Center Of Mass Velocity Research Articles

Kinematics and Dynamics Simulation Research for Roller Coaster Multi-Body System

To assess the roller coaster multi-body system security, it is need to extract the running process of kinematics, dynamics, load spectrum and other features, as basis dates of the roller coaster structural design. Based on Solidworks/motion software and in the 3D model, the calculation formula of the carrying car velocity and acceleration is derived, and the five risk points of the roller coaster track section are found by simulation in the running, and the simulation results of roller coaster axle mass center velocity are compared with theoretical calculation results, which error is less than 4.1%, indicating that the calculation and simulation have a good fit and providing the evidence for the roller coaster structure design analysis.

Lower limb joint kinetics and ankle joint stiffness in the sprint start push-off

Sprint push-off technique is fundamental to sprint performance and joint stiffness has been identified as a performance-related variable during dynamic movements. However, joint stiffness for the push-off and its relationship with performance (times and velocities) has not been reported. The aim of this study was to quantify and explain lower limb net joint moments and mechanical powers, and ankle stiffness during the first stance phase of the push-off. One elite sprinter performed 10 maximal sprint starts. An automatic motion analysis system (CODA, 200 Hz) with synchronized force plates (Kistler, 1000 Hz) collected kinematic profiles at the hip, knee, and ankle and ground reaction forces, providing input for inverse dynamics analyses. The lower-limb joints predominately extended and revealed a proximal-to-distal sequential pattern of maximal extensor angular velocity and positive power production. Pearson correlations revealed relationships (P < 0.05) between ankle stiffness (5.93 ± 0.75 N · m · deg−1) and selected performance variables. Relationships between negative power phase ankle stiffness and horizontal (r = −0.79) and vertical (r = 0.74) centre of mass velocities were opposite in direction to the positive power phase ankle stiffness (horizontal: r = 0.85; vertical: r = −0.54). Thus ankle stiffness may affect the goals of the sprint push-off in different ways, depending on the phase of stance considered.

Coordination of push-off and collision determine the mechanical work of step-to-step transitions when isolated from human walking

In human walking, each transition to a new stance limb requires redirection of the center of mass (COM) velocity from one inverted pendulum arc to the next. While this can be accomplished with either negative collision work by the leading limb, positive push-off work by the trailing limb, or some combination of the two, physics-based models of step-to-step transitions predict that total positive work is minimized when the push-off and collision work are equal in magnitude. Here, we tested the importance of the coordination of push-off and collision work in determining transition work using ankle and knee joint braces to limit the ability of a leg to perform positive work on the body. To isolate transitions from other contributors to walking mechanics, participants were instructed to rock back and forth from one leg to the other, restricting motion to the sagittal plane and eliminating the need to swing the legs. We found that reduced push-off work increased the collision work required to complete the redirection of the COM velocity during each transition. A greater amount of total mechanical work was required when rocking departed from the predicted optimal coordination of step-to-step transitions, in which push-off and collision work are equal in magnitude. Our finding that transition work increases if one or both legs do not push-off with the optimal coordination may help explain the elevated metabolic cost of pathological gait irrespective of etiology.

Movement behavior of a spider on a horizontal surface

Studying the locomotive behavior of animals has the potential to inspire the design of the mechanism and gait patterns of robots (“bio-inspired robots”). The kinematics characteristics of a spider (Ornithoctonus huwena), including movement of the legs, movement of the center of mass (COM) and joint-rotation angle, were obtained from the observation of locomotion behaviors recorded by a three-dimensional locomotion observation system. Our results showed that one set of the stance phase consists of four legs, which were leg-1 and leg-3 on one side and leg-2 and leg-4 on the other side. Additionally, two sets of the stance phase comprised eight legs alternately supporting and driving the motion of the spider’s body. The spider primarily increased its movement velocity by increasing stride frequency. In comparison to other insects, the spider, O. huwena, has superior movement stability. The velocity and height of COM periodically fluctuated during movement, reaching a maximum during alternation of leg phase, and falling to a minimum in the steady stance phase. The small change in deflection angle of the hind-leg was effective in driving locomotion, whereas each joint-rotation angle of the fore-leg changed irregularly during locomotion. This research will help in the design of bio-inspired robots, including the selection of gait planning and its control.

