Augmented feedback of COM and COP modulates the regulation of quiet human standing relative to the stability boundary

Abnormal balance control mechanisms during dynamic reaching forward and quiet standing in patients with anterior cruciate ligament reconstruction

Purpose: Postural instability and decreased balance control ability have been observed in patients after anterior cruciate ligament (ACL) reconstruction. Herein, we examined the abnormal balance control mechanisms of these patients during dynamic reaching forward and quiet standing, providing a quantitative index for rehabilitation assessment. Methods: We enrolled ACL reconstruction patients 6-8 months after surgery, and 14 gender- and age-matched healthy volunteers. The IKDC and Lysholm were applied in each patient after ACL reconstruction. All participants conducted the quiet standing and reaching forward (RF) tests at the specified locations on force plates. The ground reaction force, center of pressure (COP), and kinematics signals were recorded. The maximal reach distance (MRD), speed of RF, length of COP, peak speed of COP in anterior-posterior direction (AP-COP), and weight bearing ratio (WBR) of the affected limb were calculated in the RF test. The COP speed, COP amplitude, frequency components of COP and WBR were extracted during quiet standing. Results: We observed that the speed of RF in the patients after ACL reconstruction was significantly lower than that of controls (p < 0.05). The COP length during RF was positively correlated with the Lysholm scale in the affected limb of patients (r = 0.604, p < 0.05). The peak of AP-COP speed during RF in the affected limb of patients was significantly lower than that of the healthy controls (p < 0.05), and positively correlated with the IKDC scale (r = 0.651, p < 0.05). WBR on the affected limb of patients during RF were significantly lower than that of controls (p < 0.05). The mean (r = -0.633, p < 0.05) and peak (r = -0.643, p < 0.05) speeds of COP during quiet standing were negatively correlated with the IKDC scale value. The amplitude of AP-COP on the contralateral side of patients was significantly higher than that of controls during quiet standing (p < 0.05). Conclusion: Patients after ACL reconstruction performed decreased postural control capacity, especially in dynamic balance, and were accompanied by deficiencies in proprioception. The COP length, peak speed of COP during RF and COP speed during quiet standing could be considered as quantitative index of balance function assessment after ACL reconstruction.

Center of Mass Estimation Using a Force Platform and Inertial Sensors for Balance Evaluation in Quiet Standing

Accurate estimation of the center of mass is necessary for evaluating balance control during quiet standing. However, no practical center of mass estimation method exists because of problems with estimation accuracy and theoretical validity in previous studies that used force platforms or inertial sensors. This study aimed to develop a method for estimating the center of mass displacement and velocity based on equations of motion describing the standing human body. This method uses a force platform under the feet and an inertial sensor on the head and is applicable when the support surface moves horizontally. We compared the center of mass estimation accuracy of the proposed method with those of other methods in previous studies using estimates from the optical motion capture system as the true value. The results indicate that the present method has high accuracy in quiet standing, ankle motion, hip motion, and support surface swaying in anteroposterior and mediolateral directions. The present method could help researchers and clinicians to develop more accurate and effective balance evaluation methods.

Psychophysiological Indicators of Postural Control. Contribution of the Russian Scientific School. Part I

This article aimed to systematically review the published results of studies of psychophysiological mechanisms of posture maintenance and identify the key factors that influence the effectiveness of postural control. The recommendations of "Preferred Reporting Elements for Systematic Reviews and Meta-Analyzes" (PRISMA) were followed for the review. The results were classified, taking into account the target psychophysiological mechanisms and factors affecting postural control. The article presents the theoretical and empirical results of the Russian scientific school of research on the role of support afferentation in the sensorimotor mechanisms of cognitive and postural functions. Due to the limited number of randomized studies found, it was impossible to make meta-analytic comparisons, so the literature analysis was carried out only qualitatively. Meanwhile, our systematic review provides promising information about possible relationships between stabilometric and psychological indicators of postural control, which have theoretical significance and application in the correction and training of posture control. However, more thorough research is needed to overcome the methodological shortcomings that we have encountered in our qualitative analysis.

Age-based comparison of gait asymmetry using unilateral ankle weights

BACKGROUND

The locomotor system is proposed to be able to adapt to asymmetric conditions, which reflects the interaction between interlimb control and task constraints. However, this adaptability may be confounded by intrinsic differences between age groups.

RESEARCH QUESTION

What are the effects of mechanical asymmetry on kinematic symmetry in healthy younger and older adults?

METHODS

Two groups of 10 participants (age 18-25 and 58-65 years) walked (3 km h -1) and ran (9 km h -1) on a treadmill normally, and with unilateral ankle weights (UW). Lower-body kinematic data was collected. Symmetry Index (SI) and bilateral cyclograms were used to evaluate spatiotemporal step-cycle characteristics and joint angles.

