Average Center Of Pressure Research Articles

The Effects of Ankle Taping and a Lace-up Ankle Brace on Balance Control while Initiating Gait

[Purpose] The purpose of this study was to examine the center of pressure (COP) trajectory variables while initiating gait under three different ankle support conditions: no external ankle support, ankle taping, and the use of a lace-up ankle brace. [Subjects] The study subjects were 7 healthy males (mean [SD] age, 22.4 [1.5]; range, 22-25 years) and 8 healthy females (mean [SD] age, 22.8 [0.8]; range, 22-24 years). [Methods] Subjects stood in a predetermined position on the floor in front of a force platform and then initiated gait on the force platform at a self-paced speed under the conditions of no external ankle support, ankle taping, and lace-up ankle brace. Changes in the COP trajectories were compared among the three test conditions using one-way repeated analysis of variance. [Results] The ankle external support devices, ankle taping and the lace-up ankle brace, significantly decreased the COP displacement in the mediolateral direction as compared to the no external support condition. However, there were no significant differences in the COP displacement in the anteroposterior direction, the average COP velocity, and the total length of the COP path among the three test conditions. [Conclusion] Intervention using a prophylactic ankle support device appears to be an effective means of reducing injuries because it provides increased mechanical stability.

Lumbosacral orthoses reduce trunk muscle activity in a postural control task

Biomechanical modeling estimated that trunk muscle activity during various tasks could be reduced by 1–14% without the loss of spine stability when a lumbosacral orthosis (LSO) is worn [Cholewicki, J., 2004. The effects of lumbosacral orthoses on spine stability: what changes in EMG can be expected? Journal of Orthopedic Research 22, 1150–1155]. The present study experimentally tested these theoretical predictions in an unstable sitting task. This task required subjects to balance on a seat supported by a plastic hemisphere ( ∅ = 30 cm ) and placed on a force plate that tracked the center of pressure (CoP). The average CoP velocity quantified subjects’ performance. Healthy subjects (12 males, 11 females) balanced for 20 s in 3 trials performed with and without the LSO in random order. EMG was recorded bilaterally from rectus abdominis (RA), external oblique (EO), thoracic (TES) and lumbar erector spinae (LES), and expressed as the % of maximum voluntary activation (%MVA). There was no difference in the balance performance with and without the LSO ( p = 0.13 ) . However, EMG averaged across the trials was significantly lower in the LSO, as compared to the No LSO condition, for TES (5.8±3.2 vs. 6.4±3.7%MVA, p = 0.02 ) and LES (3.7±1.5 vs. 5.9±3.9%MVA, p = 0.01 ). No significant differences were present in the abdominal muscle activity. These results agree with earlier spine modeling simulations, which predicted the greatest reduction in muscle activity due to LSO to occur in TES and LES. It was hypothesized that such a reduction in muscle co-contraction could benefit patients with low back pain, who exhibit elevated muscular activity during postural tasks such as walking, standing and sitting.

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.

Motor Strategies Responsible for Maintaining Standing Posture After Deafferentation of the Unilateral Leg

Imai S, Hase K, Imanaka K, Suzuki E, Tanaka N, Liu M. Motor strategies responsible for maintaining standing posture after deafferentation of the unilateral leg. Objective To identify the motor strategies responsible for maintaining standing posture after deafferentation of the unilateral leg. Design Pretest-posttest, repeated-measures design. Setting A Japanese university hospital rehabilitation facility. Participants Nine healthy subjects aged 25 to 32 years. Intervention Two separate sessions that consisted of prolonged standing (control task) and standing until the H-reflex disappeared through the application of an inflated pneumatic cuff above the right knee. Main Outcome Measures Center of pressure (COP), hip and ankle joint positions, and electromyographic activities. Results In the control task, no time-related change during prolonged standing was found in any of the measured parameters. After deafferentation, the right soleus activity decreased significantly ( P<.01), so the mean velocities of the COP in the anteroposterior and mediolateral directions were greater and the average COP position under the right foot shifted backward compared with those in the previous periods ( P<.01). Also, the left tibialis anterior and soleus were activated, as was the bilateral gluteus medius. Conclusions The unilateral loss of leg and foot sensory information necessitated additional regulatory activities to stabilize the standing posture. The newly organized posture appeared to partly simulate the standing posture in patients with sensory disturbance of a unilateral leg.

