Upright Stance Research Articles

Changes in postural sway during upright stance after short-term lower limb physical inactivity: A prospective study.

Previous studies have reported that motor behavior is affected by short-term physical inactivity using cast immobilization; however, the effects of inactivity on postural sway are not well-understood. This study aimed to investigate the effects of short-term lower limb disuse on postural sway in the upright position after cast removal. Twenty-two healthy young adults were enrolled, and each participant’s lower limb on one side was fixed with a soft bandage and medical splint made from metal and soft urethane for 10 h. Fluctuations in the center of pressure (COP) were measured before and after immobilization; the total trajectory length, mean velocity, COP root mean square (RMS) area, mean medial-lateral (M-L) COP, and mean anterior-posterior (A-P) COP were selected as evaluation parameters. Compared with the postural sway before cast application, we noted an increase and shift (from the fixed to the nonfixed side) in the postural sway after cast removal. Our results therefore suggest that short-term disuse may cause acute changes in COP movements during quiet standing. Moreover, patients may maintain their standing posture by adopting a compensatory strategy involving lateral control, similar to individuals with stroke and patients who have undergone total knee arthroplasty.

Decreased vestibular efficacy contributes to abnormal balance in Parkinson's disease

Background and purposeAbnormal balance is poorly responsive to dopaminergic therapy in Parkinson's disease (PD). Decreased vestibular efficacy may contribute to imbalance in PD. The purpose of this study was to investigate the relationship between vestibular measures of dynamic posturography and imbalance in PD while accounting for confounder variables. Methods106 patients with PD underwent dynamic posturography for the 6 conditions of the sensory integration test (SOT) using the Neurocom Equitest device. All SOT measures, nigrostriatal dopaminergic denervation ((+)-[11C]DTBZ PET), brain acetylcholinesterase ([11C]PMP PET), age, duration of disease, cognitive and parkinsonian motor scores, and ankle vibration sensitivity were used as regressors in a stepwise logistic regression model comparing PD patients with versus without imbalance defined as Hoehn and Yahr (HY) stage 2.5 or higher. ResultsThe presence of imbalance was significantly associated with vestibular ratio COP RMS (P = 0.002) independently from visual ratio COP velocity (P = 0.012), thalamic acetylcholinesterase activity (P = 0.0032), cognition (P = 0.006), motor severity (P = 0.0039), age (P = 0.001), ankle vibration sensitivity (P = 0.0008), and borderline findings for somatosensory ratio COP velocity (P = 0.074) and visual ratio COP RMS (P = 0.078). Nigrostriatal dopaminergic denervation did not achieve significance. ConclusionsThe inability to efficaciously utilize vestibular information to retain upright stance is a determinant of imbalance in PD independent from visual and somatosensory processing changes and nigrostriatal dopaminergic losses. Thalamic, but not cortical, cholinergic denervation incrementally predicted balance abnormality. Further research is needed to investigate an intrinsic role of the cholinergic thalamus in multi-sensory, in particular vestibular, processing functions of postural control in PD.

Between-leg asymmetry in automatic postural responses to stance perturbations in people with Parkinson's disease

BackgroundPeople with Parkinson's disease (PwPD) showed impairments of balance control which can be aggravated by the presence of higher interlateral postural asymmetry caused by a distinct dopaminergic loss in the substantia nigra between cerebral hemispheres. Research questionWe evaluate asymmetries between the more and the less affected leg in PwPD in responses to unanticipated stance perturbations. MethodsSixteen 16 PwPD participated in the experiment that consisted of recovering a stable upright stance, keeping the feet in place, in response to a perturbation caused by a sudden release of a load equivalent to 7 % of the participant's body mass. Anterior displacement and velocity of the center of pressure (CoP), the latency of gastrocnemius medialis muscle (GM) activation onset, rate of GM activation, and normalized magnitude of muscular activation were analyzed. ResultsAnalysis revealed significantly rate (p = 0.04) and magnitude (p = 0.02) higher activation of GM in the less affected limb. No significant effects of the leg were found for GM activation latency or CoP-related variables. SignificanceThere is a higher contribution of the less affected leg in automatic postural responses in PwPD.

Task Specific and General Patterns of Joint Motion Variability in Upright- and Hand-Standing Postures.

