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Our study aimed to examine differences in postural sway variables between Parkinson’s disease (PD) patients with freezing of gait (FOG) and those without freezing of gait (NFOG), with a particular focus on their fall history. Eighty-one patients with PD participated in this study, including 32 fallers and 49 nonfallers. In the faller group, 21 patients were classified into the FOG group, while 11 patients were categorized into the NFOG group. Each patient underwent a 30 s static postural balance assessment while standing on a custom-developed force plate. The center of pressure (COP) data was filtered and analyzed to derive key COP-based outcome measures. Independent t-tests were conducted to compare the FOG and NFOG groups within both faller and nonfaller subgroups. Spearman’s rank correlation analysis was conducted to investigate the association between COP-based outcomes and Tinetti balance scores. In the nonfaller group, FOG patients exhibited significantly greater mean distance ($p<$ 0.05), particularly in the mediolateral direction ($p<$ 0.01), compared to NFOG patients. Anterior–posterior (AP) peak power was also significantly higher in the FOG group ($p<$ 0.05), along with a larger sway area ($p<$ 0.05). However, no significant differences were noted within the faller group ($p>$ 0.05). Notably, in the nonfaller NFOG group, most COP-based variables were significantly correlated with Tinetti balance scores ($r=-$0.463 – 0.535, $p<$ 0.05), with stronger correlations observed in the mediolateral direction. No significant correlations were found in other groups ($p>$ 0.05). These findings may help clinicians in better distinguish between patients with and without FOG, considering their fall history.

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 ($p<0.05$). COP fluctuation showed significant interaction of age and gender only in toe-up perturbation ($p<0.05$). Especially, age-related difference existed only in women ($p<0.001$). 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.

It is important to assess lateral balance because falls to the side frequently occur and lead to hip fracture in elderly adults. The aim of this study was to investigate lateral balance in response to lateral tilting perturbation in young and elderly adults. Fifty healthy subjects (28 young and 22 elderly) participated in this study. They were instructed to maintain postural balance against right-down and left-down perturbations. As outcome measures, the mean distance, mean velocity and peak frequency were derived from the center of pressure (COP) data in the frontal plane (medio-lateral). Independent $t$-tests were performed to compare the elderly with the young. The elderly group exhibited a greater COP mean distance and faster mean velocity compared to the young group in right-down perturbation ($p<0.05$). The COP peak frequency of the elderly was higher than that of the young in both right- and left-down perturbations ($p<0.05$). These results suggest that the elderly have a dynamic lateral balance strategy with greater and faster body sway and with more frequent adjustments of the COP in response to lateral perturbation. This study will contribute to the understanding of lateral balance strategy in the elderly for the prevention and intervention of falls.

Although the risk of foot injuries during walking increases with the foot deformity, the dynamic mechanism is not clearly understood. This study aims at the investigation of dynamic change of center of pressure (COP) in different foot types. Contrasted by previous studies, this study analyzed COP in each gait phase, i.e., loading response, mid stance, terminal stance, and pre-swing. A total of 19 young males participated in this study and the resting calcaneal stance position (RCSP) angle was measured for the classification into three foot types. All participants performed level walking with shoes on. COP trajectory was normalized by foot width and length. In the loading response and mid stance phases COP of Pes Cavus located most laterally ($p<0.05$). No difference among foot types existed at terminal stance and pre-swing phases ($p>0.05$). Foot deformity is known to occur due to the abnormality of musculoskeletal system such as lower extremities muscles, bones, and ligaments. Because the role of musculoskeletal system differs between gait phases, this may have caused phase-dependent COP difference among different foot types.

Differentiating scans without evidence of dopaminergic deficits (SWEDDs) from Parkinson’s disease (PD) is very important to avoid costly and inappropriate interventions. The postural balance strategy patients with SWEDDs are not clear. This study investigates the difference in static postural balance between patients with SWEDDs and PD. Twenty-two patients (11 SWEDDs and 11 PD groups) and 11 control groups were instructed to quietly stand on a custom-made force platform. The outcome measures were, mean distance, 95% confidence ellipse area, mean velocity, mean frequency, peak frequency and peak power derived from the center of pressure (COP) data in overall, medio-lateral (ML) and anterio-posterior (AP) directions. As statistical analysis, ANOVA and post hoc tests were conducted for comparison of patients with SWEDDs and PD. SWEDDs patients presented a smaller postural sway size and a more frequent postural sway mainly in the ML direction ($p<0.05$). On the contrary, there were no significant differences in mean velocity and peak power. Also, no significant differences existed between SWEDDs and normal subjects ($p>0.05$). The results reflect that a few COP-based outcome variables might be useful to distinguish SWEDDs patients from PD patients. This study suggests that a static postural balance test can be used for clinical screening and identification of potential SWEDDs.