RESULTS

Step-cycle characteristics were not significantly different between all comparisons. In running gait, asymmetry in range of motion (SI_RoM) of the knee and bilateral cyclogram minimum bounding box length (L) of the ankle was greater with UW compared to the normal condition. Ankle angle characteristics (SI_RoM ankle and minimum ankle angle) were more asymmetrical for older compared to younger adults, regardless of the UW condition. Interaction effects between age and UW were observed. In running gait, L knee and SI_RoM hip were greater with UW for younger adults, and smaller with UW for older adults. In walking gait, ankle angle at touchdown was greater with UW for younger adults, but smaller with UW for older adults.

SIGNIFICANCE

For both age groups walking and running with UW, symmetry appears to be preserved in step-cycle characteristics, but not in joint angle measures. While adapting to unilateral perturbation, older adults show greater asymmetry in some ankle kinematic measures compared to younger adults while running, suggesting that some kinematic solutions are altered with age, while the majority of symmetry values about the lower limbs were not.

Analysis of center of mass acceleration and muscle activation in hemiplegic paralysis during quiet standing

Hemiplegic paralysis after stroke may augment postural instability and decrease the balance control ability for standing. The center of mass acceleration (COMacc) is considered to be an effective indicator of postural stability for standing balance control. However, it is less studied how the COMacc could be affected by the muscle activities on lower-limbs in post-stroke hemiplegic patients. This study aimed to examine the effects of hemiplegic paralysis in post-stroke individuals on the amplitude and structural variabilities of COMacc and surface electromyography (sEMG) signals during quiet standing. Eleven post-stroke hemiplegic patients and the same number of gender- and age-matched healthy volunteers participated in the experiment. The sEMG signals of tibialis anterior (TA) and lateral gastrocnemius (LG) muscles of the both limbs, and the COMacc in the anterior-posterior direction with and without visual feedback (VF vs. NVF) were recorded simultaneously during quiet standing. The sEMG and COMacc were analyzed using root mean square (RMS) or standard deviation (SD), and a modified detrended fluctuation analysis based on empirical mode decomposition (EMD-DFA). Results showed that the SD and the scale exponent α of EMD-DFA of the COMacc from the patients were significantly higher than the values from the controls under both VF (p < 0.01) and NVF (p < 0.001) conditions. The RMSs of TA and LG on the non-paretic limbs were significantly higher than those on paretic limbs (p < 0.05) for both the patients and controls (p < 0.05). The TA of both the paretic and non-paretic limbs of the patients showed augmented α values than the TA of the controls (p < 0.05). The α of the TA and LG of non-paretic limbs, and the α of COMacc were significantly increased after removing visual feedback in patients (p < 0.05). These results suggested an increased amplitude variability but decreased structural variability of COMacc, associated with asymmetric muscle contraction between the paretic and the non-paretic limbs in hemiplegic paralysis, revealing a deficiency in integration of sensorimotor information and a loss of flexibility of postural control due to stroke.

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.

Real-time visual feedback about postural activity increases postural instability and visually induced motion sickness

BACKGROUND

Several studies have shown that the kinematics of standing body sway can be influenced by the provision of real time feedback about postural activity through visual displays.

RESEARCH QUESTION

We asked whether real time visual feedback about the position of the body's center of pressure (COP) might affect body sway and the occurrence of visually induced motion sickness.

METHODS

Standing participants (women) were exposed to complex visual oscillation in a moving room, a device that nearly filled the field of view. During exposure to complex visual oscillations, we provided real time feedback about displacements of the body's center of pressure through a visual display presented on a tablet computer.

RESULTS

The incidence of motion sickness was greater than in a closely related study that did not provide real time feedback. We monitored the kinematics of the body's center of pressure before and during exposure to visual motion stimuli. Body sway differed between participants who reported motion sickness and those who did not. These differences existed before any participants experienced subjective symptoms of motion sickness.

SIGNIFICANCE

Real time visual feedback about COP displacement did not reduce visually induced motion sickness, and may have increased it. We identified postural precursors of motion sickness that may have been exacerbated by the COP display. The results indicate that visual feedback about postural activity can destabilize postural control, leading to negative side effects. We suggest possible alternative types of visual displays that might help to stabilize posture, and reduce motion sickness.