The preservation of static balance in both upright- and hand-stance is maintained by the projection of center of mass (CM) motion within the region of stability at the respective base of support. This study investigated, from a degrees of freedom (DF) perspective, whether the stability of the CM in both upright- and hand-stances was predicted by the respective dispersion and time-dependent regularity of joint (upright stance—ankle, knee, hip, shoulder, neck; hand stance—wrist, elbow, shoulder, neck) angle and position. Full body three-dimensional (3D) kinematic data were collected on 10 advanced level junior female gymnasts during 30 s floor upright- and hand-stands. For both stances the amount of the dispersion of joint angle and sway motion was higher than that of the CM and center of pressure (CP) with an inverse relation to time-dependent irregularity (SampEn). In upright-standing the variability of neck motion in the anterior–posterior direction was significantly greater than that of most joints consistent with the role of vision in the control of quiet upright posture. The findings support the proposition that there are both task specific and general properties to the global CM control strategy in the balance of upright- and hand-standing induced by the different active skeletal-muscular organization and the degeneracy revealed in the multiple distributional variability patterns of the joint angle and position in 3D.

Visual occlusion effects on bipedal stance control in Chinese-speaking children with dyslexia

Visual processing of complex character configurations is especially challenging for Chinese-speaking children with dyslexia (CSCD). The purpose of this study was to compare the effects of visual occlusion on postural control between dyslexic and non-dyslexic Chinese-speaking children by examining their visual-perceptual capacity and movement coordination with scale measures. Sixteen dyslexic children (10 males and, 6 females, 9.46 ± 1.26 yrs) and sixteen non-dyslexic children (10 males and 6 females, 9.91 ± 1.18 yrs) were recruited from the campus in Taiwan. Motor and visual perceptual performance were assessed with the Movement Assessment Battery for Children, 2nd Edition (MABC-2) and the Test of Visual-Perceptual Skills, 4th Edition (TVPS-4). Root mean square (RMS) and sample entropy (SampEn) of center of pressure (COP) were characterized during a bilateral upright stance with eyes open (EO) and eyes closed (EC). The results showed significant group differences in six of the seven TVPS-4 subscales (P <.001–0.017) and one category of the MABC-2 (P =.006). In the EO condition, the children with dyslexia showed a greater RMS of COP specifically in the anterior–posterior (AP) direction than did the non-dyslexic children (P =.029). However, SampEn of COP in the two directions were not group dependent (P >.05). In the EC condition, RMS and SampEn of COP did not vary with group (P >.05). RMS of COP in the AP direction was negatively correlated with the sub-score of visual figure-ground in the TVPS-4 (r = −0.381, P =.031). In summary, postural control of Chinese-speaking children with dyslexia is more affected with eyes open than with eyes closed, and the effect is related to visual disturbance of the foreground and background.

Different Effects of 12-Week Speed or Accuracy Training on Obstacle-Crossing Foot Motion in Healthy Elderly.

Preventing falls is important in the elderly. One reason for falling is tripping or stumbling; hence, it is important to improve the crossing motion. This study aimed to compare speed- and accuracy-oriented crossing training and establish a useful training method. To investigate the effects of crossing motion training, we conducted a randomized controlled trial. Twenty healthy elderly individuals (aged 71.7 ± 1.5 years) were randomly assigned to two groups: speed training and accuracy training groups. They practiced initiating their crossing motion faster or more accurately for 12 weeks. Using a three-dimensional motion analysis system, the data on the crossing motion was captured before and after the training period. We set four conditions (normal speed, fast, leaning stance, and leaning stance and fast) and two directions (anterior and lateral) to analyze the crossing motion. The crossing motion of the speed training group became significantly faster compared to baseline (p < 0.05); however, the accuracy of the crossing motion of the accuracy training group was not statistically significant. Speed training in this study had clear effects on crossing motion. It is surprising that crossing motion training from a normal upright stance can also improve swing speed from the leaning stance. We believe that this training is easy and useful in the elderly population.

Age-dependent perturbation of the perceptual and postural vertical by visual roll vection and susceptibility to motion sickness in children