This study investigated the quantitative scaling properties of the center of pressure (COP) as well as the spatial-temporal properties of the COP to elucidate the postural control behavior of healthy elderly (HE) adults and adults with Parkinson’s disease (PD) during quiet standing. Eighteen adults with PD and eighteen HE adults participated in this study. The COP movements were recorded while participants stood on either a firm surface or on a foam pad with their eyes either opened or closed. The sway ranges in the anterior–posterior (AP) ($R_x)$ and medio-lateral (ML) ($R_y)$ directions, the total length of the trajectory ($L)$, sway area ($A)$, and scaling exponents ($\alpha )$ from detrended fluctuation analysis were computed from the measured COP data. All temporal variables of the COP in all conditions were found to be significantly larger in the PD group than in the HE group. Low scaling exponents obtained for the PD group showed this group possessed diminished postural control ability compared to the HE group. The PD group showed unpredictable open-loop control in both the AP and ML directions. This proprioceptive control became predictable and the time scale relations decreased as the postural challenges increased. The AP and ML closed-loop control of the PD group was more predictable than that of the HE group only when proprioception was distorted using intact visual input, and the visual and proprioceptive inputs were both intact.

It is important to confirm the reliability of postural balance variables because the results of balance tests would be different with different balance test protocols, i.e., stance conditions such as natural and feet-together stances. The aim of this study was to investigate the test–retest reliability of postural balance variables during the natural and feet-together stance conditions. Sixteen young healthy adults were instructed to sustain their center of body mass as stably as possible on the force plate. As balance standing conditions, natural (a comfortable self-selected stance width) and feet-together stances (placing the feet $0^{\circ }$ apart) were selected. The mean distance, mean velocity, mean frequency, and 95% confidence ellipse area were derived from the center of pressure (COP) time series in the overall, anteroposterior (AP), and mediolateral (ML) directions. To evaluate the test–retest reliability of the postural balance variables, Cronbach’s alpha coefficient was calculated in each stance condition. Mean velocity was the most reliable variable particularly in three repeated balances within both the natural and feet together stances ($\alpha =0.805$–0.921). The reliability of all variables of the three balance test was greater than that of the two balance tests, mainly in the natural stance. In contrast, the mean ML distance of the feet-together stance had poor reliability despite three balance tests ($\alpha =0.243$). These results suggest that the balance test protocol should consider the appropriate stance conditions as well as the number of balance trials to select the reliable postural balance variables.

Poor postural balance has been related to falls in older people. Although fall rates increase with age, age-related declines in postural balance of older people are still unclear. The goal of this study was to investigate age-related changes in the static postural balance ability of Korean elderly adults. Three hundred and eighty-nine normal elderly adults with ages ranging from 57 years to 87 years participated in this study. All elderly adults were instructed to stand in their preferred natural stance on a self-developed force platform. As postural sway variables, peak frequency, mean frequency, mean velocity, and mean distance were calculated from the center of pressure (COP) trajectories in both the medio-lateral (ML) and antero-posterior (AP) directions. The mean velocity in the older-elderly was significantly faster compared to those of the other age groups particularly in the AP direction ($p<0.05$). In both the ML and AP directions, the older-elderly group showed a greater mean distance compared to the other age groups ($p<0.05$). Conversely, no significant age group differences were found in mean frequency and peak frequency ($p>0.05$). Our results mean that the older-elderly group showed postural balance with faster AP sway speed and overall sway size. The results of our study help in the understanding of age-related declines in postural balance for the prevention of falls and the development of a regression model for normative postural balance ability in Korean elderly adults.

Height is associated with postural balance performance. However, there has been no study on whether the test-retest reliability of the balance performance variables is related to height. The goal of our study was to analyze the effect of body height on the test-retest reliability of center of pressure (COP) variables during natural and feet-together stances in healthy young subjects. Twenty normal young men were classified into the short height group (height range, 163–174cm) and the tall height group (height range, 176–184cm) with 10 subjects each to investigate the influence of height on test-retest reliability. All subjects performed the static balance test on a commercial force plate in feet-together and natural stance conditions three times. Four variables were calculated from the COP data. COP distance, COP area, COP velocity, and COP mean frequency were selected as the postural balance variables. Intraclass correlation coefficient (ICC) was evaluated for an analysis of the test-retest reliability of each COP variable. In both stance conditions, the COP distance and the COP area in the short height group (ICC = 0.717–0.902) showed better test-retest reliability compared to those in the tall height group (ICC = 0.146–0.688). COP velocity had the highest ICC value in both stance conditions (ICC = 0.744–0.970). Additionally, the short height group showed slightly better reliability in the feet-together stance condition, whereas the tall height group showed slightly higher ICC value in the natural stance condition. These results suggest that height should be considered for a more reliable assessment of repetitive static balance performance. Our findings might be useful for the development of an optimal static balance test protocol regardless of the effect of height.

