Influence of vision and static stretch of the calf muscles on postural sway during quiet standing

The purpose of this experimental study was to evaluate the effects of vision and stretching of the calf muscles on postural sway during quiet standing. Under pre-stretch conditions, participants stood on a force plate for 30s and the sway of the ground reaction force center of pressure was recorded. The following postural sway variables were calculated off-line: sweep speed, sway speed, standard deviation, maximal mediolateral range, maximal anteroposterior range, mean mediolateral position and mean anteroposterior position. For post-stretch conditions, participants stood quietly on a device that was utilized to impose a static 3 min ankle joint dorsiflexion stretch. Immediately thereafter, participants moved onto the force platform where postural sway parameters were again recorded. Randomized eyes-open and eyes-closed conditions were tested in both cases. Results showed that postural sway significantly increased due to stretch (sweep speed, sway speed, standard deviation, maximal anteroposterior range, mean anteroposterior position), as well as eye closure (sweep speed, sway speed, standard deviation, maximal mediolateral range, maximal anteroposterior range). The interaction between stretch and eye closure was also significant (sweep speed, sway speed, standard deviation, maximal mediolateral range), suggesting that there were only minor increases in postural sway after stretch under the eyes-open condition. It was suggested that stretching of the calf muscles has the effect of increasing postural sway, although this effect can be greatly compensated for when vision is included.     

Postural tasks are associated with center of pressure spatial patterns of three-dimensional statokinesigrams in young and elderly healthy subjects

The present study investigated the association between postural tasks and center of pressure spatial patterns of three-dimensional statokinesigrams. Young (n = 35; 27.0 ± 7.7 years) and elderly (n = 38; 67.3 ± 8.7 years) healthy volunteers maintained an undisturbed standing position during postural tasks characterized by combined sensory (vision/no vision) and biomechanical challenges (feet apart/together). A method for the analysis of three-dimensional statokinesigrams based on nonparametric statistics and image-processing analysis was employed. Four patterns of spatial distribution were derived from ankle and hip strategies according to the quantity (single; double; multi) and location (anteroposterior; mediolateral) of high-density regions on three-dimensional statokinesigrams. Significant associations between postural task and spatial pattern were observed (young: gamma = 0.548, p < .001; elderly: gamma = 0.582, p < .001). Robustness analysis revealed small changes related to parameter choices for histogram processing. MANOVA revealed multivariate main effects for postural task [Wilks’ Lambda = 0.245, p < .001] and age [Wilks’ Lambda = 0.308, p < .001], with interaction [Wilks’ Lambda = 0.732, p < .001]. The quantity of high-density regions was positively correlated to stabilogram and statokinesigram variables (p < .05 or lower). In conclusion, postural tasks are associated with center of pressure spatial patterns and are similar in young and elderly healthy volunteers. Single-centered patterns reflected more stable postural conditions and were more frequent with complete visual input and a wide base of support.     

The effects of pain and a secondary task on postural sway during standing

BackgroundPain impairs available cognitive resources and somatosensory information, but its effects on postural control during standing are inconclusive. The aim of this study was to investigate whether postural sway is affected by the presence of pain and a secondary task during standing.MethodsSixteen healthy subjects stood as quiet as possible at a tandem stance for 30s on a force platform at different conditions regarding the presence of pain and a secondary task. Subjects received painful stimulations on the right upper arm or lower leg according to a relative pain threshold [pain 7 out 10 on a Visual Analog Scale (VAS) - 0 representing “no pain” and 10 “worst pain imaginable”] using a computer pressurized cuff. The secondary task consisted of pointing to a target using a head-mounted laser-pointer as visual feedback. Center of Pressure (COP) sway area, velocity, mean frequency and sample entropy were calculated from force platform measures.FindingsCompared to no painful condition, pain intensity (leg: VAS = 7; arm VAS = 7.4) increased following cuff pressure conditions (P < .01). Pain at the leg decreased COP area (P < .05), increased COP velocity (P < .05), mean frequency (P < .05) and sample entropy (P < .05) compared with baseline condition regardless the completion of the secondary task. During condition with pain at the leg, completion of the secondary task reduced COP velocity (P < .001) compared with condition without secondary task.InterpretationPain in the arm did not affect postural sway. Rather, postural adaptations seem dependent on the location of pain as pain in the lower leg affected postural sway. The completion of a secondary task affected postural sway measurements and reduced the effect of leg pain on postural sway. Future treatment interventions could benefit from dual-task paradigm during balance training aiming to improve postural control in patients suffering from chronic pain.     

