Center Of Pressure Data Research Articles

Development of a more robust tool for postural stability analysis of laparoscopic surgeons

Physical difficulties experienced by surgeons performing minimally invasive surgery (MIS) are being given extensive attention by ergonomic researchers. Postural stability, not commonly addressed, is our prime focus. Center of pressure (COP) alone is used in the few existing postural stability studies. Using COP, we previously correlated postural stability to instrument type, task difficulty, and skill level. This study, including center of mass (COM), sway area analysis, and what we uniquely term postural stability demand (PSD), extends our investigation. Six surgeons from different experience levels were recruited to complete three fundamentals of laparoscopy (FLS(TM)) tasks. Standing on two force plates, participants performed each task as a motion capture system recorded body movements. An ellipse was created for sway area analysis of COP, the point where the ground reaction force was located, and COM, the point at which body mass was concentrated. PSD was defined as the mean distance between the COP and COM locations in the anterior-posterior (A-P) or medial-lateral (M-L) directions. Postural parameters and performance time were correlated. COM and COP sway areas positively correlated with pegboard transfer performance time (r = 0.928, p < 0.05; r = 0.864, p < 0.05) and also with circle-cutting performance time (r = 0.858, p < 0.05; r = 0.779, p = 0.06). However, COM and COP sway areas negatively correlated with endo-loop placement performance time (r = -0.925, p < 0.05; r = -0.935, p < 0.05). These results indicate unique postural controls based on skill level. During all tasks, PSD in the A-P direction strongly correlated with performance time (r = 0.829, p < 0.05; r = 0.913, p < 0.05; r = 0.880, p < 0.05), indicating that less-skilled participants experienced increased postural demands. This study demonstrated that variance in postural adjustments, as evidenced by sway area analysis, correlate to skill level and individual task. Strong correlation between PSD and performance time shows potential as a predictor of skill levels. Combining COM, COP, and PSD data produces a more robust analytic tool for identifying postural adjustments that can be correlated with skill level.

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.

Postural prioritization defines the interaction between a reaction time task and postural perturbations

Concurrent demands for postural and cognitive control processes are now known to induce interference, e.g., information processing speed may decrease during postural adjustment. It is less clear whether postural control may, at least in many situations, take precedence over cognitive control ("postural prioritization"). The purpose of this study was to determine if postural dual-task effects are the result of a postural prioritization effect. Twelve young subjects (6 female; 24.1 +/- 4.1) performed a discrete choice reaction time (RT) task in combination with a platform perturbation. To assess the effect of postural prioritization on RT and center of pressure (COP) parameters, destabilizing perturbations were randomly interspersed with non-destabilizing perturbations. Furthermore, stimulus order and the time interval of the RT stimulus relative to the platform perturbation were manipulated. COP and RT data obtained in these manipulations were compared to single-task baseline data. The results suggested that, irrespective of the degree of threat to postural stability, postural task processes are prioritized. Furthermore, anticipation of a postural stimulus negatively affects RT. However, once a perturbation commences subsequent RTs are speeded. Postural reactions were unaffected by a concurrent RT task, however. The RT stimulus acted as a cue to initiate biomechanical adaptations for an upcoming perturbation.

Influence of embedding parameters and noise in center of pressure recurrence quantification analysis

Recurrence quantification analysis (RQA) can extract the dynamics of postural control from center of pressure (CoP) data by quantifying the system's repeatability, complexity, and local dynamic stability through several variables. Computation of these variables requires the selection of suitable embedding parameters for state space reconstruction (i.e. time delay and embedding dimension); however, it is unclear how the parameters influence RQA variables when examining noisy CoP data. This study evaluated the sensitivity of RQA variables to embedding parameter values and noise level, and assessed methods of selecting embedding parameters for CoP data. Five healthy male subjects maintained quiet stance for 30 s while the anterior–posterior CoP was measured. The effect of noise was evaluated by adding uniform white noise of increasing amplitude to the raw CoP signal. The magnitude of all RQA variables decreased with increasing noise amplitude for all subjects. A sensitivity analysis was performed by systematically altering the embedding parameters for the raw data with and without a selected level of added noise. The key result was that, for all subjects, the RQA variables were sensitive to the embedding parameter values and the level of noise in the CoP data. Finally, the performance of false nearest neighbors and average displacement algorithms for choosing embedding parameters was evaluated. Both methods gave clear and consistent results for all subjects with either raw or noisy data. The results suggest that careful selection of embedding parameters is essential when using RQA to examine postural control based on noisy CoP data.

