiating gait. Although there is no functional loss, the patients have a risk of falls, postural instability, and gait impairment due to their inability to initiate gait effectively. As a result, rehabilitation is necessary even if there is no functional loss in patients with MS.

The novel Next Step test is a reliable measure of anticipatory postural adjustments made by children with cerebral palsy prior to taking a step

BACKGROUND

Children with cerebral palsy (CP) make smaller medio-lateral anticipatory postural adjustments (APAs) than typically developing peers when stepping forward to a medial target. They are also less accurate at reaching the stepping target. The Next Step test involves the biomechanical measurement of APAs and foot placement error. These may be useful outcome measures to evaluate dynamic balance in a clinical trial. The reliability of the measures must be assessed to establish their reliability as research tools.

RESEARCH QUESTION

What is the inter-rater and intra-rater reliability of stepping accuracy and measures of APAs made by children prior to taking a step?

METHODS

Typically developing (TD) (n = 14) or children with CP (n = 16) were recruited from local clinics. Children stepped to electro-luminescent targets placed medially and laterally to each foot. Stepping responses were measured using a force plate and 3D motion analysis of markers placed on the feet and pelvis. The APA was defined as the movement of the centre of pressure (COP) and the centre of mass (COM) estimated via pelvic markers, prior to lifting the lead leg. Stepping accuracy was defined as the absolute distance between the target and end foot position. Participants undertook two data collection sessions separated by at least one week. In session one, the test was measured by rater 1 who repeated this in session two, along with another data collection by a rater 2 or rater 3, after a rest period. Where data were normally distributed, they were assessed for inter-rater and intra-rater reliability using an intra-class correlation coefficient (ICC) and Bland-Altman plots. The standard error of measurement was calculated to determine the minimum difference needed to detect true change.

RESULTS

There was no between-group differences in group characteristics (age, weight, height) or in stepping velocity. We found good to excellent reliability when measuring the amplitude and velocity of medio-lateral APAs (ICC range 0.73-0.89). The reliability of antero-posterior APAs was more variable (ICC range 0.08-0.92). The minimum difference to detect a true change for peak medio-lateral motion of COP ranges from 23.7 mm to 29.6 mm and for peak velocity of medio-lateral COM estimate 41-61.9 mm. Stepping accuracy was not normally distributed.

SIGNIFICANCE

The Next Step test is a reliable measure of dynamic balance. The peak medio-lateral motion of the COP and medio-lateral velocity of the COM estimate are reliable when measured during a constrained stepping task in ambulant children with cerebral palsy.

Postural adjustments to self-triggered perturbations under conditions of changes in body orientation

We studied anticipatory and compensatory postural adjustments (APAs and CPAs) associated with self-triggered postural perturbations in conditions with changes in the initial body orientation. In particular, we were testing hypotheses on adjustments in the reciprocal and coactivation commands, role of proximal vs. distal muscles, and correlations between changes in indices of APAs and CPAs. Healthy young participants stood on a board with full support or reduced support area and held a standard load in the extended arms. They released the load in a self-paced manned with a standard small-amplitude arm movement. Electromyograms of 12 muscles were recorded and used to compute reciprocal and coactivation indices between three muscle pairs on both sides of the body. The subject's body was oriented toward one of three targets: straight ahead, 60° to the left, and 60° to the right. Body orientation has stronger effects on proximal muscle pairs compared to distal muscles. It led to more consistent changes in the reciprocal command compared to the coactivation command. Indices of APAs and CPAs showed positive correlations across conditions. We conclude that the earlier suggested hierarchical relations between the reciprocal and coactivation command could be task-specific. Predominance of negative or positive correlations between APA and CPA indices could also be task-specific.

Comparison of inertial records during anticipatory postural adjustments obtained with devices of different masses

Background

Step initiation involves anticipatory postural adjustments (APAs) that can be measured using inertial measurement units (IMUs) such as accelerometers. However, previous research has shown heterogeneity in terms of the population studied, sensors used, and methods employed. Validity against gold standard measurements was only found in some studies, and the weight of the sensors varied from 10 to 110 g. The weight of the device is a crucial factor to consider when assessing APAs, as APAs exhibit significantly lower magnitudes and are characterized by discrete oscillations in acceleration paths.

Objective

This study aims to validate the performance of a commercially available ultra-light sensor weighing only 5.6 g compared to a 168-g smartphone for measuring APAs during step initiation, using a video capture kinematics system as the gold standard. The hypothesis is that APA oscillation measurements obtained with the ultra-light sensor will exhibit greater similarity to those acquired using video capture than those obtained using a smartphone.

Materials and Methods

Twenty subjects were evaluated using a commercial lightweight MetaMotionC accelerometer, a smartphone and a system of cameras-kinematics with a reflective marker on lumbar vertebrae. The subjects initiated 10 trials of gait after a randomized command from the experimenter and APA variables were extracted: APAonset, APAamp, PEAKtime. A repeated measures ANOVA with post-hoc test analyzed the effect of device on APA measurements. Bland-Altman plots were used to evaluate agreement between MetaMotionC, smartphone, and kinematics measurements. Pearson's correlation coefficients were used to assess device correlation. Percentage error was calculated for each inertial sensor against kinematics. A paired Student's t-test compared th devices percentage error.

Results

The study found no significant difference in temporal variables APAonset and PEAKtime between MetaMotionC, smartphone, and kinematic instruments, but a significant difference for variable APAamp, with MetaMotionC yielding smaller measurements. The MetaMotionC had a near-perfect correlation with kinematic data in APAonset and APAamp, while the smartphone had a very large correlation in APAamp and a near-perfect correlation in APAonset and PEAKtime. Bland-Altman plots showed non-significant bias between smartphone and kinematics for all variables, while there was a significant bias between MetaMotionC and kinematics for APAamp. The percentage of relative error was not significantly different between the smartphone and MetaMotionC.

Conclusions

The temporal analysis can be assessed using ultralight sensors and smartphones, as MetaMotionC and smartphone-based measurements have been found to be valid compared to kinematics. However, caution should be exercised when using ultralight sensors for amplitude measurements, as additional research is necessary to determine their effectiveness in this regard.

"Posture first": Interaction between posture and locomotion in people with low back pain during unexpectedly cued modification of gait initiation motor command

The ability to adapt anticipatory postural adjustments (APAs) in response to perturbations during single-joint movements is altered in people with chronic low back pain (LBP), but a comprehensive analysis during functional motor tasks is still missing. This study aimed to compare APAs and stepping characteristics during gait initiation between people with LBP and healthy controls, both in normal (without cue occurrence) condition and when an unexpected visual cue required to switch the stepping limb. Fourteen individuals with LPB and 10 healthy controls performed gait initiation in normal and switch conditions. The postural responses were evaluated through the analysis of center of pressure, propulsive ground reaction forces, trunk and whole-body kinematics, and activation onsets of leg and back muscles. During normal gait initiation, participants with LBP exhibited similar APAs and stepping characteristics to healthy controls. In the switch condition, individuals with LBP were characterized by greater mediolateral postural stability but decreased forward body motion and propulsion before stepping. The thorax motion was associated with forward propulsion parameters in both task conditions in people with LBP but not healthy controls. No between-group differences were found in muscle activation onsets. The results suggest that postural stability is prioritized over forward locomotion in individuals with LBP. Furthermore, the condition-invariant coupling between thorax and whole-body forward propulsion in LBP suggests an adaptation in the functional use of the thorax within the postural strategy, even in poor balance conditions.

Cognitive and emotional factors influence specific domains of postural control in individuals with moderate-to-severe Parkinson's disease

INTRODUCTION

Cognition and emotional state are domains that highly interfere with postural control in individuals with Parkinson's disease (PD). This study aims to find associations between executive function, anxiety, depression, and reactive and anticipatory postural control domains in individuals with moderate-to-severe Parkinson's disease.

METHODS

In this study, 34 individuals with PD while on medication were thoroughly assessed for postural control in perturbed, quiet standing and stepping. We performed multiple linear stepwise regressions using postural variables as dependent and cognitive/emotional as independent variables.

RESULTS

The results showed that cognitive flexibility explained 23 % of anticipatory postural adjustments (APA) duration, inhibitory control explained 42 % of instability on a malleable surface, anxiety explained 21 % of APA amplitude, and 38 % of reactive postural response amplitude.

