Center Of Pressure Fluctuations Research Articles

Effect of Eight Weeks of Modified Hatha Yoga Exercises on the Balance of Children With Spastic Diplegia Cerebral Palsy Aged 7-14

Background and Aims Cerebral palsy is a common neuromuscular disease with non-progressive symptoms in children. The present study aims to investigate the effect of modified Hatha yoga exercises on the balance of children with spastic diplegia cerebral palsy. Methods This is a quasi-experimental study with a pretest/posttest design. The study population includs all children with spastic diplegia cerebral palsy (with levels 1 to 3 based on the Gross Motor Function Classification System and grades 0 to 3 based on the Modified Ashworth Scale) aged 7-14 years, who were selected from exceptional schools for the disabled students in Isfahan, Iran in 2018. Using a purposive sampling method, 14 eligible children were selected and randomly assigned into two groups of control and exercise. The exercise group performed the modified Hatha yoga exercise for eight weeks at three sessions per week; while the control group did not receive any intervention. The foot plantar center of pressure (COP) was recorded for six seconds (as the balance test) using a foot scanner in the pre-test and post-test phases. Finally, the data were analyzed using repeated measures analysis and independent t-test in SPSS software, version 23. The significance level was set at 0.05. Results The intervention significantly improved the balance of the exercise group compared to the control group. The reduction of COP fluctuations in anterior-posterior (P=0.044), mediolateral (P=0.033), and total directions (P=0.038) in the exercise group were significantly higher than in the control group (P<0.05). Conclusion The modified Hatha Yoga can be a suitable exercise to improve the postural control of children with spastic diplegia cerebral palsy. However, further research is needed to draw a definitive conclusion.

Precision aiming performance with the paretic upper limb is associated with center of pressure patterns in individuals with chronic stroke

BackgroundIndividuals with stroke commonly demonstrate upper-limb sensorimotor impairments. Upper-limb tasks occur against a background level of postural control and thus require a flexible postural control system to facilitate performance. Anterior precision aiming tasks, for example, benefit from lower medial-lateral (ML) center of pressure (COP) fluctuations (where increased fluctuations erode performance) relative to anterior-posterior (AP) fluctuations (where increased fluctuations do not strongly influence performance). After stroke, individuals may compensate for upper-limb impairments by increasing trunk movement which increases overall COP fluctuations and thus may make it more difficult to modulate COP in a task-sensitive manner. Research questionDo upper-limb task demands modulate COP movement patterns after stroke? MethodsIn this cross-sectional study, adults with chronic stroke (n = 23) and unilateral upper-limb impairments were immersed in a virtual environment displaying an anterior target. Participants aimed to maintain the position of a virtual laser pointer (via handheld controller) in the target with each hand. COP was concurrently recorded. Mixed effects models and correlations were used to detect differences in COP patterns between limbs and movement planes and evaluate associations between task performance and COP patterns, respectively. ResultsParticipants showed greater COP standard deviation and regularity in the AP compared to the ML direction. The magnitude of difference between AP and ML COP metrics was greater using the nonparetic limb. Task performance was moderately and positively associated with task-sensitive COP patterns (i.e., higher AP:ML ratios of COP metrics) using the paretic upper limb. Participants consistently demonstrated high levels of task performance and task-sensitive COP movement patterns using the nonparetic limb. SignificanceImpairments in postural control after stroke may be related to the upper limb used. It is important to recognize the role of directional COP variability and regularity in the context of a task goal after stroke.

