Influence of a visuo-spatial, verbal and central executive working memory task on postural control.

In this study, participants were required to perform different working memory (WM) tasks (a verbal task, a visuo-spatial task with two levels of difficulty and a central executive task) under different challenges to postural control (sitting, shoulder width stance and tandem stance). When a WM task was added, changes in postural sway were characterized by an increase in frequency and decrease in amplitude of sway indicating a tighter control. We found no changes in postural control between the different types of WM tasks, which might support a general capacity limitation hypothesis. However, no changes were found in performance of the WM when postural stance was modified and no changes were found in postural sway, when the difficulty level of the visuo-spatial task was modified. Consequently, the results seem to indicate that the addition of a WM task, regardless of task type or task difficulty, forces the central nervous system to choose a tighter control strategy.     

Anticipatory postural control strategies related to predictive perturbations

Balance reactions can be seen as responses to sensory information on a feedback basis, but when a balance threatening situation can be predicted, an anticipatory postural control strategy can be used. This study describes characteristics of proactive postural adjustments related to repetitive rhythmic perturbations. Furthermore, the study aimed to investigate age dependency of these anticipatory strategies. Fourteen young (age 27+/-2 years) and 10 community-dwelling elderly adults (76+/-5 years) participated in this study. Centre of pressure displacement was evaluated while the participants were standing on a moveable force plate. Perturbations consisted of alternating left/right frontal plane tilts of the platform or alternating forward/backward slides in the sagittal plane. Automation of postural control was evaluated by a dual task approach after familiarization to the perturbations. Centre of pressure displacement 200 ms before a tilt perturbation was significantly related to the direction of the perturbation. This early postural adjustment was significantly increased during the dual task condition. The dual task effect was more pronounced in the elderly, but this age-effect was not significant due to large inter-individual variation. The frequency of stepping reactions as response to slide perturbations decreased with conditioning, but increased again in the elderly during dual task condition. Conclusion: This study showed that both young and elderly use anticipatory postural control strategies to minimize the impact of predictable perturbations. The proactive postural adjustments are further enhanced when a concurrent cognitive task is introduced. Postural control seems less automated in elderly and becomes insufficient during very challenging perturbations.     

Postural control is scaled to level of postural threat

This study investigated control of posture when standing at different surface heights above ground level. Alterations in surface height were used to modify threat to postural control. Sixty-two healthy adults (mean+/-S.D.=20.3+/-1.3 years) stood quietly on a force plate 40 cm (LOW threat), 100 cm (MEDIUM threat) or 160 cm (HIGH threat) above ground level. Each standing trial was performed with eyes open for 120 s. Postural threat was presented in ascending (n=31) or descending (n=31) order with the first threat condition in each series (LOW threat for ascending group, HIGH threat for descending group) repeated. This manipulation allowed for an examination of set effects (i.e. prior experience of postural threat) on postural control. The results demonstrated scaling of postural control variables to level of postural threat. Amplitude of centre of pressure (COP) displacement decreased and frequency of COP displacement increased linearly as postural threat increased from LOW to HIGH. The central nervous system progressively tightened control of posture as postural threat increased. Initial exposure to the HIGH or LOW threat condition influenced postural control differently. The group who received the HIGH threat condition first (descending) demonstrated increased amplitude of COP displacement in the anterior-posterior direction compared with the group who received the LOW threat condition first (ascending). A 'first trial' effect was observed when standing for two consecutive trials but only at the LOW threat condition. Decreased amplitude and increased frequency of COP displacement were observed on the first trial compared to the second trial. The results of this study demonstrated that control of posture is influenced not only by the threat to posture but also by the order in which the threat to posture is experienced.     

Adaptation of vibration-induced postural sway in individuals with Parkinson's disease

Postural control requires accurate integration of visual, vestibular, cutaneous, and proprioceptive sensory information. Previous research suggests that patients with Parkinson's disease (PD) may have difficulty with this integration process, particularly involving incongruent visual information. The purpose of this study was to determine whether PD patients also show difficulty in adaptation to erroneous proprioceptive information. Postural reactions to soleus muscle vibration were explored in 8 PD patients, 8 healthy elderly, and 8 young adults. Postural sway was recorded using an electromagnetic motion analysis system in four conditions (2 vision x 2 vibration) with four trials in each condition to test subjects' adaptation to the vibrator stimulus. The results showed that PD patients did adapt to the vibration across trials, similar to those of both control groups. It was concluded that PD patients in the early stages of the disease could override inaccurate proprioceptive inputs, relying more on true vestibular and/or visual information. These results suggest that the basal ganglia are not critical for this adaptation process in postural control.     

