Center Of Pressure Measures Research Articles

Dynamic Postural Control During Trunk Bending and Reaching in Healthy Adults and Stroke Patients

Postural stability is often task-demand-dependent and often impaired in stroke patients. The purposes of this study were (1) to compare the difference in the center of pressure measures during forward bending of the trunk and reaching on postural actions between normal subjects and stroke patients and (2) to examine the effects of task demands on the center of pressure measures in both groups. Thirteen normal subjects and 23 stroke patients performed two trials of forward bending of the trunk and reaching for each of the targets at six locations at floor level while standing on a 0.5-m-long pressure mat. Center of pressure excursion, center of pressure average velocity, and bilateral limb weight-bearing ratios characterized the postural actions. Mixed-design analysis of variance was used. A significant target location by group interaction effects was found for all variables except the center of pressure average velocity. All variables except center of pressure average velocity showed significant group differences. The center of pressure excursion and bilateral limb weight-bearing ratio were smaller, and the center of pressure average velocity was larger in stroke patients than in normal subjects. The effects of target location on variables measured were prominent in stroke patients. The posturography of stroke patients was less regular than that of normal subjects. Stroke patients tended to avoid shifting their center of pressure toward the affected side, even when highly necessary. The difference of postural actions between groups depended on task demands. Small center of pressure displacement and fast center of pressure velocity characterized decreased adaptive postural actions. The amount of center of pressure shift and center of pressure velocity were also dependent on the task demands for both groups.

Effects of simulated occupational task parameters on balance

The effects of single-handed load holding, length of the base of support, and standing surface condition (narrow and wide construction beams) on balance were investigated in twenty-three healthy men between the ages of 18 and 55 years old. Balance during quiet standing was evaluated from postural sway measurements derived from center of pressure (COP) displacement. These measurements included the range or maximal displacement of the COP in the anteroposterior (AP) and mediolateral (ML) directions, the elliptical area, and mean sway velocity. Holding a load in the hand did not significantly affect postural sway measures ( p > 0.05), although the effect of surface condition was significant on all COP measures ( p < 0.001). Lengthening the base of support did not affect the ranges or elliptical area, but increased the mean velocity of sway ( p = 0.001). Changes in the dimensional characteristics of the surface condition and length of base of support affected postural sway, possibly by requiring adjustments to balance and motor control strategies. Further research is required to determine if these changes are detrimental to maintaining balance and increase the risk of falls for workers in similar environments.

Effect of tibial re-alignment surgery on single leg standing balance in patients with knee osteoarthritis

Background Standing balance is impaired in individuals with knee osteoarthritis and is associated with disease severity. The effects of surgical interventions on standing balance have received little attention. The purpose of the present study was to examine measures of balance during tests of single-limb standing before and after medial opening wedge high tibial osteotomy – a lower limb re-alignment procedure for those with varus alignment and knee osteoarthritis. Methods Standing balance was assessed in 49 individuals prior to and 12 months following medial opening wedge high tibial osteotomy. Participants performed three trials of single-limb balance lasting 10 s each while standing on a force platform. Anteroposterior and mediolateral coordinates of the centre of pressure were obtained from the force platform and used to calculate the total centre of pressure path length as well as the range and variability (standard deviation) of the anteroposterior and mediolateral coordinates. Findings Though all centre of pressure measures were lower following high tibial osteotomy, none reached statistical significance ( P > 0.05) and effect sizes were small ( d < 0.34). The largest mean improvement was 7.6% (95% confidence interval: −0.7–15.8%). Interpretation Results indicate that standing balance in individuals with knee osteoarthritis is not significantly different following high tibial osteotomy surgery. Standing balance in this patient population is a complex process not entirely dictated by disease symptoms or structural factors such as alignment.

