Changes in Postural Sway and Its Fractions in Conditions of Postural Instability

Variations in task constraints shape emergent performance outcomes and complexity levels in balancing

This study investigated the extent to which specific interacting constraints of performance might increase or decrease the emergent complexity in a movement system, and whether this could affect the relationship between observed movement variability and the central nervous system's capacity to adapt to perturbations during balancing. Fifty-two healthy volunteers performed eight trials where different performance constraints were manipulated: task difficulty (three levels) and visual biofeedback conditions (with and without the center of pressure (COP) displacement and a target displayed). Balance performance was assessed using COP-based measures: mean velocity magnitude (MVM) and bivariate variable error (BVE). To assess the complexity of COP, fuzzy entropy (FE) and detrended fluctuation analysis (DFA) were computed. ANOVAs showed that MVM and BVE increased when task difficulty increased. During biofeedback conditions, individuals showed higher MVM but lower BVE at the easiest level of task difficulty. Overall, higher FE and lower DFA values were observed when biofeedback was available. On the other hand, FE reduced and DFA increased as difficulty level increased, in the presence of biofeedback. However, when biofeedback was not available, the opposite trend in FE and DFA values was observed. Regardless of changes to task constraints and the variable investigated, balance performance was positively related to complexity in every condition. Data revealed how specificity of task constraints can result in an increase or decrease in complexity emerging in a neurobiological system during balance performance.

Body balance control deficiencies following ACL reconstruction combined with medial meniscus suture. A preliminary report

INTRODUCTION

Proprioception makes a critical contribution to body balance. The objective of this study was to evaluate static postural control after anterior cruciate ligament (ACL) reconstruction combined with medial meniscus (MM) suture, comparatively to healthy controls.

HYPOTHESIS

Body balance is adversely affected 2 months after ACL reconstruction combined with MM suture.

PATIENTS AND METHODS

Fifteen patients (12 males and 3 females) aged 20 to 35 years (mean, 26.4 ± 6.0 years) who underwent ACL reconstruction with MM suture were compared to 20 healthy, physically active controls (16 females and 4 males) aged 19 to 23 years (mean, 21.1 ± 1.8 years), most of whom were physiotherapy students. Mean age was not significantly different between the patients and controls. A balance platform was used to estimate static postural control parameters. Each participant performed four tests, two in normal bipedal stance and two in tandem stance; in each stance, one test was done with the eyes open and the other with the eyes closed. We analysed global scores on a standardised 100-point scale and mean centre of pressure (COP) displacement velocity in the sagittal and frontal planes.

RESULTS

Body balance was impaired 2 months after ACL reconstruction with MM suture. Thus, the patients had lower global scores and higher mean COP velocities in both the coronal and sagittal planes.

CONCLUSIONS

Proprioception is impaired after ACL reconstruction with MM suture. Lack of visual control significantly decreases the ability to maintain balance. A balance platform is a useful diagnostic tool for patients with ACL reconstruction and MM suture.

LEVEL OF EVIDENCE

Level II.

Relation between the Sensory and Anthropometric Variables in the Quiet Standing Postural Control: Is the Inverted Pendulum Important for the Static Balance Control?

The aim of this study was to evaluate the relation between the sensory and anthropometric variables in the quiet standing. Methods. One hundred individuals (50 men, 50 women; 20–40 years old) participated in this study. For all participants, the body composition (fat tissue, lean mass, bone mineral content, and bone mineral density) and body mass, height, trunk-head length, lower limb length, and upper limb length were measured. The center of pressure was measured during the quiet standing posture, the eyes opened and closed with a force platform. Correlation and regression analysis were run to analyze the relation among body composition, anthropometric data, and postural sway. Results. The correlation analysis showed low relation between postural sway and anthropometric variables. The multiple linear regression analyses showed that the height explained 12% of the mediolateral displacement and 11% of the center of pressure area. The length of the trunk head explained 6% of displacement in the anteroposterior postural sway. During eyes closed condition, the support basis and height explained 18% of mediolateral postural sway. Conclusion. The postural control depends on body composition and dimension. This relation is mediated by the sensory information. The height was the anthropometric variable that most influenced the postural sway.