The effect of a light finger touch on the signal complexity during quiet standing

Purpose The purpose of the study was to investigate the influence of additional tactile information (light fingertip touch) on the postural sway and regularity of center-of-pressure (COP) fluctuations. Methods Thirty-two young, healthy participants performed a quiet standing task (30s) on a force platform with and without light touch. COP time-series were analyzed using standard postural sway measures (range, root mean square error, velocity), COP regularity was measured with Sample entropy. Results Participants demonstrated significantly smaller postural sway with a light touch, but only in the anteroposterior direction. The amount of sway with additional tactile information in the sagittal plane reached the level of sway in the frontal plane without this information. Similarly, COP fluctuations were more irregular during light touch condition only in the anteroposterior direction, as evidenced by significantly higher Sample entropy. Furthermore, COP regularity decreased in the sagittal plane and reached level in the frontal plane without light touch. Conclusions These results suggest that postural sway is mostly controlled in the sagittal plane and that in the mediolateral direction the control is mostly automated. In conclusion, our results support the notion that the light touch provides additional information which enhances postural stabilization. Our results expand the relation between COP regularity and the attention invested in posture in the touch domain and prove that light touch, as an attentional demanding task, leads to increased COP irregularity. Nonlinear measures of signal regularity (i.e. SampEn) provide surplus insight into human postural control and can be used as an useful tool to traditional balance measures.

Relationship between ankle plantar flexor force steadiness and postural stability on stable and unstable platforms

This study was aimed at determining the relationship between ankle plantar flexor force steadiness and postural control during single leg standing on stable and unstable platforms. For the thirty-three healthy participants, force steadiness, at target torques of 5%, 20%, and 50% of the maximum voluntary torque (MVT) of the ankle plantar flexors, was measured. Force steadiness was calculated as the coefficient of variation of force. Single leg standing on stable and unstable platforms was performed using the BIODEX Balance System SD. The standard deviation of the anteroposterior center of pressure (COP) displacements was measured as the index for postural control. During both measurements, muscle activities of the soleus were collected using surface electromyography. On the stable platform, the COP fluctuation significantly correlated with force steadiness at 5% of MVT (r = 0.512, p = 0.002). On the unstable platform, the COP fluctuation significantly correlated with force steadiness at 20% of MVT (r = 0.458, p = 0.007). However, the extent of muscle activity observed for a single leg standing on both stable and unstable platforms was significantly greater than the muscle activity observed while performing force steadiness tasks at 5% and 20% of MVT, respectively. Postural stability during single leg standing on stable and unstable platforms may be related to one's ability to maintain constant torque at 5% and 20% of MVT regardless of the muscle activity. These results suggest that the required abilities to control muscle force differ depending on the postural control tasks.

Frail Older Individuals Maintaining a Steady Standing Position: Associations Between Sway Measurements with Frailty Status Across Four Different Frailty Instruments.

ObjectiveAn analysis of the relationships between static equilibrium parameters and frailty status and/or severity across four different frailty measures.DesignCross-sectional analysis.SettingGeriatric wards of a general hospital.ParticipantsOne hundred twenty-three geriatric inpatients comprising 70 women (56.5%) and 53 men (42.7%) with an age range of 68–95 years.MethodsThe variation in the center of pressure (CoP), ie, the length of sway, the area of sway, and the mean speed, was assessed for different positions/tasks: 1) wide standing with eyes open (WSEO); 2) wide standing with eyes closed (WSEC); 3) narrow standing with eyes open (NSEO) and 4) narrow standing with eyes closed (NSEC), using a force plate. Frailty status and/or frailty severity were evaluated using the frailty phenotype (FP), the clinical frailty scale (CFS), the 14-item frailty index based on a comprehensive geriatric assessment (FI-CGA), and a 47-item frailty index (FI).ResultsWSEO length of sway (FP, CFS, FI-CGA, FI), WSEO area of sway (FP, CFS, FI-CGA, FI), and WSEO mean speed (FP, CFS, FI-CGA, FI), WSEC length of sway (FP, FI-CGA, FI), WSEC area of sway (FP, FI-CGA, FI) and WSEC mean speed (FI-CGA, FI), NSEO length of sway (FP, FI-CGA, FI), NSEO area of sway (FP, CFS, FI-CGA, FI), and NSEO mean speed (FP, CFS, FI-CGA, FI), NSEC length of sway (FI-CGA, FI), NSEC area of sway (FI-CGA, FI) and NSEC mean speed (FI-CGA, FI) were associated with the frailty status and/or severity across the four different frailty instruments (all p < 0.05, respectively).ConclusionGreater fluctuations in CoP with increasing frailty status and/or severity were a uniform finding across various major frailty instruments.