Roll-over shapes of the able-bodied knee–ankle–foot system during gait initiation, steady-state walking, and gait termination

A few investigators have described the movement of the center of pressure (COP) of the ground reaction force and the activation patterns of the lower limb muscles during gait initiation and termination. This study examines the effective rocker (roll-over shape) behavior of the knee–ankle–foot (KAF) system during gait initiation, steady-state walking (i.e. constant speed gait), and gait termination. The KAF roll-over shapes were characterized by transforming COP data of 10 able-bodied subjects from a laboratory-based coordinate system into a leg-based coordinate system. The resulting roll-over shapes (effective rockers) were characterized using a circular arc model. The KAF roll-over shapes exhibit an overall “flexed” orientation during the first step of gait initiation and an “extended” orientation during the last step of gait termination. Understanding the behavior of the anatomical KAF system during gait initiation and termination may aid in the design of prosthetic components, i.e. mechanical devices that replace complete anatomical structures. Prostheses that intend to mimic the overall behavior of physiological KAF systems (biomimetic designs) could be manufactured using approaches that are much simpler than attempting to reconstruct the complexity of the lower limb.

The effects of load carriage and bracing on the balance of schoolgirls with adolescent idiopathic scoliosis

The balance function of children is known to be affected by carriage of a school backpack. Children with adolescent idiopathic scoliosis (AIS) tend to show poorer balance performance, and are typically treated by bracing, which further affects balance. The objective of this study is to examine the combined effects of school backpack carriage and bracing on girls with AIS. A force platform was used to record center of pressure (COP) motion in 20 schoolgirls undergoing thoraco-lumbar-sacral orthosis (TLSO brace) treatment for AIS. COP data were recorded with and without brace while carrying a backpack loaded at 0, 7.5, 10, 12.5 and 15% of the participant's bodyweight (BW). Ten participants stood on a solid base and ten stood on a foam base, while all participants kept their eyes closed throughout. Sway parameters were analyzed by repeated measures ANOVA. No effect of bracing was found for the participants standing on the solid base, but wearing the brace significantly increased the sway area, displacement and medio-lateral amplitude in the participants standing on the foam base. The medio-lateral sway amplitude of participants standing on the solid base significantly increased with backpack load, whereas significant increases in antero-posterior sway amplitude, sway path length, sway area per second and short term diffusion coefficient were found in participants standing on the foam base. The poorer balance performance exhibited by participants with AIS when visual and somatosensory input is challenged appears to be exacerbated by wearing a TLSO brace, but no interactive effect between bracing and backpack loading was found.

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.

Study of the effect of early lead exposure on postural balance by advanced signal processing methods

Measured trajectories of the Centre Of Pressure (COP) on the feet have been used to study postural balance problems. Two new signal analysis methods, the Wavelet based Multiple Fractal Formalism (WMFF) and frequency spectrum envelop characterisation method, are developed to characterise the COP data in a set of two parameters with unique physical significances. The methods are applied to re-dissect the data obtained during the Cincinnati Lead Study (CLS) to gain additional insights into the effect of early lead-exposure on children's postural balance. It is found that the frequency spectrum envelope characterisation method is especially attractive and easy to implement.

Errors in the Measurement of Center of Pressure (CoP) Computed with Force Plate Affect on 3D Lower Limb Joint Moment During Gait

The purpose of this study was to investigate the effect of errors in the center of pressure (CoP) locations on three-dimensional (3D) lower limb joint moments during the stance phase of gait. Kinematic and kinetic data were collected from walking trials of one healthy male subject (age: 22yr, height: 181cm, body mass: 80kg). Changes of 3D joint moment were calculated using inverse dynamics with 3D video analysis and ground reaction force (GRF) measurement. Thereafter, the location of the CoP was shifted toward the antero-posterior/medio-lateral direction by ±10mm, ±20mm and ±30mm, respectively, and 3D joint moments were recalculated with the shifted CoP data. As a result of the simulated shift of the CoP in the antero-posterior direction, changes of magnitude in the ankle (dorsiflexion-plantarflexion), knee (flexion-extension), and hip joint moments (flexion-extension) were observed, while the moment patterns did not change. Simulated CoP shift in the medio-lateral direction altered the magnitude of the hip joint moment (abduction-adduction, internal-external rotation). Moreover it greatly affected the change of the inversion-eversion ankle joint moment. A shift of ±10mm of CoP in the medio-lateral direction reversed joint moment pattern of ankle joint inversion-eversion. The joint moment magnitudes and patterns were more sensitive to errors in the medio-lateral direction than in the antero-posterior direction, largely due to differences in the moment arm length and the direction of the GRF. This clearly illustrates the need to accurately determine CoP position in the calculation of lower limb joint kinetics.