Influence of Anaerobic and Aerobic Exercises on the Center of Pressure During an Upright Posture

This study was designed to examine the influence of anaerobic and aerobic exercise, using a cycle ergometer, on upright standing postural control in addition to physiological and psychological responses. During an upright standing posture, 15 healthy male participants were measured for center of pressure (COP), physiological parameters (heart rate, systolic and diastolic blood pressures, and blood lactate concentration), and the ratio of perceived exertion before and after the exercises. They performed a maximal voluntary pedaling exercise for 10 seconds two times under anaerobic exercise conditions and then at 50% of maximal aerobic power for 60 minutes at 60 rpm under aerobic exercise conditions. Measurements were conducted before, immediately after and at 5, 10 and 15 minutes after the exercises. Body sway was recorded by a COP measurement device G5500 (ANIMA, Japan) with three vertical load sensors. COP sway was assessed by mean position of COP sway in the anteroposterior and mediolateral directions as well as the sway area and path length. Three COP parameters regarding sway area and velocity were significantly higher immediately after the exercises than at the other times. In conclusion, the influence of the two exercise protocols on postural control is detected by sway area and velocity. However, the exercise-induced increase of sway velocity recovers earlier than the physiological parameters (heart rate, systolic blood pressure, and blood lactate concentration). It would appear that both prolonged aerobic exercise and high-intensity anaerobic exercise have a relatively small influence on upright standing postural control in healthy young males.

Measures Of Postural Stability Are Associated With Gains And Noise In The Control System

Postural stability is reduced with age or neurological disorder and improved with certain types of training (e.g., Tai-chi) as evaluated by various measures using center of pressure (COP) during quiet standing. However, the implications of specific measures with respect to the underlying control mechanisms have remained unclear. PURPOSE: To investigate the influence of variation in the control system on the COP measures during quiet standing, using mathematical simulations. METHODS: A feedback model that represented the human body and the control system during quiet standing was built using an inverted pendulum, a proportional and derivative (PD) controller with a time delay (the neural controller) and another PD controller without a time delay (the mechanical controller). The angle of the pendulum was fed back to the controllers, and the output of the neural controller (corresponding to the motor command) was input into the neuro-muscular torque generator. The sum of the output of the neuro-muscular torque generator and of the mechanical controller represented the ankle torque. A driving noise was added to the motor command to simulate realistic body sway conditions. The gain combinations that robustly stabilized the entire system were used to simulate the COP fluctuations. The gains of the controllers and the noise amount were treated as variables. The statistical relationship between each measure and the controller gains or noise was assessed using the partial correlation analysis. Sixty thousand and five hundred combinations of the variables were tested in total. RESULTS: All gains correlated negatively with the time domain measures (r=-0.372 to-0.659) except the mean velocity, which correlated positively with each gain. The neural derivative gain correlated positively with the frequency domain measures (r =0.964 to 0.973). The proportional gains correlated with Hurst exponent positively for the short term region (r= 0.837 to 0.892) and negatively for the long term region (r=-0.828 to-0.883). The amount of noise correlated positively with time domain measures (r= 0.901 to 0.988) but not with frequency measures. CONCLUSIONS: Each COP measure is associated with the variation in the control system. As such, the COP measures have a potential for investigating or characterizing the control system of balance.

Acute Effects of Aquatic Exercise on Postural Sway in Women with Lower Extremity Arthritis

PURPOSE: It is well known that individuals with lower extremity arthritis have difficulty in postural control. Previous studies showed long-term effects of aquatic exercise on their postural control. Our recent study investigated acute effects of aquatic exercise on their postural control and found that time domain measurements of center of pressure (COP), such as total sway area, did not change after an aquatic exercise session. In this study, we investigated its acute effects on frequency domain measurements of COP. METHODS: COP while in upright position of 7 female individuals with lower extremity arthritis (60.6 ± 6.13 yrs) was measured before and after an aquatic exercise session. Written informed consent was obtained from all subjects. The exercise session was 70 min that included 20 min warm up and walking, 15 min resistance exercise and stretching in water, 10 min break, 20 min walking or swimming and 5 min cooling down. COP was recorded for 30 sec with eyes open. Frequency domain measurements (total power, mean frequency (Hz), 50% power frequency (Hz) and 95% power frequency (Hz)) of COP and velocity time series of COP in anterior-posterior and medial-lateral directions were analyzed and the pre-and the post-exercise data were statistically compared using paired t-test. RESULTS: There were significant differences (p<0.05) in 95% power frequency of COP in anterior-posterior direction, mean frequency of velocity time series in both directions and 95% power frequency of velocity time series in medial-lateral direction; these variables shifted to lower frequency after an aquatic exercise session. CONCLUSION: It was showed that compared with young and/or healthy populations, older individuals and patients with Parkinson disease or spinocerebellar degeneration had less ability to keep postural stability as well as their power spectrum of COP and velocity time series showed more power in higher frequency area. Therefore, our results focusing on spectral characteristics of COP suggested that postural control of women with lower extremity arthritis was improved by the acute effects of aquatic exercise.

