Role of lateral muscles and body orientation in feedforward postural control

The Effect of Additional Leg Supports in Control of Posture in Sitting

The objective of the study was to investigate the effect of leg supports on the anticipatory and compensatory postural adjustments of sitting subjects exposed to external perturbations in the anterior-posterior direction. Ten young participants received perturbations applied to the upper body while sitting on a stool with an anterior or posterior leg support and when using a footrest. Electromyographic activities of the trunk and leg muscles and center of pressure displacements were recorded and analyzed during the anticipatory and compensatory phases of postural control. Anticipatory activities were observed in the tibialis anterior, biceps femoris, and erector spinae muscles in the anterior leg support condition. Early onset of muscle activity was observed in the tibialis anterior, biceps femoris, rectus femoris, and erector spinae muscles in the posterior leg support condition compared to the feet support condition. Moreover, to maintain balance participants utilized co-contraction of muscles as the main mechanism of balance control in sitting regardless of the availability of the anterior or posterior leg support. There was no effect of a leg support on center of pressure displacements. The outcome of the study provides a background for future investigations of the effect of leg supports on control of balance in sitting when perturbed.

Anticipatory postural adjustments in older versus young adults: a systematic review and meta-analysis

BACKGROUND

Anticipatory postural adjustments (APAs) are a feedforward mechanism triggered in advance to a predictable perturbation, to help the individual counteract mechanical effects that the disturbance may cause. Whether or not this strategy is compromised in the elderly is not a consensus in the literature.

METHODS

In this systematic review with meta-analysis, we investigated aging effects on postural control, based on anticipatory postural adjustments (APAs). We selected 11 eligible articles of the following databases: Lilacs, SciELO, PubMed, Cochrane Central, Embase, and CINAHL, involving 324 research participants, assessing their methodological quality and extracting electromyographic, posturographic, and kinematic measurements. We included studies that investigated the occurrence of APAs in healthy younger and older adults, published before 10th August 2022, in English. Studies involving participant with conditions that may affect balance or that did not report measures of onset or amplitude of electromyography (EMG), COP, or kinematics were excluded. To analyze the aggregated results from these studies, we performed the analysis based on the outcome measures (EMG, COP, or kinematic measures) used in individual studies. We calculated differences between younger and older adult groups as the mean differences between the groups and the estimated effect. Egger's test was conducted to evaluate whether this meta-analysis had publication bias.

RESULTS

Through this review, older adults showed no significant difference in the velocity to perform a movement compared to the younger adults (MD 0.95, 95% CI -0.86, 2.76, I² = 82%), but both muscle onset and center of pressure (COP) onset were significantly more delayed in older than in younger adults: erector spinae (MD -31.44, 95% CI -61.79, -1.09, I² = 95%); rectus abdominis (RA) (MD -31.51, 95% CI -70.58, -3.57, I² = 85%); tibialis anterior (TA) (MD -44.70, 95% CI -94.30, 4.91, I² = 63%); soleus (SOL) (MD -37.74, 95% CI -65.43, -10.05, I² = 91%); gastrocnemius (GAS) (MD -120.59, 95% CI -206.70, -34.49, I² = 94%); quadriceps (Q) (MD -17.42, 95% CI -34.73, -0.12, I² = 0%); biceps femoris (BF) (MD -117.47, 95% CI -192.55, -42.70, I² = 97%); COP onset (MD -45.28, 95% CI -89.57, -0.98, I² = 93%), and COP apa (COPapa) (MD 2.35, 95% CI -0.09, 4.79, I² = 64%). These changes did not seem to be linked to the speed of movement but possibly to age-related physiological changes that indicated decreased motor control during APAs in older adults.

CONCLUSIONS

Older adults use different postural strategies that aim to increase the safety margin and stabilize the body to perform the movement, according to the requirements imposed, and this should be considered in rehabilitation protocols.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO CRD420119143198.

Validity and reliability of a smartphone-based assessment for anticipatory and compensatory postural adjustments during predictable perturbations

BACKGROUND

Postural adjustments involve displacements of the center of mass (COM), controlled by the central nervous system (CNS), to maintain equilibrium whilst standing. Postural adjustments can be anticipatory (APAs) or compensatory (CPAs), and are triggered to counteract predictable perturbations.

RESEARCH QUESTION

Is the new smartphone application, Momentum, a valid and reliable tool for the assessment of body balance, by measuring APAs and CPAs using accelerometer readings?

METHODS

20 young adults were exposed to external predictable perturbations induced at the shoulder level, whilst standing. COM linear acceleration was recorded by Momentum (extracting data from a smartphone's accelerometer) and a 3D motion capture system.

RESULTS

The key results demonstrated a very high, significant correlation (r ≥ 0.7, p < 0.05) between the two device settings in the APA parameters_, which obtained r = 0.65, denoting a high correlation. Considering the reliability, variables that are compensatory in nature are presented on a scale of good to excellent in measurement methods, kinematics, and Momentum. However, the anticipatory variables presented excellent reliability only for the kinematics.

SIGNIFICANCE

These experiments show that Momentum is a valid method for measuring COM acceleration under predictable perturbations and is reliable for compensatory events.