BackgroundThe visual contribution to the perceptual and postural vertical is mediated by a multisensory integration process and may relate to children’s susceptibility to motion sickness that is hypothesized to arise from intersensory conflicts.ObjectiveTo analyze the maturation of visual contribution to the perceptual and postural vertical in conjunction with the motion sickness susceptibility in childhood.MethodsIn 81 healthy children (aged 2–17 years; 57 females), adjustments of the subjective visual vertical and posturographically tested mediolateral displacements of body sway were measured during free upright stance and large-field visual motion stimulation in the roll plane (roll vection). Motion sickness susceptibility was assessed by taking the history of parents and children.ResultsVection-induced tilts of the visual vertical showed a linear age-dependent decrease with largest tilts in the youngest (2–7 years; median of 20°) and smallest tilts in the oldest age group (13–17 years; median of 9–10°). Analogously, postural tilts as measured by mediolateral body sway were greatest in the youngest and smallest in the oldest age group. In contrast, motion sickness susceptibility was lowest in the youngest and highest in the oldest age group and exhibited an inverse correlation with vection-induced tilts of the visual vertical.ConclusionRoll vection-induced tilts of the visual and postural vertical exhibited a similar age-dependent course with the greatest effects in the youngest and the least effects in the oldest age group, the latter of which exhibited the highest susceptibility to motion sickness.

Sample entropy discriminates balance performance of older cannabis users from non-users

BackgroundMaintaining an upright stance involves a complex interaction of sensory processing and motor outputs to adequately perform this fundamental motor skill. Aging and cannabis use independently disrupt balance performance, but our recent data did not find differences in static balance performance between older cannabis Users and older Non-Users using traditional linear measures (i.e., characteristics of the center of pressure sway). The purpose of this analysis was to determine whether an unbiased entropy measure (sample entropy) can differentiate postural control (standing posture) strategies between older cannabis Users and Non-Users when typical linear measures could not. MethodsEight medical cannabis Users and eight age- and sex-matched controls completed static posturography testing in an eyes-open condition for 60 s. Linear measures included pathlength of the anterior-posterior and medio-lateral directions and an ellipse that encapsulates 95% of the 2D area explored. The nonlinear measure was the sample entropy of the center of pressure time-series in anterior-posterior and medio-lateral directions. Group comparisons were accomplished via pairwise testing and effect size calculations. FindingsThe statistical testing revealed that sample entropy in the anterior-posterior direction was significantly larger in the Users (mean ± SD = 0.29 ± 0.08) compared to the Non-Users (0.19 ± 0.05; P = 0.01, d = 1.55). InterpretationThis finding indicates that the Users had a decreased regularity of their center of pressure signal in the anterior-posterior direction, which might reflect reduced balance adaptability and accompanies the increased fall risk observed in our recent report on these same subjects.

Neuromuscular Control Modelling of Human Perturbed Posture Through Piecewise Affine Autoregressive With Exogenous Input Models.

In this study, the neuromuscular control modeling of the perturbed human upright stance is assessed through piecewise affine autoregressive with exogenous input (PWARX) models. Ten healthy subjects underwent an experimental protocol where visual deprivation and cognitive load are applied to evaluate whether PWARX can be used for modeling the role of the central nervous system (CNS) in balance maintenance in different conditions. Balance maintenance is modeled as a single-link inverted pendulum; and kinematic, dynamic, and electromyography (EMG) data are used to fit the PWARX models of the CNS activity. Models are trained on 70% and tested on the 30% of unseen data belonging to the remaining dataset. The models are able to capture which factors the CNS is subjected to, showing a fitting accuracy higher than 90% for each experimental condition. The models present a switch between two different control dynamics, coherent with the physiological response to a sudden balance perturbation and mirrored by the data-driven lag selection for data time series. The outcomes of this study indicate that hybrid postural control policies, yet investigated for unperturbed stance, could be an appropriate motor control paradigm when balance maintenance undergoes external disruption.

Modeling and control of anterior–posterior and medial–lateral sways in standing posture

To study essential anterior–posterior and medial–lateral sways of the stance caused by rotational movements about the ankle and hip joints, a mathematical model is developed for the 3D postural kinematics and dynamics. The model is in the form of nonlinear differential–algebraic equations corresponding to a biomechanical system with holonomic constraints. A nonlinear feedback control law is further derived for stabilizing the upright stance, whilst eliminating internal torques induced by the constraints on postural movements. Numerical simulations of the model parametrized with experimental data of human body segments illustrate the performance of postural balancing with the proposed control.

Cortical Pathways During Postural Control: New Insights From Functional EEG Source Connectivity.