Muscle synergies involved in shifting the center of pressure while making a first step

We used the framework of the uncontrolled manifold (UCM) hypothesis to analyze multi-muscle synergies involved in making a step by a standing person. We hypothesized that leg and trunk muscles are organized into stable groups (muscle modes, M-modes) related to shifts of the center of pressure (COP) in the anterior-posterior and medio-lateral directions. Another hypothesis was that the magnitudes of the modes co-vary across repetitive trials to stabilize a certain magnitude of the COP shift in both directions. M-modes were defined using principal component analysis applied to indices of changes in the electromyographic (EMG) activity prior to releasing variable loads that were held by the subject using a pulley system. For the task of releasing the load behind the body three M-modes associated with a backward COP shift were defined. Four M-modes were defined for the task of releasing the load at the body side associated with a lateral COP shift. Multiple regression analysis was used to relate changes in the M-mode magnitudes to COP shifts. EMG changes prior to making a step were quantified over five 100 ms time windows before the lift-off of the stepping leg. Two components of the variance in the M-mode space computed across repetitions of a stepping task were quantified-a component that did not affect the average COP shift in a particular direction (variance within the UCM, V (UCM)), and a component that affected the COP shift (variance orthogonal to the UCM, V (ORT)). V (UCM) was significantly higher than V (ORT) for both directions of the COP shifts. This relation was observed for the M-modes in the stepping leg as well as in the support leg. The stepping leg showed a different time evolution of the ratio V (UCM)/V (ORT) such that the difference between the two variance components disappeared closer to the time of the lift-off. The findings corroborate both main hypotheses. The study supports a view that control of whole-body actions involves grouping the muscles, using fewer elemental variables to scale the muscle activity, and forming synergies in the space of the elemental variables that stabilize time profiles of important performance variables.

Reliability Of Single Leg Landing On A Portable Force Platform

Impact forces at landing are related to valgus moments at the knee and potential ACL injury risk. Reliable tests that are portable and applicable to large scale screening are important to identify high risk individuals in order to target them for appropriate interventions. PURPOSE The purpose of this study was to investigate within session reliability of measures of single leg landing in a large group of female athletes. The hypothesis tested was that within session measures of force and center of pressure (COP) would be reliable during single leg landing. METHODS 45 high school and collegiate soccer players volunteered participation. Each performed a 50cm single leg hop and were instructed to balance for 10 seconds after the landing. A portable force platform (AccuPower, AMTI) was used with force and center of pressure (COP) data. Balance during the trial was assessed by calculating the standard deviation of each variable (COPML, COPAP, FML, FAP, FV). The range of COP excursion (ML and AP) was also calculated during the balance portion of each trial. Stability algorithms utilized sequential estimation during the initial 3 seconds after impact (Colby, 1999). The time of stability (medial/lateral (ML), anterior/posterior (AP) and vertical force (V)) derived from sequential estimation has been previously shown to be reliable when collecting 10 trials. Intraclass correlation coefficients (ICC[3, k]) were performed in SPSS for each variable. RESULTS Maximum vertical ground reaction force had high reliability on both the right (r = 0.823) and left side (r = 0.877). The maximum vertical ground reaction force for the three trials on the right side was 2.40BW, 2.41BW and 2.40BW, respectively. The average (right and left side) reliability for the standard deviation was: COPML (r = 0.831), COPAP(r = 0.651), FML(r = 0.824), FAP(r = 0.714) and FV(r = 0.741). Average COP medial/lateral excursion had a reliability of r = 0.736 and anterior/posterior excursion was r = 0.607. The stability time measurement that had an ICC over 0.6 was calculating the vertical force with sequential estimation (right, r = 0.787 and left r = 0.767). CONCLUSIONS Maximum vertical force and the standard deviation and excursion measures had high reliability during three trials in female athletes. Calculation of stability time from medial/lateral and anterior/posterior force was less reliable. Calculation of a stability time from sequential estimation based on the vertical component was improved compared to the use of M/L and A/P.