Achilles tendon shear wave speed tracks the dynamic modulation of standing balance.

Standing balance performance is often characterized by sway, as measured via fluctuations of the center of pressure (COP) under the feet. For example, COP metrics can effectively delineate changes in balance under altered sensory conditions. However, COP is a global metric of whole‐body dynamics and thus does not necessarily lend insight into the underlying musculotendon control. We have previously shown that shear wave tensiometers can track wave speeds in tendon as a surrogate measure of the load transmitted by the muscle‐tendon unit. The purpose of this study was to investigate whether shear wave metrics have sufficient sensitivity to track subtle variations in Achilles tendon loading that correspond with postural sway. Sixteen healthy young adults (26 ± 5 years) stood for 10 s with their eyes open and closed. We simultaneously recorded COP under the feet and shear wave speed in the right Achilles tendon. We found that Achilles tendon shear wave speed closely tracked (r > 0.95) dynamic fluctuations of the COP in the anteroposterior direction. Achilles tendon wave speed fluctuations significantly increased during standing with eyes closed, mirroring increases in COP fluctuations. These results demonstrate that tendon wave speed can track the subtle variations in Achilles tendon loading that modulate COP in standing. Hence, shear wave tensiometry exhibits the sensitivity to investigate the muscular control of quiet standing, and may also be useful for investigating other fine motor and force steadiness tasks.

Contractile properties of superficial skeletal muscle affect postural control in healthy young adults: A test of the rambling and trembling hypothesis.

The rambling and trembling analysis separates the center of pressure (COP) fluctuations into two components: rambling (supraspinal contribution) and trembling (muscle stiffness / reflexive properties contribution). We examined whether the trembling component is correlated to the contractile properties (muscle stiffness and contraction time) of lower limb superficial skeletal muscles to experimentally test the rambling and trembling hypothesis. We hypothesized that muscle stiffness and contraction time, would be: (a) more correlated with; and (b) have a greater impact on the trembling component compared to the rambling component. Thirty-two healthy young adults were recruited for the study and tensiomyography was used to assess mechanical muscle responses to a single electrical stimulus to calculate muscle stiffness and contraction time based on radial muscle belly displacement measurements of lower limb muscles unilaterally. Moreover, upright postural control was assessed using a force plate to record ground reaction forces and moments and calculate the COP fluctuations during two 30 seconds trials. From the COP fluctuations, rambling and trembling time series were extracted, and all fluctuation time series were described using a number of different time-domain and frequency-domain parameters in both the anterior-posterior and medial-lateral directions. Our results demonstrated that both muscle stiffness and contraction time were moderately correlated with time-domain and frequency-domain parameters of the trembling component, as compared with those of the rambling component which was not as well correlated. Moreover, they also predicted the trembling component better. Overall, these results imply that postural control during quiet stance is, in part, related to intrinsic muscle stiffness in the lower extremities. Moreover, we showed that the rambling and trembling hypothesis is effective in separating postural sway fluctuations during upright posture to extract the contributions of muscle stiffness / reflexive properties (trembling), and likely the supraspinal contribution (rambling).

Evaluation of Langevin Model for Human Stabilogram Based on Reproducibility of Statistical Indicators.

Fall risk is a serious problem especially for the elderly. Fall accident causes fracture, and it leads to bedridden. Early detection of balance inability and rehabilitation training is important to decrease the risk. Quiet standing test is one of the physical tests to assess the balance inability of human. When the human is standing, he/she controls the plantar force to keep the balance. Center of pressure (COP) is the representative parameter to explain the plantar force movement. During the quiet standing test, COP fluctuates unconsciously. There are many researches that analyze the relationship between the COP fluctuation and balance inability. Many statistical indicators have been developed to assess the fluctuation specification.In contrast, several researchers have been tried to reveal the COP fluctuation mechanism by introducing stochastic mathematical model. Ornstein-Uhlenbeck process and diffusion equation are often introduced to the model. The mathematical models have been developed, and analyzed how visual input is related to the model parameters. If the mathematical model can explain the COP fluctuation completely, it becomes easier to assess the balance ability by comparing the model parameters.However, statistical indicators are still used in the clinical cases, thus there is room to discuss which statistical indicators to be used for assessment. The purpose of this research is to explore the relationship between statistical indicators and numerical model by evaluating whether the numerical model reproduces the same values with real data. The reproduced indicators and original indicators are compared, and the magnitude of the errors are evaluated.

