Development of an optomechanical measurement system for dynamic stability analysis

Balance in the human body's movement is generally associated with different synergistic pathologies. The trunk is supported by one's leg most of the time when walking. A person with poor balance may face limitation when performing their physical activities on a daily basis, and they may be more prone to having risk of fall. The ground reaction forces (GRFs), centre of pressure (COP), and centre of mass (COM) in quite standing posture were often measured for the evaluation of balance. Currently, there is still no experimental evidence or study on leg length discrepancy (LLD) during walking. Analysis of the stability parameters is more representative of the functional activity undergone by the person who has a LLD. Therefore, this study hopes to shed new light on the effects of LLD on the dynamic stability associated with VGRF, COP, and COM during walking. Eighteen healthy subjects were selected among the university population with normal BMIs. Each subject was asked to walk with 1.0 to 2.0 ms−1 of walking speed for three to five trials each. Insoles of 0.5 cm thickness were added, and the thickness of the insoles was subsequently raised until 4 cm and placed under the right foot as we simulated LLD. The captured data obtained from a force plate and motion analysis present Peak VGRF (single-leg stance) and WD (double-leg stance) that showed more forces exerted on the short leg rather than long leg. Obviously, changes occurred on the displacement of COM trajectories in the ML and vertical directions as LLD increased at the whole gait cycle. Displacement of COP trajectories demonstrated that more distribution was on the short leg rather than on the long leg. The root mean square (RMS) of COP-COM distance showed, obviously, changes only in ML direction with the value at 3 cm and 3.5 cm. The cutoff value via receiver operating characteristic (ROC) indicates the significant differences starting at the level 2.5 cm up to 4 cm in long and short legs for both AP and ML directions. The present study performed included all the proposed parameters on the effect of dynamic stability on LLD during walking and thus helps to determine and evaluate the balance pattern.

To aid in the successful execution of goal-directed walking (discrete movement from a start location to an end target) the central nervous system forms a predictive motor plan. For the motor plan to be effective, it must be adapted in response to environmental changes. Despite motor planning being inherent to goal-directed walking, it is not understood how the nervous system adapts these plans to interact with changing environments. Our objective was to understand how people adapt motor plans of center of mass (COM) trajectory during goal-directed walking in response to a consistent change in environmental dynamics. Participants preformed a series of goal-directed walking trials in a novel environment created by a cable robot that applied a lateral force field to their COM. We hypothesized that participants would adapt to the environment by forming an internal model of their COM trajectory within the force field. Our findings support this hypothesis. Initially, we found COM trajectory significantly deviated in the same direction as the applied field, relative to baseline (no field) (p = 0.002). However, with practice in the field, COM trajectory adapted back to the baseline (p = 0.6). When we unexpectedly removed the field, participants demonstrated after-effects, COM trajectory deviated in the direction opposite of the field relative to baseline (p < 0.001). Our findings suggest that when performing a goal-directed walking task, people adapt a motor plan that predicts the COM trajectory that will emerge from the interaction between a specific set of motor commands and the external environment.

102. An introductory investigation of standing postural control ‘error’ and its relation to balance in adult spinal deformity surgery patients

BackgroundPostural assessment is crucial as risk of falling is a major health problem for the elderly. The most widely used devices are force and balance plates, while center of pressure is the most studied parameter as measure of neuromuscular imbalances of the body sway. In out-of-laboratory conditions, where the use of plates is unattainable, the center of mass can serve as an alternative. This work proposes a center of mass-based posturographic measurement for free living applications. MethodsTen healthy and ten Parkinson's disease individuals (age = 26.1 ± 1.5, 70.4 ± 6.2 years, body mass index = 21.7 ± 2.2, 27.6 ± 2.8 kg/m2, respectively) participated in the study. A stereophotogrammetric system and a force plate were used to acquire the center of pressure and the 5th lumbar vertebra displacements during the Romberg test. The center of mass was estimated using anthropometric measures. Posturographic parameters were extracted from center of pressure, center of mass and 5th lumbar vertebra trajectories. Normalized root mean squared difference was used as metric to compare the trajectories; Spearman's correlation coefficient was computed among the posturographic parameters. FindingsLow values of the metric indicated a good agreement between 5th lumbar vertebra trajectory and both center of pressure and center of mass trajectories. Statistically significant correlations were found among the postural variables. InterpretationA method to perform posturography tracking the movement of the 5th lumbar vertebra as an approximation of center of mass has been presented and validated. The method requires the solely kinematic tracking of one anatomical landmark with no need of plates for free living applications.

