Center Of Pressure Feedback Research Articles

Perceptual adaptation during a balancing task in the seated posture and its theoretical model.

This paper proposes a theoretical model of the control and perception mechanism in human balance. Human balance perception is evaluated by the subjective upright posture, the posture at which a person does not feel he/she is at an incline. Our balance experiments in the seated posture showed that the subjective upright posture changed after the balancing task where the participants needed to incline to maintain their balance. This paper aimed to explain this adaptive phenomenon by reproducing the experimental results using computer simulations. Hypothesizing that "humans gradually come to recognize the posture they need to take to maintain their balance as being upright," an adaptation rule for subjective upright posture is defined, so that it approaches the averaged posture in the period of the balancing task. For the balance control, center of pressure feedback is adopted. As a result, the similar changes in subjective upright posture are simulated with a two-link model with a base link, implying that our hypothesis is one possible explanation on the mechanism for human balance control and perception.

Design and Validation of a Real-Time Visual Feedback System to Improve Minimum Toe Clearance (mTC) in Transfemoral Amputees.

Tripping is accompanied by reduced minimum toe clearance (mTC) during the swing phase of gait. The risk of fall due to tripping among transfemoral amputees is nearly 67% which is greater than the transtibial amputees. Therefore, intervention to improve mTC can potentially enhance the quality of life among transfemoral amputees. In this paper, we first develop a real-time visual feedback system with center of pressure (CoP) information. Next, we recruited six non-disabled and three transfemoral amputees to investigate the effect on mTC while participants were trained to shift the CoP anteriorly/posteriorly during heel strike. Finally, to assess the lasting effect of training on mTC, retention trials were conducted without feedback. During feedback, posterior shift in the CoP improved the mTC significantly from 4.68 ± 0.40 cm to 6.12 ± 0.68 cm (p < 0.025) in non-disabled participants. A similar significant improvement in mTC from 4.60 ± 0.55 cm to 5.62 ± 0.57 cm was observed in amputees during posterior shift of CoP. Besides mTC, maximal toe clearances, i.e., maxTC1 and maxTC2, also showed a significant increase (p < 0.025) during the posterior shift of CoP in both the participants. Moreover, during retention, mTC did not differ significantly (p > 0.05) from feedback condition in amputee, suggesting a positive effect of feedback training. The foot-to-ground angle (FGA) at mTC increased significantly (p < 0.025) during posterior shift feedback in non-disabled suggests active ankle dorsiflexion in increasing mTC. However, in amputees, FGA at mTC did not differ significantly during both anterior and posterior CoP shift feedback. The present findings suggest CoP feedback as a potential strategy during gait rehabilitation of transfemoral amputees.

Center of Pressure Feedback Modulates the Entrainment of Voluntary Sway to the Motion of a Visual Target

Visually guided weight shifting is widely employed in balance rehabilitation, but the underlying visuo-motor integration process leading to balance improvement is still unclear. In this study, we investigated the role of center of pressure (CoP) feedback on the entrainment of active voluntary sway to a moving visual target and on sway’s dynamic stability as a function of target predictability. Fifteen young and healthy adult volunteers (height 175 ± 7 cm, body mass 69 ± 12 kg, age 32 ± 5 years) tracked a vertically moving visual target by shifting their body weight antero-posteriorly under two target motion and feedback conditions, namely, predictable and less predictable target motion, with or without visual CoP feedback. Results revealed lower coherence, less gain, and longer phase lag when tracking the less predictable compared to the predictable target motion. Feedback did not affect CoP-target coherence, but feedback removal resulted in greater target overshooting and a shorter phase lag when tracking the less predictable target. These adaptations did not affect the dynamic stability of voluntary sway. It was concluded that CoP feedback improves spatial perception at the cost of time delays, particularly when tracking a less predictable moving target.