Comparison of posturographic outcomes between two different devices

The Interactive Balance System (IBS), a posturography device for assessing posture control, is widely used in clinical and rehabilitation settings. However, data on the validity of the device are unavailable. Fluctuations of the center of pressure (COP) were measured in 24 healthy participants (age: 29 ± 5 (mean ± SD) years, 12 females) synchronously using the IBS, which was rigidly mounted on a Kistler platform. Four different bipedal conditions were examined: eyes open or closed on stable or soft surfaces. Time series were compared using congruity (CON, proportion of the measurement time during which values of both devices changed similarly in direction), whereas IBS-specific postural outcomes were correlated with traditional postural control outcomes of the Kistler force platform. The time-displacement curves showed similar shapes for CON (>0.9) for each of the four standing conditions without differences between male and female participants (P > 0.39). The path length results of both devices showed very high linear associations, explaining on average 92% (medio-lateral) or 96% (anterior-posterior) of the common variance. The Kistler path length of the anterior-posterior direction revealed nearly perfect linear associations with the stability index of the IBS (r2 > 0.99). The results of this study indicate that the IBS provides valid posturographic results. Since the medial-lateral and anterior-posterior trajectories of the IBS can be used to calculate COP fluctuations, comparisons between different measurement systems are possible.

AGE-RELATED DIFFERENCE IN DYNAMIC POSTURAL BALANCE AGAINST TILTING PERTURBATION IN MEN AND WOMEN

The aim of this study was to investigate how age and gender affect the dynamic postural balance during tilting perturbation. Fifty healthy subjects (15 young men, 13 young women, 11 elderly men and 11 elderly women) performed balance test on a movable force plate that tilted toe-up and toe-down. As outcome measures, maximum excursion and fluctuation were calculated from center of pressure (COP) data in the sagittal plane (anteroposterior). Two-way analysis of variance (ANOVA) and post-hoc comparisons were performed for the outcome measures with the independent factors of age and gender. The elderly had a greater COP maximum excursion as compared to the young during both perturbations ([Formula: see text]). COP fluctuation showed significant interaction of age and gender only in toe-up perturbation ([Formula: see text]). Especially, age-related difference existed only in women ([Formula: see text]). These results suggest that elderly women have dynamic balance strategy with great and fluctuated sway in response to toe-up perturbation. The age-related changes in dynamic balance among women may be related to the greater fall rate of elderly women.

Influence of musical groove on postural sway.

Timescales of postural fluctuation reflect underlying neuromuscular processes in balance control that are influenced by sensory information and the performance of concurrent cognitive and motor tasks. An open question is how postural fluctuations entrain to complex environmental rhythms, such as in music, which also vary on multiple timescales. Musical groove describes the property of music that encourages auditory-motor synchronization and is used to study voluntary motor entrainment to rhythmic sounds. The influence of groove on balance control mechanisms remains unexplored. We recorded fluctuations in center of pressure (CoP) of standing participants (N = 40) listening to low and high groove music and during quiet stance. We found an effect of musical groove on radial sway variability, with the least amount of variability in the high groove condition. In addition, we observed that groove influenced postural sway entrainment at various temporal scales. For example, with increasing levels of groove, we observed more entrainment to shorter, local timescale rhythmic musical occurrences. In contrast, we observed more entrainment to longer, global timescale features of the music, such as periodicity, with decreasing levels of groove. Finally, musical experience influenced the amount of postural variability and entrainment at local and global timescales. We conclude that groove in music and musical experience can influence the neural mechanisms that govern balance control, and discuss implications of our findings in terms of multiscale sensorimotor coupling. (PsycINFO Database Record

Trunk muscle co-activation using functional electrical stimulation modifies center of pressure fluctuations during quiet sitting by increasing trunk stiffness.