Effects of dual-task training on balance and executive functions in Parkinson's disease: A pilot study

The aim of this study was to analyze the efficacy of cognitive-motor dual-task training compared with single-task training on balance and executive functions in individuals with Parkinson's disease. Fifteen subjects, aged between 39 and 75 years old, were randomly assigned to the dual-task training group (n = 8) and single-task training group (n = 7). The training was run twice a week for 6 weeks. The single-task group received balance training and the dual-task group performed cognitive tasks simultaneously with the balance training. There were no significant differences between the two groups at baseline. After the intervention, the results for mediolateral sway with eyes closed were significantly better for the dual-task group and anteroposterior sway with eyes closed was significantly better for the single-task group. The results suggest superior outcomes for the dual-task training compared to the single-task training for static postural control, except in anteroposterior sway with eyes closed.

Postural sway and perceived comfort in pointing tasks

In this study, we explored relations between indices of postural sway and perceived comfort during pointing postures performed by standing participants. The participants stood on a force plate, grasped a pointer with the dominant (right) hand, and pointed to targets located at four positions and at two distances from the body. We quantified postural sway over 60-s intervals at each pointing posture, and found no effects of target location or distance on postural sway indices. In contrast, comfort ratings correlated significantly with indices of one of the sway components, trembling. Our observations support the hypothesis that rambling and trembling sway components involve different neurophysiological mechanisms. They also suggest that subjective perception of comfort may be more important than the actual posture for postural sway.

Toward a dynamical theory of body movement in musical performance

Musicians sway expressively as they play in ways that seem clearly related to the music, but quantifying the relationship has been difficult. We suggest that a complex systems framework and its accompanying tools for analyzing non-linear dynamical systems can help identify the motor synergies involved. Synergies are temporary assemblies of parts that come together to accomplish specific goals. We assume that the goal of the performer is to convey musical structure and expression to the audience and to other performers. We provide examples of how dynamical systems tools, such as recurrence quantification analysis (RQA), can be used to examine performers' movements and relate them to the musical structure and to the musician's expressive intentions. We show how detrended fluctuation analysis (DFA) can be used to identify synergies and discover how they are affected by the performer's expressive intentions.

The effect of a silicone wristband in dynamic balance

The effect of a wristband on the dynamic balance of young adults was assessed. Twenty healthy young adults wore a commercial Power BalanceT or fake silicone wristband. A 3D accelerometer was attached to their lumbar region to measure body sway. They played the video game Tightrope (Wii video game console) with and without a wristband; body sway acceleration was measured. Mean balance sway acceleration and its variability were the same in all conditions, so silicone wristbands do not modify dynamic balance control.

Ellipse area calculations and their applicability in posturography

The quantification of postural sway is considered to be an essential part of posturography and is important for research and clinical utility. A widely used method to calculate the scatter of center of pressure data is an ellipse that encloses about 100(1-α)% of the observations. However, underlying definitions and terminologies have been misused in many cases. Hence, outcomes of different studies are proved to be incommensurable. In order to attain inter-study comparability, standardization of calculation methods has to be advanced. This work features a comprehensive and consistent overview of the methods for elliptic area approximation contrasting general principles of confidence and prediction regions. As a result, we recommend the usage of the prediction ellipse, as far as we demonstrate that confidence ellipses emerge to be inappropriate for posturographic scatter evaluation. Furthermore, we point at problems that come along with different sample sizes.

The influence of cervical spine flexion-rotation range-of-motion asymmetry on postural stability in older adults

STUDY DESIGN

Cross-sectional design.

OBJECTIVE

The purpose of this study was to isolate the contribution of cervical spine range-of-motion (ROM) asymmetry to postural control in a group of older adults with neck pain.

SUMMARY OF BACKGROUND DATA

Previous research has suggested that the upper cervical spine plays an important role in postural stability. However, it remains unclear whether the upper cervical spine rotation ROM asymmetry is associated with postural stability.

METHODS

Using the cervical range-of-motion device to assess upper cervical spine ROM via the cervical flexion-rotation test, we classified 54 older adults with neck pain (30 females; mean [standard deviation] age, 66 [5] yr) into the (1) symmetrical group (SYM; n = 20; ≤5° side-to-side difference) or (2) asymmetrical group (ASYM; n = 34; >5° difference). Standing postural control was characterized by the center-of-pressure (CoP) movements measured using a Balance Board. Other measures included habitual, fast-paced gait speed and neck pain. Both groups were compared on the various measures using Welch t tests.