BackgroundThe 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.MethodsThirty-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 (COPAP) and mediolateral (COPML) directions. A motion analysis system was used to calculate the coordinates of the center of mass (COM) in both directions (COMAP and COMML). The coordinates of the COG in the AP direction (COGAP) 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 COPAP and/or COGAP moved forward and vice versa. The COP + COG group received the real-time COPAP and COGAP feedback simultaneously, whereas the COP group received the real-time COPAP 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 COPAP and COGAP and reduce the COGAP fluctuation, whereas the COP group was required to reduce the COPAP 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.ResultsIn the post-session, the velocity and root mean square of COMAP 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 COMAP velocity and COP − COM parameters.ConclusionsThe results suggest that the novel visual feedback training that incorporates the COPAP–COGAP interaction reduces postural sway better than the training using the COPAP alone during quiet standing. That is, even COPAP fluctuation around the COGAP would be effective in reducing the COMAP velocity.

Postural instability marks a prevalent symptom of Parkinson’s disease (PD). It often manifests in increased body sway, which is commonly assessed by tracking the Center of Pressure (CoP). Yet, in terms of postural control, the body’s Center of Mass (CoM), and not CoP is what is regulated in a gravitational field. The aim of this study was to explore the effect of early- to mid-stage PD on these measures of postural control in response to unpredictable visual perturbations. We investigated three cohorts: (i) 18 patients with early to mid-stage PD [Hoehn & Yahr stage (1–3); 1.94 ± 0.70]; (ii) a group of 15 age-matched controls (ECT); and (iii) a group of 12 young healthy adults (YCT). Participants stood on a force plate to track their CoP, while the movement of their entire body was recorded with a video-based motion tracking system to monitor their CoM. A moving room paradigm was applied through a head-mounted virtual reality headset. The stimulus consisted of a virtual tunnel that stretched in the anterior-posterior direction which either remained static or moved back and forth in an unpredictable fashion.We found differences in mean sway amplitude (MSA) and mean velocities of CoP and CoM between the groups under both conditions, with higher MSA of CoP and CoM for PD and higher mean velocities of both variables for PD and ECT when compared with YCT. Visual perturbation increased mean CoP velocity in all groups but did not have effects on mean CoM velocity or MSA. While being significantly lower for the young adults, the net effect of visual perturbation on mean CoP velocity was similar between patients with PD and age-matched controls. There was no effect of the visual perturbation on mean CoM velocity for any of the groups.Our simultaneous assessment of CoP and CoM revealed that postural control is reflected differently in CoM and CoP. As the motion of CoM remained mostly unaffected, all groups successfully counteracted the perturbation and maintained their balance. Higher CoP velocity for PD and ECT revealed increased corrective motion needed to achieve this, which however was similar in both groups. Thus, our results suggest increased effort, expressed in CoP velocity, to be an effect of age rather than disease in earlier stages of PD.

Vertical ground reaction force(vGRF) has been widely known as a qualitative features of human walking. While a human walks, the vGRF exhibits a double peak profile in which two maximal values appear from the heel contact to the toe off. So far, various studies have been mainly conducted in the field of biomechanics to elucidate the mechanism by which the double peak profile of the vGRF can be generated. In these researches, inverted pendulum models that allow the change in the length of the support leg have been considered as one of the minimal models. It is, however, known that the center of mass(COM) trajectory of human waling coincides well with the COM trajectory of the inverted pendulum with constant support leg length because changes in the length of legs during walking is small compared to the length. In this research, we propose a new minimal model for human walking based on the inverted pendulum model with fixed support leg length which can interpret the double peak profile of vGRF by considering double support phase. As a result of the verification by the numerical simulation, the proposed model was successfully confirmed that the double peak profile of vGRF can be reproduced in accordance with the COM trajectory of the inverted pendulum model with fixed support leg length.

ABSTRACTThe aim of this study was to advance current movement analysis methodology to enable a technique analysis in sports facilitating (1) concurrent comparison of the techniques between several athletes; (2) identification of potentially beneficial technique modifications and (3) a visual representation of the findings for feedback to the athletes. Six elite cross-country skiers, three world cup winners and three national elite, roller ski skated using the V2 technique on a treadmill while their movement patterns were recorded using 41 reflective markers. A principal component analysis performed on the marker positions resulted in multi-segmental “principal” movement components (PMs). A novel normalisation facilitated comparability of the PMs between athletes. Additionally, centre of mass (COM) trajectories were modelled. We found correlations between the athletes’ performance levels (judged from race points) and specific features in the PMs and in the COM trajectories. Plausible links between COM trajectories and PMs were observed, suggesting that better performing skiers exhibited a different, possibly more efficient use of their body mass for propulsion. The analysis presented in the current study revealed specific technique features that appeared to relate to the skiers’ performance levels. How changing these features would affect an individual athlete’s technique was visualised with animated stick figures.