BackgroundThe purpose of this study was to examine the impact of functional electrical stimulation (FES) induced co-activation of trunk muscles during quiet sitting. We hypothesized that FES applied to the trunk muscles will increase trunk stiffness. The objectives of this study were to: 1) compare the center of pressure (COP) fluctuations during unsupported and FES-assisted quiet sitting - an experimental study and; 2) investigate how FES influences sitting balance - an analytical (simulation) study.MethodsThe experimental study involved 15 able-bodied individuals who were seated on an instrumented chair. During the experiment, COP of the body projected on the seating surface was calculated to compare sitting stability of participants during unsupported and FES-assisted quiet sitting. The analytical (simulation) study examined dynamics of quiet sitting using an inverted pendulum model, representing the body, and a proportional-derivative (PD) controller, representing the central nervous system control. This model was used to analyze the relationship between increased trunk stiffness and COP fluctuations.ResultsIn the experimental study, the COP fluctuations showed that: i) the mean velocity, mean frequency and the power frequency were higher during FES-assisted sitting; ii) the frequency dispersion for anterior-posterior fluctuations was smaller during FES-assisted sitting; and iii) the mean distance, range and centroidal frequency did not change during FES-assisted sitting. The analytical (simulation) study showed that increased mechanical stiffness of the trunk had the same effect on COP fluctuations as the FES.ConclusionsThe results of this study suggest that FES applied to the key trunk muscles increases the speed of the COP fluctuations by increasing the trunk stiffness during quiet sitting.

Correlation between functional balance and mobility tests and center of pressure fluctuation parameters in quiet standing with and without cognitive task in children with hemiparetic cerebral palsy

Introduction: Because of the effect of person, task and environment on the postural control of children with Hemiparetic Cerebral Palsy (HCP), different clinical and laboratory methods have been designed for postural control evaluation. The purpose of this study was to investigate the correlation between clinical and laboratory tests of mobility and balance in children with HCP. Methods: In this correlational study, twenty eight children with HCP by average age of 10.09 (±2.34) years were selected by simple non-probability sampling method. Timed Up and Go (TUG), Functional Reach (FR) and Bruininks Oseretsky Test of Motor Proficiency (BOTMP) and mean velocity, path length, total phase plane, area parameters of center of pressure fluctuations in 3 levels of postural difficulty (quiet standing on rigid surface with open and closed eyes and foam with closed eyes) and two levels of cognitive difficulty (with and without cognitive task) recorded by force plate were used as clinical and laboratory assessments respectively. Results: Among three postural tasks, the only in standing on foam with closed eyes was seen the significantly moderate to very high correlation between the all center of pressure sway measures and TUG, FR and BOTMP tests. The cognitive task in standing on foam surface condition cause to increase correlation between clinical and laboratory measures. Conclusion: The proper correlation was obtained between functional balance and mobility tests and center of pressure sway parameters in standing on foam surface with closed eyes condition. Key words: Postural control, Functional balance and mobility tests, Force plate parameters, Children, hemiparesia, cerebral palsy

A characteristic pattern in the postural sway of unilateral vestibular impaired patients