Assessing the predictive validity of cultural intelligence over time

The study examines four core components of cultural intelligence (CQ) - Behavioral, Motivational, Cognitive and Meta-cognitive - as predictors of cross-cultural adaptation problems in a longitudinal study of international students in New Zealand and tests the hypothesis that Motivational CQ predicts better psychological and sociocultural outcomes over time. One hundred and four students completed measures of CQ during a pre-term orientation program and assessments of adaptation problems approximately three months later. In line with the hypothesis, bi-variate correlations indicated that Motivational CQ was related to fewer psychological symptoms (r = −.30, p < .01) and sociocultural adaptation problems (r = −.27, p < .01). However, hierarchical regression analysis, controlling for age, gender, length of residence abroad and region of origin, revealed that while Motivational CQ was a significant (negative) predictor (ß = −.36, p < .01) of psychological symptoms, the overall amount of variance explained (14.6%) in the model was not significant. In contrast, region of origin (ß = .37, p < .01) was the only significant predictor of sociocultural adaptation problems with international students from Western countries reporting fewer difficulties than those from other regions. The results are discussed in relation to contemporary theories of motivation along with recommendations for future research.     

Tonic postural lean after-effects influenced by support surface stability and dynamics

Tonic neuromuscular processes are evident during lean after-effects, which occur after prolonged standing on a fixed ramp. Postural processes underlying lean after-effects were examined here using dynamic surface conditions. Three tilt adaptation conditions were tested with eyes-closed (n = 11). Tilt adaptation conditions involved standing for 120 s on a fixed toes-up ramp (7°) or on a toes-up sinusoidally tilted surface (7° ± 3°), which was followed by 120 s of standing on either a fixed horizontal surface or sway-referenced surface. All participants showed postural after-effects (p < .003). Specifically, standing on a fixed horizontal surface after sine-tilt adaptation, resulted in forward leaning which decayed over 120 s back to baseline. Standing on a sway-referenced surface after tilt-adaptation, initially showed no lean after-effect, however over the course of the trial the center-of-pressure shifted backward (p < .02). This after-effect during sway-reference conditions was also evident in the sway-induced surface tilt, which increased in dorsiflexion (p < .002), rather than decaying back to baseline. Thus, adaptation occurs on a dynamically tilted surface, while reliability of the surface as a stable reference affects the return of the center-of-pressure and surface tilt to baseline. These findings relate to changes in flexor/extensor muscle tonic set-point which also occur following a prolonged voluntary isometric contraction.     

Differences in Rapid Initiation and Termination of Voluntary Postural Sway Associated with Ageing and Falls-Risk

The authors examined differences between young adults (n = 25) and healthy older adults (n = 48) in reaction time and the relations between center of pressure (COP) and center of mass (COM) motions during rapid initiation and termination of voluntary postural sway. Older adults were divided into low and high falls-risk groups based on Physiological Profile Assessment scores of sensorimotor function. Low falls-risk older adults had slower reaction times during anteroposterior sway initiation and decreased COP–COM separation during anteroposterior and medialateral sway initiation and anteroposterior continuous voluntary sway compared with young adults. High falls-risk older adults had slower initiation and termination reaction times in all response directions and decreased COP–COM separation during sway initiation and continuous voluntary sway in the anteroposterior and medialateral directions compared with young adults. Compared with low falls-risk older adults, high falls-risk older adults had slower initiation and termination reaction times in all response directions and decreased COP–COM separation during medialateral continuous voluntary sway. Reaction time and COP–COM measures significantly predicted group status in discriminant models with sensitivities and specificities of 72–100%. Overall, these findings highlight important associations of age-related declines in sensorimotor function related to an increased risk of falling with slower postural reaction time and reduced postural stability.     

