Does the anthropometric model influence whole-body center of mass calculations in gait?

A comparison of minimum segment models for the estimation of centre of mass position and velocity for slip recovery during a bathtub transfer task

BACKGROUND

Exploring the use of minimum marker sets is important for balancing the technical quality of motion capture with challenging data collection environments and protocols. While minimum marker sets have been demonstrated to be appropriate for evaluation of some motion patterns, there is limited evidence to support model choices for abrupt, asymmetrical, non-cyclic motion such as balance disturbance during a bathtub exit task.

RESEARCH QUESTION

How effective are six models of reduced complexity for the estimation of centre of mass (COM) displacement and velocity, relative to a full-body model.

METHODS

Eight participants completed a bathtub exit task. Participants received a balance perturbation as they crossed the bathtub rim, stepping from a soapy wet bathtub to a dry floor. Six reduced models were developed from the full, 72-marker, 12 segment 3D kinematic data set. Peak displacement and velocity of the body COM, and RMSE (relative to the full-body model) for displacement and velocity of the body COM were determined for each model.

RESULTS

Main effects were observed for peak right, left, anterior, posterior, upwards and downwards motion, and peak left, anterior, posterior, upwards and downwards velocity. Time-varying (RMSE) was smaller for models including the thighs than models not containing the thighs. In contrast, inclusion of upper arm, forearm, and hand segments did not improve model performance. The model containing the sacrum marker only consistently performed the worst across peak and RMSE metrics.

SIGNIFICANCE

Findings suggest a simplified centre of mass model may adequately capture abrupt, asymmetrical, non-cyclic tasks, such as balance disturbance recovery during obstacle crossing. A reduced kinematic model should include the thighs, trunk and pelvis segments, although models that are more complex are recommended, depending on the metrics of interest.

Three-dimensional acceleration of the body center of mass in people with transfemoral amputation: Identification of a minimal body segment network

BACKGROUND

The analysis of biomechanical parameters derived from the body center of mass (BCoM) 3D motion allows for the characterization of gait impairments in people with lower-limb amputation, assisting in their rehabilitation. In this context, magneto-inertial measurement units are promising as they allow to measure the motion of body segments, and therefore potentially of the BCoM, directly in the field. Finding a compromise between the accuracy of computed parameters and the number of required sensors is paramount to transfer this technology in clinical routine.

RESEARCH QUESTION

Is there a reduced subset of instrumented segments (BSN) allowing a reliable and accurate estimation of the 3D BCoM acceleration transfemoral amputees?

METHODS

The contribution of each body segment to the BCoM acceleration was quantified in terms of weight and similarity in ten people with transfemoral amputation. First, body segments and BCoM accelerations were obtained using an optoelectronic system and a full-body inertial model. Based on these findings, different scenarios were explored where the use of one sensor at pelvis/trunk level and of different networks of segment-mounted sensors for the BCoM acceleration estimation was simulated and assessed against force plate-based reference acceleration.

RESULTS

Trunk, pelvis and lower-limb segments are the main contributors to the BCoM acceleration in transfemoral amputees. The trunk and shanks BSN allows for an accurate estimation of the sagittal BCoM acceleration (Normalized RMSE ≤ 13.1 %, Pearson's correlations r ≥ 0.86), while five segments are necessary when the 3D BCoM acceleration is targeted (Normalized RMSE ≤ 13.2 %, Pearson's correlations r ≥ 0.91).

SIGNIFICANCE

A network of three-to-five segments (trunk and lower limbs) allows for an accurate estimation of 2D and 3D BCoM accelerations. The use of a single pelvis- or trunk-mounted sensor does not seem advisable. Future studies should be performed to confirm these results where inertial sensor measured accelerations are considered.