Soleus H-reflex and its relation to static postural control

The Hoffmann reflex (H-reflex) test has been extensively used to investigate the responsiveness of Ia afferent spinal loop in animal and human studies. The H-reflex response is influenced by multiple neural pathways and the assessment of H-reflex variation is a useful tool in understanding the neural mechanisms in control of movement. Recently, several studies have examined the relationship between the H-reflex modulation and postural stability. For example, it has been reported that the amplitude of soleus (SOL) H-reflex is depressed in relation to increased body sway during upright standing on a soft surface compared to that on a solid surface. It has been suggested that the SOL H-reflex modulation under such condition is predominately affected by the presynaptic inhibitory mechanisms for avoiding oversaturation of the spinal motoneurons. It has also been reported that after balance training, the SOL H-reflex amplitude is down-modulated in parallel with improvement in balance control, suggesting a functional adaptation at the supraspinal levels. The aim of this review is to examine the current literature on the relationship between H-reflex modulation and postural control for a better understanding of the physiological mechanisms involved in control of posture in humans.     

Otolith contributions to postural control in man: short latency motor responses following sound stimulation in a case of otolithic Tullio phenomenon

The effect of unilateral sound stimulation (95 dB(A), 490 Hz) on antigravity muscles subserving postural balance, and on neck and forearm muscles was recorded in a patient with sound-induced oscillopsia, vertigo, and postural imbalance (otolithic Tullio phenomenon) using surface EMG electrodes; postural sway was measured while the patient stood upright with his eyes closed on a Kistler platform. The tibialis anterior and gastrocnemius muscles of the ipsilateral leg were coactivated at latencies as short as 47 ms (mean 58 ± 10 ms), with a weaker activation of the contralateral lower leg muscles following about 10 ms later. Neck muscles, triceps brachii, and forearm extensor muscles ipsilateral to the stimulated ear were activated at latencies as long as those of the lower leg muscles or often longer. Changes in posture, e.g. turning the head in either direction, balancing on one foot, and sitting or resting supine, as well as visual input modulated these early motor responses. They are thus of functional significance for counterregulating body perturbations and maintaining equilibrium in man. We conclude that otolithic input contributes directly to the control of posture and gait by fast descending pathways.     

Adaptation to walking with an exoskeleton that assists ankle extension

The goal of this study was to investigate adaptation to walking with bilateral ankle-foot exoskeletons with kinematic control that assisted ankle extension during push-off. We hypothesized that subjects would show a neuromotor and metabolic adaptation during a 24 min walking trial with a powered exoskeleton. Nine female subjects walked on a treadmill at 1.36 ± 0.04 ms^?1 during 24 min with a powered exoskeleton and 4 min with an unpowered exoskeleton. Subjects showed a metabolic adaptation after 18.5 ± 5.0 min, followed by an adapted period. Metabolic cost, electromyography and kinematics were compared between the unpowered condition, the beginning of the adaptation and the adapted period. In the beginning of the adaptation (4 min), a reduction in metabolic cost of 9% was found compared to the unpowered condition. This reduction was accompanied by reduced muscular activity in the plantarflexor muscles, as the powered exoskeleton delivered part of the necessary ankle extension moment. During the adaptation this metabolic reduction further increased to 16%, notwithstanding a constant exoskeleton assistance. This increased reduction is the result of a neuromotor adaptation in which subjects adapt to walking with the exoskeleton, thereby reducing muscular activity in all leg muscles. Because of the fast adaptation and the significant reductions in metabolic cost we want to highlight the potential of an ankle-foot exoskeleton with kinematic control that assists ankle extension during push-off.     

Methods for evaluation of postural control adaptation

New methods were developed to determine the dynamic changes of postural control during the initial exposure to large perturbances of stance. The adjustments of postural control over time in measured anteroposterior torque, were investigated in ten normal subjects. Perturbations of stance were evoked by two high intensity vibrators applying pseudorandom stimulation either to the calf muscles or the paravertebral muscles of the neck. The new methods use a system identification approach, which distinguishes between feedback control, adaptation of postural responses and adaptation to stimulus. This approach makes it possible to quantify motion dynamics and complexity, stimulus impact and adjustments of postural control. Quantification of the different adaptive responses could be useful for diagnostic purposes, in evaluating treatment efficacy and patient progress in rehabilitation programs.     