Predicting incident falls: Relationship between postural sway and limits of stability in older adults

BackgroundWe have previously shown that objective measurements of postural sway predicts fall risk, although it is currently unknown how limits of stability (LOS) might influence these results.Research question: How integrated postural sway and LOS measurements predict the risk of incident falls in a population-based sample of older adults.Methods:The sample for this prospective observational study was drawn from the Healthy Ageing Initiative cohort and included data collected between June 2012 and December 2016 for 2396 men and women, all 70 years of age. LOS was compared to postural sway with measurements during eyes-open (EO) and eyes-closed (EC) trials, using the previously validated Wii Force Plate. Fall history was assessed during baseline examination and incident falls were collected during follow-up at 6 and 12 months. Independent predictors of incident falls and additional covariates were investigated using multiple logistic regression models.Results:During follow-up, 337 out of 2396 participants (14%) had experienced a fall. Unadjusted regression models from the EO trial revealed increased fall risk by 6% (OR 1.06, 95% CI 1.02–1.11) per each centimeter squared increase in sway area and by 16% (OR 1.16, 95% CI 1.07–1.25) per 1-unit increase in Sway-Area-to-LOS ratio. Odds ratios were generally lower when analyzing EC trials and only slightly attenuated in fully adjusted models.Significance:Integrating postural sway and LOS parameters provides valid fall risk prediction and a holistic analysis of postural stability. Future work should establish normative values and evaluate clinical utility of these measures.     

A Meta-Analysis of the Convergent Validity of Self-Control Measures

There is extraordinary diversity in how the construct of self-control is operationalized in research studies. We meta-analytically examined evidence of convergent validity among executive function, delay of gratification, and self- and informant-report questionnaire measures of self-control. Overall, measures demonstrated moderate convergence (r_random = .27 [95% CI = .24, .30]; r_fixed = .34 [.33, .35], k = 282 samples, N = 33,564 participants), although there was substantial heterogeneity in the observed correlations. Correlations within and across types of self-control measures were strongest for informant-report questionnaires and weakest for executive function tasks. Questionnaires assessing sensation seeking impulses could be distinguished from questionnaires assessing processes of impulse regulation. We conclude that self-control is a coherent but multidimensional construct best assessed using multiple methods.     

Effects of Visual Manipulation in Sit-to-Stand Movement in Children With Cerebral Palsy

The authors sought to verify the effects of vision on sit-to-stand (STS) movement performance by means of postural sway in children with cerebral palsy (CP) and typical children (TC). Participants were 42 TC and 21 children with CP. STS movement was assessed with eyes open and with eyes closed. Area and velocity of center of pressure sway were analyzed in each of the 3 STS phases. We observed greater postural sway during STS movement with eyes closed. Children with CP presented greater postural sway than TC did. Both groups exhibited greater postural instability with absence of vision expressing the role of vision to keep postural stability. Moreover, the greater postural instability was observed in children with CP.     

Age-related Changes in Postural Sway During Sit-to-stand in Typical Children and Children with Cerebral Palsy

Objective: To assess age-related changes in postural sway during sit-to-stand (STS) in typical children (TC) and children with mild cerebral palsy (CP). Methods: Thirty-five TC and 23 children with mild CP were allocated in four different age groups: 5–6, 7–9, 10–12, and 13–15 years; they all performed STS movements over a force plate. Anterior-posterior and medial-lateral amplitude of center of pressure (CoP) displacement, area and velocity of CoP sway were analyzed and compared between the age groups for TC and children with CP. Results: TC at 5 to 6 years of age showed higher values of anterior-posterior CoP displacement and Area of CoP sway than at 10–12 years, during the stabilization phase. There were no age-related changes for CP. Conclusion: TC change their postural sway during the last STS phase over the years, reducing their body sway. Children with CP did not show age-related changes in sway during STS, reflecting a distinct rhythm of postural control development in this population.     