In bipedal robot locomotion, generation of Center of Mass (CoM) trajectory forms an essential component in gait pattern generation using approximate models. Towards this, rolling sphere model as an approximate model has been widely studied for biped walk. In this work, the rolling sphere model is used to generate realizable bipedal walk by improving an existing approach. In doing so, a consistent CoM trajectory is derived with a desirable Zero Moment Point (ZMP) making the resulting gait more practical. The efficacy of the formulation has been validated through simulation using parameters of humanoid robot NAO.

There are a number of studies concerning difference of postural control following left or right hemisphere lesions. Few studies, however, compare the role of the right and left hemisphere in learning new postural tasks. This study aimed to address this question. Twenty patients with hemiparesis after ischemic stroke in the middle cerebral artery territory (11 with a right and 9 with a left hemispheric lesion) were investigated. All subjects were trained using two different tasks during ten training sessions. In both tasks, the subjects stood on a force platform and were taught to change the position of the center of pressure (COP) presented as a cursor on a monitor screen in front of the patient. The subjects were instructed to align the COP with the target and then move the target by shifting the COP in the indicated direction. In the "Balls" task, the position of the target (a ball) varied randomly, so the subject had to learn a general strategy of voluntary COP control. In "Bricks", the subject always had to move the target in a single direction (downward) from the top to the bottom of the screen, so that a precise postural coordination had to be learned. The number of correctly performed trials for a session was scored. The task performance and its rate were analyzed and compared with respect to the lesion lateralization between two patient groups. The voluntary control of the COP position and learning course were initially impaired in all groups of patients in both tasks. In "Balls", there were no differences between the two groups of patients. In contrast, in "Bricks", there was a greater initial deficit in patients with right hemisphere lesions, while the rate of postural learning and the final performance level did not differ between the groups. With a lower initial deficit and similar rate of learning, the maximal level of the task performance was reached earlier (on the 5th day of training) in patients with left hemisphere lesions. This group stopped improving its performance during follow-up training. The results suggest that the motor structures of the right hemisphere are more involved in the precise control of COP trajectory, but not in learning. There is no difference between hemispheres in the initial performance and learning of the general strategy of voluntary COP control. Possibly, the control of specific COP trajectory needs more sensory feedback that is associated with greater involvement of the right hemisphere. This might be a reason for the greater initial impairment of this task after lesions in the right hemisphere.

Many researchers have referred to the human body as an inverted pendulum. The inverted pendulum model, relating the center of mass (COM), the controlled variable, with the center of pressure (COP), the controlling variable, shows that the differences between COP and COM are proportional to the horizontal accelerations of the body. Using the three-dimensional motion analysis synchronized with three force plates, the inverted pendulum model was applied to four different movements (quiet standing, intended postural sway, falling and walking) in twenty healthy volunteers who have no musculoskeletal problems and balance disorders in order to investigate the relationship among COP, COM and the horizontal acceleration of the body. We also performed the crosscorrelation analysis between (COP-COM) and the horizontal acceleration of the body. Results revealed that (COP-COM) had high negative correlations with the horizontal acceleration in quiet standing and postural sway. In addition, high negative correlations between (COP-COM) and horizontal accelerations of the body were found before falling occurs, but high positive correlations were found once falling occurs. However, during walking, high negative correlations between (COPCOM) and midial/lateral accelerations of the body were found. Therefore, the correlation between (COP-COM) and horizontal accelerations of the body based on the inverted pendulum model can be used to determine the postural stability in falling.

Instability is a significant risk factor for falls in individuals with a bilateral labyrinthine deficit. The purpose of this case report is to describe an intervention that we found to improve balance in a patient with bilateral labyrinthine deficit using a training paradigm based on the sensory reweighting hypothesis. The participant was a female and 10 years post-onset of bilateral labyrinthine deficit. The participant was instructed to focus on the motion of her hips and knees while standing on a dynamic platform that was either stationary or matched to the excursion of her center of mass (COM) but in the opposite direction and with gradually increasing amplitude. She was tested for her ability to maintain her balance under conditions of sensory conflict both before the training and on two periods after training. Decreases in anteroposterior and mediolateral motion of the COM were observed between the pretest and both posttests with a stationary and a moving platform when in the dark and under conditions of sensory conflict. Using the method of approximate entropy, we found that the complexity of the center of pressure (COP) response increased in both the anteroposterior and medolateral directions from the pretest to both posttests when on the platform matched to the COM motion. Results indicated that training on a dynamic platform diminished the destabilizing effect of conflicting sensory signals. Additionally, a relationship was observed between decreased COM motion and increased complexity in COP, which represents a more self-organized system. This finding suggests that improved stability may be associated with an increased complexity in the COP trajectory.