Postural control is a complex feedback system that relies on vast array of sensory inputs in order to maintain a stable upright stance. The brain cortex plays a crucial role in the processing of this information and in the elaboration of a successful adaptive strategy to external stimulation preventing loss of balance and falls. In the present work, the participants postural control system was challenged by disrupting the upright stance via a mechanical skeletal muscle vibration applied to the calves. The EEG source connectivity method was used to investigate the cortical response to the external stimulation and highlight the brain network primarily involved in high-level coordination of the postural control system. The cortical network reconfiguration was assessed during two experimental conditions of eyes open and eyes closed and the network flexibility (i.e. its dynamic reconfiguration over time) was correlated with the sample entropy of the stabilogram sway. The results highlight two different cortical strategies in the alpha band: the predominance of frontal lobe connections during open eyes and the strengthening of temporal-parietal network connections in the absence of visual cues. Furthermore, a high correlation emerges between the flexibility in the regions surrounding the right temporo-parietal junction and the sample entropy of the CoP sway, suggesting their centrality in the postural control system. These results open the possibility to employ network-based flexibility metrics as markers of a healthy postural control system, with implications in the diagnosis and treatment of postural impairing diseases.

Long term correlation and inhomogeneity of the inverted pendulum sway time-series under the intermittent control paradigm

In this study the extended detrended fluctuation analysis (EDFA) was applied to the sway data generated from an inverted pendulum (IP) model, intermittently controlled at the ankle. The time series taken into account was the center of pressure (COP), since it represents the widest used time series in posturography, and it constitutes a natural link between model and data-based analysis approaches for studying the dynamics of the human balance maintenance. COP time-series were obtained by varying the intermittent control parameters (ICP) in a uniform distribution range that ensures IP stability to quantify changes in the long-term correlation and inhomogeneity of the time-series. Globally, EDFA coefficients (α and β) showed to be sensitive to the variations of derivative control gain (D), whereas for proportional gain (P) and ρ parameters no significant trends were observed. However, relations between EDFA coefficients and ρ arose whether derivative gain is examined within a low and high regions of value. For low D gains, both α and β showed a significant correlation with ρ, which disappears when higher D values were considered. Thus EDFA coefficients can provide useful insights about the long-term correlation and local characteristics of COP timeseries, which are strictly related to the control policy adopted for maintaining balance. This supports the validity of the intermittent motor control paradigm for the human upright stance and suggests the use of EDFA in real posturography applications, in order to extract meaningful information regarding the properties of COP timeseries for different groups of patients.

Differential activation of the plantar flexor muscles in balance control across different feet orientations on the ground

The ankle plantar flexor muscles act synergistically to control quiet and dynamic body balance. Previous research has shown that the medial (MG) and lateral (LG) gastrocnemii, and soleus (SOL) are differentially activated as a function of motor task requirements. In the present investigation, we evaluated modulation of the plantar flexors' activation from feet orientation on the ground in an upright stance and the ensuing reactive response to a perturbation. A single group of young participants (n = 24) was evaluated in a task requiring initial stabilization of body balance against a backward pulling load (5% or 10% of body weight) attached to their trunk, and then the balance was suddenly perturbed, releasing the load. Four feet orientations were compared: parallel (0°), outward orientation at 15° and 30°, and the preferred orientation (M = 10.5°). Results revealed a higher activation magnitude of SOL compared to MG-LG when sustaining quiet balance against the 10% load. In the generation of reactive responses, MG was characterized by earlier, steeper, and proportionally higher activation than LG-SOL. Feet orientation at 30° led to higher muscular activation than the other orientations, while the activation relationship across muscles was unaffected by feet orientation. Our results support the conclusion of task-specific differential modulation of the plantar flexor muscles for balance control.

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

BackgroundVoluntary 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 questionHow does VR modulate early, anticipatory, and compensatory postural adjustments during a dynamic task of leg lifting while avoiding an obstacle? MethodsFirst, 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. ResultsThe 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. SignificanceThe 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 mental fatigue on static upright stance and functional balance in older adults

BackgroundCognitive influences on balance control may contribute to greater instability and falls in older adults. In support of this, old age exacerbates the effects of a concurrent cognitive task on balance. Mental fatigue is a psychobiological state experienced following prolonged demanding cognitive activity. However, its effects on static upright stance and functional balance in older adults is unclear. It is also unclear how the effects of mental fatigue and a concurrent task interact. MethodsBalance was assessed in ten younger and ten older participants before and immediately after 25 min of the incongruent Stroop colour-word test (mental fatigue) and leisurely reading (control), presented in a randomized counterbalanced order on separate days. Static and functional balance was assessed using the centre of pressure path length when standing still and the ‘Timed Up & Go’ test, respectively. These balance assessments were completed with and without a concurrent backward counting task (i.e. dual-task and single-task, respectively). ResultsUnder subjectively-confirmed conditions of mental fatigue, sway path length when standing still was 32% greater than the control condition in older adults but unchanged in younger adults. This age-dependant effect of mental fatigue on static balance was similar in single-task and dual-task trials. Mental fatigue did not significantly affect functional balance performance in either age group. ConclusionsThese findings are the first to show mental fatigue to impair static balance control in older adults. Therefore, whether due to everyday activities or a symptom of disease, mental fatigue may contribute to poor balance in older adults.