Effect Of Strength Training On Static Balance In Persons With Multiple Sclerosis - A Preliminary Study

Multiple sclerosis (MS) is the most common cause of nontraumatic neurological disability in young adults. MS lesions can occur in both the gray and white matter of the central nervous system and due to the variable distribution of demyelination, people with MS may experience a wider variety of symptoms than any other neurological disease, including problems with strength and balance. PURPOSE To provide preliminary data evaluating the effect of a 16-week progressive resistance training program on postural sway characteristics in individuals with MS. METHODS Four female and one male MS subjects (age: 41.2 (12.6) y; height: 1.66 (0.08) m; mass: 70.2 (11.7) kg; EDSS = 3.6 (1.7)) participated in a progressive strength training program 2 times/wk for 16 wks under supervision. To assess balance pre- and post-training, subjects were monitored while standing on a force platform (sampled at 40 Hz) as steady as possible in 4 different stance conditions: feet 15.2 cm apart (F), feet 15.2 cm apart on a foam pad (P), semitandem (S), and tandem (T). Two trials lasting 20 sec were conducted for each stance. The center-of-pressure (COP) was tracked for all trials and the average COP path length, and the amplitudes in the mediolateral (ML) and antero-posterior (AP) directions over the two trials were calculated for each stance. Nonparametric Wilcoxon matched-pairs signed-ranks tests were performed to examine the training effect (α = 0.05). RESULTS A significant decrease was found for the path length in the P stance (p = .043) and the medio-lateral amplitude in the T stance (p = .043) after training (Table 1).Table 1: - Mean (SD) values for different dependent variables for different stances. * denotes p <0.05.CONCLUSION Our data suggests that strength training in subjects with MS is associated with reduced sway, improved stability, and possibly reduced predisposition for falling.

Restoration of weight-shifting capacity in patients with postacute stroke: A rehabilitation cohort study

de Haart M, Geurts AC, Dault MC, Nienhuis B, Duysens J. Restoration of weight-shifting capacity in patients with postacute stroke: a rehabilitation cohort study. Arch Phys Med Rehabil 2005;86:755-62. Objectives To identify and interrelate recovery characteristics of voluntary weight shifting after stroke and to examine whether the assessment of weight shifting adds information about balance recovery compared with the assessment of quiet standing. Design Exploratory study using an inception cohort with findings related to reference values from healthy elderly persons. Setting Dutch rehabilitation center. Participants Thirty-six inpatients (mean age, 61.8y; mean time poststroke, 10wk) with a first hemispheric intracerebral infarction or hematoma who were admitted to retrain standing balance and walking. Intervention Individualized therapy. Main outcome measures Center of pressure (COP) displacements were registered during voluntary frontal-plane weight shifting guided by visual COP feedback using a dual-plate force platform. Besides the speed (number of weight shifts) and imprecision (normalized average lateral COP displacement per weight shift), the weight-transfer time asymmetry and the spatiotemporal distribution were determined. Assessments took place as soon as patients could stand unassisted for at least 30 seconds and at 2, 4, 8, and 12 weeks later. Results During the 12-week training period, the stroke patients increased both their speed (2.3 hits/30s; 95% confidence interval [CI], 1.1–3.4) and precision (37.7mm/hit; 95% CI, 10.4–65.0) of weight shifting. Although the speed appeared to stabilize at a suboptimal level after 8 weeks, precision reached normal reference values after 12 weeks. Both older age (≥65y) and the presence of visuospatial hemineglect negatively affected weight-shifting speed but not its relative improvement in time. During the training period, a small degree of weight-transfer time asymmetry persisted (mean change, .07; 95% CI, –.21 to .36), with an average of 23% slower weight shifts toward the paretic leg, but the spatiotemporal distribution remained symmetrical. The correlations between weight-shifting and quiet-standing control at the end of training were moderate (Spearman ρ range, .50–.77). Conclusions Even subjects with severe stroke who are selected for inpatient rehabilitation are able to improve their speed and precision of weight shifting by reducing the weight-transfer time toward both legs in a proportionate manner. The observed correlations between weight shifting and quiet standing indicate that the assessment of weight-shifting capacity provides unique information about balance recovery after stroke.