RESULTS

Although the ASYM group had 26% greater anteroposterior postural sway than the SYM (P < 0.01), both groups did not differ on postural sway velocity, gait speed, and neck pain intensity. Analyzing the frequency content of the postural sway using wavelet analysis (a modern, nonlinear signal processing method) shed further light: the standing postural sway in the ASYM group was skewed toward lower frequency movement (ultralow [< 0.10Hz] frequency content, anteroposterior: 6.7% in ASYM, 4.7% in SYM, P = 0.01; medial-lateral: 4.2% in ASYM, 3.4% in SYM, P = 0.045).

CONCLUSION

The ASYM group seemed to have compensated for their altered somatosensory input to achieve similar functional levels as the SYM group. Given what is known about the association between ultralow frequency postural sway and visual input, we speculated that the postural strategy adopted by the ASYM group was adaptive and that this group may be relying on the visual system to achieve these compensations.

The effects of aging on the rambling and trembling components of postural sway: effects of motor and sensory challenges

The effects of healthy aging on postural sway and its rambling and trembling components were studied. Young and elderly subjects stood quietly for 1 min in different postures, and with eyes open and closed. We found that age-related changes in postural sway and its components were similar to those observed in young participants in challenging conditions. These changes may therefore be viewed as secondary to the increased subjective perception of the complexity of postural tasks. Contrary to our expectations, stronger effects of age were seen in characteristics of rambling, not trembling. The commonly accepted hypothesis that older persons rely on vision more was not supported by this study: we found no significant interaction effects of age and vision on any of the sway characteristics. It was concluded that the reported higher reliance on vision in older persons may be task-specific. The results are compatible with the ideas that much of the age-related changes in postural sway emerge at the level of exploring the limits of stability and using the drift-and-act strategy. Our results suggest that the dominant view on rambling and trembling as reflecting supraspinal and peripheral mechanisms, respectively, may be too simplistic.

Getting Your Sea Legs

Sea travel mandates changes in the control of the body. The process by which we adapt bodily control to life at sea is known as getting one's sea legs. We conducted the first experimental study of bodily control as maritime novices adapted to motion of a ship at sea. We evaluated postural activity (stance width, stance angle, and the kinematics of body sway) before and during a sea voyage. In addition, we evaluated the role of the visible horizon in the control of body sway. Finally, we related data on postural activity to two subjective experiences that are associated with sea travel; seasickness, and mal de debarquement. Our results revealed rapid changes in postural activity among novices at sea. Before the beginning of the voyage, the temporal dynamics of body sway differed among participants as a function of their (subsequent) severity of seasickness. Body sway measured at sea differed among participants as a function of their (subsequent) experience of mal de debarquement. We discuss implications of these results for general theories of the perception and control of bodily orientation, for the etiology of motion sickness, and for general phenomena of perceptual-motor adaptation and learning.

Influência da hidrocinesioterapia no equilíbrio postural de idosas institucionalizadas

O objetivo deste estudo foi avaliar a influência de um programa de hidrocinesioterapia em piscina rasa no equilíbrio postural de idosas institucionalizadas. Participaram do estudo 8 idosas institucionalizadas com média de idade de 62,95 ± 2,87 anos. O equilíbrio postural foi avaliado por meio de uma plataforma de força. Foram realizadas 3 tentativas em cada condição do teste de organização sensorial (TOS) previamente e posteriormente a intervenção de hidrocinesioterapia em piscina rasa. As variáveis analisadas foram: amplitude de deslocamento do centro de pressão (COP) na direção ântero-posterior, médio-lateral e velocidade de deslocamento do COP. Foi aplicado o teste Wilcoxon para a comparação entre as avaliações pré e pós a intervenção. Foi observada uma melhora no controle postural após a intervenção, visto que ocorreu diminuição do deslocamento do centro de pressão, principalmente na direção ântero-posterior. Para o grupo de idosas institucionalizadas participantes deste estudo, a hidrocinesioterapia realizada em piscina rasa foi capaz de melhorar o equilíbrio postural.