Effects of Galvanic Vestibular Stimulation on Visual Verticality and Standing Posture Differ Based on the Polarity of the Stimulation and Hemispheric Lesion Side in Patients With Stroke.

Introduction: There is growing evidence supporting the relationship of vertical misperception and poor balance control with asymmetrical standing posture in patients with stroke. Although the vestibular system has been shown to be responsible for vertical misperception and balance disorders, the effect of galvanic vestibular stimulation (GVS) on both vertical misperception and postural asymmetry after stroke remains elusive. The aim of this study was to investigate the effects of GVS on visual verticality and postural asymmetry after stroke and to clarify whether the effects differ depending on the polarity of the stimulation and hemispheric lesion side.Methods: We measured the subjective visual vertical (SVV) and body weight distribution on each foot in an upright stance in 24 patients with a hemispheric stroke (10 with a left hemisphere lesion and 14 with a right hemisphere lesion) and nine age-matched healthy controls. During the measurements, bipolar GVS (1.5 mA) was applied over the bilateral mastoid processes in three stimulation conditions: contralesional-anodal and ipsilesional-cathodal vestibular stimulation, ipsilesional-anodal and contralesional-cathodal vestibular stimulation, and no stimulation. To examine whether GVS modulates visual verticality and standing posture, SVV and weight-bearing in the three conditions were analyzed.Results: During no stimulation, the SVV deviated to the contralesional side in patients with a right hemisphere lesion, while more weight-bearing was observed on the ipsilesional limb than on the contralesional limb in both patient groups than in the controls. The SVV was modulated by reversing the polarity of GVS in all the groups when the cathodal stimulus side was either ipsilateral or contralateral to the lesion while the ipsilesional-cathodal vestibular stimulation reduced weight-bearing asymmetry in only the patients with a right hemisphere lesion.Conclusions: These findings demonstrate that the effects of GVS on the SVV and standing posture differ depending on the polarity of GVS and the hemispheric lesion side. Patients with a right hemisphere lesion have difficulty maintaining their preferred standing posture under visual verticality modulation evoked by GVS. The application of GVS may clarify whether the vestibular system has neural redundancy after stroke to suppress any effects of the stimulation, including modulation of the visual verticality, on balance.

Effects of Caffeine Ingestion on Human Standing Balance: A Systematic Review of Placebo-Controlled Trials.

Caffeine ingestion may influence balance control via numerous mechanisms. Although previously investigated using various study designs and methods, here we aimed to create the first evidence-based consensus regarding the effects of caffeine on the control of upright stance via systematic review (PROSPERO registration CRD42021226939). Embase, PubMed/MEDLINE, SPORTDiscus and Web of Science databases were searched on 27 January 2021 to identify placebo-controlled trials investigating caffeine-induced changes in human standing balance. Reference lists of eligible studies were also searched. Overall, nine studies involving a total of 290 participants were included. All studies were moderate to strong in quality according to the QualSyst tool. Balance-related outcome measures were collected across a range of different participant ages, stances and sensory conditions. The results show that younger participants’ balance was generally unaffected by caffeine ingestion. However, a significant balance impairment was observed following caffeine ingestion in all studies involving older participants (average age >65 years). Our results therefore suggest an age-dependent effect of caffeine ingestion on human standing. Further research into this effect is warranted as only one study has directly compared younger and older adults. Nonetheless, an important implication of our findings is that caffeine ingestion may increase fall risk in older adults. Furthermore, based on our findings, caffeine ingestion should be considered as a potential confounding factor when assessing human standing balance, particularly in older adults.