Examining Postural Control without Feedback in Individuals with and without Chronic Ankle Instability

Lateral ankle sprains are common orthopedic injuries that often result in chronic ankle instability (CAI). CAI is characterized by many residual symptoms, including pain, episodes of giving way, recurrent injury, impaired postural control and gait, decrease in strength, structural laxity, and decreased physical activity. Impairments in postural control have been identified using both static balance tests like the single leg test on Biodex Stability System and dynamic balance tests, like the star excursion balance test. However, it is not known if there are differences in postural control between individuals with and without a history of ankle sprain when tested without feedback. PURPOSE: To examine postural control and stability characteristics of persons with and without CAI during the Athlete Single Leg Stability Test on the Biodex Balance System (BBS). METHODS: 18 persons with CAI, 15 Copers, and 18 healthy controls (mean age: 22 years for all the groups) completed two 20s trials for each leg without center of pressure feedback at each of the following BBS levels: Static, Levels 12, 8 and 4. Each level corresponds to the degree of tilt of the platform surface with a lower number corresponding to lesser stability or more tilt. A 3x4 ANOVA was run with the dependent variables: overall stability index (OSI) and sway area. OSI is the mean distance of the center of pressure from the center of the platform. RESULTS: Significant level main effect was observed for both OSI and sway area (both P< = 0.001). At Level 12, OSI (1.2) and sway area (1.1cm2) were significantly less compared to other levels (all P < =0.001). At Level 8, OSI (1.8) and sway area (2 cm2) were significantly less compared to Static and Level 4 (both P <=0.002). Static OSI (2.3) and sway area (4.9cm2) values were similar to those at Level 4 (OSI: 2.6; sway area: 10.9>) values were similar to those at Level 4 (OSI: 2.6; sway area: 10.9cm2). No significant group main effect or interaction was observed for either of the variables. CONCLUSION: The Athlete Single Leg Stability Test on BBS without feedback is not sensitive in detecting differences among those with and without CAI. Further testing should examine if standing with eyes closed on the static level on BBS provides a more sensitive way to challenge proprioceptive input in this population. Use of OSI as a dependent variable should be used with caution.

Interpreting lateral dynamic weight shifts using a simple inverted pendulum model

Seventy-five young, healthy adults completed a lateral weight-shifting activity in which each shifted his/her center of pressure (CoP) to visually displayed target locations with the aid of visual CoP feedback. Each subject's CoP data were modeled using a single-link inverted pendulum system with a spring-damper at the joint. This extends the simple inverted pendulum model of static balance in the sagittal plane to lateral weight-shifting balance. The model controlled pendulum angle using PD control and a ramp setpoint trajectory, and weight-shifting was characterized by both shift speed and a non-minimum phase (NMP) behavior metric. This NMP behavior metric examines the force magnitude at shift initiation and provides weight-shifting balance performance information that parallels the examination of peak ground reaction forces in gait analysis. Control parameters were optimized on a subject-by-subject basis to match balance metrics for modeled results to metric values calculated from experimental data. Overall, the model matches experimental data well (average percent error of 0.35% for shifting speed and 0.05% for NMP behavior). These results suggest that the single-link inverted pendulum model can be used effectively to capture lateral weight-shifting balance, as it has been shown to model static balance.

Visual Reliance for Balance Control in Older Adults Persists When Visual Information Is Disrupted by Artificial Feedback Delays

Sensory information from our eyes, skin and muscles helps guide and correct balance. Less appreciated, however, is that delays in the transmission of sensory information between our eyes, limbs and central nervous system can exceed several 10s of milliseconds. Investigating how these time-delayed sensory signals influence balance control is central to understanding the postural system. Here, we investigate how delayed visual feedback and cognitive performance influence postural control in healthy young and older adults. The task required that participants position their center of pressure (COP) in a fixed target as accurately as possible without visual feedback about their COP location (eyes-open balance), or with artificial time delays imposed on visual COP feedback. On selected trials, the participants also performed a silent arithmetic task (cognitive dual task). We separated COP time series into distinct frequency components using low and high-pass filtering routines. Visual feedback delays affected low frequency postural corrections in young and older adults, with larger increases in postural sway noted for the group of older adults. In comparison, cognitive performance reduced the variability of rapid center of pressure displacements in young adults, but did not alter postural sway in the group of older adults. Our results demonstrate that older adults prioritize vision to control posture. This visual reliance persists even when feedback about the task is delayed by several hundreds of milliseconds.