On the impact of the erroneous identification of inertial sensors' locations on segments and whole-body centers of mass accelerations: a sensitivity study in one transfemoral amputee

The kinematics of the body center of mass (bCoM) may provide crucial information supporting the rehabilitation process of people with transfemoral amputation. The use of magneto-inertial measurement units (MIMUs) is promising as it may allow in-the-field bCoM motion monitoring. Indeed, bCoM acceleration might be obtained by fusing the estimated accelerations of body segments' centers of mass (sCoM), the formers being computed from the measured accelerations by segment-mounted MIMUs and the known relative position between each pair of MIMU and underlying sCoM. This paper investigates how erroneous identifications of MIMUs positions impact the accuracy of estimated 3D sCoM and bCoM accelerations in transfemoral amputee gait. Using an experimental design approach, 2¹⁵ simulations of erroneous identifications of MIMUs positions (up to 0.02 m in each direction) were simulated over seven recorded gait cycles of one participant. MIMUs located on the trunk and sound lower limbs were shown to explain up to 77% of the variance in the accuracy of the estimated bCoM acceleration, presumably due to the higher mass and/or angular velocity of these segments during gait of lower-limb amputees. Therefore, a special attention should be paid when identifying the positions of MIMUs located on segments contributing the most to the investigated motion. Sensitivity of the estimated vertical body center of mass acceleration to erroneous identifications of MIMU positions in the anteroposterior (AP), mediolateral (ML), and vertical (V) directions, expressed in percentage of the total variance of the estimation accuracy.

Self-selection of gestational lumbopelvic posture and bipedal evolution

BACKGROUND

Not all pregnant women seem to select the more curved lumbopelvic posture that their sexual dimorphic anatomy allows even though many previous researchers have assumed lumbopelvic curvature to be standard during pregnancy. This study is vital to understanding coevolution of lumbopelvic sexual dimorphism and bipedalism, and understanding some clinical implications of intervening in gestational posture changes.

RESEARCH QUESTIONS

Are there anthropometric changes that correspond with selection of lumbopelvic curvature change during pregnancy? What are the biomechanical costs and benefits of gestational lumbopelvic curvature change?

METHODS

Twenty pregnant women were tested at five different times in the 2nd and 3rd trimesters of pregnancy. Lumbopelvic posture, standing kinetics and gait kinetics were measured longitudinally. Additionally, we modeled the effects on standing and gait without lumbopelvic postural changes, but with anthropometric changes, for each individual.

RESULTS

We found greater lumbopelvic angulation to correspond with a shorter body height (6 cm difference between groups, p = 0.048) and deeper 2nd trimester abdomen (2 cm difference between groups, p = 0.013). Lumbopelvic angulation lowers support requirements (in standing and walking (6% lower support impulse, p = 0.056), but at the cost of shifting the propulsive actions to a less efficient pulling action rather than pushoff (13 % reduction in pushoff time, p = 0.001). We observed minimal effects on walking kinematics and balance control.

SIGNIFICANCE

Our findings suggest the evolutionary advantage of the female lumbopelvic unit is the adaptability it provides to adjust for the individual needs of the pregnant woman. We discuss multiple potential contributing factors that may have shaped hominin female lumbopelvic evolution and are involved in self-selecting lumbopelvic posture.

A Neural-Network-Based Methodology for the Evaluation of the Center of Gravity of a Motorcycle Rider

A correct reproduction of a motorcycle rider’s movements during driving is a crucial and the most influential aspect of the entire motorcycle–rider system. The rider performs significant variations in terms of body configuration on the vehicle in order to optimize the management of the motorcycle in all the possible dynamic conditions, comprising cornering and braking phases. The aim of the work is to focus on the development of a technique to estimate the body configurations of a high-performance driver in completely different situations, starting from the publicly available videos, collecting them by means of image acquisition methods, and employing machine learning and deep learning techniques. The technique allows us to determine the calculation of the center of gravity (CoG) of the driver’s body in the video acquired and therefore the CoG of the entire driver–vehicle system, correlating it to commonly available vehicle dynamics data, so that the force distribution can be properly determined. As an additional feature, a specific function correlating the relative displacement of the driver’s CoG towards the vehicle body and the vehicle roll angle has been determined starting from the data acquired and processed with the machine and the deep learning techniques.