CONCLUSION

Our results highlight the impact of emotional and cognitive states on particular domains of postural control in individuals with PD while on medication. These results may have significant implications for future treatments, mainly considering the predictors for postural control domains, which were consistent with the assumption that impairments in affective and executive domains underlie posture. As we have shown that cognitive and emotional states influence postural control domains in individuals with PD, this should be taken into account in rehabilitation protocols.

Older adults can rely on an auditory cue to generate anticipatory postural adjustments prior to an external perturbation

To minimize the potential postural disturbance induced by predictable external perturbations, humans generate anticipatory postural adjustments (APAs) using visual information about a perturbation. However, it is unknown whether older adults can generate APAs relying on auditory information. Ten older adults received external perturbations (a) with visual information but no auditory information available, (b) without neither visual nor auditory information, (c) with both visual and auditory information available, and (d) participated in training with only auditory information available. In addition, they were tested again after 1 week of washout period. Electromyography activities of eight leg and trunk muscles and ground reaction forces were recorded and analyzed during the anticipatory and compensatory phases. Outcome measures included the latencies and integrals of muscle activities, and center-of-pressure displacements. After a short period of training, participants were able to rely on the auditory cue only to generate APAs close to that when the visual information was available. In addition, after 1 week of washout period, they were able to partially retain the skill to rely on auditory cues to generate APAs. The outcome provides a foundation for future studies focusing on utilizing auditory cues to optimize postural control in individuals who have balance or vision deficit.

Perceptual distortion in virtual reality and its impact on dynamic postural control

BACKGROUND

Voluntary movement such as lifting a foot in preparation to stepping acts as a self-initiated perturbation that disturbs postural equilibrium. To maintain and restore equilibrium, humans utilize early, anticipatory, and compensatory postural adjustments. Despite technological progress in accessible virtual reality (VR) devices, little is known on the usage of VR in control and maintenance of balance while standing.

RESEARCH QUESTION

How does VR modulate early, anticipatory, and compensatory postural adjustments during a dynamic task of leg lifting while avoiding an obstacle?

METHODS

First, the postural adjustments in a single-leg obstacle avoidance were compared between real and VR settings, where a statistical reanalysis was performed with data subsets that minimize the difference of foot elevation speed. Second, the effect of simple foot elevation was examined to identify the fundamental nature of leg lifting motion as a self-initiated perturbation. Lastly, perceptual distortion in VR was assessed by evaluating how the spatial scale of the virtual scene used in the single-leg obstacle avoidance experiment was recognized by participants.

RESULTS

The VR setting reduced the activities of lower leg muscles on the supporting side not only in the compensatory phase but also in the preparatory early and anticipatory phases. On the other hand, simple foot elevation resulted in a significant increase of muscle activities with lifting height only found in the compensatory phase. Furthermore, it is suggested that the VR induced perceptual distortion in estimating the sizes of the virtual objects.

SIGNIFICANCE

The findings provide more definitive evidence that VR presentation modulates the components of postural adjustments for maintaining upright stance while being perturbed. One of the potential psychophysical factors is perceptual distortion in VR, and this provides critical information for further development of VR based training system.

Effects of unilateral and bilateral lower extremity fatiguing exercises on postural control during quiet stance and self-initiated perturbation

Postural control can be more difficult during muscle fatigue. Anticipatory postural adjustments (APAs) and compensatory postural adjustments (CPAs) are the two main postural strategies controlled by the central nervous system. Unchanged or early anticipatory onset together with altered activation magnitude during the APAs phase is observed in the trunk and thigh muscles following unilateral and bilateral fatiguing exercises. Thus far, no studies have compared the effect of such exercises on APAs and CPAs. This study compared the effects of these exercises performed at the same relative workload on center of pressure (COP)-based postural stability measures during quiet stance and electromyography (EMG)-based APAs and CPAs during self-initiated perturbation. Fifteen young male subjects completed the two separated fatiguing sessions; 50% of maximal voluntary contraction force obtained from the unilateral (dominant) and bilateral legs with five sets of 20 times lower limb exercise was respectively applied as unilateral and bilateral fatiguing protocols. Spatio-temporal COP parameters (sway velocity, total displacement, and envelope area) were used to evaluate postural stability, and spectral analysis was performed to estimate the distributions in COP power spectrum. EMG activities of transversus abdominis/internal oblique (TrA/IO) and lumbar multifidus (LMF) were recorded and analyzed during the APAs and CPAs phases. Increased sway velocity and total displacements occurred following both unilateral and bilateral fatiguing exercises; however, the envelope area was not affected. Further, early anticipatory onset of TrA/IO was found after bilateral than after unilateral fatiguing exercise. Co-activation index of the TrA/IO-LMF muscle pair during the CPAs phase increased following both fatiguing sessions. The results partly confirmed previously reported fatigue effects induced by unilateral and bilateral exercises on postural stability. It was observed that APAs onsets were altered differently following a unilateral or bilateral fatiguing exercise, whereas the alterations of CPAs were independent of fatigue conditions. Repetitive unilateral or bilateral fatiguing exercises in patients or athletes may differently alter the anticipatory component of postural control.

The effect of extensible and non-extensible lumbosacral orthoses on anticipatory postural adjustments in participants with low back pain and healthy controls

BACKGROUND

Wearing a lumbosacral orthosis (LSO) is known to influence spine mechanics, but less is known about how LSOs affect motor control. Whether the use of a LSO can negatively affect motor control of the lumbar spine is still under debate.

OBJECTIVE

The current study examined the immediate effects of two flexible LSOs (extensible and non-extensible) on the anticipatory postural adjustments that prepare the spine for a predictable perturbation.

DESIGN

A comparative study using a repeated measures design in a laboratory setting.

METHODS

Healthy controls (n = 20) and participants with low back pain (n = 40) performed a rapid arm flexion/extension cycle with and without these LSOs. The latency between the activations of the shoulder and different back (iliocostalis lumborum) and abdominal (rectus abdominis, internal and external obliques) muscles, as measured with surface electromyography, was used as the outcome.

RESULTS

The effects, which were comparable between groups and between LSOs, were mixed, with some muscles showing significantly (p ˂ 0.05) earlier activation and others showing delayed activation with the use of a LSO, relative to the control condition. The corresponding effect sizes were low to average (Hedges's g range: 0.17-0.48).

CONCLUSIONS

These findings suggest a change in the motor program before task initiation, which might be generalizable to other activities of daily living or work. However, none of the effects were large, making it difficult to provide clear conclusions with regard to their clinical relevance. It remains to be tested whether these immediate adaptations in motor planning can induce long term detrimental effects to the control of lumbar stability.

Role of the peripheral nervous system for an appropriate postural preparation during gait initiation in patients with a chronic inflammatory demyelinating polyneuropathy: A pilot study

BACKGROUND

Gait initiation is an automatized motor program that is preceded by anticipatory postural adjustments (APAs). During attentional tasks, these APAs can be modulated, producing multiple APAs. However, the role of the peripheral nervous system in the regulation of these APAs is unknown.

RESEARCH QUESTION

The objective of our study was to investigate whether APAs are also regulated by peripheral nervous afferents.

METHODS

We assessed 21 patients suffering from chronic inflammatory demyelinating neuropathy and 20 healthy controls. Participants initiated gait with the right or left leg either freely (in the standard condition) or according to a visual trigger (i.e., the select condition). Kinetic and kinematic parameters of APAs and step initiation were recorded.

RESULTS

The select condition was related to a higher rate of multiple APAs compared to the standard condition, and was more attention-consuming in both groups. The group with a neuropathy showed longer APAs than the control group, associated with a longer time to recover from multiple APAs. Consequently, the step execution time was delayed in patients with a peripheral neuropathy.

SIGNIFICANCE

The impairment of the peripheral nervous system is therefore responsible for an alteration of the mechanisms underlying the recovery from multiple APAs during gait initiation. Our results are in favor of a role of proprioceptive afferents in the early peripheral regulation of motor errors. Further study on gait initiation in peripheral nervous disease could be helpful to better explore sensory-motor coupling in tasks requiring balance control.