Role of angular position of the seat in control of posture in response to external perturbation

Ability of the human body to regain balance after being externally perturbed is important in the maintenance of vertical posture. The goal of this study was to investigate trunk and leg muscle response to external perturbation while sitting on a stool with varying seat inclinations. Ten healthy subjects were required to receive a perturbation applied to the upper body while sitting on an adjustable stool with 0°, 10° forward or 10° backward inclination of the seat and without footrest and leg support. Electromyographic activities of the trunk and leg muscles and center of pressure displacements were recorded and analyzed during the anticipatory (APA) and compensatory (CPA) phases of postural control. APAs and CPAs were generated in response to an external perturbation. Delays in the onset of anticipatory muscle activity were seen when seated on the inclined seat compared to sitting on the horizontal seat (p < 0.05). To maintain balance after a perturbation, participants activated the trunk and thigh muscles, the activity of which was modulated to a greater degree than that of leg muscles. Moreover, they utilized co-contraction of muscles as the main mechanism of balance control in sitting. Furthermore, there was no effect of a seat inclination on COP displacements. The outcome provides a background for future investigations of the effect of seat inclination on control of balance in sitting.

A Study on the Relationship between Postural Control and Pain-Related Clinical Outcomes in Patients with Chronic Nonspecific Low Back Pain

Objectives

To explore the relationship between postural control and pain-related clinical outcomes in patients with chronic nonspecific low back pain (cNLBP).

Methods

Participants with cNLBP and healthy individuals were recruited. Muscle activities were recorded during internal and external perturbation tasks. Postural control capacity was assessed by muscle onset time and integrals of electromyography (iEMGs) of postural muscles during the phases of anticipatory postural adjustments (APAs) and compensatory postural adjustments (CPAs). Correlation analysis was employed to investigate the relationship between postural control capacity, pain, and disability.

Results

Twenty-seven patients with cNLBP and 27 healthy participants were recruited. Gastrocnemius (GA) muscle onset time was earlier in the cNLBP group than in the control group in the internal perturbation task. The onset time of GA and erector spinae (ES) of the cNLBP group was later than that of the controls in the external perturbation task. Disability level moderately correlated with the iEMGs of rectus abdominis (RA), GA, and external oblique (EO) during APAs. Pain score moderately correlated with the iEMGs of RA, EO, and ES during CPAs of perturbation tasks.

Conclusion

cNLBP participants had altered muscle activation strategy to maintain postural stability in response to perturbation. This study further discovered that pain-related disabilities of cNLBP participants were likely related to the APAs capacity, whereas the pain intensity may relate to the CPAs capacity. Pain and disability may therefore be related to the control process of the posture-related muscles.

Effect of jaw clenching on head acceleration during a predictable load impact

BACKGROUND

Jaw clenching is considered to reduce head acceleration while receiving a strong impact on the body during sport activities.

OBJECTIVE

The present study aimed to clarify the effect of jaw clenching on reduction of head acceleration during a predictable load impact to the body.

METHODS

Seven healthy participants were exposed to a predictable load impact with and without jaw clenching. We recorded the electromyographic activity of the masseter (MA) and digastricus (DIG) muscles, occlusal pressure and head acceleration throughout the experiment.

RESULTS

When participants were not instructed to clench their jaws, they naturally positioned their jaws without occlusal contact at the time of pendulum impact by co-contracting the jaw opener and closer muscles. When participants were instructed to clench their jaws, neither the activity of the jaw opener muscle nor the head acceleration differed at the time of pendulum impact when compared with when participants were not instructed to clench their jaws.

CONCLUSIONS

A slightly distanced jaw position (co-contracting the jaw opener and closer muscles without occlusal contact) might serve inherently safety for reduction of head acceleration during predictable body impact, while jaw clenching does not contribute to reduction of head acceleration in response to pendulum impact more than the distanced jaw position does. Notably, DIG activation to minimise the head acceleration in response to pendulum impact was similar in clenching and no clenching positions. This suggests that DIG may play a crucial role in the reduction of head acceleration, regardless of MA muscle activity.

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.

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.

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

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

Anticipatory postural adjustments and spatial organization of motor cortex: evidence of adaptive compensations in healthy older adults

During anticipated postural perturbations induced by limb movement, the central nervous system generates anticipatory postural adjustments (APAs) in the trunk and hip musculature to minimize disturbances to equilibrium. Age-related changes in functional organization of the nervous system may contribute to changes in APAs in healthy older adults. Here we examined if altered APAs of trunk/hip musculature in older adults are accompanied by changes in the representation of these muscles in motor cortex. Twelve healthy older adults, 5 with a history of falls and 7 nonfallers, were compared with 13 young adults. APAs were assessed during a mediolateral arm raise task in standing. Temporal organization of postural adjustments was quantified as latency of APAs in the contralateral external oblique, lumbar paraspinals, and gluteus medius relative to activation of the deltoid. Spatial organization was quantified as extent of synergistic coactivation between muscles. Volume and location of the muscle representations in motor cortex were mapped using transcranial magnetic stimulation. We found that older adults demonstrated significantly delayed APAs in the gluteus medius muscle. Spatial organization of the three muscles in motor cortex differed between groups, with the older adults demonstrating more lateral external oblique representation than the other two muscles. Separate comparisons of the faller and nonfaller subgroups with young adults indicated that nonfallers had the greatest delay in gluteus medius APAs and a reduced distance between the representational areas of the lumbar paraspinals and gluteus medius. This study indicates that altered spatial organization of motor cortex accompanies altered temporal organization of APA synergies in older adults.

NEW & NOTEWORTHY Anticipatory postural adjustments are a critical component of postural control. Here we demonstrate that, in healthy older adults with and without a history of falls, delayed anticipatory postural adjustments in the hip musculature during mediolateral perturbations are accompanied by altered organization of trunk/hip muscle representation in motor cortex. The largest adaptations are evident in older adults with no history of falls.