A Comprehensive Approach To The Center Of Pressure Differences Between Yoginis And Controls

Standard measures such as root mean square (RMS) and mean power frequency (MPF) adequately examine the magnitude of the center of pressure (COP), while nonlinear measures examine the structure of the COP time series. Combining these measures provides a comprehensive analysis of balance control (e.g. comparison of balance of dancers and track athletes, Schmit, Regis, and Riley, 2005). Increased COP amplitude could be due to decreased balance skills or increased exploratory behavior; nonlinear analyses can help differentiate between these interpretations. We expect that yoga practitioners will demonstrate different balance strategies when compared to controls. PURPOSE: To compare balance of yoginis and controls with traditional measures and non linear measures of COP. METHODS: Twenty females participated (27.9 ± 11.48 yrs.), ten yoginis and ten controls. Two randomly-presented focus conditions were observed (1) no instruction, where the participant stood quietly, (2) external fixation, where the participant was asked to focus on an × placed approximately three feet in front of them on the floor. Subjects stood either on two legs or one leg. RESULTS: The results for the anterior posterior sway direction are reported. No three way interactions (group × leg × vision) or two way interactions (group × leg; group × vision; leg × vision) were observed for the standard or the nonlinear measures. The standard measures revealed main effects for RMS for leg and vision conditions (p<.01 leg and p<.01 vision) as well as for MPF for leg condition (p<.01). The nonlinear measures revealed main effects for entropy across groups (p<.01), percent recurrence and percent determinism for the leg condition (p<.01). CONCLUSION: The magnitude and the frequency of the COP measure was not different across the two groups, however, the yoga group COP was less chaotic (decreased entropy) and had trends of increased noise (decreased percent recurrence). These changes are consistent with increased behavioral flexibility. A more complete understanding of postural control was observed when linear and non-linear measures were combined.

Reliability of COP-based postural sway measures and age-related differences

The objectives of this study were to assess the within-day and between-day reliability of several center of pressure (COP)-based measures of postural sway and identify whether there were age-related differences in reliability. Thirty-two healthy individuals (16 younger and 16 older) participated. COP was recorded during quiet upright stance on 4 different days, and a variety of measures determined: mean velocity, median power frequency, RMS distance, sway area, and two fractal measures derived from Hurst rescaled range analysis ( H R/S) and detrended fluctuation analysis (DFA). Intraclass correlation coefficient (ICC) and standard error of measurement (SEM) were used to quantify reliability. Mean velocity was the most reliable measure. DFA exponents had relatively better reliability than H R/S exponents. In general, within-day reliability was better than between-day. In comparison with younger participants, older participants exhibited better relative reliability (ICC) for all COP measures and comparable absolute reliability (SEM) except for mean velocity and sway area. These results may be useful in guiding the future selection and interpretation of COP-based measures.

Direct parameterization of postural stability during quiet upright stance: Effects of age and altered sensory conditions

The purpose of this study was to determine the relationship between measures of local dynamic stability (LDS) during upright stance and both descriptive measures of postural sway and a scaling index ( α) derived from detrended fluctuation analysis. Center of pressure (COP) time series were obtained from healthy participants (16 young and 16 older) during upright quiet stance. Vision and somatosensation were altered by eye closure and standing on a compliant surface, respectively. A non-linear time-series analysis method was used to compute three LDS parameters from the COP data: A which was defined as the COP excursion range in state space, and τ S and τ L which were defined as the divergence rates over short- and long-term timescales, respectively. LDS parameters, descriptive COP measures, and α had generally consistent sensitivities to age and/or altered sensory conditions. Age x sensory condition interactions, however, had distinct effects on LDS parameters compared to the other COP-based measures. Older individuals exhibited faster divergence rates while having similar magnitudes of A, compared to young individuals. These results suggest that older individuals stiffen the musculoskeletal system via increased muscle activity, perhaps as an age-related postural adaptation. In addition, correlations between LDS parameters and other COP measures were relatively small ( r 2⩽0.29). Hence, LDS parameters ( A, τ S and τ L) provide distinct information on postural control and stability, supplementing other COP-based measures.