Preserved flexibility of dynamic postural control in individuals with Parkinson's disease

BACKGROUND

Continuous oscillation of the support base requires anticipatory and reactive postural adjustments to maintain a stable balance. In this context, postural control flexibility or the ability to adjust balance mechanisms following the requirements of the environment is needed to counterbalance the predictable, continuous perturbation of body balance. Considering the inflexibility of postural responses in individuals with Parkinson's disease (PD), maintaining stability in the support base's continuous oscillations may be challenging. Varying the frequency of platform oscillation is an exciting approach to assess the interactions between reactive and anticipatory adjustments.

RESEARCH QUESTION

This study aimed to analyze postural responses of individuals with PD on an oscillatory support base across different frequencies.

METHODS

Thirty participants with moderate PD diagnosis (M = 64.47 years, SD = 8.59; Hoehn and Yahr scale 3) and fifteen healthy age-matched controls (M = 65.8 years, SD = 4.2) were tested. Subjects maintained a dynamic balance on a platform oscillating in sinusoidal translations. Four oscillation frequencies were evaluated in different trials that ranged from 0.2 to 0.8 Hz in steps of 0.2 Hz.

RESULTS

Analysis showed similar performance between PD and healthy participants, with modulation of amplitudes of head displacement, center of pressure, center of mass and feet-head coordination to platform oscillation frequency.

DISCUSSION

Our findings suggest a preserved ability of individuals with PD to dynamically control body balance on a support base with predictable oscillatory translations.

Biomechanical characteristics of handstand walking initiation

BACKGROUND

The initiation in human locomotion is defined as the transition between upright stance and steady-state gait. While past literature abundantly investigated the initiation in bipedal gait, the initiation of handstand walking remains unexplored.

RESEARCH QUESTION

The current study aims to characterise the centre of pressure (CoP) and centre of mass (CoM) trajectory of handstand walking initiation as well as the spatiotemporal and kinematic parameters and balance strategy of this task. Also, the study examined the CoP trajectory similarity within- and between-participants using a coefficient of multiple correlation analysis.

METHODS

Nineteen gymnasts took part in this study. Handstand walking initiation trials were recorded using force plates and a stereophotogrammetric system. CoM and CoP trajectories were analysed during the Baseline, Preparation and Execution phases of the motor task.

RESULTS

We found that to successfully perform the handstand walking initiation, a shift of the CoM forward and towards the stance hand is required as a result of a lateral and posterior CoP shift. All participants performed a similar CoP pattern in the mediolateral direction, whereas two anteroposterior CoP displacement strategies were identified across participants based on different timing execution of posterior CoP shift. While CoP and CoM kinematic differences were identified during the Preparation Phase due to the adopted strategy, no significant difference was found in the Execution Phase for the spatiotemporal and kinematic characteristics.

SIGNIFICANCE

A better understanding of the required CoP/CoM patterns and balance control provides the basis for further neuromechanics research on the topic and could contribute to individualise training protocols to improve the learning of the task.

Quantitative analysis of voluntary movement and anticipatory postural adjustments: a functional approach

Daily living activities and tasks like standing forward reaching present complex Anticipatory Postural Adjustments (APAs), and an objective, repeatable, subject- and task-dependent procedure to detect Voluntary Movements (VM) and APAs onsets is still missing. This paper proposes a new approach to the VMs study, based on a functional mechanical interpretation of the movement performing, which allows defining kinematic and dynamic APAs. A protocol for the identification of VMs and APAs onsets in the reaching movement is presented. Acquired data on 9 healthy young subjects enable a preliminary validation of this method suitability as support for an objective quantification of APAs.

Effect of jaw clenching on postural adjustments to a predictable external perturbation

This study investigated the effect of jaw clenching on anticipatory postural adjustments (APAs) and reactive postural adjustments (RPAs). Eight healthy adults were exposed to predictable external perturbations with and without submaximal jaw clenching. We recorded the three-dimensional body kinematics, ground reaction forces, and electromyography (EMG) of 12 muscles. EMG onset, integrated EMG, and positions of the center-of-mass (COM) and center-of-pressure (COP) during the anticipatory and reactive phases of postural adjustments were computed and compared for the jaw clenching and control conditions. Jaw clenching resulted in an earlier onset of APAs and a greater amplitude of trunk and lower limb EMG activities in the APAs and RPAs. There were no differences in the COM and COP displacements after the perturbation. Jaw clenching is effective for enhancing APAs and RPAs, although it may not increase postural stability following predictable external perturbation in healthy adults.

Development of temporal and spatial characteristics of anticipatory postural adjustments during gait initiation in children aged 3-10 years

This study aimed to analyze the development of direction specificities of temporal and spatial control and the coordination pattern of anticipatory postural adjustment (APA) along the anteroposterior (AP) and mediolateral (ML) directions during gait initiation (GI) in children aged 3-10 years. This study included 72 healthy children aged 3-10 years and 14 young adults. The child population was divided into four groups by age: 3-4, 5-6, 7-8, and 9-10 years. The GI task included GI using the dominant limb. The peak center of feet pressure (COP) shifts during APAs (APA_peak), initiation time of COP shifts (APA_onset), and the COP vectors in the horizontal plane were calculated to evaluate the direction specificity of spatial, temporal, and coordination control, respectively. A difference in direction specificity development was found for the APA_peak. The APA_peak in the mediolateral axis, but not in the anteroposterior axis, was significantly higher in the 7-8 years age group than in other groups. Although APA_onset was not found for direction specificity, a significant difference between the adult and children groups (5-6 years, 7-8 years, and 9-10 years) was observed in the direction of the COP vector. In conclusion, the developmental process of the spatial, temporal, and coordination control of APAs during GI varied with age. Furthermore, the spatial control and coordination pattern of APAs was found to be direction specific. All components of APAs, namely temporal and spatial control, coordination pattern, and direction specificities, should be analyzed to capture the developmental process of anticipatory postural control.

Effect of local and general fatiguing exercises on disturbed and static postural control

This study compared the effect of local and general fatiguing exercise on disturbed and static postural control performances. Surface electromyography and center of pressure signals were respectively recorded during self-initiated perturbation test and static postural stability test from 7 young male subjects. Local fatiguing exercise was performed using intermittent isometric knee extensions at the level of 40% of maximal voluntary torques. General fatiguing exercise was implemented with rowing ergometer at a speed of 200 ± 5 m/min. Results of disturbed postural tests showed no significant change of anticipatory postural adjustment (APAs) organizations in individual muscles following both fatiguing exercises, but observed larger APAs coactivations in trunk and dorsal muscle pairs following local than general fatiguing exercise, and larger compensatory postural adjustments (CPAs) coactivation in dorsal muscle pair after both fatiguing exercises. In addition, the results of static postural tests indicated efficient static postural stability accompanying the down-weighting of visual input and the up-weighting of vestibular/somatosensory component following both fatiguing exercises. These findings evidenced a general compensation in the central nervous system in response to the neuromuscular deficiencies induced by local fatiguing exercise and put forward the function of sensory recalibration in maintaining postural stability under fatigue conditions.

Effects of a lower limb muscular fatigue on posture-movement interaction during a lower limb pointing task

PURPOSE

The aim of the present study was to investigate the effects of muscular fatigue on the interaction between posture and movement during a lower limb pointing task.

METHODS

Participants (n = 16), aged 18-30 years, kicked a ball toward a target in four conditions of fatigue: No muscular fatigue (NF), fatigue in the kicking (FM) or postural limb (FP) alone, and fatigue in both limbs (FMP). The mean amplitude and speed of the centre of foot pressure (CoP) and centre of mass (CoM) displacements were estimated through a force platform and an optoelectronic system, respectively. In addition, surface electromyography (EMG) of the biceps femoris, rectus femoris, medial gastrocnemius, and peroneus longus was recorded to investigate the anticipatory postural adjustments (APAs).

RESULTS

Muscular fatigue yielded a decreased kicking accuracy (p < 0.001) and an increased time to perform the movement (p < 0.001), mainly during the backswing motion. In addition, significant increases in the mean amplitude and speed of the CoP and CoM displacement were found in the anteroposterior (AP) and mediolateral (ML) axes (ps < 0.001), especially when both limbs were fatigued. The EMG analysis confirmed that fatigue modified the way APAs were generated. During fatigue, postural muscle activity increased, but was delayed with respect to movement onset (ps < 0.001). This pattern of response was more consistent when both limbs were fatigued (p < 0.001).