The high rate of knee osteoarthritis has raised the need for accurate diagnostic methods. In this study, we propose a precise detection method using the center of pressure data obtained from the patients. The introduced automatic detection pipeline is based on the two modern algorithms of grey wolf and BAT. The extracted statistical features and the obtained data from healthy individuals and patients are processed with the grey wolf binary algorithm. The results are fed into the binary bat algorithm to select important features and increase the pipeline accuracy. Then the groups are classified using a four-layer neural network. We show that the proposed method with a simple four-layer neural network offers fantastic accuracy in high-speed processing large data and classifies the high-dimensional knee osteoarthritis center of pressure data with appropriate precision, recall, specificity, and F1 values. The proposed method has direct applications in knee osteoarthritis diagnostics in clinics.

When examining postural sway measures of single-leg squat (SLS), there is a lack of consensus on how many trials are required to obtain reliable and clinically relevant data. Forty adults with chronic low back pain performed five consecutive trials of SLS for each side on a portable force plate. The left and right sides were categorized into problem and non-problem sides by Clinical Pilates assessment. SLS performance was characterized by terminal knee flexion angle, squat duration, peak vertical force and postural sway parameters. Data across five trials were first examined with repeated measures analysis of variance; variables with significant differences were further analyzed using intraclass correlation coefficients (ICC). Using all trials as a reference, the reliability of other trial combinations was assessed to examine the potential effects of learning (2-5 squats, 3-5 squats, 4-5 squats), fatigue (1-2 squats, 1-3 squats, 1-4 squats) and steady-state (2-4 squats). For the non-problem side, postural sway measures were highly reliable ($\mathrm{\text{ICC}}\ge 0.9$) regardless of the number of trials analyzed. For the problem side, analyzing the 1-4 squats combination offered consistently reliable results across all postural measures ($\mathrm{\text{ICC}}\ge 0.72$). Thus, it is recommended to analyze the first four consecutive trials to obtain reliable postural sway measures.

The aim of this paper is to reduce the energy consumption of a humanoid by analyzing electrical power as input to the robot and mechanical power as output. The analysis considers motor dynamics during standing up and sitting down tasks. The motion tasks of the humanoid are described in terms of joint position, joint velocity, joint acceleration, joint torque, center of mass (CoM) and center of pressure (CoP). To reduce the complexity of the robot analysis, the humanoid is modeled as a planar robot with four links and three joints. The humanoid robot learns to reduce the overall motion torque by applying Q-Learning in a simulated model. The resulting motions are evaluated on a physical NAO humanoid robot during standing up and sitting down tasks and then contrasted to a pre-programmed task in the NAO. The stand up and sit down motions are analyzed for individual joint current usage, power demand, torque, angular velocity, acceleration, CoM and CoP locations. The overall result is improved energy efficiency between 25–30% when compared to the pre-programmed NAO stand up and sit down motion task.

Wrestling is one of the oldest and most popular competitive sports in the world, however, knowledge of the biomechanics of wrestling is not well established and the biomechanical risk factors of injuries unclear. The purpose of this study was to investigate the joint kinematics of the lower limbs and the center of pressure (COP) movements in Greco-Roman style (GR) and free style (FS) wrestlers during tackle defense. Eighteen male college wrestlers participated in the current study: 10 majored in GR (height: 171.1 ± 8.0 cm; weight: 73.9 ± 11.5, kg) and 8 in FS (height: 169.0 ± 5.2 cm; weight: 71.8 ± 11.4 kg). The wrestlers received tackle attacks from three different directions while their kinematic data measured by a 3D motion capture system and ground reaction forces from two AMTI forceplates. The wrestlers who majored in GR style tended to resist tackle attacks longer than the FS group. Compared to the GR group, the FS wrestlers tended to have greater A/P excursions of the COP with significant greater knee flexion. This flexed knee strategy may be related to the rule of the game and the training the FS wrestlers received. Significantly increased joint angles in the transverse and frontal planes at the knee and ankle found in the current study may be related to the risk of knee and ankle injuries commonly observed in wrestlers. Strengthening of the muscles of the lower extremity may be helpful for reducing these injuries during competitions.