Synergies and Motor Equivalence in Voluntary Sway Tasks: The Effects of Visual and Mechanical Constraints

The authors used two analyses developed within the framework of the uncontrolled manifold hypothesis to quantify multimuscle synergies during voluntary body sway: analysis of intertrial variance and analysis of motor equivalence with respect to the center of pressure (COP) trajectory. Participants performed voluntary sway tasks in the anteroposterior direction at 0.33 and 0.66 Hz. Muscle groups were identified in the space of muscle activations and used as elemental variables in the synergy analyses. Changing mechanical and vision feedback–based constraints led to significant changes in indices of sway performance such as COP deviations in the uninstructed, mediolateral direction and indices of spontaneous postural sway. In contrast, there were no significant effects on synergy indices. These findings show that the neural control of performance and of its stability may involve different control variables and neurophysiological structures. There were strong correlations between the indices of motor equivalence and those computed using the intercycle variance analysis. This result is potentially important for studies of patients with movement disorders who may be unable to perform multiple trials (cycles) at any given task, making analysis of motor equivalence of single trials a viable alternative to explore changes in stability of actions.     

The effects of perturbation type and direction on threat-related changes in anticipatory postural control

The purpose of this study was to determine whether the type and direction of postural perturbation threat differentially affect anticipatory postural control. Healthy young adults stood on a force plate fixed to a translating platform and completed a series of rise-to-toes movements without (No Threat) and with (Threat) the potential of receiving a postural perturbation to either their feet (15 participants) or torso (16 participants). Each type of perturbation threat was presented along the anteroposterior (A-P) or mediolateral (M-L) axis. For each condition, the A-P center of pressure (COP) signal and tibialis anterior (TA) and soleus (SOL) electromyographical (EMG) recordings were used to quantify the anticipatory postural adjustment (APA). Results indicated that across both threat types and directions, postural threat induced a 40.2% greater TA activation (p < 0.001), a 18.5% greater backward COP displacement (p < 0.001) and a 23.9% greater backward COP velocity (p < 0.001), leading to larger and faster APAs than the No Threat condition. Subsequently, a 7.7% larger forward COP displacement (p = 0.001), a 20.4% greater forward COP velocity (p < 0.001) and 43.2% greater SOL activation (p = 0.009) were observed during the execution phase of the rise-to-toes for the Threat compared to the No Threat condition. Despite these threat effects, there were no differences in the magnitude or velocity of APAs between the threat directsion conditions. Since the type and direction of perturbation-induced postural threat had minimal differential effects on anticipatory postural control, these factors are unlikely to explain the discrepancy of previous findings.     

Interstimulus Intervals and Sensory Modality Modulate the Impact of a Cognitive Task on Postural Control

The present literature not only reveals the use of a wide variety of cognitive tasks but variability in their interaction with postural control. The question then arises, as to, whether postural control is sensitive to specific features of a cognitive task. The present experiment assessed the impact of cognitive tasks with interstimulus intervals (ISI) of varied duration and sensory modality on postural control in young adults. Seventeen participants (23.71 ± 1.99 years old) were instructed to stand on a force platform while concurrently performing cognitive tasks with ISIs of two and 5 s. The tasks were presented both, auditorily and visually. The visual tasks consisted of counting the total occurrence of a single digit. The auditory tasks consisted of counting the total occurrence of a single letter. Performing the cognitive tasks with an ISI of 2 s resulted only in an increase in the anteroposterior mean power frequency. Presenting the tasks visually also significantly reduced area of 95% confidence ellipse and AP and mediolateral sway variability. These results may suggest that ISIs can modify postural performance by altering the allocation of attentional focus. Also, presenting tasks using a visual sensory modality appears to yield lower postural sway.     

Anticipatory and compensatory postural adjustments in sitting in children with cerebral palsy

The aim of this study was to examine postural control in children with cerebral palsy performing a bilateral shoulder flexion to grasp a ball from a sitting posture. The participants were 12 typically developing children (control) without cerebral palsy and 12 children with cerebral palsy (CP). We analyzed the effect of ball mass (1 kg and 0.18 kg), postural adjustment (anticipatory, APA, and compensatory, CPA), and groups (control and CP) on the electrical activity of shoulder and trunk muscles with surface electromyography (EMG). Greater mean iEMG was seen in CPA, with heavy ball, and for posterior trunk muscles (p < .05). The children with CP presented the highest EMG and level of co-activation (p < .05). Linear regression indicated a positive relationship between EMG and aging for the control group, whereas that relationship was negative for participants with CP. We suggest that the main postural control strategy in children is based on corrections after the beginning of the movement. The linear relationship between EMG and aging suggests that postural control development is affected by central nervous disease which may lead to an increase in muscle co-activation.     