Effects of attentional focus and age on suprapostural task performance and postural control

PURPOSE

Suprapostural task performance (manual tracking) and postural control (sway and frequency) were examined as a function of attentional focus, age, and tracking difficulty. Given the performance benefits often found under external focus conditions, it was hypothesized that external focus instructions would promote superior tracking and reduced postural sway for both age groups, most notably as a function of tracking difficulty.

METHOD

Postural sway, frequency of postural adjustments, and tracking accuracy under two levels of task difficulty were assessed for younger (M(age) = 20.98 years) and older (M(age) = 70.80 years) participants while they manually tracked a pursuit-rotor target. Participants received instructions to focus on either their actions (internal focus) or the effect of their actions (external focus).

RESULTS

Analyses revealed a beneficial effect of an external focus on suprapostural performance on the less-difficult (0.5 Hz) tracking task, and this performance was associated with a modest improvement in medial-lateral postural sway.

CONCLUSION

The findings offer limited support for external focus-of-attention benefits under a mildly challenging tracking task. While older adults tend to adopt a conservative postural control strategy regardless of tracking task difficulty, external focus instructions on a suprapostural task promoted a modest, beneficial shift in postural control.

The reliability of a rambling-trembling analysis of center of pressure measures

The center of pressure (COP) measures are most commonly used in postural studies (e.g., COP range, sway path length and mean COP velocity). Relatively new method based on the equilibrium point hypothesis that addresses more explicitly the dynamic nature of COP and represents more subtle aspects of postural control is the rambling (RM) and trembling (TR) decomposition of a stabilogram. With this method, the COP trajectory can be decomposed into deviations resulting from supraspinal (i.e., RM) and spinal processes (i.e., TR). The trajectory generated by shifting COP is referred to as RM and its difference from the original COP signal is referred to as TR. Although there is still scientific discussion as to whether this method gives valuable additional information about postural control processes, it is gaining popularity. Unfortunately, there is lack of standardization in the methodology of data collection for this approach, particularly with respect to the number of trial repetitions and their duration. The objective of this study was to assess the reliability of RM-TR measurements. Fifteen young, healthy male subjects performed ten 120-s trials of quiet standing. Measurements of RM, TR and COP were calculated using the first 30, 60, 90 and 120-s of each trial. An ANOVA-based ICC model was used to estimate the reliability. The results of this study suggest that at least three 60-s trials should be used when using selected measures of RM-TR.

The influence of anthropometric factors on postural balance: the relationship between body composition and posturographic measurements in young adults

OBJECTIVE

The aim of the present study was to evaluate the influence of anthropometric characteristics and gender on postural balance in adults. One hundred individuals were examined (50 males, 50 females; age range 20-40 years).

METHODS

The following body composition measurements were collected (using bone densitometry measurements): fat percentage (% fat), tissue (g), fat (g), lean mass (g), bone mineral content (g), and bone mineral density (g/cm²). In addition, the following anthropometric measurements were collected: body mass (kg), height (cm), length of the trunk-cephalic region (cm), length of the lower limbs (cm) and length of the upper limbs (cm). The following indices were calculated: body mass index (kg/m²), waist-hip ratio and the support base (cm²). Also, a postural balance test was performed using posturography variables with open and closed eyes.

RESULTS

The analysis revealed poor correlations between postural balance and the anthropometric variables. A multiple linear regression analysis demonstrated that the whole group (female and male) height explained 12% of the medial-lateral displacement, 10% of the speed of oscillation, and 11% of the displacement area. The length of the trunk-cephalic length explained 6% of the displacement in the anteroposterior direction. With eyes closed, the support base and height explained 18% of the medial displacement, and the lateral height explained 10% of the displacement speed and 5% of the scroll area.

CONCLUSION

Measured using posturography, the postural balance was only slightly influenced by the anthropometric variables, both with open and closed eyes. Height was the anthropometric variable that most influenced postural balance, both in the whole group and separately for each gender. Postural balance was more influenced by anthropometric factors in males than females.