The statistical properties of the center of pressure (COP) change over time, and -invariant methods of the COP analysis is not sufficient to monitor the changes. Therefore, dynamic temporal information of the COP signals has been important in assessing the postural stability. The purpose of this study was to evaluate the characteristic pattern of time–frequency dynamics during the upright stance in patients with unilateral vestibular dysfunction (UVD). This study included 20 dizzy patients with UVD and age-matched 20 control subjects without any vestibular disorder. The COP signals were collected using a single standard force platform and a spectral analysis including the time–frequency dynamics was carried out on the basis of the maximum entropy method (MEM) by using a segment time series analysis. The power spectral density (PSD) analysis exhibited an exponential decreasing shape (1/f −β) when plotted on a double logarithmic scale. The average value of β in the low frequency range of less than 1Hz in the UVD group was significantly lower than that in the control group (p<0.05) in the medial-lateral (ML) direction under eyes closed condition. In the segment time series analysis, the peak frequency of the COP signals in the patients with UVD gradually converged to the frequency range of 0.1–0.2Hz in the ML direction under EC condition. Our results suggest that the segment time series analysis of the COP signals can derive a characteristic pattern related to the UVD. It may indicate an increased reflexivity of the COP fluctuation by the UVD, resulting in inducing the instability.

Decreased dynamical complexity during quiet stance in children with Autism Spectrum Disorders

BackgroundPostural control deficits in individuals with Autism Spectrum Disorders (ASD) are widely acknowledged; however, the underlying biomechanical features of these deficits remain unknown. Nonlinear analyses provide insight into the nature of how movement is controlled and have the potential to provide new insight into the postural control abnormalities associated with ASD. The purpose of this study was to further investigate postural control deficits in children with ASD through linear and nonlinear analyses of center of pressure (COP) data. MethodsWe evaluated COP data during quiet standing for 16 children with ASD and 17 age-matched typically developing (TD) children. The magnitude of COP fluctuations (COP ranges, velocity, and sway area) and complexity of postural control dynamics, quantified by multiscale entropy (MSE), were compared across groups. ResultsChildren with ASD displayed larger fluctuations in their COP data, observed in COP ranges (95.5% mediolaterally and 46.9% anteroposteriorly, p<0.05 respectively) and COP sway area (885%, p<0.05). Children with ASD also displayed less complexity in their COP data, observed in the MSE complexity index (CI) (32.4% mediolaterally and 35.7% anteroposteriorly, p<0.05 respectively). ConclusionsThe present study successfully revealed that children with ASD have more repetitive patterns in their COP data, indicating a less complex control of posture, on multiple time scales, during quiet stance. These findings suggest a more regular or restricted control of posture and may be an initial step in linking postural instability to stereotypic behavior and the neurobiology of ASD.

Frontal Plane Instability Following Rapid Voluntary Stepping: Effects of Age and a Concurrent Cognitive Task

Quick step execution may prevent falls when balance is lost. Lateral steps often emerge as a consequence of frontal plane instability arising after the first rapid step. In this study, we suggest a new analysis, focusing on the variability of the frontal plane fluctuations of center of pressure (CoP), that is, mediolateral instability, and their changes over time during and immediately following rapid voluntary stepping in older and younger adults in single- and dual-task conditions. This may be useful in understanding age-related alterations in the locomotor control system. Seventeen older adults, who live independently in the community, and 16 younger adults performed rapid forward voluntary stepping under single- and dual-task conditions. The average mediolateral CoP fluctuations, that is, the average distance the CoP travels from side to side in the frontal plane over time, standard deviation, and the coefficient of variation of mediolateral CoP fluctuation were extracted and calculated from CoP data during and immediately following rapid voluntary stepping using a force plate. We found an age-related increase in the coefficient of variation that represents the variability of frontal plane fluctuations and no significant differences in the average and standard deviations of frontal plane fluctuations. Cognitive task had no influence on measures of frontal plane fluctuations in both age groups. The study showed frontal plane instability during and immediately following rapid stepping in older persons. This may be a factor contributing to lateral balance loss and the large number of lateral falls seen in the older population.

Recurrence Quantification Analysis of Human Postural Fluctuations in Older Fallers and Non-fallers

We investigate postural sway data dynamics in older adult fallers and non-fallers. Center of pressure (COP) signals were recorded during quiet standing in 28 older adults. The subjects were divided in two groups: with and without history of falls. COP time series were analyzed using recurrence quantification analysis (RQA) in both anteroposterior and mediolateral (ML) directions. Classical stabilometric variables (path length and range) were also computed. The results showed that RQA outputs quantifying predictability of COP fluctuations and Shannon entropy of recurrence plot diagonal line length distribution, were significantly higher in fallers, only for ML direction. In addition, the range of ML COP signals was also significantly higher in fallers. This result is in accordance with some findings of the literature and could be interpreted as an increased hip strategy in fallers. The RQA results seem coherent with the theory of loss of complexity with aging and disease. Our results suggest that RQA is a promising approach for the investigation of COP fluctuations in a frail population.