Knowledge of the control of the body's dynamic stability in patients with knee osteoarthritis (OA) is helpful for the management of these patients and for the evaluation of treatment outcomes. The purpose of the current study was to investigate the dynamic stability of patients with knee OA during level walking using variables describing the motion of the body's center of mass (COM) and its relationship to the center of pressure (COP). Kinematic and kinetic data during level walking were obtained from 10 patients with bilateral knee OA and 10 normal controls using a motion analysis system and two forceplates. Compared to the normal controls, patients with knee OA exhibited normal COM positions and velocities at key instances of gait but with significant changes in COM accelerations. In the sagittal plane, adjustments to the anterioposterior acceleration of the COM throughout the complete gait cycle were needed for better control of the COM during the more challenging latter half of single leg stance. Diminished A/P COM–COP separation was also used to maintain body stability with reduced joint loadings. In the frontal plane, this was achieved by increasing the acceleration of the body's COM towards the stance leg. The more jerky motion of the body's COM observed may be a result of reduced ability associated with knee OA in the control of the motion of the COM. Strengthening of the muscles of the lower extremities, as well as training of the control of the COM through a dynamic balance training program, are equally important for the dynamic stability of patients with knee OA.

There is a lack of studies concerning standing stability in subjects with incomplete spinal cord injury (SCI). The aims of this study were to analyze the changes of center of pressure (COP), joint moment, joint angle, and muscle activities in patients with incomplete SCI during quiet stance with and without arm support. Ten cervical–thoracic cord incompletely injured patients (mean age: 47.8 ± 13.4 years old) participated in this study. They performed quiet standing with each leg on separate force plate (AMTI, USA) for 5 s with and without holding the walker to obtain the kinetic data. The kinematic data of joint angles were recorded by Motion Analysis System (Vicon 250, Oxford, UK). The electromyographic activities of trunk muscles and quadriceps were recorded by surface electrodes. The results indicated that the COP sway area and the normalized COP sway area significantly (P < 0.01) increased from 94.9 ± 61.3 mm² to 254.6 ± 91.8 mm² and from 0.0013 ± 0.0007 to 0.0037 ± 0.0012, respectively, in patients with incomplete SCI, while standing with arms released from walker. The ankle and knee moments also increased significantly (P < 0.05) during stance without support. However, the joint angles and recorded muscle activities were not significantly different between those with and without support. The conclusion was that the proprioceptive feedback of postural sway, and the motor control strategies of knee–ankle muscles would be important for patients with incomplete SCI during stance without arm support.

Tai-Chi Chuan (TCC) is thought to be a low-impact and effective exercise to improve balance capability in the elderly. However, the effects of TCC exercise on balance improvement remain controversial. The purpose of the current study was to investigate the effects of long-term Yang-Style TCC training on balance variables such as stable standing time and center of pressure (COP) movement patterns. Fifteen long-term Yang-Style TCC practitioners and fifteen age-matched adults performed different static balance tests each for 30 seconds. For each test, the time-varying COP positions were measured by two forceplates. The sway area of the COP was described by an equivalent ellipse, the two principal axes of which were obtained by using principal component analysis. The results showed that elderly subjects with long-term Yang-Style TCC training were able to maintain stable standing longer than those without TCC training, with reduced COP sway area during challenging tasks such as single-leg stance and tandem stance. It is suggested that long-term TCC exercise is a good choice in a training program aimed at reducing the risk of falling in the elderly.

The degeneration of sensory and motor systems due to aging could affect the elderly’s posture and increase the risk of falling. The strategies applied to maintain postural stability might be different between ages, especially in the condition requiring both proprioception and vision sensorimotor coupling. This study proposed a novel sensorimotor assessment protocol to evaluate the postural control ability across the aging process, by using the computerized dynamic posturography and the virtual reality (VR) system. Ten young and 20 elderly healthy adults without fall experience were recruited, and were assessed on a continuous-perturbed platform with or without the VR-based visual interference in a random sequence. Measured variables of the center of pressure as well as the weight-bearing ratio were analyzed and compared. Results showed that the postural sway was significantly larger in all subjects under the VR condition, but the young subjects could rapidly adjust the body to regain postural stability in a rhythmic and symmetric manner; whereas, the elderly adults performed less effectively in postural response. We suggested that the application of the multiple sensation disturbances with VR could effectively evaluate the postural control ability among the healthy elderly. The proposed assessing protocol is also recommended for training the sensorimotor integration to improve the dynamic postural control ability.