Shoulder and elbow requirements during sagittal reach as a result of changing anthropometry throughout pregnancy

During pregnancy, anthropometric and physiological changes can result in difficulty reaching for and lifting everyday objects. The aims of this study were to determine the changes in sagittal plane anterior reach space (SPARS) and shoulder/elbow strength requirements throughout pregnancy. Seventeen participants were tested through a longitudinal observational cohort study between 16 and 36 weeks gestation in four-week intervals. A 25% decrease in SPARS was observed at the L3-4 torso height. Combined with arm mass increases, shoulder and elbow moment requirements at the minimum and maximum static reach distances significantly increased. However, inverse dynamics analysis determined that mass gains in the arm alone only minimally impact dynamic shoulder moments. Additionally, torso flexion increases throughout pregnancy demonstrates that women are attempting to compensate for decreased SPARS, possibly indicating the additional perceptual importance of reach space in accommodations for pregnant workers.

Estimation of 3D Body Center of Mass Acceleration and Instantaneous Velocity from a Wearable Inertial Sensor Network in Transfemoral Amputee Gait: A Case Study

The analysis of the body center of mass (BCoM) 3D kinematics provides insights on crucial aspects of locomotion, especially in populations with gait impairment such as people with amputation. In this paper, a wearable framework based on the use of different magneto-inertial measurement unit (MIMU) networks is proposed to obtain both BCoM acceleration and velocity. The proposed framework was validated as a proof of concept in one transfemoral amputee against data from force plates (acceleration) and an optoelectronic system (acceleration and velocity). The impact in terms of estimation accuracy when using a sensor network rather than a single MIMU at trunk level was also investigated. The estimated velocity and acceleration reached a strong agreement (ρ > 0.89) and good accuracy compared to reference data (normalized root mean square error (NRMSE) < 13.7%) in the anteroposterior and vertical directions when using three MIMUs on the trunk and both shanks and in all three directions when adding MIMUs on both thighs (ρ > 0.89, NRMSE ≤ 14.0% in the mediolateral direction). Conversely, only the vertical component of the BCoM kinematics was accurately captured when considering a single MIMU. These results suggest that inertial sensor networks may represent a valid alternative to laboratory-based instruments for 3D BCoM kinematics quantification in lower-limb amputees.

Development of Machine Learning Algorithms for the Determination of the Centre of Mass

The study of the human body and its movements is still a matter of great interest today. Most of these issues have as their fulcrum the study of the balance characteristics of the human body and the determination of its Centre of Mass. In sports, a lot of attention is paid to improving and analysing the athlete’s performance. Almost all the techniques for determining the Centre of Mass make use of special sensors, which allow determining the physical magnitudes related to the different movements made by athletes. In this paper, a markerless method for determining the Centre of Mass of a subject has been studied, comparing it with a direct widely validated equipment such as the Wii Balance Board, which allows determining the coordinates of the Centre of Pressure. The Motion Capture technique was applied with the OpenPose software, a Computer Vision method boosted with the use of Convolution Neural Networks. Ten quasi-static analyses have been carried out. The results have shown an error of the Centre of Mass position, compared to that obtained from the Wii Balance Board, which has been considered acceptable given the complexity of the analysis. Furthermore, this method, despite the traditional methods based on the use of balances, can be used also for prediction of the vertical position of the Centre of Mass.

Untangling biomechanical differences in perturbation-induced stepping strategies for lateral balance stability in older individuals

When recovering balance from a lateral perturbation, younger adults tend to stabilize balance with a single lateral sidestep while older adults often take multistep responses. Using multiple steps to recover balance is consistently associated with increased fall risk, altered body center of mass (CoM) control and instability. The aim of this study was to compare the spatio-temporal stepping characteristics and the margin of stability (MoS) of single lateral sidesteps (LSS1) with the first and second steps of a two-step protective step sequence. Two-step sequences begin with either a cross-over step to the front or back, or a medial step followed by a lateral sidestep. Seventy-one older adults received random lateral waist-pull perturbations to either side. We hypothesized that LSS1 would be more stable (larger MoS) than either step in a two-step sequence. With some exceptions, utilizing a two-step sequence was associated with a reduced CoM velocity and distance between the base of support and CoM and decreased stability in the frontal plane following limb loading of the first and second step. There were no differences in the time available to arrest the extrapolated CoM at the end of a single lateral sidestep or the final step of a two-step sequence. Two-step sequences involving a cross-over step include more complex stepping trajectories and also challeng stability in the sagittal plane requiring a multidimensional balance correction. These results indicate important step type differences in center of mass control in recovering balance with a single lateral sidestep as opposed to a two-step sequence among older adults.