Inconsistent anticipatory postural adjustments (APAs) in rugby players: a source of injuries?

Background

We are developing since 2010 with Thales and the Fédération Française de Rugby (FFR) M-Rex, a new kind of rugby scrum simulator. The study questioned whether it could improve safety and protect players from injury by using it as a tool for training/coaching the packs.

Aim

To explore the anticipatory postural adjustments (APAs) during the engagement of the ruck, because these predictive neck and back muscles contractions protect the spinal cord at the time of impacts, which is crucial to prevent injuries.

Methods

We quantified the kinematics and the EMG activities in high-level front row players during their initial engagement, when scrummaging with M-Rex. All studies were performed with one player interacting with the robot, at first, and then with the three players acting together.

Results

For most of the tested high-level players, the APA latencies were highly variable from trial to trial even though the engagement resulted in similar impacts. At time, the onset of the electromyography activity in the neck and back muscles showed latencies inferior to 50 ms or even close to zero prior to the impact, which rendered muscle contractions inefficient as APAs. We were also unable to identify clear muscular synergies underlying the APAs because of their great variability on a trial-to-trial basis. Finally, the APAs were not related to the amplitude of the ensuing impact and were asymmetric in most trials. All these characteristics held true, whether the player was playing alone or with two other frontline players.

Conclusion

Our result suggest that APAs should be systematically tested in high-level rugby players as well as in any high-level sport men at risk of neck and back injuries. Because APAs can be efficiently trained, our study paves the way to design individual position-specific injury prevention programme.

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.

Anticipatory and Compensatory Postural Adjustments in Response to External Lateral Shoulder Perturbations in Subjects with Parkinson's Disease

The purpose of this study was to investigate the anticipatory (APA) and compensatory (CPA) postural adjustments in individuals with Parkinson’s disease (PD) during lateral instability of posture. Twenty-six subjects (13 individuals with PD and 13 healthy matched controls) were exposed to predictable lateral postural perturbations. The electromyographic (EMG) activity of the lateral muscles and the displacement of the center of pressure (COP) were recorded during four time intervals that are typical for postural adjustments, i.e., immediately before (APA1, APA2) and after (CPA1 and CPA2) the postural disturbances. The magnitude of the activity of the lateral muscles in the group with PD was lower only during the CPA time intervals and not during the anticipatory adjustments (APAs). Despite this finding, subjects with PD exhibit smaller COP excursions before and after the disturbance, probably due to lack of flexibility and proprioceptive impairments. The results of this study suggest that postural instability in subjects with PD can be partially explained by decreased postural sway, before and after perturbations, and reduced muscular activity after body disturbances. Our findings can motivate new studies to investigate therapeutic interventions that optimize the use of postural adjustment strategies in subjects with PD.

Both anticipatory and compensatory postural adjustments are adapted while catching a ball in unstable standing posture

The main objective of this study was to analyze the role of balance exercises on anticipatory (APA) and compensatory (CPA) postural adjustments in different conditions of postural stability. Sixteen subjects were required to catch a ball while standing on rigid floor, trampoline and foam cushion surfaces. Electromyographic activities (EMG) of postural muscles were analyzed during time windows typical for APAs and CPAs. Overall there were a reciprocal activation of the muscles around the ankle and co-activations between ventral and dorsal muscles of the thigh and trunk during the catching a ball task. Compared to the rigid floor, the tibialis anterior activation was greater during the trampoline condition (CPA: p = 0.006) and the soleus muscle inhibition was higher during foam cushion condition (APA: p = 0.001; CPA: p = 0.007). Thigh and trunk muscle activities were similar across the conditions. These results advance the knowledge in postural control during body perturbations standing on unstable surfaces.

Trunk muscles contribute as functional groups to directionality of reaching during stance

Muscle activity preceding the onset of voluntary movement has been shown to reduce centre of mass (CoM) displacement and stabilise the body during self-induced 'perturbations'. However, based on recent findings in the lower limb, where preparatory muscle activity creates the dynamics necessary for the initiation of movement, this study sought to investigate whether trunk musculature acted consistently to minimise the displacement of the CoM, or in contrast, contribute to the movement. While standing, nine healthy participants made single-step (point-to-point) reaching movements to 13 visual targets throughout a 180° range (target interval = 15°). Full-body kinematics and electromyographic activity from 'focal' arm and 'postural' trunk muscles were analysed for a preparatory phase of 250-ms preceding movement onset (termed pPA). Akin to lower limb findings, direction-specific patterns of anticipatory trunk muscle activity accompanied the onset of rotational kinematics and CoM acceleration in the direction of the desired target. When arranged in terms of peak activation, we found functionally relevant groupings aligned to either ipsi-, central or contra-lateral reaching directions. Contrary to traditional approaches, which focus on CoM stabilisation, this spatial recruitment was in favour of assisting initiation of movement. Such activity suggests that the central nervous system may rely on synergic patterns of muscle activation within an undistinguishable and shared focal/postural motor command for functional voluntary movements.