CONCLUSION

The present results suggested an additive effect of fatigue and a functional adaptation and subsequent decrease in the overall variability of APAs, indicating that postural and motor processes are interdependent.

Effect of predictability of the magnitude of a perturbation on anticipatory and compensatory postural adjustments

Balance maintenance in response to a perturbation could be affected by the predictability of the magnitude of the body disturbance. We investigated anticipatory (APAs) and compensatory (CPAs) postural adjustments in response to perturbations of predictable and unpredictable magnitudes. Twenty young adults received series of perturbations of small or large magnitudes the order of which was varied. Electromyographic activity of six leg and trunk muscles and displacements of the center-of-pressure (COP) were recorded. The muscle onset time, integrals of muscle activity, and COP displacements in the anterior-posterior direction were analyzed during the APA and CPA phases. The results indicated that when the participants were exposed to the repeated perturbation magnitude, it became predictable and they generated APAs more precisely according to the magnitudes of the perturbation. Moreover, when the magnitude of perturbation changed unpredictably, the participants overestimated or underestimated the magnitudes of the perturbation, as they generated APAs based on their prior experience of dealing with the perturbation. The optimal adjustment of APAs occurred after five trials of repeated perturbations. The findings imply that the process of APAs and CPAs generation depends on the accuracy of the predictability of perturbation magnitudes.

Gait initiation in progressive supranuclear palsy: brain metabolic correlates

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Dysfunctional gait initiation in progressive supranuclear palsy relates to poor feedforward motor control.

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Hypometabolism of the caudate nucleus impairs programming of anticipatory postural adjustments.

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Thalamic hypometabolism correlates with the center of mass kinematic resultants of anticipatory postural adjustments.

The initiation of gait is a highly challenging task for the balance control system, and can be used to investigate the neural control of upright posture maintenance during whole-body movement. Gait initiation is a centrally-mediated motion achieved in a principled, controlled manner, including predictive mechanisms (anticipatory postural adjustments, APA) that destabilize the antigravitary postural set of body segments for the execution of functionally-optimized stepping. Progressive supranuclear palsy (PSP) is a neurodegenerative disease characterized by early impairment of balance and frequent falls. The neural correlates of postural imbalance and falls in PSP are largely unknown. We biomechanically assessed the APA at gait initiation (imbalance, unloading, and stepping phases) of 26 patients with PSP and 14 age-matched healthy controls. Fourteen of 26 enrolled patients were able to perform valid gait initiation trials. The influence of anthropometric and base-of-support measurements on the biomechanical outcome variables was assessed and removed. Biomechanical data were correlated with clinical findings and, in 11 patients, with brain metabolic abnormalities measured using positron emission tomography and 2-deoxy-2-[18F]fluoro-D-glucose. Patients with PSP showed impaired modulation of the center of pressure displacement for a proper setting of the center of mass momentum and subsequent efficient stepping. Biomechanical measurements correlated with “Limb motor” and “Gait and midline” subscores of the Progressive Supranuclear Palsy Rating Scale. Decreased regional glucose uptake in the caudate nucleus correlated with impaired APA programming. Hypometabolism of the caudate nucleus, supplementary motor area, cingulate cortex, thalamus, and midbrain was associated with specific biomechanical resultants of APA. Our findings show that postural instability at gait initiation in patients with PSP correlates with deficient APA production, and is associated with multiple and distinctive dysfunctioning of different areas of the supraspinal locomotor network. Objective biomechanical measures can help to understand fall-related pathophysiological mechanisms and to better monitor disease progression and new interventions.

Center of pressure excursion and muscle activation during gait initiation in individuals with and without chronic ankle instability

The aim of the current study was to determine differences in center of pressure (COP)excursion and muscle activation during gait initiation (GI) in those with and without chronic ankle instability (CAI). Thirty-four participants, 17 per group, volunteered to participate. Participants were asked to stand barefoot on a force plate before initiating gait upon hearing an auditory cue. Reaction time, anticipatory postural adjustment phase time, as well as normalized peak COP excursion during the anticipatory postural adjustment phase was calculated. Response time of Soleus and Tibialis Anterior muscles were concurrently recorded via electromyography. The results demonstrate a longer reaction time and shorter anticipatory postural adjustment phase time in the CAI group (p < 0.05). No significant between group differences in peak normalized COP excursion were noted (p > 0.05). Muscle onset patterns differed between groups as those with CAI demonstrated earlier Soleus activation compared to the control group (p < 0.05). The results suggest that those with CAI have an altered GI motor control strategy as evidenced by reduced or absent Soleus muscle inhibition during APA phase of GI relative to controls. The APA phase is controlled by the secondary motor area, therefore, the presence of motor control alterations in CAI patients may be due to a supra-spinal alterations.

Somatosensory cortical facilitation during step preparation restored by an improved body representation in obese patients

BACKGROUND

The anticipatory postural adjustments (APA) associated with step initiation are impaired in obese patients (e.g. longer duration, greater lateral center of pressure excursion). This could arise from the known altered internal representation of the body in obese individuals as this representation is crucial for enhancing the processing of foot cutaneous inputs prior to step initiation and for setting the APA.

RESEARCH QUESTION

The purpose of the study was to examine if the processing of foot cutaneous inputs and the preparation of the APA when planning a step are impaired in obese patients due to their damaged body internal representation (BIR). We also investigated whether these sensorimotor processes will be restored after a 15-day intervention program composed of motor and cognitive activities engaging the BIR without aiming weight loss.

METHODS

We compared, prior to (D1) and after (D15) the program, the amplitude of the cortical response evoked by foot cutaneous stimulation (SEP) occurring either during quiet standing or during the planning of a step in 18 obese patients (mean body mass index, BMI: 35). The APA were analyzed by measuring the amplitude and latency of the lateral force exerted on the ground.

RESULTS AND SIGNIFICANCE

The SEP amplitude was not significantly different between the standing and stepping tasks at D1, but increased in the stepping task at D15. This enhanced sensory processing was associated with an increased activation of the posterior parietal cortex, suggesting a stronger involvement of the body representation during the planning of the stepping movement after the program. These cortical changes could have contributed to the changes in the temporal dimension of the APA observed at D15. These results suggest that programs targeting different dimensions of the BIR could be beneficial in improving the dynamic balance in obesity.

Postural control in preparation to a step during support surface perturbation

We investigated changes in the anticipatory activity of the leg and trunk muscles in preparation for a step during support surface perturbation. Eight healthy subjects performed stepping tasks under three conditions: normal, forward perturbation, and backward perturbation. R and C indices were calculated for the reciprocal and co-activation patterns of muscle pairs within the time intervals typical of anticipatory postural adjustments. When the support surface perturbation occurred, anticipatory muscle activations were predominantly in the C indices in the leg muscles. Significant differences in the maximum displacement of the centre of pressure were seen between conditions (FS vs NS; BS vs FS). The results suggest that activation of the leg muscles rather than the trunk muscles was modified to ensure equilibrium for taking a step in response to support surface perturbation.

What is the influence of severity levels of knee osteoarthritis on gait initiation?

BACKGROUND

The anticipatory postural adjustments required for gait initiation have not yet been investigated in older adults with different levels of severity of knee osteoarthritis. This study aimed to evaluate the anticipatory postural adjustments adopted by older adults with different severity levels of knee osteoarthritis during gait initiation.

METHODS

Sixty-seven older adults with knee osteoarthritis (mild, moderate, and severe levels) and 11 healthy older adults control were evaluated bilaterally with a force plate to analyze gait initiation. The center of pressure trajectory during gait initiation was divided into four phases: three anticipatory postural adjustments, and a locomotor phase. The length, duration, and velocity of each phase were calculated.

FINDINGS

The results showed that during the right and left limbs swing forward, the severe and moderate knee osteoarthritis groups presented a significant reduction in the length of anticipatory postural adjustment phases, locomotion, duration, and velocity (P < 0.05). The severe knee osteoarthritis group presented a significantly higher body mass index (P < 0.003) than the other groups. However, just the healthy group presented a correlation between body mass index and anticipatory postural adjustments.