Adaptation of balancing behaviour during continuous perturbations of stance. Supra-postural visual tasks and platform translation frequency modulate adaptation rate

When humans are administered continuous and predictable perturbations of stance, an adaptation period precedes the steady state of balancing behaviour. Little information is available on the modulation of adaptation by vision and perturbation frequency. Moreover, performance of supra-postural tasks may modulate adaptation in as yet unidentified ways. Our purpose was to identify differences in adaptation associated to distinct visual tasks and perturbation frequencies. Twenty non-disabled adult volunteers stood on a platform translating 10 cm in antero-posterior (AP) direction at low (LF, 0.18 Hz) and high frequency (HF, 0.56 Hz) with eyes open (EO) and closed (EC). Additional conditions were reading a text fixed to platform (EO-TP) and reading a text stationary on ground (EO-TG). Peak-to-peak (PP) displacement amplitude and AP position of head and pelvis markers were computed for each of 27 continuous perturbation cycles. The time constant and extent of head and pelvis adaptation and the cross-correlation coefficients between head and pelvis were compared across visual conditions and frequencies. Head and pelvis mean positions in space varied little across conditions and perturbation cycles but the mean head PP displacements changed over time. On average, at LF, the PP displacement of the head and pelvis increased progressively. Adaptation was rapid or ineffective with EO, but slower with EO-TG, EO-TP, EC. At HF, the head PP displacement amplitude decreased progressively with fast adaptation rates, while the pelvis adaptation was not apparent. The results show that visual tasks can modulate the adaptation rate, highlight the effect of the perturbation frequency on adaptation and provide evidence of priority assigned to pelvis stabilization over visual tasks at HF. The effects of perturbation frequency and optic flow and their interaction with other sensory inputs and cognitive tasks on the adaptation strategies should be investigated in impaired individuals and considered in the design of rehabilitation protocols.

Anthropometry, standing posture, and body center of mass changes up to 28 weeks postpartum in Caucasians in the United States

BACKGROUND

Anthropometric models are used when body center of mass motion is calculated for assessment of dynamic balance. It is currently unknown how body segments and posture change in the postpartum period. Therefore, this study was conducted to evaluate the longitudinal changes in anthropometry, center of mass, and standing posture postpartum.

METHODS

Seventeen pregnant women were tested at nine different times: 16-20 weeks and 36-40 weeks gestation, and then in 4-week intervals from childbirth to 28 weeks postpartum. Anthropometry was measured and then participants conducted a static standing and static laying trial. Force plate data and motion capture data were used in combination with anthropometry to calculate the masses of individual segments and the body center of mass. Change over time was determined through a linear mixed model analysis.

RESULTS

Anthropometric changes related to the abdomen or fluid retention during pregnancy immediately regress to early pregnancy levels following childbirth. However, other changes related to breast tissue and fat deposits persist postpartum. As such, masses of different segments affect an anthropometric model for center of mass calculation, and body center of mass changes in the lateral and anterior directions postpartum. Vertical body center of mass position was unaffected.

SIGNIFICANCE

Increased postpartum breast mass may be the cause of persistent lordotic curvature changes in the lumbar spine. There is potential that this affects postpartum back pain. Future research should explore how body center of mass changes postpartum for individuals that do not breast feed, and thus may not have significant breast mass postpartum.

Walking balance on a treadmill changes during pregnancy

BACKGROUND

Altered standing balance during pregnancy has been previously reported. To date, body center of mass (bCOM) motion has not been used to track balance changes in this population. We recently compared three methods to determine the torso center of mass (tCOM) location (via force plate acquired center of pressure calculation, using Pavol surface anthropometry measurements, and a combination of the two) to use in calculating the bCOM during pregnancy.

RESEARCH QUESTION

This current research explored two questions: (1) does walking balance change during pregnancy, and (2) do the methods for identifying tCOM location affect the resulting balance measures?