Smartphone accelerometry to assess postural control in individuals with multiple sclerosis

BackgroundFalls are a major health concern for people with Multiple Sclerosis (pwMS), and impaired postural control is an important predictor of falls. Lab-based technology to measure posture is precise but expensive, and clinical tests may not capture underlying impairments. An alternative solution is to leverage smartphone accelerometry as it is affordable, ubiquitous, and portable.Research question: Can smartphone accelerometry measure postural control compared to a force plate and research grade accelerometer in pwMS, and can smartphone accelerometry discriminate between assisted device and non-assisted device users?Methods27 pwMS (12 assisted device users, 15 non-assisted device users) stood on a force plate while holding a smartphone with an attached research grade accelerometer against their chest. Participants performed two, 30 s trials of: eyes open, eyes closed, semi-tandem, tandem, and single leg. Acceleration and center of pressure were extracted, and Root Mean Square (RMS) and 95 % confidence ellipse were calculated. Spearman’s correlations were performed, and receiving operating characteristic (ROC) curves and the Area Under the Curve (AUC) were calculated.ResultsThere were moderate to high correlations between the smartphone and accelerometer for RMS (ρ = 0.85 – 1.0; p = 0.001 – <0.001) and 95 % area ellipse (ρ = 0.92 – 0.99; p = <0.001). There were weak to moderate correlations between the smartphone and force plate for RMS (ρ = 0.38 – 0.92; p = 0.06 – <0.001) and 95 % area ellipse (ρ = 0.69 – 0.90 p = 0.002 – <0.001). To discriminate between assisted device usage, ROC curves for smartphone outputs were constructed, the AUC was high and statistically significant (p < 0.001 – 0.02).SignificanceThere is potential to leverage smartphone accelerometery to measure postural control in pwMS. These finding provide preliminary results to support the development of a mobile health application to measure fall risk for pwMS.     

A method to model anticipatory postural control in driver braking events

Human body models (HBMs) for vehicle occupant simulations have recently been extended with active muscles and postural control strategies. Feedback control has been used to model occupant responses to autonomous braking interventions. However, driver postural responses during driver initiated braking differ greatly from autonomous braking. In the present study, an anticipatory postural response was hypothesized, modelled in a whole-body HBM with feedback controlled muscles, and validated using existing volunteer data. The anticipatory response was modelled as a time dependent change in the reference value for the feedback controllers, which generates correcting moments to counteract the braking deceleration. The results showed that, in 11 m/s² driver braking simulations, including the anticipatory postural response reduced the peak forward displacement of the head by 100 mm, of the shoulder by 30 mm, while the peak head flexion rotation was reduced by 18°. The HBM kinematic response was within a one standard deviation corridor of corresponding test data from volunteers performing maximum braking. It was concluded that the hypothesized anticipatory responses can be modelled by changing the reference positions of the individual joint feedback controllers that regulate muscle activation levels. The addition of anticipatory postural control muscle activations appears to explain the difference in occupant kinematics between driver and autonomous braking. This method of modelling postural reactions can be applied to the simulation of other driver voluntary actions, such as emergency avoidance by steering.     

A method to model anticipatory postural control in driver braking events

Human body models (HBMs) for vehicle occupant simulations have recently been extended with active muscles and postural control strategies. Feedback control has been used to model occupant responses to autonomous braking interventions. However, driver postural responses during driver initiated braking differ greatly from autonomous braking. In the present study, an anticipatory postural response was hypothesized, modelled in a whole-body HBM with feedback controlled muscles, and validated using existing volunteer data. The anticipatory response was modelled as a time dependent change in the reference value for the feedback controllers, which generates correcting moments to counteract the braking deceleration. The results showed that, in 11 m/s² driver braking simulations, including the anticipatory postural response reduced the peak forward displacement of the head by 100 mm, of the shoulder by 30 mm, while the peak head flexion rotation was reduced by 18°. The HBM kinematic response was within a one standard deviation corridor of corresponding test data from volunteers performing maximum braking. It was concluded that the hypothesized anticipatory responses can be modelled by changing the reference positions of the individual joint feedback controllers that regulate muscle activation levels. The addition of anticipatory postural control muscle activations appears to explain the difference in occupant kinematics between driver and autonomous braking. This method of modelling postural reactions can be applied to the simulation of other driver voluntary actions, such as emergency avoidance by steering.     