Relative efficacy of various strategies for visual feedback in standing balance activities

Seventy-nine young, healthy adults were led through static balance and weight-shifting activities in order to study the effects of visual feedback on balance. Based on their performance, the relative effects of various feedback properties were analyzed: (1) arrangement [direct center of pressure (CoP) vs. lateral weight distribution feedback], (2) numbers (presence vs. absence of numeric feedback), and (3) dimensionality (1D vs. 2D CoP information). In the static balance activity, subjects were instructed to maintain equal weight across both feet; in the dynamic weight-shifting activity, subjects were instructed to shift their weight to each displayed target location. For static balance, lateral symmetry and sway were measured by classical parameters using CoP, center of gravity (CoG), and the difference between the two (CoP-CoG). Weight-shifting balance performance was measured using the time required to shift between target CoP positions. Results indicated that feedback arrangement had a significant effect on static sway and dynamic weight shifting, with direct CoP feedback resulting in better balance performance than lateral weight distribution. Also, numbers had a significant effect on static sway, reducing lateral sway compared to feedback without numbers. Finally, 2D CoP feedback resulted in faster performance than 1D CoP feedback in dynamic weight shifting. These results show that altering different properties of visual feedback can have significant effects on resulting balance performance; therefore, proper selection of visual feedback strategy needs to take these effects into consideration.

Active control of approach-oriented posture is influenced by emotional reactions.

Emotions adaptively prepare the body to interact with the environment through execution of motor actions, but the extent to which emotional states modulate force control during directionally targeted movement tasks remains unknown. We sought to determine how emotions influence active control of approach-oriented posture. Participants (N = 43; 25 females) stood on a force plate and displaced their center of pressure (COP) anteriorly to a target at 50% of their maximum voluntary lean. After 7 s of real-time COP feedback, a picture representing 6 discrete categories (attack, mutilation, contamination, erotic couples, happy faces, and neutral objects) replaced the target and remained on the screen. Participants were instructed to maintain the target COP position throughout the trial. Deviation of the COP position relative to the target (root-mean-square error; RMSE) and error direction (constant error; CE) were evaluated during the feedback and picture portions of the trial. RMSE increased for all affective conditions following feedback occlusion. Following picture onset, lean error exhibited when viewing attack pictures was more anterior (i.e., greater CE) compared with the mutilation, contamination, and erotica conditions. Additionally, participants leaned more anteriorly (i.e., greater CE) during the happy faces condition compared with the mutilation and erotica conditions. Collectively, results indicate that the maintenance of an anterior COP position in the anterior direction is primarily modulated by the motivational direction of emotional stimuli.

Exploratory behavior during stance persists with visual feedback

Recent evidence showing center of pressure (COP) displacements increase following an external stabilization of the center of mass (COM) supports the theory that postural sway may be exploratory and serve as a means of acquiring sensory information. The aim of the current study was to further test this theory and rule out potential confounding effects of sensory illusions or motor drift on prior observations. Participants stood as still as possible in an apparatus which allowed movements of the COM to be stabilized ("locked") without subject awareness, and they were provided real-time visual feedback of their COM or COP throughout the trial. If there was an influence of sensory illusions or motor drift, we hypothesized that the change in COP displacement with locking would be reduced when participants were provided visual confirmation of COM stabilization (COM feedback), or when they were aware of the position of the COP throughout the trial (COP feedback). Confirming our previous results, increases in COP displacement were observed when movements of the COM were stabilized. In addition, our results showed that increases in COP displacement could not be explained by the presence of sensory illusions or motor drift, since increases in COP were observed despite being provided convincing evidence that the COM had been stabilized, and when participants were aware of their COP position throughout the trial. These results provide further support for an exploratory role of postural sway. The theoretical basis of current clinical practices designed to deal with balance control deficits due to age or disease is typically based on the opinion that increases in sway are a consequence of a failing balance control system. Our results suggest that this may not be the case, and if sway is in fact exploratory, a serious re-evaluation of current clinical practices may be warranted.