Older adults utilize less efficient postural control when performing pushing task

The ability to maintain balance deteriorates with increasing age. The aim was to investigate the role of age in generation of anticipatory (APA) and compensatory (CPA) postural adjustments during pushing an object. Older (68.8 ± 1.0 years) and young adults (30.1 ± 1.4 years) participated in the experiment involving pushing an object (a pendulum attached to the ceiling) using both hands. Electrical activity of six leg and trunk muscles and displacements of the center of pressure (COP) were recorded and analyzed during the APA and CPA phases. The onset time, integrals of muscle activity, and COP displacements were determined. In addition, the indexes of co-activation and reciprocal activation of muscles for the shank, thigh, and trunk segments were calculated. Older adults, compared to young adults, showed less efficient postural control seen as delayed anticipatory muscle onset times and delayed COP displacements. Moreover, older adults used co-activation of muscles during the CPA phase while younger subjects utilized reciprocal activation of muscles. The observed diminished efficiency of postural control during both anticipatory and compensatory postural adjustments observed in older adults might predispose them to falls while performing tasks involving pushing. The outcome provides a background for future studies focused on the optimization of the daily activities of older adults.

Anticipatory and compensatory postural adjustments in individuals with multiple sclerosis in response to external perturbations

Deficit in balance control is a common and often an initial disabling symptom of multiple sclerosis (MS). The aim of the study was to investigate the organization of anticipatory and compensatory postural adjustments in individuals with MS dealing with external perturbations. Ten individuals with MS and ten age-and-gender matched healthy controls were exposed to external perturbations applied at the shoulder level. The perturbations were either predictable or unpredictable as subjects stood with eyes open or closed. Electrical activity of six leg and trunk muscles as well as displacements of the center of pressure (COP) were recorded and quantified within the time intervals typical of anticipatory (APAs) and compensatory (CPAs) postural adjustments. Individuals with MS demonstrated delayed anticipatory onsets of muscle activity and smaller anticipatory COP displacements as compared to healthy control subjects. The deficiency of the APAs was associated with increased displacements of the COP during the balance restoration phase. The results demonstrate the underlying impairment in anticipatory postural control of individuals with MS. The study outcome provides a background for development of rehabilitation strategies focused on balance restoration in people with MS.

Improvement of anticipatory postural adjustments for balance control: effect of a single training session

Humans use anticipatory and compensatory postural strategies to maintain and restore balance when perturbed. Inefficient generation and utilization of anticipatory postural adjustments (APAs) is one of the reasons for postural instability. The aim of the study was to investigate the role of training in improvement of APAs and its effect on subsequent control of posture. Thirteen healthy young adults were exposed to predictable external perturbations before and after a single training session consisting of catches of a medicine ball thrown at the shoulder level. 3-D body kinematics, EMG activity of thirteen trunk and lower limb muscles, and ground reaction forces were recorded before and immediately after a single training session. Muscle onsets, EMG integrals, center of pressure (COP), and center of mass (COM) displacements were analyzed during the anticipatory and compensatory phases of postural control. The effect of a single training session was seen as significantly early muscle onsets and larger anticipatory COP displacements. As a result, significantly smaller peak COM displacements were observed after the perturbation indicating greater postural stability. The outcome of this study provides a background for examining the role of training in improvement of APAs and its effect on postural stability in individuals in need.

Diminished sub-maximal quadriceps force control in anterior cruciate ligament reconstructed patients is related to quadriceps and hamstring muscle dyskinesia

The aim of this study was to determine the effects of anterior cruciate ligament reconstruction (ACLR) on sub-maximal quadriceps force control with respect to quadriceps and hamstring muscle activity. Thirty ACLR individuals together with 30 healthy individuals participated. With real-time visual feedback of muscle force output and electromyographic electrodes attached to the quadriceps and hamstring muscles, subjects performed an isometric knee extension task where they increased and decreased their muscle force output at 0.128Hz within a range of 5-30% maximum voluntary capacity. The ACLR group completed the task with more error and increased medial hamstring and vastus medialis activation (p<0.05). Moderate negative correlations (p<0.05) were observed between quadriceps force control and medial (Spearman's rho=-0.448, p=0.022) and lateral (Spearman's rho=-0.401, p=0.034) hamstring activation in the ACLR group. Diminished quadriceps sub-maximal force control in ACLR subjects was reflective of medial quadriceps and hamstring dyskinesia (i.e., altered muscle activity patterns and coordination deficits). Within the ACLR group however, augmented hamstring co-activation was associated with better quadriceps force control. Future studies should explore the convergent validity of quadriceps force control in ACLR patients.

The effect of aging on anticipatory postural control

The aim of the study was to investigate the differences in anticipatory postural adjustments (APAs) between young and older adults and its effect on subsequent control of posture. Ten healthy older adults and thirteen healthy young adults were exposed to predictable external perturbations using the pendulum impact paradigm. Electromyographic activity of the trunk and leg muscles, the center of pressure (COP), and center of mass (COM) displacements in the anterior-posterior direction were recorded and analyzed during the anticipatory and compensatory postural adjustments (CPAs) phases of postural control. The effect of aging was seen as delayed anticipatory muscle activity and larger compensatory muscle responses in older adults as compared to young adults. Moreover, in spite of such larger reactive responses, older adults were still more unstable, exhibiting larger COP and COM peak displacements after the perturbation than young adults when exposed to similar postural disturbances. Nonetheless, while APAs are impaired in older adults, the ability to recruit muscles anticipatorily is largely preserved; however, due to their smaller magnitudes and delayed onsets, it is likely that their effectiveness in reducing the magnitude of CPAs is smaller. The outcome of the study lends support toward investigating the ways of improving anticipatory postural control in people with balance impairments due to aging or neurological disorders.

Isolated and combined effects of asymmetric stance and pushing movement on the anticipatory and compensatory postural control

OBJECTIVE

To investigate effects of symmetric and asymmetric stance and pushing movement on anticipatory and compensatory postural adjustments (APAs and CPAs).