INTERPRETATION

Our results demonstrated that older adults with severe and moderate levels of knee osteoarthritis adopt longer lasting and slower anticipatory postural adjustment phases, lower locomotion, and lower center of pressure displacement during gait initiation, suggesting that this population has adaptive strategy in performing gait initiation, which is significantly changed by the knee osteoarthritis severity level.

The role of an auditory cue in generating anticipatory postural adjustments in response to an external perturbation

Anticipatory postural adjustments (APAs) are usually generated to minimize the potential postural disturbance induced by predictable external perturbations. Visual information about a perturbation is important for the generation of APAs, but whether people can rely on auditory information to generate APAs is unknown. The aim of this study was to investigate the role of an auditory cue in generating APAs when visual information is not available. Fifteen young adults participated in the study. They received external perturbations a) with visual information but no auditory information available, b) without neither visual nor auditory information, c) with both visual and auditory information available, and d) with only auditory information available. Electromyography (EMG) activities of eight leg and trunk muscles and displacements of the center-of-pressure (COP) were recorded and analyzed during the anticipatory and compensatory (CPAs) phases. Outcome measures included the latencies and integrals of muscle activities, COP displacements, and indices of co-contraction and reciprocal activation of muscles. The results showed that after a short training, participants were able to rely only on the auditory cue to generate APAs comparable to that when the visual information was available. In addition, a training effect was found such that the participants demonstrated stronger APAs and less demands for CPAs through the training trials. The outcome provides a foundation for future studies focusing on the utilization of auditory cues for postural control in older adults and individuals who have vision deficit.

Task-invariance and reliability of anticipatory postural adjustments in healthy young adults

BACKGROUND

Anticipatory postural adjustments (APAs) occur in the trunk during tasks such as rapid limb movement and are impaired in individuals with musculoskeletal and neurological dysfunction. To understand APA impairment, it is important to first determine if APAs can be measured reliably and which characteristics of APAs are task-invariant.

RESEARCH QUESTION

What is the test-retest reliability of latency, amplitude and muscle activation patterns (synergies) of trunk APAs during arm-raise and leg-raise tasks, and to what extent are these APA characteristics invariant across tasks at the individual and group levels?

METHODS

15 young adults (mean age: 23.7 (±3.2) years) performed six trials of a rapid arm raise task in standing and a leg raise task in supine on two occasions. Latency, amplitude and coactivation of APAs in the erector spinae and external/internal oblique musculature were measured, and APA synergies were identified with principle components analysis. Test-retest reliability across the two sessions was calculated with intraclass correlation coefficients. Task-invariance was assessed at the individual level with correlation and at the group level with tests of equivalence.

RESULTS

Most variables demonstrated acceptable test-retest reliability. Synergies and many features of APA activation varied across tasks, although at the individual level, motor performance time and amplitude of lumbar erector spinae activation were significantly correlated across tasks. Average pre-motor reaction time, external oblique latency, contralateral oblique amplitude and internal oblique coactivation were equivalent across tasks.

SIGNIFICANCE

Characteristics of trunk muscle APAs quantified during a single task may not be representative of an anticipatory postural control strategy that generalizes across tasks. Therefore, APAs must be assessed during multiple tasks with varying biomechanical demands to adequately investigate mechanisms contributing to movement dysfunction. The reliability analysis in this study facilitates interpretation of group differences or changes in APA behavior in response to intervention for the selected tasks.

Postural control processes during standing and step initiation in autism spectrum disorder

Background

Individuals with autism spectrum disorder (ASD) show a reduced ability to maintain postural stability, though motor control mechanisms contributing to these issues and the extent to which they are associated with other gross motor activities (e.g., stepping) are not yet known.

Methods

Seventeen individuals with ASD and 20 typically developing (TD) controls (ages 6–19 years) completed three tests of postural control during standing. During the neutral stance, individuals stood with their feet shoulder width apart. During the Romberg one stance, they stood with feet close together. During the circular sway, participants stood with feet shoulder width apart and swayed in a circular motion. The standard deviation (SD) of their center of pressure (COP) in the mediolateral (ML) and anteroposterior (AP) directions and the COP trajectory length were examined for each stance. We also assessed mutual information (MI), or the shared dependencies between COP in the ML and AP directions. Participants also completed a stepping task in which they stepped forward from one force platform to an adjacent platform. The amplitude and duration of anticipatory postural adjustments (APAs) were examined, as were the maximum lateral sway, duration, and velocity of COP adjustments following the initial step. We examined stepping variables using separate one-way ANCOVAs with height as a covariate. The relationships between postural control and stepping measures and ASD symptom severity were assessed using Spearman correlations with scores on the Autism Diagnostic Observation Schedule–Second Edition (ADOS-2) and the Autism Diagnostic Interview-Revised (ADI-R).

Results

Individuals with ASD showed increased COP trajectory length across stance conditions (p = 0.05) and reduced MI during circular sway relative to TD controls (p = 0.02). During stepping, groups did not differ on APA amplitude (p = 0.97) or duration (p = 0.41), but during their initial step, individuals with ASD showed reduced ML sway (p = 0.06), reduced body transfer duration (p < 0.01), and increased body transfer velocity (p = 0.02) compared to controls. Greater neutral stance COP_ML variability (r = 0.55, p = 0.02) and decreased lateral sway (r = − 0.55, p = 0.02) when stepping were associated with more severe restricted and repetitive behaviors in participants with ASD.

Conclusions

We found that individuals with ASD showed reduced MI during circular sway suggesting a reduced ability to effectively coordinate joint movements during dynamic postural adjustments. Additionally, individuals with ASD showed reduced lateral sway when stepping indicating that motor rigidity may interfere with balance and gait. Postural control and stepping deficits were related to repetitive behaviors in individuals with ASD indicating that motor rigidity and key clinical issues in ASD may represent overlapping pathological processes.

Peroneus longus muscle exhibits pre-programmed anticipatory activity before unilateral abduction of the lower limb while standing: a pilot study

[Purpose] This study aimed to develop a method for assessing anticipatory postural adjustments associated with voluntary movements in individuals with functional ankle instability. We examined whether the peroneus longus muscle exhibits anticipatory activation before unilateral abduction of the lower limb in individuals without disability. [Participants and Methods] Twelve healthy young adults participated in this study. Participants maintained a standing posture with 95 ± 2.5% of their weight on the left side and with the thenar of their right foot in contact with a small wooden board fixed to a force platform. Thereafter, they abducted their right lower limb by approximately 35° at maximum speed; during this time, electromyographic activities of the focal and postural muscles were recorded. [Results] The peroneus longus, external oblique, and erector spinae muscles on the left side of the body were activated before the right gluteus medius muscle, which is a focal muscle of abduction of the right lower limb. The activation timing of the left peroneus longus was the fastest among these postural muscles. [Conclusion] These findings suggest that the peroneus longus muscle plays an important role in anticipatory postural adjustments associated with unilateral abduction of the lower limb and that an ankle strategy is adopted in anticipatory postural adjustments during this task.

Anticipatory postural control differs between low back pain and pelvic girdle pain patients in the absence of visual feedback

PURPOSE

The aim of this study was to examine the effect of vision on anticipatory postural control (APA) responses in two groups of clinically diagnosed chronic low back pain patients, those with Posterior Pelvic Girdle pain and those with Non-Specific Low Back Pain compared to a matched group of healthy controls during the modified Trendelenburg task.

METHODS

Seventy-eight volunteer participants (60 females and 18 males) gave informed consent to take part in this study. 39 with confirmed LBP or PGP lasting longer than 12 weeks and 39 healthy matched controls performed 40 single leg lift tasks (hip flexion to 90° as quickly as possible) with their non-dominant lower limb. A force plate was used to determine the medial-lateral displacement of the center of pressure, and the initiation of weight shift; kinematics was used to determine initiation of leg lift; and electromyography was used to determine onset times from the external oblique (EO), internal oblique (IO) and lumbar multifidus (MF), gluteus maximus (GM) and biceps femoris (BF).

RESULTS

The PGP group showed significantly longer muscle onset latencies in the BF, EO MF with visual occlusion (F_2,746 = 4.51, p < .0001).

CONCLUSION

The muscle onset delays identified between the two LBP sub-groups suggests that pain may not be the primary factor in alteration of APA response. The PGP group show a greater reliance on vision which may signal impairment in multiple feedback channels.