METHODS

Fifteen pregnant women were recruited to perform 60-second trial of treadmill walking at 4-week intervals from 12 weeks gestation until delivery. Walking balance was measured as bCOM motion within the base of support. Gestation time and anthropometric model (force plate, Pavol, and combination) were repeated-measures independent variables in a general linear mixed model analysis.

RESULTS

There was a significant decrease in walking balance during pregnancy. As gestation progressed, we observed non-linear changes in the bCOM motion within the base of support over time, with some changes starting early in pregnancy and others not starting until late 2^nd trimester. The anthropometric model used to locate the bCOM significantly influences balance measures. The results of this study indicate that the force plate method is more appropriate for locating the tCOM in the anterior and lateral directions.

SIGNIFICANCE

The results of this study will inform clinicians and patients about the gestational stage-associated changes in balance during pregnancy that increase the risk of falling and injury. Researchers should also carefully consider the method for locating the bCOM.

Modeling margin of stability with feet in place following a postural perturbation: Effect of altered anthropometric models for estimated extrapolated centre of mass

BACKGROUND

Maintaining the centre of mass (CoM) of the body within the base of support is a critical component of upright balance; the ability to accurately quantify balance recovery mechanisms is critical for many research teams.

RESEARCH QUESTION

The purpose of this study was to investigate how exclusion of specific body segments in an anthropometric CoM model influenced a dynamic measure of postural stability, the margin of stability (MoS), following a support-surface perturbation.

METHODS

Healthy young adults (n = 10) were instrumented with kinematic markers and a safety harness. Sixteen support-surface translations, scaled to ensure responses did not involve a change in base of support, were then issued (backwards, forwards, left, or right). Whole-body CoM was estimated using four variations of a 13-segment anthropometric model: i) the full-model (WFM), and three simplified models, ii) excluding upper limbs (NAr); iii) excluding upper and lower limbs (HTP); iv) pelvis CoM (CoMp). The CoM calculated for each variant was then used to estimate extrapolated CoM (xCoM) position and the resulting MoS within the plane of postural disturbance.

RESULTS

Comparisons of simplified models to the full model revealed significant differences (p < 0.05) in MoS for all models in each perturbation condition; however, the largest differences were following sagittal plane based perturbations. Poor estimates of WFM MoS were most evident for HTP and CoMp models; these were associated with the greatest values of RMS/maximum error, poorest correlations, etc. The simplified models provided low-error approximates for frontal plane perturbations.

SIGNIFICANCE

Findings suggest that simplified calculations of CoM can be used by researchers without reducing MoS measurement accuracy; however, the degree of simplification should be context-dependent. For example, CoMp models may be appropriate for questions pertaining to frontal plane MoS; sagittal plane MoS necessitates inclusion of lower limb and HTP segments to prevent underestimation of postural stability.

Comparison of segmental analysis and sacral marker methods for determining the center of mass during level and slope walking

BACKGROUND

A human's center of mass (COM) is a widely used parameter in both clinical and practical applications. The segmental analysis method for determining the COM is considered the gold standard but is difficult to apply in a real environment.

RESEARCH QUESTION

The purpose of this study was to confirm the efficacy of an alternative COM determination method-the sacral marker method-by comparing segmental analysis and sacral marker method results in three dimensions during level or slope walking.

METHODS

Ten healthy young subjects (age = 24.0 ± 4.5 yr, height = 174.5 ± 5.9 cm, and weight = 66.9 ± 9.4 kg) participated in the study. Each participant was monitored using a Helen Hayes full-body marker set and asked to walk on level and sloped (7°) terrain. The markers' trajectories were subsequently recorded. Each participant's COM was determined using segmental analysis and sacral marker methods via calculation and direct measurement, respectively.

RESULTS

Comparative results indicated no significant differences (p > 0.05) between the segmental analysis and sacral marker method results for the COM displacement, velocity, or acceleration in the fore-aft and vertical directions. Conversely, significant differences (p < 0.05) between the two methods were observed for the COM displacement and acceleration in the medial-lateral direction, suggesting kinematic differences.

SIGNIFICANCE

Based on this latter finding, caution should be exercised when determining COM kinematics using the sacral marker method.