Different centre of pressure patterns within the golf stroke II: Group-based analysis

Although the golf coaching literature stresses the importance of weight transfer during the swing, research has been conflicting or lacking statistical support. A potential problem with previous studies is that no attempt was made to account for different movement strategies in the golf swing. This study evaluated the relationship between centre of pressure measures and club head velocity within two previously identified swing styles, the “Front Foot” and “Reverse” styles. Thirty-nine Front Foot golfers and 19 Reverse golfers performed swings with a driver while standing on two force plates. From the force plate data, centre of pressure displacement, velocity, range, and timing parameters were calculated. Correlation and regression analysis indicated that a larger range of centre of pressure and a more rapid centre of pressure movement in the downswing was associated with a larger club head velocity at ball contact for the Front Foot group. For the Reverse golfers, positioning the centre of pressure further from the back foot at late backswing and a more rapid centre of pressure transfer towards the back foot at ball contact was associated with a larger club head velocity at ball contact. This study has highlighted the importance of identifying different movement strategies before evaluating performance measures, as different parameters were found to be important for the Front Foot and Reverse styles.

Reliability and comparison of electromyographic and kinetic measurements during a standing reach task in children with and without cerebral palsy

The purposes of this study of children with cerebral palsy (CP) and with typical development (TD) were to determine: (1) within session test–retest reliability of electromyography (EMG) of postural muscles [using motor coordination patterns (cMCP)] and center of pressure (COP) measures, (2) relationships between cMCP and COP measures, and (3) differences between children with CP and with TD. Data were collected twice during a standing reach task in 20 pairs of age and gender matched children, 8–14-years-old. Test–retest reliability was poor for cMCPs ( κ = 0.16–0.21), but good for kinetic measures (ICC = 0.69–0.87). Both groups demonstrated strong reliance on anterior–posterior neuromuscular control with greater variability in the children with CP. The group with TD showed greater control in anterior–posterior COP movements than did the group with CP. Anterior–posterior COP measures were associated with more effective cMCPs in both groups. These data support the concept of the center of gravity as a potential control parameter working with a flexible neuromuscular system for postural control.

Step Prediction During Perturbed Standing Using Center Of Pressure Measurements

The development of a sensor that can measure balance during quiet standing and predict stepping response in the event of perturbation has many clinically relevant applica- tions, including closed-loop control of a neuroprothesis for standing. This study investigated the feasibility of an algorithm that can predict in real-time when an able-bodied individual who is quietly standing will have to make a step to compensate for an external perturbation. Anterior and posterior perturbations were performed on 16 able-bodied subjects using a pul- ley system with a dropped weight. A linear relationship was found between the peak center of pressure (COP) velocity and the peak COP displacement caused by the perturbation. This result suggests that one can predict when a person will have to make a step based on COP velocity measurements alone. Another important feature of this finding is that the peak COP velocity occurs considerably before the peak COP displacement. As a result, one can predict if a subject will have to make a step in response to a perturbation sufficiently ahead of the time when the subject is actually forced to make the step. The proposed instability detection algorithm will be implemented in a sensor system using insole sheets in shoes with minitur- ized pressure sensors by which the COPv can be continuously measured. The sensor system will be integrated in a closed-loop feedback system with a neuroprosthesis for standing in the near future.