The aim of this study was to investigate the immediate effect of wearing the functional insoles with different slopes of forefoot wedges on postural stability in young adults during quiet stance. In this study, the functional insole was composed of a forefoot wedge and a medial arch support. Twelve healthy young adults (six males and six females) participated. Each subject wore sneakers with and without functional insole and stood as still as possible on a force plate with feet together, arms by side and head facing ahead for 60s, while eyes open and eyes closed, respectively. The functional insole was applied in the random sequence of no insole, wearing insole with a medial arch and a four-degree forefoot wedge, as well as wearing insole with a medial arch and an eight-degree forefoot wedge. The sway areas as well as the maximal excursions of the center of pressure (COP) in anterior–posterior (AP) and medial–lateral (ML) directions were used to evaluate the static postural stability. During stance with feet together and eyes closed, the sway area and maximal excursion of the COP in the AP direction were significantly decreased when wearing an eight-degree forefoot wedge functional insole. Since the reduced displacements of the COP indicated better postural control, it was suggested that the functional insole with an eight-degree forefoot wedge and a medial arch support might be beneficial to improve the postural stability in patients with impaired balance control, especially for whom having high risk of forward falls.

Falling is one of the leading causes of accidental injury and death among elderly adults and construction workers, with costs exceeding US$31 billion each year. Having good balance reduces the likelihood of falling — therefore it is important to determine which possible factors might influence balance. The purpose of this study was to determine if consuming three different types of breakfast altered blood glucose levels in such a way that young healthy individual’s balance control was compromised. Balance was then measured while the subjects completed single- and dual-task standing trials with eyes open and closed. Although changing blood glucose did alter quiet standing balance — as measured by the separation distance between the COG and COP, the velocity of the COM, and the total distance traveled by the COG and COP along the anterior–posterior (AP) and medial–lateral (ML) axes — the results were contradictory to what was hypothesized. Subjects with lower blood glucose swayed less than those with higher blood glucose. This could potentially be due to the habitual skipping of breakfast in young adults. Though the changing of blood glucose did influence quiet standing balance of young healthy adults, it was not in a way which increased the risk of falling.

The purpose of this study is to evaluate the sway areas of the center of pressure (COP) and the inclination angles between the center of mass (COM) and COP among elderly subjects with and without training in Chinese traditional exercises, including Tai Chi (TC) and Yuanji-Dance (YD). Additionally, this study investigates the electromyographic (EMG) activities during walking across these groups. The current study employed a cross-sectional study design. Twenty-seven healthy participants aged 45–70 years old were recruited from the local community in this study. TC group and YD group should have regular exercise training for at least two years, respectively. In the first part of this study, each subject of three groups (TC, YD and CON) was asked to walk at a self-selected speed on an 8 m walkway for gait analysis. Maximum center of mass–center of pressure (COM–COP) inclination angles in the sagittal plane and frontal plane were calculated. The EMG activity was recorded bilaterally from four muscles of the lower limbs including quadriceps, hamstring, tibialis anterior and gastrocnemius. In the second part, each subject was guided to perform four tandem static balance tests with the dominant leg leading on a force plate. The time-varying COP positions and the sway area of the COP were calculated from the ground reaction force data. A one-way ANOVA was conducted to compare walking variables between groups, and a two-way repeated measures ANOVA was used to evaluate the effects of group (between-group) and conditions (within-group) on the variables of the static tests. There were no significant differences in the maximum COM–COP inclination angles in anterior– posterior ($p$ = 0.06) and medial–lateral ($p$ = 0.15) directions as well as the EMG activities ($p$ > 0.05) among the TC, YD and CON groups. For static balance test, the TC group showed decreased excursion of COP in anterior– posterior direction and sway area compared with the CON group ($p$ < 0.01). For the YD group, decreased excursion of COP in the anterior–posterior direction during tandem stance on the soft surface was found ($p$ < 0.01). Moreover, the YD group showed decreased sway areas of COP compared with the CON group ($p$ < 0.01). The findings show that the TC group demonstrated smaller sway areas compared to the YD group, indicating that TC training may be especially effective in enhancing the static standing balance. However, there were no significant differences in the anterior–posterior and medial–lateral COM–COP inclination angles during walking among the TC, YD, and control groups. This result suggests that the training effects of TC and YD may not significantly impact the dynamic balance or the muscle activities observed during level walking.