Obesity, mechanical and strength relationships to postural control in adolescence

There is preliminary evidence that BMI is positively correlated with movement variability of standing posture. However, this negative effect of obesity on postural control may be mediated by the change in other body scale variables (e.g., mechanical and fitness) that also occur with changes in BMI. This study investigated the influence of selected body scale (height, body mass, BMI), body composition (body fat percentage), mechanical (moment of inertia - MI) and strength (S) variables as predictors of the control of postural motion in adolescents. 125 healthy adolescents (65 boys, 60 girls) with a wide range of BMI (13.8-31.0 kg/m(2)) performed a battery of tests that assessed body composition, anthropometry, muscular strength and postural control. Multiple measures of postural motion variability were derived for analysis with body scale, mechanical and lower extremity strength variables separately for boys and girls. BMI, height and body mass, considered both separately and collectively, were poor and/or inconsistent predictors of variability in all three posture tasks. However, the ratio of lower extremity strength to whole body moment of inertia showed the highest positive correlation to most postural variability measures in both boys and girls and these effects were strongest in the less stable tasks of single leg standing and recovery of stance. Our findings support the hypothesis that diminished lower extremity strength to mechanical constraint ratio compromises the robustness of the strength to body scale relation in movement and postural control.     

The influence of foam surfaces on biomechanical variables contributing to postural control

The individual receptor systems of the somatosensory system include the muscle receptors, joint receptors and mechanoreceptors. This study was intended to investigate how standing on foam surfaces would change the multiple biomechanical variables associated with the functions of each of these individual receptor systems in the foot. Mechanical inputs to these receptors, including the changes in plantar pressures, ankle joint rotation and velocity, as well as postural responses from the gastrocnemius and tibialis anterior muscles, were measured during an abrupt toes-up movement of the supporting base. These measurements were compared for three different surface conditions (i.e. hard surface, 1″, and 2″ foams, respectively). A group of 15 healthy, young male subjects participated in the study. The results showed that standing on foam (or soft surface) would affect the inputs to both joint receptors and cutaneous mechanoreceptors in the foot, but not the muscle receptors in the early phase of the platform movement. Furthermore, the medium and long latency responses in the leg muscles appeared to be delayed significantly when standing on foam, while the short latency response in the gastrocnemius muscle was unchanged.     

Motor dual task with eyes closed improves postural control in patients with functional motor disorders: A posturographic study

BackgroundFunctional motor disorders (FMD) are highly disabling neurological conditions in which postural control deficits increase the risk of falls and disability in performing daily living activities. Scattered evidence suggests that such disturbances may depend on abnormal attentional focus and might improve with distraction.Research questionHow do motor and cognitive dual tasks performed under two different sensory conditions shape postural control in patients with FMD.MethodsThis posturographic study involved 30 patients with FMD (age, 45.20 ± 14.57 years) and 30 healthy controls (age, 41.20 ± 16.50 years). Postural parameters were measured with eyes open, and eyes closed in quiet stance (single task) and on a motor dual task (m-DT) and a calculation (cognitive) dual task (c-DT). The dual task effect (DTE, expressed in percentage) on motor and cognitive performance was calculated for sway area, length of Center of Pressure (CoP), and velocity of CoP displacement.ResultsThere was a statistically significant three-way interaction between task, condition, and group for the DTE on sway area (p = 0.03). The mean sway area DTE on the motor task in the eyes-closed condition was increased by 70.4 % in the healthy controls, while it was decreased by 1% in the patient group (p = 0.003). No significant three-way interaction was observed for the DTE on length of CoP and velocity of CoP displacement.SignificanceThis study provides novel preliminary evidence for the benefit of a simple motor dual task in the eyes closed condition as a way to improve postural control in patients with FMD. These findings are relevant for the management of postural control disorders in patients with FMD.     

Effect of hyperbaric air on long-term memory organization and recall.

Three experiments are reported which investigated the effects of hyperbaric air on long-term memory. In the first, word lists were learned at 1 and 8.6 ATA using a variable input-free recall paradigm. It was found that learning was affected but not clustered memory organization, and it was concluded that disorganization of memory is not a factor contributing to the learning deficit found with hyperbaric air. In the second and third experiments it was found that the recall of words, which had been learned when non-narcotic, was disrupted at 10 ATA by hyperbaric air and that this disruption was not overcome by providing memory-cues at the time of recall. Two possible explanations for these results are discussed. A hypothesis is put forward to reconcile the results of various hyperbaric memory experiments by pointing out that a relationship between stress and learning found with nitrous oxide could be applicable to these studies also.     