METHODS

Ten healthy volunteers stood symmetrically (feet parallel) or asymmetrically (one foot forward and the other backward) and pushed a handle with both hands or right or left hand. Bilateral EMG activity of the trunk and leg muscles and center of pressure (COP) displacements in the anterior-posterior (AP) and medial-lateral (ML) directions were recorded and analyzed during the APAs and CPAs.

RESULTS

Isolated asymmetry of stance was associated with larger muscle activity of the backward leg while isolated asymmetry of pushing movement induced larger trunk muscle activity on the contralateral side. A combined asymmetry of stance and pushing movement resulted in the increase or decrease of the thigh muscle activity and ML COP displacement depending on whether both asymmetries were induced on the same side of the body or on opposite sides.

CONCLUSIONS

Both isolated and combined asymmetries affect APAs and CPAs in pushing. Using combined asymmetry of stance and arm movement might be beneficial in performing pushing activity.

SIGNIFICANCE

The outcome of the study provides a basis for studying postural control in individuals with unilateral impairment while performing daily tasks involving pushing.

Compensatory but not anticipatory adjustments are altered in older adults during lateral postural perturbations

OBJECTIVE

This study investigated anticipatory postural adjustments (APAs) and compensatory postural adjustments (CPAs) and their relationship in older adults during lateral postural perturbations.

METHODS

Unpredictable and predictable postural disturbances were induced by a swinging pendulum that impacted at the shoulder level of two groups of older adults, non-fallers (20) and fallers (20), and in a group of young control subjects (20). The electromyographic (EMG) activity of the postural muscles and the center of pressure (COP) displacement were recorded and quantified within the time intervals typical for APAs and CPAs.

RESULTS

Both groups of older adults (non-fallers and fallers) showed higher magnitude of EMG activity in the lateral muscles and increased COP displacement, particularly, during the CPAs time interval when compared to the young group. Older adults, however, were able to change the electrical activity of the muscles during the predictable task by generating APAs with similar magnitudes of those found in young subjects.

CONCLUSIONS

Compensatory but not anticipatory adjustments are altered in older adults during predictable lateral postural perturbations.

SIGNIFICANCE

These findings provide new data on the role of APAs and CPAs in their relationship in older adults during external lateral perturbations and may advance current rehabilitative management strategies to improve balance control in older individuals.

Biomechanical and neurophysiological mechanisms related to postural control and efficiency of movement: a review

Understanding postural control requires considering various mechanisms underlying a person's ability to stand, to walk, and to interact with the environment safely and efficiently. The purpose of this paper is to summarize the functional relation between biomechanical and neurophysiological perspectives related to postural control in both standing and walking based on movement efficiency. Evidence related to the biomechanical and neurophysiological mechanisms is explored as well as the role of proprioceptive input on postural and movement control.

Ajustes posturais antecipatórios e compensatórios ao pegar uma bola em condição de estabilidade e instabilidade postural

Indivíduos jovens pré-selecionam suas estratégias de ajuste postural antes que uma perturbação externa ocorra, com base nas características da tarefa. Entretanto, ainda não é bem conhecido de que maneira o sistema nervoso central lida com os ajustes posturais antecipatórios e compensatórios, mediante alguns treinos de equilíbrio que são comumente usados na prática clínica da Fisioterapia. Treze adultos jovens receberam 20 perturbações posturais externas de pegar uma bola arremessada pelo experimentador sobre condições de estabilidade e instabilidade postural (sobre uma espuma). As atividades eletromiográficas dos músculos tríceps braquial (músculo focal), retoabdominal e paravertebral lombar foram avaliadas nas janelas de tempo típicas dos ajustes posturais antecipatórios e compensatórios, assim como o pico do deslocamento do centro de pressão corporal após a perturbação. A magnitude das integrais da atividade eletromiográfica do músculo tríceps braquial foi significativamente menor em condição de instabilidade postural, não houve diferença estatística entre as condições para as integrais da atividade eletromiográfica dos músculos retoabdominal e paravertebral lombar. O pico do deslocamento anteroposterior do centro de pressão corporal foi similar entre as duas condições. Treino de equilíbrio associado à perturbação externa da postura sobre espuma pode não ser mais eficaz ou eficiente do que sobre uma superfície estável, quando se pretende alterar a atividade dos músculos posturais do tronco. Além disso, este tipo de intervenção pode corroborar para diminuir a ativação antecipatória (ajustes posturais antecipatórios) do músculo focal.

Effect of core stability exercises on feed-forward activation of deep abdominal muscles in chronic low back pain: a randomized controlled trial

STUDY DESIGN

A randomized controlled trial.

OBJECTIVE

To investigate feed-forward activation or timing of abdominal muscle activation in response to rapid shoulder flexion after 8 weeks with core stability exercises, sling exercises, or general exercises in chronic nonspecific low back pain (LBP) patients.

SUMMARY OF BACKGROUND DATA

Delayed onset in abdominal muscles has been associated with LBP. Low load exercises to volitionally activate the transversus abdominis were introduced to restore trunk muscle activation deficits. More forceful co-contraction exercises have been advocated by others. This study explored whether abdominal muscle onset changed after low-load core stability exercises, high-load sling exercises, or general exercises.

METHODS

Subjects (N = 109) with chronic nonspecific LBP of at least 3 months' duration were randomly assigned to 8 weekly treatments with low-load core stability exercises, high-load stabilizing exercises in slings, or general exercises in groups. Primary outcome was onset recorded bilaterally by m-mode ultrasound imaging in the deep abdominal muscles in response to rapid shoulder flexion.