Motor strategies for initiating downward-oriented movements during standing in adults

Sitting down and squatting are routine activities in daily living that lower the body mass by flexing the trunk and legs, but they obviously require different motor strategies for each goal posture. The former action must transfer the supporting surface onto a seat, whereas the latter must maintain the center of mass within the same base of both feet. By comparing the performance of both maneuvers, the mechanisms involved in initiating the downward-oriented movements and the process of optimizing the performance during their repetitions were studied. Twelve healthy subjects were asked to perform sitting-down and squatting actions immediately when a light cue was given, but at a natural speed. Electromyograms, angular movements of the joints of the right leg, and center of pressure (COP) displacement were recorded before and during each task. The initial mechanisms to initiate the break from the upright posture and the changes of postural adjustments during repetitive movements were analyzed separately. The sitting-down movement was achieved by a stereotyped motor strategy characterized by a gastrocnemius muscle burst coupled with deactivation of the erector spinae muscle. The former produced a transient COP displacement in the forward direction, and simultaneous unlocking of the trunk prevented a fall backward. By contrast, because of the absence of any need to produce momentum in a given direction, a variety of motor strategies were available to initiate squatting. The direction of initial COP displacement to initiate squatting varied with the muscles involved in unlocking the upright posture. During repetition of sitting down, the average COP position of the initial standing posture in the preparatory period was immediately shifted forward after the second trial. Simultaneously, the erector spinae muscle was deactivated earlier in the later trials. These resulted in a decreased momentum in the backward direction while the subjects were transferring themselves onto the seat. In the squatting task, however, these changes could not be identified, except for a slight flexed position of the knee during standing in the first trial. These findings suggest that in the case of transferring the body-mass to another supporting base the central nervous system immediately adjusts the size of the initial impetus to optimize the performance.

Stabilometric analysis after percutaneous repair of ruptured Achilles tendon

Summary Fourteen patients treated with percutaneous repair following rupture of the Achilles tendon, without residual functional deficit despite increased Achilles tendon thickness and sural triceps hypotrophy, underwent stabilometric analysis to detect any unsettled mechanism in postural control. The analysis was performed using a platform equipped with sensors to detect sole pressures, which was connected to a computer that calculates the average centre of pressure and a series of parameters related to centre of pressure sway. As no statistically significant differences were found in the parameters between the patients and a control group of 14 healthy volunteers, a study was undertaken to better understand the differences in reaction between the two limbs, particularly as regards the leg-foot system. V(Y) is a parameter that measures the average speed of the centre of pressure sway on the anterior-posterior axis. In all 14 patients it was greater in the operated limb, suggesting worse control on the anterior-posterior axis.

First-order statistics of human stabilogram

We investigated the statistical properties of the centre of pressure (COP) vector during quiet standing, as measured on a force platform. To derive a statistical model for the data, the COP vector, locally averaged COP, and deviations from the average COP were analysed. Whereas indefinite multimodal distributions for the components of the COP were observed, the density functions of deviation from the averaged COP had reproducible properties. The anterior–posterior and mediolateral components of the deviations had in common that they were correlated variables with a non-circular statistic. The main axes of the equiprobability density curves were rotated with respect to the natural anterior–posterior and mediolateral directions. By means of more detailed statistical analysis of the data, two distinct populations of subjects were identified. In some cases we have found Gaussian distributions of the components of deviation from the average COP. In most cases the statistics was non-Gaussian. The average contrast, a new dimensionless characteristic quantity, independent of platform calibration, was introduced as a useful indicator of non-Gaussian effects in the postural control system.