Noise Stimuli Improve the Accuracy of Target Aiming: Possible Involvement of Noise-Enhanced Balance Control

Balance stability is important for human beings. Previous studies have shown that mechanical noise applied to the soles of the feet via vibrating insoles can improve human balance control. In this study we designed an experi- mental procedure called standard aiming target test to quanti- tatively evaluate the effect of vibrating insole on specific task of aiming during standing. Ten subjects were asked to aim at a target sheet using laser gun, and the traces of their aimed light spots and the center of pressure (COP) were recorded simul- taneously. The same test was repeated twice for each individ- ual with vibrating insole off and on, respectively. To assess dynamic patterns in aiming spot and COP signals, we calcu- lated theentropiesinthemagnitudeincrementsofbothsignals at different time scales, i.e., multiscale entropies (MSE).We found that the vibrating insole significantly improved the aiming performance and led to increased entropy in the fluc- tuations of aiming spot displacement over a range of time scales from 0.001 to 0.05 s. In addition, subjects with larger entropies in the aiming spot fluctuations had better aiming performances. Furthermore, we found that the entropy of the light spot signal was positively correlated with the entropy of the magnitude time series of COP fluctuations. These results suggest that vibrating insoles is beneficial for the complex postural control process during target aiming, possibly via their dynamic influences on the balance regulatory system.

Synergistic co-activation in multi-directional postural control in humans

To examine the muscle synergies of multi-directional postural control, we calculated the target-directed variance fraction (η) and net action direction of each muscle using the electromyogram-weighted averaging (EWA) method. Subjects stood barefoot on a force platform and maintained their posture by producing a center of pressure (COP) in twelve target directions. Surface electromyograms were recorded from 6 right-sided muscles: tibialis anterior (TA), soleus (SOL), lateral gastrocnemius (LG), medial gastrocnemius (MG), fibularis longus (FL), and gluteus medius (GM). η was calculated from COP with duration of 20-s, during which the COP was relatively constant. The EWA method was applied to the EMG and the two COP components to estimate the net action direction of each muscle. The results showed that η values in all directions did not cross the 0.8 threshold. This suggests that human postural control is achieved by synergistic co-activation. The EWA revealed that the net action directions of TA, SOL, LG, MG, and GM were 277.6°, 71.1°, 87.7°, 94.0°, and 2.2°, respectively. This suggests that postural maintenance by muscle synergy can be attributed to the relevant muscles having various action directions. These results demonstrate that muscle synergies can be investigated using COP fluctuations.

Large postural fluctuations but unchanged postural sway dynamics during tiptoe standing compared to quiet standing

The purpose of this study was to detect the characteristics of center of pressure (COP) movement during tiptoe standing (TS) compared to quiet standing (QS). Eight healthy subjects were asked to perform QS and TS on a force platform. During standing, surface electromyograms (EMGs) were recorded from the soleus (SOL), flexor hallucis brevis (FHB), medial gastrocnemius (MG), lateral gastrocnemius (LG), and tibialis anterior (TA) muscles. The path length and rectangular area of the COP trajectory were significantly larger during TS than during QS. In contrast, irrespective of standing condition, the scaling coefficients in the short and long regions were above and below 0.5, respectively. The coherence spectrum between the COP and EMG from the SOL and FHB muscles was statistically significant during TS at frequencies up to 17Hz, while that for the QS was only significant below 1Hz. In conclusion, the control of COP movement during TS was similar to that during QS despite large COP fluctuations during TS. Our results suggest that unstable posture during TS is compensated for by the activities of the SOL and FHB muscles, which enhance postural control.