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

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

How visual information links to multijoint coordination during quiet standing

The link between visual information and postural control was investigated based on a multi-degree-of-freedom model using the framework of the uncontrolled manifold (UCM) hypothesis. The hypothesis was that because visual information specifies the position of the body in space, it would couple preferentially into those combinations of degrees of freedom (DOFs) that move the body in space and not into combinations of DOFs that do not move the body in space. Subjects stood quietly in a virtual reality cave for 4-min trials with or without a 0.2, 2.0 Hz, or combined 0.2 and 2.0 Hz visual field perturbation that was below perceptual threshold. Motion analysis was used to compute six sagittal plane joint angles. Variance across time of the angular motion was partitioned into (1) variance associated with motion of the body and (2) variance reflecting the use of flexible joint combinations that keep the anterior-posterior positions of the head (HD(POS)) and center of mass (CM(POS)) invariant. UCM analysis was performed in the frequency domain in order to link the sensory perturbation to each variance component at different frequencies. As predicted, variance related to motion of the body was selectively increased at the 0.2-Hz drive frequency but not at other frequencies of sway for both CM(POS) and HD(POS). The dominant effect with the 2.0-Hz visual drive also was limited largely to variance related to motion of the body.

Shifting the balance: evidence of an exploratory role for postural sway

Humans and other species are unable to stand perfectly still; their bodies continuously sway during stance even during concentrated efforts to avoid such movement. Traditionally, this phenomenon has been viewed as an inability of the central nervous system (CNS) to maintain perfect equilibrium because of its reliance on feedback from sensory signals to control corrective ground-reaction forces. Using a novel method to minimize movements of the body during stance without subject awareness, we have made the unique discovery that ground-reaction forces are generated independent of body sway, as evidenced by observations of increased centre of pressure variability when postural sway is minimized experimentally. Contrary to traditional views, our results suggest that postural sway may be used by the CNS as an exploratory mechanism to ensure that continuous dynamic inputs are provided by multiple sensory systems. This novel paradigm has the potential to significantly shift long-standing views on balance, and questions the theoretical basis behind conventional treatment strategies for balance deficits associated with age and disease.

Gender-specific effect of obesity on balance

Obesity modifies the body geometry by adding mass to different regions and it influences the biomechanics of activities of daily living. Weight influences postural stability, but there is no consensus as to whether the different fat distribution in males and females produces gender-related effects on balance. The aim of this study was to investigate the effect of body weight increases on postural performance in males and females. A total of 22 obese females (BMI: 41.1+/-4.1 kg/m2) and 22 obese males (BMI: 40.2+/-5 kg/m2) were analyzed during a static posture trial on a force platform in standardized conditions. Twenty healthy subjects (10 females, 10 males) constituted the control group. We computed the following parameters related to the center of pressure (CoP): velocity and displacements along the antero-posterior (AP) and medio-lateral axis (ML). We found several statistically significant differences between healthy and obese men, in particular regarding the AP and ML CoP parameters, which were correlated to body weight (r=0.36-0.58). The comparison between healthy and obese females pointed out statistically significant differences in AP parameters and no significant differences in ML displacements. Body weight was found to correlate with AP parameters (r=0.36-0.74), but not with ML displacements. The increased body mass seems to produce AP instability in both genders and ML destabilization only in males. Rehabilitation programs should take these findings into account by adopting specific interventions to improve ML control in obese males, and through weight loss and strengthening of ankle flexors/extensors in both genders.

Effect of low back pain on postural stability in younger women: influence of visual deprivation

SUMMARY

This study investigated the effect of low back pain (LBP) on body balance during normal and visual deprivation during standing in a LBP group (10 women) and a control group (10 women). A 3-D force plate was used to measure the center of pressure (COP) anteroposterior and mediolateral displacements, and resultant velocity. ANOVA was used to compare situations. LPB group presented higher amplitudes of COP for anterioposterior direction (p<0.01) in conditions of open (3.07 ± 0.53 cm) and closed eyes (3.70 ± 0.71 cm) than healthy women (1.39 ± 0.17 cm and 1.75 ± 0.36 cm, for open and closed eyes, respectively). Similar results were found for COP involving mediolateralsway. The resultant COP velocity was larger for LBP group (p<0.05) when visual information was removed (3.03 ± 0.68 m/s and 3.63 ± 1.33 m/s for LBP and healthy women, respectively). LBP influenced the stability of young women during quiet standing, and the visual deprivation appears to reinforce LBP effects.