Modulation of anticipatory postural adjustments using a powered ankle orthosis in people with Parkinson's disease and freezing of gait

BACKGROUND

Freezing of gait (FOG) during gait initiation in people with Parkinson's disease (PD) may be related to a diminished ability to generate anticipatory postural adjustments (APAs). Externally applied perturbations that mimic the desired motion of the body during an APA have been demonstrated to shorten and amplify APAs; however, no portable device has been tested. In this study, a portable powered ankle-foot orthosis (PPAFO) testbed was utilized to investigate the effect of mechanical assistance, provided at the ankle joint, on the APAs during gait initiation.

RESEARCH QUESTION

Does mechanical assistance provided at the ankle joint improve APAs during gait initiation in people with PD and FOG?

METHODS

Thirteen participants with PD and FOG initiated gait across five test conditions: two self-initiated (uncued) conditions in walking shoes [Baseline-Shoes], and the PPAFO in unpowered passive mode [Baseline-PPAFO_Passive]; three "go" cued conditions that included an acoustic tone with the PPAFO in unpowered passive mode [Acoustic + PPAFO_Passive], the mechanical assistance from the PPAFO [PPAFO_Active], and the acoustic tone paired with mechanical assistance [Acoustic + PPAFO_Active]. A warning-cue preceded the imperative "go" cue for all the cued trials. Peak amplitudes and timings of the vertical ground reaction forces (GRFs) and center of pressure (COP) shifts from onset to toe-off were compared across conditions.

RESULTS

Mechanical assistance significantly increased the peak amplitudes of the GRFs and COP shifts, reduced APA variability, and decreased the time to toe-off relative to the passive conditions.

SIGNIFICANCE

These findings demonstrate the potential utility of mechanical assistance at the ankle joint (with or without an acoustic cue) as a method to generate more consistent, shortened, and amplified APAs in people with PD and FOG.

Postural adjustments in anticipation of predictable perturbations allow elderly fallers to achieve a balance recovery performance equivalent to elderly non-fallers

BACKGROUND

In numerous laboratory-based perturbation experiments, differences in the balance recovery performance of elderly fallers and non-fallers are moderate or absent. This performance may be affected by the subjects adjusting their initial posture in anticipation of the perturbation.

RESEARCH QUESTIONS

Do elderly fallers and non-fallers adjust their posture in anticipation of externally-imposed perturbations in a laboratory setting? How does this impact their balance recovery performance?

METHODS

21 elderly non-fallers, 18 age-matched elderly fallers and 11 young adults performed both a forward waist-pull perturbation task and a Choice Stepping Reaction Time (CSRT) task. Whole-body kinematics and ground reaction forces were recorded. For each group, we evaluated the balance recovery performance in the perturbation task, change in initial center of mass (CoM) position between the CSRT and the perturbation task, and the influence of initial CoM position on task performance.

RESULTS

The balance recovery performance of elderly fallers was equivalent to elderly non-fallers (p > 0.5 Kolmogorov-Smirnov test). All subject groups anticipated forward perturbations by shifting their CoM backward compared to the CSRT task (young: 2.1% of lower limb length, elderly non-fallers: 2.7%, elderly fallers: 2.2%, Hodges-Lehmann estimator, p < 0.001 Mann-Whitney U). This backward shift increases the probability of resisting the traction without taking a step.

SIGNIFICANCE

The ability to anticipate perturbations is preserved in elderly fallers and may explain their preserved balance recovery performance in laboratory-based perturbation tasks. Therefore, future fall risk prediction studies should carefully control for this postural strategy, by interleaving perturbations of different directions for example.

Effects of vision and cognitive load on anticipatory and compensatory postural control

This study assessed the effects of vision and cognitive load on anticipatory postural adjustments (APAs) and compensatory postural adjustments (CPAs) in response to an externally triggered postural perturbation. A ball-hitting test was repeated under different visual conditions (eyes open, EO; eyes closed, EC) and cognitive loads (no load, 3-subtraction task, time-limited 3-subtraction task). Data were collected separately for I) surface electromyography from the right side of the biceps brachii (BIC) and erector spinae (ES) to detect the latency and response intensity (RI); and II) displacement of the centre of pressure (ΔCOP) to detect the standard deviation (ΔCOP_SD) and maximum value (ΔCOP_max) in the anterior-posterior direction. Compared with the results under the EC condition, the ES latency was shorter and the RI of the BIC was lower under the EO condition. Accordingly, the ΔCOP_SD and ΔCOP_max were increased in the APAs phase and decreased in the CPAs phase. Cognitive load had no effect on APAs and CPAs or on ΔCOP in the APAs phase. However, ΔCOP_max was decreased in the CPAs phase during the EC condition. In conclusion, vision played an important role in APAs and CPAs for muscle activation and ΔCOP. Cognitive load had no effect on neuromuscular APAs or CPAs except when the postural perturbation occurred when visually unexpected.

Development of postural control during single-leg standing in children aged 3-10 years

BACKGROUND

The ability to control the center of mass (COM) during single-leg standing (SLS) is imperative for individuals to walk independently. However, detailed biomechanical features of postural control during SLS performed by children remain to be comprehensively investigated.

RESEARCH QUESTION

We aimed to investigate the development of postural control during SLS in children aged 3-10 years.

METHODS

Forty-eight healthy children (26 boys and 22 girls) aged 3-10 years and 11 young adults participated in this experiment. The child population was divided into four groups by age: 3-4, 5-6, 7-8, and 9-10 years. The SLS task included standing on a single leg as long and as steady as possible for up to 30 s. A three-dimensional motion capture system and two force plates were used for calculating the COM and center of pressure (COP). The task was divided into three phases (accelerated, decelerated, and steady) on the basis of the relationship between COM and COP.

RESULTS

COP-COM distances in the 5-6 years' and 7-8 years' groups were significantly increased during the acceleration phase when compared with those in the adult group. Furthermore, COP-COM distances during the decelerated phase were significantly higher in all children's groups compared with those in the adult group. Lastly, COP-COM distance during the steady phase was significantly higher in the 3-4 year age group than in the 9-10 year age and adults groups.

SIGNIFICANCE

These results suggest that postural control during the acceleration and steady phases mature by 9 years. Conversely, children ∼10 years did not attain adult-like levels of postural control during the decelerated phase. The developmental process for postural control at each phase possibly plays a significant role in the basic biomechanics of movement and does not display a monotonic pattern.

Effect of the severity of manual impairment and hand dominance on anticipatory and compensatory postural adjustments during manual reaching in children with cerebral palsy

AIM

To investigate the role of the severity of manual impairment and of hand dominance on postural sway during anticipatory [APA] and compensatory [CPA] postural adjustments in a seated manual reaching task performed by children with cerebral palsy (CP) and typical children (TC).

METHODS

We tested 26 TC (mean age 9.5 ± 2.1 years) and 29 children with CP (age 9.6 ± 3 years) classified based on manual impairment levels as mild (Manual Ability Classification System [MACS] I; n = 18) or moderate-to-severe (MACS II-III, n = 11). Participants were instructed to reach towards a target using their dominant vs. non-dominant arm while sitting on a force-plate. Center of pressure (CoP) sway was analyzed during APA and CPA.

RESULTS

For all groups, using the non-dominant arm determined greater amplitude and velocity of CoP sway in CPA. Children with moderate-to-severe manual impairment showed greater sway during APA and CPA compared to mild impairment and TC groups.

CONCLUSION

More severe manual impairment resulted in higher sway during the anticipatory and compensatory phases of the reaching task. Using the non-dominant arm resulted in greater compensatory adjustments during reaching.