Evaluation of instrumented shoes for ambulatory assessment of ground reaction forces

Currently, force plates or pressure sensitive insoles are the standard tools to measure ground reaction forces and centre of pressure data during human gait. Force plates, however, impose constraints on foot placement, and the available pressure sensitive insoles measure only one component of force. In this study, shoes instrumented with two force transducers measuring forces and moments in three dimensions were evaluated. Technical performance was assessed by comparing force measurement and centre of pressure reconstructions of the instrumented shoes against a force plate. The effect of the instrumented shoes on gait was investigated using an optical tracking system and a force plate. Instrumented shoes were compared against normal shoes and weighted shoes. The ground reaction force measured with force plate and instrumented shoes differed by 2.2 ± 0.1% in magnitude and by 3.4 ± 1.3° in direction. The horizontal components differed by 9.9 ± 3.8% in magnitude and 26.9 ± 10.0° in direction. Centre of pressure location differed by 13.7 ± 2.4 mm between measurement systems. A MANOVA repeated measures analysis on data of seven subjects, revealed significant differences in gait pattern between shoe types ( p ≤ 0.05). A subsequent univariate analysis showed significant differences only in maximum ground reaction force but these could not be attributed to specific shoe types by pair-wise comparison. This study indicates that shoes instrumented with force transducers can be a valuable alternative to current measurement systems if accurate sensing of position and orientation of the force transducers is improved. They are applicable in ambulatory settings and suitable for inverse dynamics analysis.

Control of lateral balance in walking: Experimental findings in normal subjects and above-knee amputees

In walking the human body is never in balance. Most of the time the trunk is supported by one leg and the centre of mass (CoM) ‘falls’ to the contralateral side. In dynamical situations the velocity of the CoM should be acknowledged as well in the ‘extrapolated centre of mass’ (XcoM). Centre of pressure (CoP) position was recorded by a treadmill with built-in force transducers. Lateral CoM and XcoM position were computed by filtering the CoP data. Subjects were six above-knee amputees and six matched healthy controls. They walked at approximately 0.75, 1, and 1.25 m/s for 2 min. Amputees showed asymmetric gait with shorter stance (60%) at the prosthetic side versus 68% at the non-prosthetic side and a wider stride (13 ± 4 cm, mean ± S.D.) compared to controls (9 ± 3 cm). At foot placement CoP was just lateral to the XcoM. The margin between average CoP and XcoM at foot contact was only 1.6 ± 0.7 cm in controls, 2.7 ± 0.5 cm in amputees at the prosthetic side and 1.9 ± 0.6 cm at the non-prosthetic side. Next to this ‘stepping strategy’, CoP position was corrected after initial contact by modulating the lateral foot roll-off (‘lateral ankle strategy’) in non-prosthetic legs up to about 2 cm. A simple mechanical model, the inverted pendulum model, can explain that: (1) a less precise foot placement (greater CoP–XcoM margin) results in a wider stride, (2) this effect can be reduced by walking with a higher cadence, and (3) a greater margin at one side, as with a leg prosthesis, should be compensated by a shorter stance duration at the same side to achieve a straight path. This suggests that not in all cases symmetric gait should be an aim of rehabilitation.

Detecting altered postural control after cerebral concussion in athletes with normal postural stability

Objective: To determine if approximate entropy (ApEn), a regularity statistic from non-linear dynamics, could detect changes in postural control during quiet standing in athletes with normal postural stability after cerebral...

The effect of backpack weight on the standing posture and balance of schoolgirls with adolescent idiopathic scoliosis and normal controls

Concerns have been raised regarding the effect of carrying a backpack on adolescent posture and balance, but the effect of backpack loading combined with other factors affecting balance, such as adolescent idiopathic scoliosis (AIS), has not been determined. This study examines the effects of backpack load on the posture and balance of schoolgirls with AIS and normal controls. The standing posture of 26 schoolgirls with mild AIS (mean age 13, Cobb angle 10-25 degrees ) and 20 age-matched normal schoolgirls were recorded without a backpack and while carrying a standard dual-strap backpack loaded at 7.5%, 10%, 12.5% and 15% of the subject's bodyweight (BW). Kinematics of the pelvis, trunk and head were recorded using a motion analysis system and centre of pressure (COP) data were recorded using a force platform. Reliable COP data could only be derived for 13 of the subjects with AIS. Increasing backpack load causes a significantly increased flexion of the trunk in relation to the pelvis and extension of the head in relation to the trunk, as well as increased antero-posterior range of COP motion. While backpack load appears to affect balance predominantly in the antero-posterior direction, differences between groups were more evident in the medio-lateral direction, with AIS subjects showing poor balance in this direction. Overall, carrying a backpack causes similar sagittal plane changes in posture and balance in both normal and AIS groups. Load size or subject group did not influence balance, but the additive effect of backpack carrying and AIS on postural control alters the risk of fall in this population. Therefore, load limit recommendations based on normal subjects should not be applicable to subjects with AIS.