Sensory reweighting as a method of balance training for labyrinthine loss

BACKGROUND AND PURPOSE

Instability is a significant risk factor for falls in individuals with a bilateral labyrinthine deficit. The purpose of this case report is to describe an intervention that we found to improve balance in a patient with bilateral labyrinthine deficit using a training paradigm based on the sensory reweighting hypothesis.

CASE DESCRIPTION

The participant was a female and 10 years post-onset of bilateral labyrinthine deficit. The participant was instructed to focus on the motion of her hips and knees while standing on a dynamic platform that was either stationary or matched to the excursion of her center of mass (COM) but in the opposite direction and with gradually increasing amplitude. She was tested for her ability to maintain her balance under conditions of sensory conflict both before the training and on two periods after training.

OUTCOMES

Decreases in anteroposterior and mediolateral motion of the COM were observed between the pretest and both posttests with a stationary and a moving platform when in the dark and under conditions of sensory conflict. Using the method of approximate entropy, we found that the complexity of the center of pressure (COP) response increased in both the anteroposterior and medolateral directions from the pretest to both posttests when on the platform matched to the COM motion.

SUMMARY

Results indicated that training on a dynamic platform diminished the destabilizing effect of conflicting sensory signals. Additionally, a relationship was observed between decreased COM motion and increased complexity in COP, which represents a more self-organized system. This finding suggests that improved stability may be associated with an increased complexity in the COP trajectory.

Twelve-week biomechanical ankle platform system training on postural stability and ankle proprioception in subjects with unilateral functional ankle instability

BACKGROUND

Functional ankle instability is defined as the subjective sensation of giving way or feeling joint instability after repeated episodes of ankle sprain. The purpose of this study was to examine the effects of 12-week biomechanical ankle platform system training on static postural stability and ankle reposition sense in subjects with unilateral functional ankle instability.

METHODS

Twelve university students (4 females and 8 males) with unilateral functional ankle instability volunteered as subjects. The active and passive reposition senses were assessed using an isokinetic dynamometer. The mean radius of the center of pressure excursion was recorded during single-leg standing with a force platform. A 12-week training program and a progression test for controlling the platform in certain directions and advancing to next training level was given to each subject. Repeated-measures 2-way analyses of variance were conducted to determine differences in postural stability and ankle proprioception between each limb before and after the training period.

FINDINGS

The mean radius of center of pressure on unilateral standing and the absolute error from pre-selected ankle angle in the functional ankle instability limb were significantly reduced after 12 weeks of training.

INTERPRETATION

These improvements in postural stability appear to reflect improved neuromuscular ability along with enhanced functional joint stability, as ankle proprioception also demonstrated the same positive improvements after training.

Modulation of cortical activity as a result of voluntary postural sway direction: an EEG study

There is increasing evidence demonstrating the role of the cerebral cortex in human postural control. Modulation of EEG both in voltage and frequency domains has been observed preceding and following self-paced postural movements and those induced by external perturbations. The current study set out to provide additional evidence regarding the role of cerebral cortex in human postural control by specifically examining modulation of EEG as a function of postural sway direction. Twelve neurologically normal subjects were instructed to produce self-paced voluntary postural sways in the anterior-posterior (AP) and medial-lateral (ML) directions. The center of pressure dynamics and EEG both in voltage and frequency domains were extracted by averaging and Morlet wavelet techniques, respectively. The amplitude of movement-related cortical potentials (MRCP) was significantly higher preceding ML sways. Also, time-frequency wavelet coefficients (TF) indicated differential modulation of EEG within alpha, beta and gamma bands as a function of voluntary postural sway direction. Thus, ML sway appear to be more difficult and energy demanding tasks than the AP sway as reflected in differential modulation of EEG. These results are discussed within the conceptual framework of differential patterns of brain activation as a result of postural task complexity.

Is voluntary control of natural postural sway possible?

The authors explored whether standing human participants could voluntarily decrease the amplitude of their natural postural sway when presented with explicit visual feedback and a target. Participants (N = 9) stood quietly, without any feedback and with feedback on the center of pressure coordinate or the head orientation. They were unable to decrease sway amplitude when presented with visual feedback and a target. Decreasing target size led to contrasting effects on the 2 fractions of sway: rambling and trembling. The smaller target was associated with a decrease in rambling and an increase in trembling. Those observations suggest that sway represents a superposition of at least 2 independent processes. They also suggest that providing visual feedback on a variable tied to body sway may not be an effective way to decrease postural sway in young healthy people.