RESULTS

No or small changes were found in onset after treatment. Baseline adjusted between group differences showed a 15 ms (95% confidence interval [CI], 1-28; P = 0.03) and a 19 ms (95% CI, 5-33; P < 0.01) improvement with sling relative to core stability and general exercises, respectively, but on 1 side only. There was no association between changes in pain and onset over the intervention period (R ≤ 0.02).

CONCLUSION

Abdominal muscle onset was largely unaffected by 8 weeks of exercises in chronic LBP patients. There was no association between change in onset and LBP. Large individual variations in activation pattern of the deep abdominal muscles may justify exploration of differential effects in subgroups of LBP.

Trunk muscle control in response to (un)expected turns in cart pushing

Before altering the travel direction in normal gait, anticipatory activation in trunk muscles is observed, followed by a top-down sequence of rotation of body segments. Turning while pushing a cart is a more challenging task for the trunk because of its low stiffness in pushing while walking and the interaction with the high inertia of the cart. 12 healthy subjects pushed a 200 kg cart at shoulder and hip height while making turns (gradual, sharp and unexpected sharp). The normalized electromyogram amplitudes of left (right) lateral and anterior external oblique muscle, and right (left) internal oblique muscle were averaged to represent left (right) trunk rotator muscle activity. The baseline values of trunk rotator muscle activity before the turn and the peak values after the turn were determined. Additionally, peak values of hand forces, twisting moments and twisting motions were assessed. Before turning, higher trunk rotator muscle activity than in straight pushing without turning was only observed before making a turn in the gradual or sharp turn conditions. After the turn, clockwise twisting motion was associated with a clockwise twisting moment induced by the reaction forces at the left hand. Anticipatory activation was initially absent in the unexpected sharp turn, while bilateral trunk rotator muscle activity increased after the turn, indicating co-contraction. In the unexpected turn condition the combination of an uncontrolled twisting motion with delayed muscle activation may increase the potential risk of low-back injury.

The effect of an exercise ball on trunk muscle responses to rapid limb movement

The use of exercise balls as an aid to facilitate improvements in posture in patients with trunk weakness is widely advocated. However, mechanisms underlying any effect on postural mechanisms have received little attention. This study compared the increases in trunk EMG activity in response to limb movement when seated on an exercise ball or on a chair in 16 healthy, moderately active subjects. At the sound of an auditory cue, the subjects carried out either hip flexion or arm flexion (unilateral or bilateral), as fast as possible, whilst sitting on an exercise ball or a standard chair. The amplitude of EMG activity was recorded from selected trunk muscles (erector spinae, external obliques, internal obliques and rectus abdominis) and either an upper limb muscle (deltoid) or a lower limb muscle (rectus femoris). There were minimal differences in amplitudes of EMG activity in any of the trunk muscles between the conditions (ball or chair) following the upper limb movements. These results suggest that there is no benefit in simple arm flexion movements whilst seated on the exercise ball in comparison to a chair. The onset and amplitude of the rectus abdominis (RA) and external obliques (EO) were significantly different between conditions in the hip flexion protocol. However, they do suggest significant benefit in decreasing RA and EO muscle activity onsets and increasing amplitude in the hip flexion condition. These results may have implications for rehabilitation of those with trunk muscle deficits such as stroke.

Anticipatory postural adjustments in children with hemiplegia and diplegia

Anticipatory postural adjustments (APAs) play an important role in the performance of many activities requiring the maintenance of standing posture. However, little is known about if and how children with cerebral palsy (CP) generate APAs. Two groups of children with CP (hemiplegia and diplegia) and a group of children with typical motor development performed arm flexion and extension movements while standing on a force platform. Electromyographic activity of six trunk and leg muscles and displacement of center of pressure (COP) were recorded. Children with CP were able to generate anticipatory postural adjustments and produce directionally specific APAs and COP displacements similar to those described in adults and typically developing children. However, children with diplegia were unable to generate APAs of the same magnitude as children with typical development and hemiplegia and had higher baseline muscle activity prior to movement. In children with diplegia, COP was posteriorly displaced and peak acceleration was smaller during bilateral extension compared to children with hemiplegia. The outcomes of the study highlight the role of APAs in the control of posture of children with CP and point out the similarities and differences in anticipatory control in children with diplegia and hemiplegia. These differences may foster ideas for treatment strategies to enhance APAs in children with CP.

Early and late components of feed-forward postural adjustments to predictable perturbations

OBJECTIVES

The purpose was to investigate two types of feed-forward postural adjustments associated with preparation to predictable external perturbations.

METHODS

Nine subjects stood on a wedge, toes-up or toes-down while a pendulum impacted their shoulders. EMGs of leg and trunk muscles were analyzed within the framework of the uncontrolled manifold hypothesis.

RESULTS

Early postural adjustments (EPAs) were seen 400-500 ms and anticipatory postural adjustments (APAs), 100-150 ms prior to the impact. EPAs and APAs were also seen in the time profiles of muscle modes representing muscle groups with linear scaling of the activation levels. Center of pressure shifts were stabilized by co-varied adjustments in muscle mode magnitudes across trials. The index of these multi-muscle synergies showed two drops (anticipatory synergy adjustments, ASAs), prior to EPA and APA in each subject. The findings were consistent between the two conditions.

CONCLUSIONS

The results show that feed-forward postural adjustments represent a sequence of two phenomena, EPAs and APAs. Each of those is preceded by ASAs that reduce stability of a variable that is to be adjusted during the EPAs and APAs. The findings fit a hierarchical scheme with synergic few-to-many mappings at each level of the hierarchy based on the referent body configuration hypothesis.