Corner height influences center of mass kinematics and path trajectory during turning

Introduction Commercially available dynamic response (DR) prosthetic feet have unique ankle designs, which provide different degrees of ankle motion. Differences in ankle motion between the prosthetic and anatomical foot creates an asymmetry in the locomotor system and affects its functions of shock absorption, progression, and stabilization. Ankle motion during gait can be assessed by quantifying the center of pressure (CoP) movement during the stance period of gait. Although previous studies have compared gait parameters between DR and non-DR feet, no studies have examined difference in CoP trajectories among common designs of DR feet. Methods LP-Variflex, Talux, and Variflex prosthetic feet were used as test feet to represent the common DR feet, that is, low-profile (LP), multi-axial (MX), and J-shaped (JS) ankle designs, respectively. Five subjects with unilateral transtibial amputation (TTA), six subjects with unilateral transfemoral amputation (TFA), and three healthy controls participated in the study. Subjects were prosthetic users for at least 18 months and were classified as K-level 3 or K-level 4 functional level. Each subject received standardized functional prosthetic gait training and accommodation with each test foot. For data collection, subjects walked on level ground using standardized shoes and CoP data were collected using in-sole sensors. The testing order of feet was randomized. The A-P and M-L location of CoP was determined at the following gait phases: initial contact (2% of gait cycle), early midstance (15% of gait cycle), terminal stance (35% of gait cycle), preswing (50% of gait cycle), and before toe-off (60% of gait cycle). The maximum A-P and M-L CoP excursion was also calculated. The A-P displacement of the CoP was normalized to the foot length, and the M-L displacement was normalized to the foot width. Analysis of variance was used to calculate differences in CoP between test feet, and a comparison-wise alpha of P &lt; 0.1 was considered as significant. Results In the TTA group, there was a significant difference between feet in A-P CoP location during early midstance (15% gait cycle). For the MX foot, the CoP was located at 21.3% ± 3.1% of the foot length, which was significantly greater than the LP and JS feet (15.6% ± 2% and 14.1% ± 3.4% foot length, respectively). During the period from terminal stance to toe-off (35%–60% gait cycle), the split keel design of JS and LP feet had greater M-L movement compared with that of the MX foot. In the TFA group, there were no differences in CoP trajectory between test feet. The maximum A-P excursion for all prosthetic feet was significantly greater than control subjects (80.4% ± 0.5% and 69.1% ± 2.7% foot length, respectively). The maximum M-L excursion for all prosthetic feet was significantly lesser than control subjects (14% ± 3.7% and 26.2% ± 3.7% foot width, respectively). Conclusions Differences in ankle design among different DR prosthetic feet can influence the asymmetry and functions of the locomotor system. In the TTA group, the MX foot resulted in a faster transition from initial contact to midstance, compared with the JS and LP feet. The split keel design of JS and LP feet closely mimicked the motion of the anatomical foot in late-stance period and provided controlled weight transfer from the prosthetic limb to the intact limb. In the TFA group, ankle design did not have a significant effect on CoP movement. All prosthetic feet had greater A-P CoP excursion and lower M-L excursion compared with the anatomical foot. Additional studies investigating the effects of DR foot/ankle designs on distinct functions of the locomotor system are needed to guide prosthetic foot prescription.

The uncontrolled manifold (UCM) approach has been widely used in recent studies to examine variability in daily tasks; however, it has not yet been used to study running or the effects of expertise. Therefore, the aim of this study was to analyse the synergy structure stabilizing the centre of mass (CoM) trajectory in experts compared to novices during running at two different speeds using a subject-specific 3D model. A total of 25 healthy young adults (13 experts, 12 novices) participated in the study. All subjects ran at 10 and 15 km h−1 on a treadmill. In each case, kinematics of 20 consecutive gait cycles were recorded and the effects of expertise and gait cycle phase on the synergy structure were investigated at both speeds. Specifically, the variance affecting the CoM , the variance not affecting the CoM , and their ratio were analysed. Descriptively, in both groups there was a synergy stabilizing the CoM trajectory in running. However, the ANOVA showed no differences in between the two groups. In novices, and were significantly higher compared to experts at the 15 km h−1 condition. In both groups, there was more variability in the stance phase compared to the flight phase in the majority of cases. The results indicate that experts adopted a more consistent running style. The stride-to-stride variability was diminished but not abolished. This difference was only visible at the 15 km h−1 condition. Furthermore, variability was less constrained in the stance phase compared to the flight phase.