Muscle synergies involved in shifts of the center of pressure while standing on a narrow support

We investigated multi-muscle synergies during preparation to push a load forward and their changes with different support conditions. We hypothesized that the subjects show unchanged mode structure and would be able to form multi-mode COP stabilizing synergies while standing on an unstable board. Eight healthy subjects participated in the study. Standing subjects performed load-pushing tasks under two conditions, "normal support" and "ML narrow support". Electromyographic (EMG) signals of 12 postural muscles were recorded and analyzed. The participants also performed standard tasks associated with releasing a load. These trials were used to identify muscle groupings (M-modes) associated with shifts of the center of pressure (COP) and relations between small changes in the M-modes and COP shifts in different support conditions. The subjects showed unchanged mode structure across different support conditions. The framework of the uncontrolled manifold hypothesis was used to partition the EMG variance across load-pushing trials into two components that kept constant and changed the COP coordinates in the anterior-posterior (AP) direction. This analysis has allowed us associate changes in the contribution of muscles with COP shifts under different support conditions. Different time profiles of the synergies were observed related to the COP shifts across conditions. This outcome supports a view that indices of multi-muscle (multi-M-mode) synergies can show anticipatory changes in preparation for a predictable perturbation.

Postural preparation to making a step: is there a 'motor program' for postural preparation?

We tested the hypothesis that a sequence of mechanical events occurs preceding a step that scales in time and magnitude as a whole in a task-specific manner, and is a reflection of a "motor program." Young subjects made a step under three speed instructions and four tasks: stepping straight ahead, down a stair, up a stair, and over an obstacle. Larger center-of-pressure (COP) and force adjustments in the anterior-posterior direction and smaller COP and force adjustments in the mediolateral direction were seen during stepping forward and down a stair, as compared with the tasks of stepping up a stair and over an obstacle. These differences were accentuated during stepping under the simple reaction time instruction. These results speak against the hypothesis of a single motor program that would underlie postural preparation to stepping. They are more compatible with the reference configuration hypothesis of whole-body actions.

The Influence of Gender and Somatotype on Single-Leg Upright Standing Postural Stability in Children

The purpose of this study was to investigate the influence of gender and somatotypes on single-leg upright standing postural stability in children. A total of 709 healthy children from different schools were recruited to measure the anthropometric somatotypes and the mean radius of center of pressure (COP) on a force platform with their eyes open and eyes closed. The results were that (a) girls revealed significantly smaller mean radius of COP distribution than boys, both in the eyes open and eyes closed conditions, and (b) the mesomorphic, muscular children had significantly smaller mean radius of COP distribution than the endomorphic, fatty children and the ectomorphic, linear children during the eyes closed condition. The explanation for gender differences might be due to the larger body weight in boys. The explanation for somatotype differences might be due to the significantly lower body height and higher portion of muscular profile in the mesomorphic children.

Stepping from a narrow support

The study addresses postural preparation to stepping. In particular, it tests a hypothesis that such preparation involves adjustments in the activity of ankle plantarflexors to produce shifts of the center of pressure. We investigated the initiation of a step from quiet stance when the subjects stood on boards with a decreased dimension of the support area in the anterior-posterior direction ("unstable boards"). Stepping from an unstable board was associated with decreased preparatory shifts of the center of pressure (COP) in the anterior-posterior direction from about 3 cm to 0.9 cm and further to 0.1cm when the support narrowed from comfortable standing to 3.3 cm and to nearly 0 cm. There was a smaller decrease in the COP shift in the medio-lateral direction. When the subjects stood on a board which rested on a very narrow ridge ("zero-support"), they showed an increase in the magnitude of changes in the horizontal force immediately prior to making a step. There was a general increase in the level of activation of leg and trunk muscles during stepping from unstable boards. The modulation of the activity of ankle plantarflexors increased during stepping from unstable boards. We conclude that, to initiate a step, COP shifts and changes in shear force can be modulated independently of each other in a constraint-specific manner. The results speak against the hypothesis that modulation of ankle plantarflexor activity during postural adjustments is directly related to the production of COP shifts.