SIGNIFICANCE

The results show the complexity of the postural preparation to action. Potentially, they have implications for the current strategies of rehabilitation of patients with neuro-motor disorders characterized by impaired postural control.

The effect of decreased visual acuity on control of posture

OBJECTIVES

The goal of this study was to investigate the effect of visual acuity on the anticipatory (APAs) and compensatory (CPAs) components of postural control.

METHODS

Ten individuals participated in the experiments involving perturbations induced by a pendulum while their visual acuity was altered. The different visual acuity conditions were no glasses, blurred vision induced by wearing glasses with positive or negative lenses, and no vision. EMG activity of trunk and leg muscles and ground reaction forces were recorded during the typical anticipatory and compensatory periods.

RESULTS

In the no vision condition the subjects did not generate APAs, which resulted in the largest displacements of the center of pressure (COP) after the perturbation (p<0.01). In all other visual conditions APAs were present showing a distal to proximal order of muscle activation. The subjects wearing positive glasses showed earlier and larger anticipatory EMGs than subjects wearing negative glasses or no glasses at all.

CONCLUSIONS

The study outcome revealed that changes in visual acuity induced by wearing differently powered eye glasses alter the generation APAs and as a consequence, affect the compensatory components of postural control.

SIGNIFICANCE

The observed changes in APAs and CPAs in conditions with blurred vision induced by positive and negative glasses suggest the importance of using glasses with an appropriate power. This outcome should be taken into consideration in balance rehabilitation of individuals wearing glasses.

Postural control in response to a perturbation: role of vision and additional support

The purpose of the study was to investigate the availability of vision and additional support on anticipatory (APA) and compensatory (CPA) postural adjustments and their interaction. Eight healthy adults were exposed to external perturbations induced at the shoulder level while standing with and without holding onto a walker in full vision and while blindfolded. Electrical activity of the trunk and leg muscles and center of pressure (COPAP) displacement were recorded and quantified within the time intervals typical of APA and CPA. The results showed that with full vision, there was no difference in both APA and CPA in standing with and without holding onto a walker. With subjects holding onto a walker, CPA in standing blindfolded were comparable to CPA in full vision; this was seen in changes in the electrical activity of most of the muscles at the individual muscle, joint, and the muscle group levels as well as in COPAP displacement. The findings suggest that (1) in conditions where vision is available, vision overrules simultaneously available proprioceptive information from the support, (2) while in conditions where vision is not available, proprioceptive information from the support or support itself could be substituted for vision. It is possible to suggest that using a non-stabilizing support could be a valuable strategy to improve postural control when visual information is not available or compromised.

Two stages and three components of the postural preparation to action

Previous studies of postural preparation to action/perturbation have primarily focused on anticipatory postural adjustments (APAs), the changes in muscle activation levels resulting in the production of net forces and moments of force. We hypothesized that postural preparation to action consists of two stages: (1) Early postural adjustments (EPAs), seen a few hundred ms prior to an expected external perturbation and (2) APAs seen about 100 ms prior to the perturbation. We also hypothesized that each stage consists of three components, anticipatory synergy adjustments seen as changes in covariation of the magnitudes of commands to muscle groups (M-modes), changes in averaged across trials levels of muscle activation, and mechanical effects such as shifts of the center of pressure. Nine healthy participants were subjected to external perturbations created by a swinging pendulum while standing in a semi-squatting posture. Electrical activity of twelve trunk and leg muscles and displacements of the center of pressure were recorded and analyzed. Principal component analysis was used to identify four M-modes within the space of muscle activations using indices of integrated muscle activation. This analysis was performed twice, over two phases, 400-700 ms prior to the perturbation and over 200 ms just prior to the perturbation. Similar robust results were obtained using the data from both phases. An index of a multi-M-mode synergy stabilizing the center of pressure displacement was computed using the framework of the uncontrolled manifold hypothesis. The results showed high synergy indices during quiet stance. Each of the two stages started with a drop in the synergy index followed by a change in the averaged across trials activation levels in postural muscles. There was a very long electromechanical delay during the early postural adjustments and a much shorter delay during the APAs. Overall, the results support our main hypothesis on the two stages and three components of the postural preparation to action/perturbation. This is the first study to document anticipatory synergy adjustments in whole-body tasks. We interpret the results within the referent configuration hypothesis (an extension of the equilibrium-point hypothesis): The early postural adjustment is based primarily on changes in the coactivation command, while the APAs involve changes in the reciprocal command. The results fit an earlier hypothesis that whole-body movements are controlled by a neuromotor hierarchy where each level involves a few-to-many mappings organized to stabilize its overall output.

The role of anticipatory postural adjustments in compensatory control of posture: 2. Biomechanical analysis

The central nervous system (CNS) utilizes anticipatory (APAs) and compensatory (CPAs) postural adjustments to maintain equilibrium while standing. It is known that these postural adjustments involve displacements of the center of mass (COM) and center of pressure (COP). The purpose of the study was to investigate the relationship between APAs and CPAs from a kinetic and kinematic perspective. Eight subjects were exposed to external predictable and unpredictable perturbations induced at the shoulder level while standing. Kinematic and kinetic data were recorded and analyzed during the time duration typical for anticipatory and compensatory postural adjustments. When the perturbations were unpredictable, the COM and COP displacements were larger compared to predictable conditions with APAs. Thus, the peak of COM displacement, after the pendulum impact, in the posterior direction reached 28+/-9.6mm in the unpredictable conditions with no APAs whereas it was 1.6 times smaller, reaching 17+/-5.5mm during predictable perturbations. Similarly, after the impact, the peak of COP displacement in the posterior direction was 60+/-14 mm for unpredictable conditions and 28+/-3.6mm for predictable conditions. Finally, the times of the peak COM and COP displacements were similar in the predictable and unpredictable conditions. This outcome provides additional knowledge about how body balance is controlled in presence and in absence of information about the forthcoming perturbation. Moreover, it suggests that control of posture could be enhanced by better utilization of APAs and such an approach could be considered as a valuable modality in the rehabilitation of individuals with balance impairment.