Anticipatory and compensatory postural adjustments in response to loading perturbation of unknown magnitude

In response to sudden postural perturbations, the posture control system uses anticipatory and compensatory postural adjustments (APAs and CPAs) to maintain balance and equilibrium. APAs strengthen as the perturbation magnitude increases, while CPAs remain constant because APAs make the necessary adjustments. However, the magnitude of a postural perturbation cannot always be fully known. This research focused on postural adjustments in response to perturbations with unknown magnitude. Participants caught falling sandbags of three weights on a tray held in their hands. Participants were told about the weight used for the upcoming trial in the KNOWN condition and not told in the UNKNOWN condition. Surface electromyography (sEMG) of the lumbar muscles and displacement of the center of pressure (COP) were recorded synchronously. The results showed that APAs and CPAs were stronger in the UNKNOWN condition than in the KNOWN condition. Meanwhile, in the UNKNOWN condition, the activity of the lumbar muscles and displacements of the COP showed no difference between weight levels. The lumbar erector spinae (LES) and lumbar multifidus (LMF) activated earlier in the UNKNOWN condition than for the heaviest weight in the KNOWN condition. The outcome of this study indicates that APAs and CPAs of lumbar muscles and displacements of the COP are affected by the knowledge of postural perturbations. The central nervous system (CNS) coped with load perturbations of unknown magnitude with redundancy response strategy, based on the maximum assumption of perturbation magnitude.

Anticipatory and compensatory postural adjustments in people with low back pain: a systematic review and meta-analysis

BACKGROUND CONTEXT

Despite altered anticipatory (APAs) and compensatory postural adjustments (CPAs) being hypothesized to contribute to the onset and persistence of low back pain (LBP), results from studies comparing people with and without LBP are conflicting.

PURPOSE

This systematic review aimed to determine whether APAs or CPAs are altered in the presence of acute and chronic LBP.

STUDY DESIGN

A systematic review of studies was carried out.

PATIENT SAMPLE

No patient sample was required.

OUTCOME MEASURES

Between group standardized mean differences and 95% confidence intervals for APAs ad CPAs METHODS: A comprehensive search was conducted for articles comparing people with LBP (acute or chronic) to healthy controls for the onset or amplitude of muscle activity, center of pressure (COP), or kinematic responses to expected or unexpected perturbations. Two independent reviewers extracted data and assessed the methodological quality of relevant studies. Differences between people with and without LBP were calculated as standardized mean differences, and included in a meta-analysis if outcomes were homogeneous. Otherwise, a narrative synthesis was conducted.

RESULTS

Twenty-seven studies were included, of which the majority examined muscle onsets in response to expected and unexpected perturbations. Only two studies compared people with and without acute LBP, and results for these studies were conflicting. The results show delayed muscle onsets in response to expected and unexpected perturbations for people with chronic LBP when compared with healthy controls. No conclusive evidence for differences between people with and without chronic LBP for COP or kinematic responses.

CONCLUSIONS

There is currently no convincing evidence of differences between people with and without acute LBP for APAs or CPAs. Conversely, delayed muscle onsets in people with chronic LBP suggest APAs and CPAs are altered in this population. However, the functional relevance of these delayed muscle onsets (eg, COP and kinematics) is unknown.

Early and anticipatory postural adjustments in healthy subjects under stable and unstable sitting conditions

Clinicians frequently incorporate unstable sitting devices into training plans for improving proximal postural muscle control; however, the effect of unstable sitting conditions on postural adjustments during dynamic activities has not been fully explored. The aim of this study was to characterize early postural adjustments (EPAs) and anticipatory postural adjustments (APAs) under stable and unstable sitting conditions. Using a cross-sectional laboratory study design, 13 healthy college student volunteers used their dominant hand to reach forward and push a target under stable and unstable sitting conditions; subjects sat on an air-filled rubber cushion for the unstable condition. EPAs and APAs were quantified by recording muscle activation of the trunk and lower extremity muscles using electromyography (EMG). The center of pressure (COP) was measured using a force plate. The resulting EMG integral of the ipsilateral gastrocnemius muscle was larger during the EPA phase and smaller during the APA phase under unstable conditions (p = 0.014 and p = 0.041, respectively). COP amplitude in the anterior-posterior direction, path length, and velocity, was larger during the APA phase (p = 0.035, p = 0.023, and 0.023, respectively). This suggests greater distal muscle activation during EPAs in unstable sitting conditions, specifically in the ipsilateral gastrocnemius muscle. In addition, APAs adjusted by reducing the activity of the ipsilateral gastrocnemius muscle and increasing the anterior-posterior shift in the COP to compensate for the expected additional perturbation due to an unstable surface.

Brain network connectivity associated with anticipatory postural control in children and adults

Internal models provide a coherent framework for understanding motor behavior. Examples for the use of internal models include anticipatory postural adjustments (APAs), where the individual anticipates and cancels out the destabilizing effect of movement on body posture. Yet little is known about the functional changes in the brain supporting the development of APAs. Here, we addressed this issue by relating individual differences in APAs as assessed during bimanual load lifting to interindividual variation in brain network interactions at rest. We showed that the strength of the connectivity between three main canonical brain networks, namely the cingulo-opercular, the fronto-parietal and the somatosensory-motor networks, is an index of the ability to implement APAs from late childhood (9- to 11-year-old children). We also found an effect of age on the relationship between APAs and coupling strength between these networks, consistent with the notion that APAs are near but not yet fully mature in children. We discuss the implications of these findings for our understanding of learning disorders with impairment in predictive motor control.

Anticipatory postural adjustments as a function of response complexity in simple reaction time tasks

The central nervous system preplans postural responses to successfully perform complex multi-joint movements. These responses have been termed anticipatory postural adjustments (APAs), and they constitute a general type of response to stabilize posture prior to movement initiation. APA sequences are elicited with shorter latency when a startling acoustic stimulus is applied, demonstrating their preplanned nature. Increasing task complexity using a simple reaction time (RT) paradigm has been shown to delay limb movement RT as a result of additional planning or sequencing requirements; however, the effect of task complexity on APA dynamics is unclear. The purpose of the present study was to investigate if task complexity modulates APA onset in a manner analogous to that observed in the primary effector. 13 participants completed 150 trials of simple (1-target) and complex (2- or 3-target) arm movements while standing on a force plate. Results indicated participants had significantly faster arm movement RTs in the simple versus the most complex condition. Similar to the primary effector, APA RTs were longer in the most complex (3-target) movement compared to both the 1-target and 2-target movements. Furthermore, APA excursion velocities were scaled to the complexity of the upcoming movement: the rate of APAs increased from simplest to most complex movements. These findings clearly demonstrate APAs are sensitive to task complexity, further elucidating their preplanned role in stabilizing posture which enables the successful completion of intended movements.

The degree of postural automaticity influences the prime movement and the anticipatory postural adjustments during standing in healthy young individuals

Less attention to a balance task reduces the center of foot pressure (COP) variability by automating the task. However, it is not fully understood how the degree of postural automaticity influences the voluntary movement and anticipatory postural adjustments. Eleven healthy young adults performed a bipedal, eyes closed standing task under the three conditions: Control (C, standing task), Single (S, standing + reaction tasks), and Dual (D, standing  +  reaction + mental tasks). The reaction task was flexing the right shoulder to an auditory stimulus, which causes counter-clockwise rotational torque, and the mental task was arithmetic task. The COP variance before the reaction task was reduced in the D condition compared to that in the C and S conditions. On average the onsets of the arm movement and the vertical torque (Tz, anticipatory clockwise rotational torque) were both delayed, and the maximal Tz slope (the rate at which the torque develops) became less steep in the D condition compared to those in the S condition. When these data in the D condition were expressed as a percentage of those in the S condition, the arm movement onset and the Tz slope were positively and negatively, respectively, correlated with the COP variance. By using the mental-task induced COP variance reduction as the indicator of postural automaticity, our data suggest that the balance task for those with more COP variance reduction is less cognitively demanding, leading to the shorter reaction time probably due to the attention shift from the automated balance task to the reaction task.

Schizotypal traits and forearm motor control against self-other produced action in a bimanual unloading task

The present study investigated the relation between schizotypy and motor control against self- or other-produced action. We used an unloading task to focus on the timing component of anticipatory motor control. In the task, a weight was removed from a participants' hand by the participants themselves or by an experimenter (voluntary versus imposed unloading). Postural disturbance at the removal timing was measured as an index of predictive function in motor control. We hypothesized that the postural disturbance in the voluntary unloading would be positively related to schizotypal traits; however, the results did not support this theory. The results showed almost zero correlation between the schizotypy scores and the postural disturbance in the voluntary unloading condition. In contrast, the schizotypy scores positively correlated with the postural disturbance in the imposed unloading condition. These findings were replicated across two participant groups and two schizotypy scales. Further analyses on subscales of the schizotypy questionnaire found moderate levels of positive correlation between each subscale for Cognitive-Perceptual and Disorganization factors and the disturbance. Accordingly, the present study did not support the idea that non-pathological individuals with high schizotypal traits have deficits in prediction of self-produced actions, at least for a temporal domain. Instead, the results suggested that individuals with high schizotypal traits, particularly for the positive and disorganization symptoms, are not good at responding to others-produced actions. The schizophrenic symptoms were discussed in terms of the failure in the processes executed after calculating prediction of sensory consequences and dysfunction in internal models for "other people".