The influence of ankle support on postural control

Postural control assessments are commonly administered to athletes as part of a pre-season screening. Establishing a baseline level of function permits the clinician to compare post-injury results to normal functioning during the return to play decision-making process. In the athletic setting, follow-up tests may be completed on the sideline immediately following injury. We sought to examine the effect of commonly administered external ankle joint support on postural control using the balance error scoring system (BESS) and the NeuroCom sensory organization test (SOT). Nineteen volunteers free from balance issues completed three sessions with varied ankle support: bilateral prophylactic ankle taping, laced bracing device, or barefoot. Each session included an initial balance assessment on the BESS and SOT, a 20 min treadmill walk, and post-walk balance test. Fewer errors, indicating improved balance, were committed on the BESS during the barefoot condition than the braced ankle condition (p = 0.044) at the pre-walk assessment. During the post-walk assessment, fewer errors were committed during the barefoot condition compared to the braced ankle condition (p = 0.034) and the taped ankle condition (p = 0.037). All ankle support conditions showed similar improvements in balance between the pre and post-walk assessments on the BESS (p < 0.001) and SOT composite balance score (p = 0.009). These findings indicate that ankle support devices may influence postural control on the BESS, but not on the NeuroCom SOT. Clinicians using the BESS as a balance assessment device at multiple time points should be consistent in the application of ankle support devices.     

Kinematic analysis of postural control in gymnasts vs. athletes practicing different sports

Purpose

The aim of this study was to examine the postural control difference between gymnasts and other athletes practicing different sports.

Methods

Twenty-eight elite-level male athletes (7 gymnasts, 7 volleyball players, 7 windsurfers, and 7 monofin swimmers) participated in this study. Two-dimensional kinematic analysis of mediolateral and anteroposterior sway velocity of the center of mass (COM) on single plane balance board in bipedal and unipedal conditions, and on hard and foam surfaces was performed using two cameras (Sony, DCR-PC108^E, 50 Hz). Data were digitized using the video-based data analysis system SkillSpector.

Results

Findings showed significant between groups differences regardless of the postural stance i.e., unipedal or bipedal (d = 1.76–6.82, p < 0.01). In this regards, windsurfers presented the best postural performance in terms of sway velocity of the COM followed by gymnasts, volleyball players, and monofin swimmers. Furthermore, regardless of the postural stance, significant differences between surfaces (d = 0.61–1.03, p < 0.05) were established. In this context, COM’s sway velocity was higher for hard compared with foam surfaces for all disciplines (d = 0.75–0.66, p < 0.05) except monofin swimmers who presented higher COM’s sway velocity in foam compared to hard surfaces. Further, in the anteroposterior stance, windsurfers and gymnasts showed significantly lower floor contacts number compared with the other groups (d = 1.76–2.39, p < 0.01). However, volleyball players and monofin swimmers showed comparable postural performance in the mediolateral plane.

Conclusions

The windsurfers presented the best postural control followed by gymnasts, volleyball players, and monofin swimmers irrespective of the surface’s nature (i.e., hard or foam) and postural stance (i.e., bipedal or unipedal). Therefore, it is recommended to incorporate windsurfing practice to promote postural control of gymnastic athletes.

     

Altered postural control in anticipation of postural instability in persons with recurrent low back pain

Insight into the mechanisms of altered postural control in persons with low back pain (LBP) could lead to better interventions for patients with LBP. This study investigated (1) whether persons with recurrent LBP have an altered body inclination, and (2) whether anticipation of postural instability further alters body inclination. Thirty-three young healthy individuals and 56 young persons with recurrent LBP participated in this study. The upright standing posture was evaluated by means of two piezo-resistive electrogoniometers and a force platform for the conditions as follows: (1) quiet stance with and without vision, and (2) in anticipation of postural instability due to a ballistic arm movement or ankle muscle vibration. No differences in body inclination were observed when visual information was available between the two groups (P > 0.05). However, significant more forward inclination was seen in the persons with recurrent LBP when vision was occluded (+7.4%) and in anticipation of postural instability (+19%) (P < 0.05) compared to the healthy individuals. The results suggest that young persons with recurrent LBP have an altered body inclination that might be caused by anticipation of postural instability. The adopted forward inclined posture may potentially be a factor in the recurrence of LBP.