Postural feedback scaling deficits in Parkinson's disease

Many differences in postural responses have been associated with age and Parkinson's disease (PD), but until now there has been no quantitative model to explain these differences. We developed a feedback control model of body dynamics that could reproduce the postural responses of young subjects, elderly subjects, and subjects with PD, and we investigated whether the postural impairments of subjects with PD can be described as an abnormal scaling of postural feedback gain. Feedback gains quantify how the nervous system generates compensatory joint torques based on kinematic responses. Seven subjects in each group experienced forward postural perturbations to seven different backward support surface translations ranging from 3- to 15-cm amplitudes and with a constant duration of 275 ms. Ground reaction forces and joint kinematics were measured to obtain joint torques from inverse dynamics. A full-state feedback controller with a two-segment body dynamic model was used to simulate joint kinematics and kinetics in response to perturbations. Results showed that all three subject groups gradually scaled postural feedback gains as a function of perturbation amplitudes, and the scaling started even before the maximum allowable ankle torque was reached. This result implies that the nervous system takes body dynamics into account and adjusts postural feedback gains to accommodate biomechanical constraints. PD subjects showed significantly smaller than normal ankle feedback gain with low scaling and larger hip feedback gain, which led to an early violation of the flat-foot constraint and unusually small (bradykinetic) postural responses. Our postural feedback control model quantitatively described the postural abnormality of the patients with PD as abnormal feedback gains and reduced ability to modify postural feedback gain with changes in postural challenge.

The role of anticipatory postural adjustments in compensatory control of posture: 1. Electromyographic analysis

Anticipatory (APAs) and compensatory (CPAs) postural adjustments are the two principal mechanisms that the central nervous system uses to maintain equilibrium while standing. We studied the role of APAs in compensatory postural adjustments. Eight subjects were exposed to external predictable and unpredictable perturbations induced at the shoulder level, while standing with eyes open and closed. Electrical activity of leg and trunk muscles was recorded and analyzed during four epochs representing the time duration typical for anticipatory and compensatory postural control. No anticipatory activity of the trunk and leg muscles was seen in the case of unpredictable perturbations; instead, significant compensatory activation of muscles was observed. When the perturbations were predictable, strong anticipatory activation was seen in all the muscles: such APAs were associated with significantly smaller compensatory activity of muscles and COP displacements after the perturbations. The outcome of the study highlights the importance of APAs in control of posture and points out the existence of a relationship between the anticipatory and the compensatory components of postural control. It also suggests a possibility to enhance balance control by improving the APAs responses during external perturbations.

Reaching to multiple targets when standing: the spatial organization of feedforward postural adjustments

We examined the spatial organization of feedforward postural adjustments produced prior to and during voluntary arm reaching movements executed while standing. We sought to investigate whether the activity of postural muscles before and during reaching was directionally tuned and whether a strategy of horizontal force constraint could be observed. To this end, eight human subjects executed self-paced reach-to-point movements on the random illumination of one of 13 light targets placed within a 180 degrees array centered along the midline of the body. Analysis was divided into two periods: a first corresponding to the 250 ms preceding the onset of the reaching movements (termed pPA period) and a second 250-ms period immediately preceding target attainment (the aPA period). For both periods, electromyographic activity of the lower limb muscles revealed a clear directional tuning, with groups of muscles being activated for similar directions of reach. Analysis of horizontal ground reaction forces supported the existence of a force constraint strategy only for the pPA period, however, with those in the aPA period being more widely dispersed. We suggest that the strategy adopted for feedforward pPAs is one where the tuned muscle synergies constrain the forces diagonally away from the center of mass (CoM) to move it within the support base. However, the need to control for final finger and body position for each target during the aPA phase resulted in a distribution of vectors across reaching directions. Overall, our results would support the idea that endpoint limb force during postural tasks depends on the use of functional muscle synergies, which are used to displace the CoM or decelerate the body at the end of the reach.

Visual feedback training improves postural adjustments associated with moving obstacle avoidance in elderly women

The study examined the impact of visually guided weight shifting (WS) practice on the postural adjustments evoked by elderly women when avoiding collision with a moving obstacle while standing. Fifty-six healthy elderly women (70.9+/-5.7 years, 87.5+/-9.6 kg) were randomly assigned into one of three groups: a group that completed 12 sessions (25 min, 3s/week) of WS practice in the Anterior/Posterior direction (A/P group, n=20), a group that performed the same practice in the medio/lateral direction (M/L group, n=20) and a control group (n=16). Pre- and post-training, participants were tested in a moving obstacle avoidance task. As a result of practice, postural response onset shifted closer to the time of collision with the obstacle. Side-to-side WS resulted in a reduction of the M/L sway amplitude and an increase of the trunk's velocity during avoidance. It is concluded that visually guided WS practice enhances elderly's ability for on-line visuo-motor processing when avoiding collision eliminating reliance on anticipatory scaling. Specifying the direction of WS seems to be critical for optimizing the transfer of training adaptations.