Learning effects of dynamic postural control by auditory biofeedback versus visual biofeedback training

Augmented sensory biofeedback (BF) for postural control is widely used to improve postural stability. However, the effective sensory information in BF systems of motor learning for postural control is still unknown. The purpose of this study was to investigate the learning effects of visual versus auditory BF training in dynamic postural control. Eighteen healthy young adults were randomly divided into two groups (visual BF and auditory BF). In test sessions, participants were asked to bring the real-time center of pressure (COP) in line with a hidden target by body sway in the sagittal plane. The target moved in seven cycles of sine curves at 0.23Hz in the vertical direction on a monitor. In training sessions, the visual and auditory BF groups were required to change the magnitude of a visual circle and a sound, respectively, according to the distance between the COP and target in order to reach the target. The perceptual magnitudes of visual and auditory BF were equalized according to Stevens' power law. At the retention test, the auditory but not visual BF group demonstrated decreased postural performance errors in both the spatial and temporal parameters under the no-feedback condition. These findings suggest that visual BF increases the dependence on visual information to control postural performance, while auditory BF may enhance the integration of the proprioceptive sensory system, which contributes to motor learning without BF. These results suggest that auditory BF training improves motor learning of dynamic postural control.

Adaptation effects in static postural control by providing simultaneous visual feedback of center of pressure and center of gravity

Background

The benefit of visual feedback of the center of pressure (COP) on quiet standing is still debatable. This study aimed to investigate the adaptation effects of visual feedback training using both the COP and center of gravity (COG) during quiet standing.

Methods

Thirty-four healthy young adults were divided into three groups randomly (COP + COG, COP, and control groups). A force plate was used to calculate the coordinates of the COP in the anteroposterior (COP_AP) and mediolateral (COP_ML) directions. A motion analysis system was used to calculate the coordinates of the center of mass (COM) in both directions (COM_AP and COM_ML). The coordinates of the COG in the AP direction (COG_AP) were obtained from the force plate signals. Augmented visual feedback was presented on a screen in the form of fluctuation circles in the vertical direction that moved upward as the COP_AP and/or COG_AP moved forward and vice versa. The COP + COG group received the real-time COP_AP and COG_AP feedback simultaneously, whereas the COP group received the real-time COP_AP feedback only. The control group received no visual feedback. In the training session, the COP + COG group was required to maintain an even distance between the COP_AP and COG_AP and reduce the COG_AP fluctuation, whereas the COP group was required to reduce the COP_AP fluctuation while standing on a foam pad. In test sessions, participants were instructed to keep their standing posture as quiet as possible on the foam pad before (pre-session) and after (post-session) the training sessions.

Results

In the post-session, the velocity and root mean square of COM_AP in the COP + COG group were lower than those in the control group. In addition, the absolute value of the sum of the COP − COM distances in the COP + COG group was lower than that in the COP group. Furthermore, positive correlations were found between the COM_AP velocity and COP − COM parameters.

Conclusions

The results suggest that the novel visual feedback training that incorporates the COP_AP–COG_AP interaction reduces postural sway better than the training using the COP_AP alone during quiet standing. That is, even COP_AP fluctuation around the COG_AP would be effective in reducing the COM_AP velocity.

Error augmentation feedback for lateral weight shifting

This study examines the effect of error augmentation of center of pressure (CoP) visual feedback on the performance of a lateral weight shifting task. Error augmentation emphasizes deviations from a standard CoP trajectory generated from existing data of over 2000 weight shifts collected with young, healthy subjects. Thirty-six subjects completed nine lateral weight shifting sessions, of which four were training sessions between each of the five testing sessions. Half of the subjects received error augmentation feedback during the training sessions, while the other half received the unaltered, control feedback. The change in visual feedback did not affect the final steady state weight shifting performance. Instead, error augmentation feedback was found to drive subjects to their steady-state performance sooner than unaltered visual feedback. The emphasis on deviations from the standard trajectory with error augmentation appears to lead to reduced variation in shifting. This finding may be useful in generating novel therapies that improve the efficiency of balance rehabilitation.

Real-time visual feedback of COM and COP motion properties differentially modifies postural control structures

The experiment was setup to investigate the control of human quiet standing through the manipulation of augmented visual information feedback of selective properties of the motion of two primary variables in postural control: center of pressure (COP) and center of mass (COM). Five properties of feedback information were contrasted to a no feedback dual-task (watching a movie) control condition to determine the impact of visual real-time feedback on the coordination of the joint motions in postural control in both static and dynamic one-leg standing postures. The feedback information included 2D COP or COM position and macro variables derived from the COP and COM motions, namely virtual time-to-contact (VTC) and the COP-COM coupling. The findings in the static condition showed that the VTC and COP-COM coupling feedback conditions decreased postural motion more than the 2D COP or COM positional information. These variables also induced larger sway amplitudes in the dynamic condition showing a more progressive search strategy in exploring the stability limits. Canonical correlation analysis (CCA) found that COP-COM coupling contributed less than the other feedback variables to the redundancy of the system reflected in the common variance between joint motions and properties of sway motion. The COP-COM coupling had the lowest weighting of the motion properties to redundancy under the feedback conditions but overall the qualitative pattern of the joint motion structures was preserved within the respective static and dynamic balance conditions.