Joint coordination during quiet stance: effects of vision

Stabilization of the center of mass (CM) is an important goal of the postural control system. Coordination of several joints along the human "pendulum" is required to achieve this goal. We studied the coordination among body segments with respect to horizontal CM stabilization during a quiet stance task and the effects of vision on CM stability. Subjects were asked to stand quietly on a narrow wooden block supporting only the mid-foot, with either open (EO) or closed (EC) eyes on separate trials. Instant equilibrium points (IEPs) in the center of pressure (CP) trajectory were determined when the horizontal component of the ground reaction force was zero and the CP data were decomposed into their rambling and trembling components. The joint angle, CM and CP data were divided into short cycles (time-normalized to 100 data points) or longer segments (time-normalized to 1000 data points) of equal length beginning and ending in an IEP. Motor abundance with respect to patterns of joint coordination was evaluated using the uncontrolled manifold (UCM) approach. Here, a UCM is a subspace spanning all joint combinations resulting in a given CM position. All combinations of joint angles that lie within this subspace are equivalent with respect to that CM position while joint angle combinations lying in a subspace orthogonal to the UCM lead to deviation from that CM position. UCM analysis was performed on data organized either across time within longer segments or at each point in time across multiple segments or across multiple cycles. Regardless of method of analysis, most of the variance in joint space was constrained to be within the UCM, preserving the mean CM position in both the EO and EC conditions. Joint configuration variance was significantly higher in the EC than in the EO condition although this increase occurred primarily within the UCM rather than in the orthogonal subspace that would have led to variation of the CM position. These results demonstrate the ability of the control system to selectively "channel" motor variability into directions in joint space that stabilize the CM position. This effect was enhanced when the task was made more challenging in the absence of vision. There was also a significant relationship between joint variance that led to a change in the CM position and, in particular, the rambling component of the CP path, lending some support to the idea that the CNS prescribes a certain stable trajectory of the CP during quiet stance that leads to a small controlled movement of the CM.

화면 움직임과 Cybersickness의 관계에 관한 연구

This paper investigates the effects of scene movements on cybersickness to develop the guidelines of scene movements in virtual environments. The types of scene movements were made for both scene navigations(through the axes of X: lateral, Y: fore & after, and Z: vertical) and scene rotations(by pitch, roll and yaw). And there were each three levels of speed; 2.7, 4.5 and 6.3 /s(for navigation), and 10, 20 and 30 /s(for rotation) were conducted. Twelve participants were exposed to each scene for 15 minutes, and three tests were performed to measure the degree of sickness. Before and after subjects were exposed to virtual environments, they were requested to describe their sickness symptoms by means of answering the Simulator Sickness Questionnaire(SSQ). And the postural stability tests, in which the Center of Pressure(COP) of subjects were traced and recorded by a `force platform`, were conducted. During the exposure on virtual environments, the subjects were requested to rate the degree of nausea. For both navigation and rotation, the effects of speeds and axes were significant in the SSQ scores and the nausea ratings, while it was not in the COP. The correlation between the SSQ scores and the COP data was not found. Therefore, it was inappropriate to use COP as a measure of cybersickness. The degree of sickness increased, except for the case of the yaw, as the speed increased. The sickness was most severe in the scene navigation through the axis X and in the scene rotation by the yaw.

Determining the centre of pressure during walking and running using an instrumented treadmill

In this paper, a new method of determining spatial and temporal gait parameters by using centre of pressure (CoP) data is presented. A treadmill is used which was developed to overcome limitations of regular methods for the analysis of spatio-temporal gait parameters and ground reaction forces during walking and running. The design of the treadmill is based on the use of force transducers underneath a separate left and right plate, which together form the treadmill walking surface. The results of test procedures and measurements show that accurate recordings of vertical ground reaction force can be obtained. These recordings enable a separate analysis of vertical ground reaction forces during double support phases in walking, and the analysis of changes in the centre of pressure (CoP) position during subsequent foot placements. From the CoP data, temporal gait parameters (e.g. duration of left/right support and swing phases) and spatial gait parameters (i.e. left/right step lengths and widths) can be derived.