Influence of dual-task constraints on the interaction between posture and movement during a lower limb pointing task

One of the challenges regarding human motor control is making the movement fluid and at a limited cognitive cost. The coordination between posture and movement is a necessary requirement to perform daily life tasks. The present experiment investigated this interaction in 20 adult men, aged 18-30 years. The cognitive costs associated to postural and movement control when kicking towards a target was estimated using a dual-task paradigm (secondary auditory task). Results showed that addition of the attentional demanding cognitive task yielded a decreased kicking accuracy and an increased timing to perform the movement, mainly during the backswing motion. In addition, significant differences between conditions were found for COP and COM displacement (increased amplitude, mean speed) on the anteroposterior axis. However, no significant differences between conditions were found on the mediolateral axis. Finally, EMG analysis showed that dual-task condition modified the way anticipatory postural adjustments (APAs) were generated. More specifically, we observed an increase of the peroneus longus activity, whereas the temporal EMG showed a decrease of its latency with respect to movement onset. These results suggested a functional adaptation resulting in an invariance of overall APAs, emphasizing that cognitive, postural, and motor processes worked dependently.

Balance control during gait initiation: State-of-the-art and research perspectives

It is well known that balance control is affected by aging, neurological and orthopedic conditions. Poor balance control during gait and postural maintenance are associated with disability, falls and increased mortality. Gait initiation - the transient period between the quiet standing posture and steady state walking - is a functional task that is classically used in the literature to investigate how the central nervous system (CNS) controls balance during a whole-body movement involving change in the base of support dimensions and center of mass progression. Understanding how the CNS in able-bodied subjects exerts this control during such a challenging task is a pre-requisite to identifying motor disorders in populations with specific impairments of the postural system. It may also provide clinicians with objective measures to assess the efficiency of rehabilitation programs and better target interventions according to individual impairments. The present review thus proposes a state-of-the-art analysis on: (1) the balance control mechanisms in play during gait initiation in able bodied subjects and in the case of some frail populations; and (2) the biomechanical parameters used in the literature to quantify dynamic stability during gait initiation. Balance control mechanisms reviewed in this article included anticipatory postural adjustments, stance leg stiffness, foot placement, lateral ankle strategy, swing foot strike pattern and vertical center of mass braking. Based on this review, the following viewpoints were put forward: (1) dynamic stability during gait initiation may share a principle of homeostatic regulation similar to most physiological variables, where separate mechanisms need to be coordinated to ensure stabilization of vital variables, and consequently; and (2) rehabilitation interventions which focus on separate or isolated components of posture, balance, or gait may limit the effectiveness of current clinical practices.

Adaptation of feedforward movement control is abnormal in patients with cervical dystonia and tremor

OBJECTIVE

It is under debate whether the cerebellum plays a role in dystonia pathophysiology and in the expression of clinical phenotypes. We investigated a typical cerebellar function (anticipatory movement control) in patients with cervical dystonia (CD) with and without tremor.

METHODS

Twenty patients with CD, with and without tremor, and 17 healthy controls were required to catch balls of different load: 15 trials with a light ball, 25 trials with a heavy ball (adaptation) and 15 trials with a light ball (post-adaptation). Arm movements were recorded using a motion capture system. We evaluated: (i) the anticipatory adjustment (just before the impact); (ii) the extent and rate of the adaptation (at the impact) and (iii) the aftereffect in the post-adaptation phase.

RESULTS

The anticipatory adjustment was reduced during adaptation in CD patients with tremor respect to CD patients without tremor and controls. The extent and rate of adaptation and the aftereffect in the post-adaptation phase were smaller in CD with tremor than in controls and CD without tremor.

CONCLUSION

Patients with cervical dystonia and tremor display an abnormal predictive movement control.

SIGNIFICANCE

Our findings point to a possible role of cerebellum in the expression of a clinical phenotype in dystonia.

Anticipatory postural adjustment patterns during gait initiation across the adult lifespan

Gait initiation involves a complex sequence of anticipatory postural adjustments (APAs) during the transition from steady state standing to forward locomotion. APAs have four core components that function to accelerate the center of mass forwards and towards the initial single-support stance limb. These components include loading of the initial step leg, unloading of the initial stance leg, and excursion of the center of pressure in the posterior and lateral (towards the stepping leg) directions. This study examined the incidence, magnitude, and timing of these components and how they change across the lifespan (ages 20-79). 157 individuals performed five trials of self-paced, non-cued gait initiation on an instrumented walkway. At least one component of the APA was absent in 24% of all trials. The component most commonly absent was loading of the initial step leg (absent in 10% of all trials in isolation, absent in 10% of trials in conjunction with another missing component). Trials missing all four components were rare (1%) and were observed in both younger and older adults. There was no significant difference across decades in the incidence of trials without an APA, the number or type of APA components absent, or the magnitude or timing of the APA components. These data demonstrate that one or more components of the APA sequence are commonly absent in the general population and the spatiotemporal profile of the APA does not markedly change with ageing.

Dual task interference on postural sway, postural transitions and gait in people with Parkinson's disease and freezing of gait

Freezing of gait (FoG) is associated with less automatic gait and more impaired cognition, balance and postural transitions compared to people with PD who do not have FoG. However, it is unknown whether dual-task cost during postural sway, postural transitions (such as gait initiation and turning), and gait are more in subjects with Parkinson's disease (PD) who have freezing of gait (FoG+) compared to those who do not have FoG (FoG-). Here, we hypothesized that the effects of a cognitive dual task on postural sway, postural transitions and gait would be larger in FoG+ than FoG-. Thirty FoG- and 24 FoG+ performed an Instrumented Stand and Walk test in OFF medication state, with and without a secondary cognitive task (serial subtraction by 3s). Measures of postural sway, gait initiation, turning, and walking were extracted using body-worn inertial sensors. FoG+ showed significantly larger dual task cost than FoG- for several gait metrics, but not during postural sway or postural transitions. During walking, FoG+ exhibited a larger dual task cost than FoG- resulting in shorter stride length and slower stride velocity. During standing, FoG+ showed a larger postural sway compared to FoG- and during gait initiation, FoG+, but not FoG-, showed a longer first step duration during the dual-task condition compared to single-task condition (interaction effect, p=0.04). During turning, both groups showed a slower turn peak speed in the dual-task condition compared to single task condition. These findings partly support our hypothesis that dual task cost on walking is greater in FoG+ than FoG-.

Step initiation interferes with working memory in nondisabled patients with the earliest multiple sclerosis-A dual-task study

Balance and cognition are affected by multiple sclerosis (MS). Cognitive-motor interference (CMI) is important for balance impairment in MS, however little is known about CMI at the earliest stages of the disease. Step initiation (SI) with anticipatory postural adjustments (APAs) has been linked to postural instability and falls in subjects with MS, therefore we aimed to assess CMI between SI and the two storage systems of working memory in patients with clinically isolated syndrome (presented as optic neuritis-ON) suggestive of MS. Twenty patients with normal/near normal visual acuity and 20 age-, weight-, height-, sex- and education-matched control subjects were included. APAs were studied using center of pressure measures in three conditions: SI alone, SI+Brooks' spatial- and SI+2-back verbal working memory task. Decrements (% change) in performance on cognitive tasks and in APA parameters were calculated. CMI was assessed combining the two decrements scores. Performance on both cognitive tasks was more affected by dual-tasking in patients compared to healthy subjects. In both groups APA parameters were not influenced by dual-tasking. CMI was higher in patients compared to healthy subjects. Our results suggest that the disease affects CMI in its earliest stages. Since both cognitive tasks were similarly affected by dual-tasking in patients and controls central executive seems to play the major role in CMI between SI and working memory. Patients prioritizing motor over cognitive task for balance maintenance suggests reduced divided attention capacity as a cause of increased CMI in the earliest MS.