Upright standing after stroke: How loading-unloading mechanism participates to the postural stabilization

Postural strategies employed by hemiparetic stroke patients need to be better understood to guide rehabilitation. Of the two complementary mechanisms used to stabilize the standing posture, loading-unloading (LU) and pressure distribution (PD), it is hypothesized that the former would be predominantly used. To this aim, posturographic assessments, through a dual force-platform, were performed in 30 Hemiparetics tested 3 months after a unilateral stroke, and 30 matched healthy Controls. Original indices (from 0 to 1) were calculated to assess LU and PD contributions. The results show that along the mediolateral axis, the LU contribution was very high and similar in Hemiparetics and in Controls (0.80 ± 0.07 vs 0.76 ± 0.09 a.u; p > 0.05), indicating a predominant hip involvement. Along the anteroposterior axis, the PD contribution was very close to 1 in controls (0.96 ± 0.03 a.u.) indicating an exclusive ankle involvement. Despite a lower contribution in Hemiparetics (0.88 ± 0.11 a.u.; p < 0.01), the indices were surprisingly always above 0.5, meaning that ankle movements remain predominant for controlling postural sways along the anteroposterior axis in all patients even those with severe clinical deficits. However the PD contribution appeared larger in patients with light or moderate deficits of the sensitivity (r = −0.532; p < 0.01) or the motor command (r = −0.513; p < 0.01). These results indicate that postural stabilization of hemiparetic persons remains controlled by a PD mechanism along the anteroposterior axis, even in those combining poor distal motor command and deep sensory loss. This ankle control, piloted by the more-loaded non-paretic limb, would therefore be preferred to a hip control through lateral trunk motion. This should be considered when defining the objectives of the postural rehabilitation after stroke.     

Development of postural stability limits: Anteroposterior and mediolateral postural adjustment mechanisms do not follow the same maturation process

There is increasing evidence that indicates a critical transition period for the maturation of postural control from the ages of 6–7 years. Some studies suggest that this transitional period may be explained by a change from a ballistic toward a sensory strategy, but the cause remains unknown. The purpose of this study was to investigate the influence of the transition period on dynamic postural control in a natural self-initiated leaning task under different sensory conditions. We evaluated the center of pressure (COP) displacement during maximum leaning in four directions (forward, backward, rightward, leftward) under three sensory conditions (eyes open, eyes closed and eyes closed standing on a foam). Three groups were tested: young children (4 years old), older children (8–10 years old) and adults (21–42 years old). The maximum COP excursion along the anteroposterior and mediolateral axes and the COP amplitude were analyzed. Young children showed smaller maximum anteroposterior and mediolateral COP excursion than other groups. Older children also exhibited a significantly smaller maximum excursion along the mediolateral direction but performed similar to adults along the anteroposterior direction. In a similar manner, the analysis of the COP amplitude did not indicate any differences between the groups along the anteroposterior axis. In contrast, along the mediolateral axis, the results showed developmental differences. Furthermore, the effect of sensory conditions was similar across the children's groups. Our results suggest an important plasticity period for the maturation of postural control mechanisms. Notably, our findings support the idea that the postural mechanisms controlling the anteroposterior axis reach maturity before the mechanisms involved in controlling the mediolateral axis.     