Development of a virtual reality environment for somatosensory and perceptual stimulation in the balance assessment of children

In this study, we developed a balance assessment system in which the visual stimulus was generated by a virtual reality (VR) technique and somatosensation was obtained from a movable platform. By standing on the movable platform, the center of pressure (COP) was measured to express the subject's postural control in response to varied visual stimuli. From the COP data, a singular value decomposition technique was used to derive the sway area and direction, represented in a polar form. Our system demonstrated the feasibility of using a VR environment in postural control trials and provided more realistic somatosensory and visual inputs.

Assessment of the reliability of a technique to measure postural sway in horses.

To assess the reliability of the center-of-pressure (COP) values obtained from a force platform for analysis of postural sway in horses. Six 2-year-old horses that were free from lameness and neurologic disease. Horses stood stationary with all 4 hooves on a force platform; COP data were collected at 1,000 Hz and 3-dimensional kinematics collected at 60 Hz for 10 seconds. Five trials were recorded at each of 3 time periods (15-minute intervals) or at 1 time period on 3 separate days. Mean values for each set of 5 trials and actual, normalized, and relative COP variables were calculated. The reliability was quantified by use of agreement boundary. The COP results within and across days were similar and provided small agreement boundary limits (eg, across days, in order of least relative reliability: area, +/- 62 mm2; mediolateral range, +/- 8 mm; radius, +/- 2 mm; craniocaudal range, +/- 4 mm; and velocity, +/- 3 mm/s). Head height possessed the greatest relative intraday reliability (12%) but a high agreement boundary limit (+/- 0.15 m). CONCLUSIONS AND CLINICAL RELEVANCE; The use of a force platform to analyze postural sway in a group of young healthy horses was found to produce reliable results and may provide a simple and sensitive measure for assessing balance deficiencies in horses. Agreement boundaries provide 95% confidence intervals for use as limits of error and variability in measurements that, if exceeded, may signify meaningful effects.

The effect of a simulated knee joint effusion on postural control in healthy subjects

Palmieri RM, Ingersoll CD, Cordova ML, Kinzey SJ, Stone MB, Krause BA. The effect of a simulated knee joint effusion on postural control in healthy subjects. Arch Phys Med Rehabil 2003;84:1076-9. Objective:To determine the effects of a simulated knee joint effusion on center of pressure (COP) path and mean power frequency (MPF) during standing. Design:Quasi-experimental design. Setting:Sports injury research laboratory in a university setting. Participants:Twenty healthy volunteers, 10 of whom (age, 20.1±2.4y; height, 168.0±8.1cm; weight, 70.4±13.3kg) were assigned to an effusion group and 10 of whom (age, 25±3.8y; height, 169.4±8.9cm; weight, 74.7±7.7kg) were assigned to a control group. Interventions:COP data were collected before and after a 60-mL injection of sterile saline into the knee joint space. Main Outcome Measures:COP path and mediolateral and anteroposterior MPF. Results:COP path decreased after the effusion (pre-effusion mean, 92.2±21.9cm; posteffusion mean, 77.27±23.0cm). No change was found within the control group for COP path (P>.05). No differences were detected before or after joint effusion when the MPF was examined (P>.05). Conclusions:Possible explanations for the improved postural control after the effusion include additional somatosensory feedback, an augmented neural drive to the soleus, and/or increased capsular tension.

A simple method to determine body segment masses in vivo: reliability, accuracy and sensitivity analysis

Objective. To show that force plates can be used to quickly acquire subject-specific segment mass data. Design. In vivo measurements were performed on subjects belonging to three populations: female varsity swimmers, female varsity volleyball players, and male college students. Segmental masses were measured using a force plate technique, and were compared with published data. Background. Patients from populations for which data from the literature are not applicable (e.g. pathological, aging females, obese, children, etc.) would benefit from a direct measure of inertial parameters for accurate joint moment calculations. Methods. Eight female varsity volleyball players, 17 female varsity swimmers, and 10 male college students were measured anthropometrically. They then lay on a board placed on a force plate and the center of pressure was recorded while the subjects adopted various prescribed limb positions. Their limb masses were subsequently calculated from the center of pressure data given estimated center of mass locations. Results. The method was highly reproducible with an average reliability coefficient of 0.83 and yielded results similar to those of published methods. Significantly different mass distributions were found between the two female populations tested ( P<0.025). Conclusions. The method can quickly provide subject-specific limb segment mass information. Relevance Measuring subjects’ segment masses individualizes clinical assessments and may be necessary for those from special populations to avoid erroneous biomechanical conclusions.