Inverse relations in the patterns of muscle and center of pressure dynamics during standing still and movement postures

The aim of this study was to investigate the postural center of pressure (COP) and surface muscle (EMG) dynamics of young adult participants under conditions where they were required to voluntarily produce random and regular sway motions in contrast to that of standing still. Frequency, amplitude and regularity measures of the COP excursion and EMG activity were assessed, as were measures of the coupling relations between the COP and EMG outputs. The results demonstrated that, even when standing still, there was a high degree of regularity in the COP output, with little difference in the modal frequency dynamics between standing still and preferred motion. Only during random conditions was a significantly greater degree of irregularity observed in the COP measures. The random-like movements were also characterized by a decrease in the level of synchrony between COP motion on the anterior-posterior (AP) and medio-lateral (ML) axes. In contrast, at muscle level, the random task resulted in the highest level of regularity (decreased ApEn) for the EMG output for soleus and tibialis anterior. The ability of individuals to produce a random motion was achieved through the decoupling of the COP motion in each dimension. This decoupling strategy was reflected by increased regularity of the EMG output as opposed to any significant change in the synchrony in the firing patterns of the muscles examined. Increased regularity across the individual muscles was accompanied by increased irregularity in COP dynamics, which can be characterized as a complexity tradeoff. Collectively, these findings support the view that the dynamics of muscle firing patterns does not necessarily map directly to the dynamics at the movement task level and vice versa.

Comparison of standing balance between female collegiate dancers and soccer players

Study design This study was designed as a comparison study of two cohorts. Objectives The hypothesis of this study was that soccer players and dancers have different balance abilities and that these differences could be objectively measured using center of pressure measurements. Background Center of pressure (COP) measurements are reproducible and have been validated in the literature for assessing standing balance. The literature does not provide sensitive enough techniques for discriminating between two groups of athletes with excellent standing balance. Methods and measures A Matscan pressure mat (Tekscan, Boston, MA) was used to compare COP change variability between 32 female collegiate soccer players and 32 dancers. COP was used to calculate sway index, center acquisition time, sway path length and sway velocity as measures of standing balance. Results The dancers had significantly better balance scores ( p < 0.05) in 5 of 20 balance tests. Results for the remaining 15 balance tests were not significantly different. Conclusion These data show that standing balance characteristics of dancers and soccer players can be objectively measured using COP data. Dancers have certain standing balance abilities that are better than those of soccer players. The COP measurements in this study can be used as a tool in future studies investigating standing balance in different groups of athletes.

Kinematics Analysis of Knee Joint Motion in Closed Chain Movement among Different Sports Events: A Pilot Study

Accelerometry (ACC) shows promise as an easily implemented clinical measure of balance. The purpose of the study was to estimate test–retest reliability of ACC measures and determine the relationship between ACC measured at the pelvis and underfoot center of pressure (COP) measures during sensory organization test (SOT) conditions. Eighty-one subjects were recruited from the community with no known orthopedic or vestibular deficits (19–85 years). Subjects completed three consecutive, ninety second trials for each of the six SOT conditions, while wearing the accelerometer. ACC and COP time series were described by calculating the normalized path length, root mean square (RMS), and peak-to-peak values. The test–retest reliability of the three measures within each SOT condition was estimated over three trials using the intraclass correlation coefficient. ACC and COP test–retest reliability were similar, ranging from 0.63 to 0.80 using ACC and 0.42 to 0.81 using COP for the measure of normalized path length. Linear regression between ACC and COP measures showed significant correlation under almost every SOT condition using both single and average measures across trials. The degree of association between COP and ACC was equivalent when using the first trial or the 3-trial average, suggesting that one trial may be sufficient. The use of accelerometry may have value in estimating balance function and minimizing clinical evaluation time.

Correction of the inertial effect resulting from a plate moving under low-friction conditions

The purpose of the present study was to develop a set of equations that can be employed to remove the inertial effect introduced by the movable platform upon which a person stands during a slip induced in gait; this allows the real ground reaction force (GRF) and its center of pressure (COP) to be determined. Analyses were also performed to determine how sensitive the COP offsets were to the changes of the parameters in the equation that affected the correction of the inertial effect. In addition, the results were verified empirically using a low friction movable platform together with a stationary object, a pendulum, and human subjects during a slip induced during gait. Our analyses revealed that the amount of correction required for the inertial effect due to the movable component is affected by its mass and its center of mass (COM) position, acceleration, the friction coefficient, and the landing position of the foot relative to the COM. The maximum error in the horizontal component of the GRF was close to 0.09 (body weight) during the recovery from a slip in walking. When uncorrected, the maximum error in the COP measurement could reach as much as 4 cm. Finally, these errors were magnified in the joint-moment computation and propagated proximally, ranging from 0.2 to 1.0 Nm/body mass from the ankle to the hip.