Broad stance conditions change postural control and postural sway

Intuitively, a broad stance (i.e., standing with the feet farther apart than usual) should significantly improve postural stability. However, this intuition was not confirmed in quiet stance. Hence, a motion analysis system (markers attached to the trunk and head) and a force platform were used to investigate 13 healthy, young adults who performed 8 trials in standard and broad stances. In broad stance, the medialateral center of pressure (COP) sway mean power frequency was expected to be greater, whereas the variability (standard deviation) of COP, head, and trunk sway and the mean velocity of head and trunk sway was expected to be significantly lower. Accordingly, adoption of a broad stance significantly increased the medialateral mean power frequency of COP sway; decreased the standard deviation of medialateral COP, trunk, and head sway; and decreased the medialateral mean velocity of head sway. A broad stance was also associated with lower variability for head and COP sways in the anteroposterior axis. Unexpectedly, an effect of trial repetition was found for the variability of medialateral trunk sway. This was probably due to the break halfway through the study. In practical terms, broad stance conditions can improve postural control in the medialateral and anteroposterior axes.     

Effects of sensory manipulations on the dynamical structure of center-of-pressure trajectories of children with cerebral palsy during sitting

Aim: To investigate the effects of manipulating visual information and the compliance of the support surface on the area of sway and dynamical trajectories of center-of-pressure (CoP) in children with CP and children with typical development during static sitting. Methods: 32 typical children, 14 children with mild CP and 12 with moderate-to-severe CP were tested for CoP sway during static sitting under four sensory conditions: (1) eyes open on a rigid surface; (2) eyes closed on a rigid surface; (3) eyes open on foam; (4) eyes closed on foam. Results: Children with moderate-to-severe CP showed greater regularity and local stability of dynamical CoP trajectories and lower complexity in their motor patterns than typical children and children with mild CP. Moreover, removing vision and sitting on a compliant surface reduced the regularity of CoP trajectories. Conclusion: Children with CP were able to adjust the structure and complexity of their postural control responses to sensory challenges, although the structure of their postural responses was poorer than in typical children.     

Test–retest reliability of the soleus H-reflex excitability measured during human walking

The purpose of the study was to investigate with what accuracy the soleus H-reflex modulation and excitability could be measured during human walking on two occasions separated by days. The maximal M-wave (Mmax) was measured at rest in the standing position. During treadmill walking every stimulus elicited an M-wave of 25 ± 10% of Mmax in the soleus muscle and a supra-maximal stimulus elicited a maximal M-wave 60 ms after the first stimulus. Both Mmax during rest and during walking were later used for normalization. When normalized to resting Mmax, the peak reflex amplitude during walking was 5% lower on Day 2 than on Day 1 (p = .32). However, when the peak H-reflex was normalized to Mmax in every sweep, Day 2 showed a significant 15% lower amplitude (p = .037). The same pattern was found for the mean H-reflex. Spearman’s Rho was .92 when normalized to resting Mmax but .88 when normalized to Mmax in every sweep. The Pearson product was used to identify one participant at a time on Day 1 among all seven participants on Day 2. For both normalization procedures 5 of 7 participants were identified by this test. Since 5 of 7 participants were recognized between days, it must be recommended to use 10-15 participants for training or intervention studies as far as the H-reflex pattern of modulation during movement is concerned.     

Dynamical structure of center-of-pressure trajectories with and without functional taping in children with cerebral palsy level I and II of GMFCS

Postural control during quiet standing was examined in typical children (TD) and children with cerebral palsy (CP) level I and II of GMFCS. The immediate effect on postural control of functional taping on the thighs was analyzed. We evaluated 43 TD, 17 CP children level I, and 10 CP children level II. Participants were evaluated in two conditions (with and without taping). The trajectories of the center of pressure (COP) were analyzed by means of conventional posturography (sway amplitude, sway-path-length) and dynamic posturography (degree of twisting-and-turning, sway regularity). Both CP groups showed larger sway amplitude than the TD while only the CP level II showed more regular COP trajectories with less twisting-and-turning. Functional taping didn’t affect sway amplitude or sway-path-length. TD children exhibited more twisting-and-turning with functional taping, whereas no effects on postural sway dynamics were observed in CP children. Functional taping doesn’t result in immediate changes in quiet stance in CP children, whereas in TD it resulted in faster sway corrections. Children level II invest more attention in postural control than level I, and TD. While quiet standing was more automatized in children level I than in level II, both CP groups showed a less stable balance than TD.