Stiffness control of balance in quiet standing.

Our goal was to provide some insights into how the CNS controls and maintains an upright standing posture, which is an integral part of activities of daily living. Although researchers have used simple performance measures of maintenance of this posture quite effectively in clinical decision making, the mechanisms and control principles involved have not been clear. We propose a relatively simple control scheme for regulation of upright posture that provides almost instantaneous corrective response and reduces the operating demands on the CNS. The analytic model is derived and experimentally validated. A stiffness model was developed for quiet standing. The model assumes that muscles act as springs to cause the center-of-pressure (COP) to move in phase with the center-of-mass (COM) as the body sways about some desired position. In the sagittal plane this stiffness control exists at the ankle plantarflexors, in the frontal plane by the hip abductors/adductors. On the basis of observations that the COP-COM error signal continuously oscillates, it is evident that the inverted pendulum model is severely underdamped, approaching the undamped condition. The spectrum of this error signal is seen to match that of a tuned mass, spring, damper system, and a curve fit of this "tuned circuit" yields omega n the undamped natural frequency of the system. The effective stiffness of the system, Ke, is then estimated from Ke = I omega n2, and the damping B is estimated from B = BW X I, where BW is the bandwidth of the tuned response (in rad/s), and I is the moment of inertia of the body about the ankle joint. Ten adult subjects were assessed while standing quietly at three stance widths: 50% hip-to-hip distance, 100 and 150%. Subjects stood for 2 min in each position with eyes open; the 100% stance width was repeated with eyes closed. In all trials and in both planes, the COP oscillated virtually in phase (within 6 ms) with COM, which was predicted by a simple 0th order spring model. Sway amplitude decreased as stance width increased, and Ke increased with stance width. A stiffness model would predict sway to vary as Ke-0.5. The experimental results were close to this prediction: sway was proportional to Ke(-0.55). Reactive control of balance was not evident for several reasons. The visual system does not appear to contribute because no significant difference between eyes open and eyes closed results was found at 100% stance width. Vestibular (otolith) and joint proprioceptive reactive control were discounted because the necessary head accelerations, joint displacements, and velocities were well below reported thresholds. Besides, any reactive control would predict that COP would considerably lag (150-250 ms) behind the COM. Because the average COP was only 4 ms delayed behind the COM, reactive control was not evident; this small delay was accounted for by the damping in the tuned mechanical system.

PS-24-9 Coupling between voluntary tapping, stepping and cardiac rhythms in healthy human subjects

Biomechanical modeling estimated that trunk muscle activity during various tasks could be reduced by 1–14% without the loss of spine stability when a lumbosacral orthosis (LSO) is worn [Cholewicki, J., 2004. The effects of lumbosacral orthoses on spine stability: what changes in EMG can be expected? Journal of Orthopedic Research 22, 1150–1155]. The present study experimentally tested these theoretical predictions in an unstable sitting task. This task required subjects to balance on a seat supported by a plastic hemisphere (∅=30cm) and placed on a force plate that tracked the center of pressure (CoP). The average CoP velocity quantified subjects’ performance. Healthy subjects (12 males, 11 females) balanced for 20 s in 3 trials performed with and without the LSO in random order. EMG was recorded bilaterally from rectus abdominis (RA), external oblique (EO), thoracic (TES) and lumbar erector spinae (LES), and expressed as the % of maximum voluntary activation (%MVA). There was no difference in the balance performance with and without the LSO (p=0.13). However, EMG averaged across the trials was significantly lower in the LSO, as compared to the No LSO condition, for TES (5.8±3.2 vs. 6.4±3.7%MVA, p=0.02) and LES (3.7±1.5 vs. 5.9±3.9%MVA, p=0.01). No significant differences were present in the abdominal muscle activity. These results agree with earlier spine modeling simulations, which predicted the greatest reduction in muscle activity due to LSO to occur in TES and LES. It was hypothesized that such a reduction in muscle co-contraction could benefit patients with low back pain, who exhibit elevated muscular activity during postural tasks such as walking, standing and sitting.