The capacity to adapt to changing balance threats: A comparison of children with cerebral palsy and typically developing children

Objectives: This study investigated differences in reactive balance abilities of typically developing children and those with spastic diplegia. Recovery from balance threats was compared by: (i) Platform velocity and amplitude thresholds: Speed and size of platform movement at which children required assistance to remain upright, (ii) percentage of trials with feet-in-place vs. loss of balance, and (iii) center of pressure measures. Participants included 8 children with spastic diplegic cerebral palsy, 15 developmentally matched children (similar walking stages) and 21 age-matched control children.Methods: Backward platform movements graded as easy, moderate and difficult were unexpectedly imposed on children standing on a moveable platform.Results: Children with cerebral palsy (CP) had lower platform velocity thresholds, greater percentages of loss of balance trials, increased distances and increased frequency of directional changes in center-of-pressure (COP) trajectories than control children. Older children with CP fell more often than those under 5 years. Greatest differences between children with and without CP were found in comparisons based on age rather than developmental levels.Conclusions: Using balance perturbations that challenged children with CP to the limits of their balance abilities effectively identified age performance differences and differences compared to typically developing children. Implications for rehabilitation programs are presented.

Development of an instrumented pole test for use as a gait laboratory quality check

Gait analysis is becoming an increasingly common procedure in the diagnosis of movement disorders. Core equipment includes 3D tracking cameras and force platforms. Important clinical decisions are made with the assistance of gait data, therefore it is essential to have confidence in its quality. Calibration has a major role to play here and this paper describes the development of a daily spot check pole test for use in a gait laboratory. The work develops previous such tests, by the addition of a force transducer, removing the need to assume an idealised force system and allowing amplifier gain ranges to be checked. Repeatability results are presented, along with the response to 14 simulated error modes. The test provides useful data on system performance and is able to identify and diagnose failures. Failure thresholds can be tightened as accuracy improves, particularly in the area of centre of pressure measurement. Spot checks, such as this, along with full calibration tests, will become essential as new standards are introduced into clinical practice.

The Effect of Gluteus Medius and Rectus Femoris Fatigue on EMG and Center of Pressure Measurements

The Effect of Gluteus Medius and Rectus Femoris Fatigue on EMG and Center of Pressure Measurements

Age-Related Differences in the Effect of a Perceived Threat to Stability on Postural Control

Studies indicate that the strategy for postural control may be affected by psychological factors, and that young adults respond to perceived threat to stability by increasing their ankle stiffness. The objective of this study was to compare the postural control strategy adopted by young and old adults when faced with postural threat induced by manipulating surface height. Sixty elderly (mean age 77.5+/-4.4 years) and 20 young volunteers (mean age 21.5+/-3.7 years) participated in the study. Movement of the center of pressure (COP) was recorded for 60 seconds with a portable force-plate, as participants stood with eyes open and closed, both at ground level and 85 cm above ground level. Analysis of variance and Tukey-Kramer tests were conducted to determine the effects of age, postural threat, and vision on mean power frequency (MPF), on amplitude variability both in the anterior-posterior (AP) and medio-lateral (ML) directions, and on mean COP sway velocity, with the significance level set at p=.05. Postural threat did not affect COP measures for the young adults, whereas it induced greater changes in MPF in the AP and ML directions in the elderly participants. A similar trend was observed in the elderly group for amplitude variability in the ML direction and for mean COP sway velocity. In comparison with young adults, elderly adults tend to have an exaggerated postural response to conditions that are perceived as threatening to stability. This response involves primarily increases in MPF, with limited compensatory decreases in amplitude variability observed only in the ML direction.