EFFECT OF LOWER LIMB EXERCISE ON POSTURE STABILITY AND BRAIN ACTIVITY DURING WHOLE BODY VIBRATION FOR THE ELDERLY

This study aimed to investigate the effect of lower limb exercise on electroencephalogram (EEG) activity and posture stability during whole body vibration (WBV) for healthy elderly people. Subjects were composed of 20 elderly people who were over 60 years old and had no disease. They were subjected to WBV of 30[Formula: see text]Hz and strength intensity of 20 (total 5 sets, 20[Formula: see text]s/set, rest time: 20[Formula: see text]s, except for the final set). The subjects were grouped into two groups. One group of 11 subjects was WBVSt (WBV of upright stance) and the other group of nine subjects was WBVSq (WBV of squat posture). In the WBVSt group, the subjects were requested to maintain a standing position, whereas those in the WBVSq group were asked to perform 60∘-knee flexion squat at 2[Formula: see text]s intervals during WBV stimulation. In this study, the electromyography (EMG) was measured on the right and left erector spinae, rectus femoris, vastus lateralis, vastus medialis, tibialis anterior to evaluate lower limb muscle activity and postural stability during timed up and go (TUG) test before and after WBV. The EEG was measured on frontal (F3, F4, F7, F8), central (Cz, C3, C4), temporal (T3, T4, T5, T6), parietal (P3, P4), and occipital (O1, O2) lobes to evaluate brain activity during the check of mini-mental state examination for dementia screening (MMSE-DS) before and after WBV. The results of this study showed improved postural stability and muscle activation for the healthy elderly during lower limb and WVB exercises. The WBVSq group showed higher changes in TUG-test ([Formula: see text]) and iEMG analysis by EMG ([Formula: see text]) compared with the WBVSt group. However, there was no significant change in MMSE-DS ([Formula: see text]). Because of the increase in relative SMR power by EEG, the central (C4), frontal (F7) and parietal (P3, P4) lobes were activated in WBVSt group ([Formula: see text]); however, all regions, except the parietal lobe, were activated in WBVSq group ([Formula: see text]). Lower limb exercise with WBV stimulation improved postural stability, lower limb muscle, and brain activation for the elderly. These results may be helpful to the protocol of exercise using WBV for the healthy elderly.

Cortical correlates in upright dynamic and static balance in the elderly

Falls are the second most frequent cause of injury in the elderly. Physiological processes associated with aging affect the elderly’s ability to respond to unexpected balance perturbations, leading to increased fall risk. Every year, approximately 30% of adults, 65 years and older, experiences at least one fall. Investigating the neurophysiological mechanisms underlying the control of static and dynamic balance in the elderly is an emerging research area. The study aimed to identify cortical and muscular correlates during static and dynamic balance tests in a cohort of young and old healthy adults. We recorded cortical and muscular activity in nine elderly and eight younger healthy participants during an upright stance task in static and dynamic (core board) conditions. To simulate real-life dual-task postural control conditions, the second set of experiments incorporated an oddball visual task. We observed higher electroencephalographic (EEG) delta rhythm over the anterior cortex in the elderly and more diffused fast rhythms (i.e., alpha, beta, gamma) in younger participants during the static balance tests. When adding a visual oddball, the elderly displayed an increase in theta activation over the sensorimotor and occipital cortices. During the dynamic balance tests, the elderly showed the recruitment of sensorimotor areas and increased muscle activity level, suggesting a preferential motor strategy for postural control. This strategy was even more prominent during the oddball task. Younger participants showed reduced cortical and muscular activity compared to the elderly, with the noteworthy difference of a preferential activation of occipital areas that increased during the oddball task. These results support the hypothesis that different strategies are used by the elderly compared to younger adults during postural tasks, particularly when postural and cognitive tasks are combined. The knowledge gained in this study could inform the development of age-specific rehabilitative and assistive interventions.

Parkinson's disease-related changes in the behavioural synergy between eye movements and postural movements.

Patients with Parkinson's disease (PD patients) have been shown to exhibit abnormally low levels of synergy in their posture control. The goal of this study was to determine how synergic interactions between vision and posture are affected in PD patients. These synergic interactions were expected to be impaired because PD affects the basal ganglia, which are involved in the modulation of both types of movement. Twenty patients (mean age: 60) on levodopa and 20 age-matched-controls (mean age: 61) performed a precise visual task (searching for targets in an image) and an unprecise control task (randomly looking at an image) in which images were projected onto a large panoramic display. Lower back, upper back, head and eye movements were recorded simultaneously. To test behavioural synergies, Pearson correlations between eye and postural movements were analysed. The relationships between eye movements and upper and lower back movements were impaired in the patients. The age-matched controls did not show any significant correlations between eye and postural movements. Overall, our results showed that the PD patients failed to adjust and control their postural stability for success in the visual task. The impaired synergy between eye and postural movements was not related to clinical variables-probably because our patients had early-stage PD. Our results showed that impairments in synergy can occur very early in PD. Hence, the analysis of this synergy might provide a better understanding of postural instability, visual task performance in the upright stance, and perhaps the risk of falls in PD patients.