Quantitative analysis of static sitting posture in chronic stroke

Unsupported sitting requires postural stability of the trunk which is also necessary for almost all activities in daily living, yet there is a lack of research dealing with the persistence of trunk impairment post-stroke using quantitative methodologies. Therefore, the purpose of this study was to investigate unsupported sitting in individuals with chronic stroke by analyzing center of pressure (COP) signals from a force platform. Ten healthy control subjects and ten chronic stroke subjects sat on a chair without a footrest that was placed on top of a force platform. Trials consisted of eyes closed, staring at a target, and COP feedback. COP signals were analyzed using spatial and temporal techniques. Compared to controls, stroke group had larger sway area and larger displacements in all conditions ( p < 0.05) and less sample entropy ( p < 0.05) in eyes closed and target conditions. In feedback conditions, both groups had decreased sway area and maximum displacements along with stroke group having increased sample entropy ( p < 0.05). Our data suggest that trunk control, necessary for unsupported sitting, is impaired well into the chronic stage of stroke onset. Further investigations of sitting should be conducted for better understanding balance deficits under conditions localized to the trunk musculature.

Postural Effects of the Scaled Display of Visual Foot Center of Pressure Feedback Under Different Somatosensory Conditions at the Foot and the Ankle

Vuillerme N, Bertrand R, Pinsault N. Postural effects of the scaled display of visual foot center of pressure feedback under different somatosensory conditions at the foot and the ankle. Objectives To assess the effects of the scaled display of visual foot center of pressure (COP) feedback on upright postural control under different somatosensory conditions at the foot and the ankle. Design Before and after intervention trials. Setting University medical bioengineering laboratory. Participants Young healthy adults (N=8; mean age, 23±2.5y; mean body weight, 76.8±11.2kg; mean height, 179.8±6.8cm). Interventions Participants were asked to stand upright, as immobile as possible, in 3 visual conditions: a stationary cross feedback (SC-FB) condition and 2 different foot COP feedback (COP-FB) conditions involving increasing scale displays of 2:1 (COP-FB2) and of 10:1 (COP-FB10). These latter conditions correspond to the ratio between the COP displacement on the screen and the actual COP displacement measured by the force platform. This postural task was executed on 2 (firm, foam) support surface conditions. In the foam condition, a 2-cm thick foam support surface was placed under the participants' feet to alter the quality and/or quantity of somatosensory information at the foot and the ankle. Main Outcome Measure COP displacements were recorded using a force platform. Results In the firm support surface condition, no significant difference was observed between the COP-FB2 and the SC-FB conditions, whereas the COP-FB10 condition yielded decreased COP displacements relative to the SC-FB condition. In the foam support surface condition, both the COP-FB2 and the COP-FB10 conditions yielded decreased COP displacements relative to the SC-FB condition, with a greater stabilizing effect in the COP-FB10 than COP-FB2 condition. Conclusions The postural effects of the scale display of visual COP feedback differed depending on the somatosensory conditions at the foot and the ankle. These findings suggest that increased reliance on augmented sensory information for controlling upright posture in conditions of altered somatosensory input from the foot and ankle could have implications in clinical and rehabilitative areas.

22.24 Advantage of center of pressure feedback on static element performance for gymnasts and coaches

22.24 Advantage of center of pressure feedback on static element performance for gymnasts and coaches

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.

From tibialis anterior to Tai Chi: biofeedback and beyond.

This keynote presentation highlights events that have contributed to scientific explorations of one research clinician. Steve Wolf traces his scientific roots to early studies in single motor unit control under the guidance of his primary mentor, John Basmajian, MD. This work led to subsequent studies on the role of EMG feedback in predicting successful outcomes in upper extremity use and in ambulatory capabilities among patients with chronic stroke. These findings are contrasted to further efforts to condition entire reflexes rather than individual muscles through use of operant-conditioning paradigms. The findings from applications of EMG biofeedback to stroke patients became the basis for minimal motor criteria in the treatment of the impaired upper extremities of patients with chronic stroke, using "forced use" or "constraint-induced movement therapy." Last, investigations into center of pressure feedback using computerized balance machines resulted in a series of experiments that ultimately led to the finding that Tai Chi as an exercise form for older adults can have a substantially favorable effect in delaying the onset of fall events.