Submaximal leaping in the grey mouse lemur

In arboreal animals such as the grey mouse lemur ( Microcebus murinus Miller, 1777), leaping is the most frequent strategy for predator avoidance. The aim of this study was to characterise the locomotor adaptation in response to the structural constraint of the habitat (i.e., position of the landing substrate). Thus, we characterised the push-off phase by inducing the lemurs to leap up to a range of heights from horizontal to their own individual highest performance. Using uniplanar high-frequency cineradiographs collected in a sagittal plane, the relative contributions of the centre of mass (CoM) velocity vector magnitude and orientation to leaping performance were evaluated. The kinematics of the push-off phase showed that for low landing heights, leaping performance was essentially due to hip and knee extensions. Higher leaps seemed to be related to an increase in ankle contribution. At all leaping heights, the proximal-to-distal sequence of the hind limb joints controlled the orientation and magnitude of the M. murinus CoM velocity vector while pushing off. Finally, the analysis of the velocity vector at the onset of take-off suggested that the optimal solution for predator avoidance was to leap for horizontal distance and not for vertical distance.

Effects of obesity on balance recovery from small postural perturbations

Obesity is a major and growing health concern associated with a risk of falls. While obese individuals exhibit increased sway during quiet standing, most falls result from some type of postural perturbation. This study investigated effects of obesity on balance recovery from small forward postural perturbations. Altogether, 20 males, 10 normal weight (BMI: 21.9 ± 1.4) and 10 obese (BMI: 33.2 ± 2.3), received force (pendulum) and position (release from a static lean angle) perturbations. Obese individuals showed less centre of mass (COM) displacements and slower COM velocities than normal weight individuals following force perturbations. However, when force was normalised by body weight, and when released from identical lean angles, no differences in COM performance were found. Despite differences in quiet standing due to obesity in the absence of any externally applied perturbations, these results show no differences in balance recovery from small externally applied perturbations. Additional research is needed to determine the source of increased fall risk in the obese. Statement of Relevance: Obesity is associated with an increased risk of falls. Understanding how obesity affects balance is important for designing safe workplace environments and tasks. Despite prior research showing poorer balance in the obese during quiet standing, these results show no negative effects of obesity on balance recovery after small postural perturbations.

Concentration statistics for transport in heterogeneous media due to stochastic fluctuations of the center of mass velocity

We study concentration statistics for tracer transport in the flow through a heterogeneous porous medium. Spatial heterogeneity and temporal variability cause fluctuations in spatial characteristics of the tracer distribution such as the center of mass velocity v( t) and the spreading rate D e ( t), which leads to fluctuations in the concentration. These fluctuations can be quantified statistically by the one-point probability density function (PDF) of concentration and by its simplest characteristics, the concentration mean and variance. We focus on the impact of variability in the center of mass velocity and treat D e ( t) as deterministic. The fluctuating v( t) is modeled as a Gaussian stochastic process. Based on this stochastic description, we derive explicit expressions for the concentration variance and the full one-point concentration PDF. Their behavior is mainly controlled by the ratio of the spatial variance of the tracer distribution κ e ( t) in single realizations and the variance κ m ( t) of the plume centroids between realizations. We explore the behavior of concentration variance and PDF for two velocity correlation models, one that leads to a diffusive increase of centroid variance κ m ( t) and one that yields an asymptotically constant κ m ( t). The obtained results give insight into the dynamics of concentration statistics in heterogeneous media and the mechanisms by which they are controlled.

A collisional perspective on quadrupedal gait dynamics

The analysis of terrestrial locomotion over the past half century has focused largely on strategies of mechanical energy recovery used during walking and running. In contrast, we describe the underlying mechanics of legged locomotion as a collision-like interaction that redirects the centre of mass (CoM). We introduce the collision angle, determined by the angle between the CoM force and velocity vectors, and show by computing the collision fraction, a ratio of actual to potential collision, that the quadrupedal walk and gallop employ collision-reduction strategies while the trot permits greater collisions. We provide the first experimental evidence that a collision-based approach can differentiate quadrupedal gaits and quantify interspecific differences. Furthermore, we show that this approach explains the physical basis of a commonly used locomotion metric, the mechanical cost of transport. Collision angle and collision fraction provide a unifying analysis of legged locomotion which can be applied broadly across animal size, leg number and gait.

Kinetic analysis of stair descent: Part 1. Forwards step-over-step descent

This study examined lower extremity biomechanics during the initiation of stair descent from an upright, static posture. Seventeen healthy subjects (aged 23 ± 2.4 years) descended a five-step, steel-reinforced, wooden laboratory staircase (34° decline). Ten trials of stair descent were separated into two blocks of five trials. Beginning from an upright posture, subjects descended the staircase at their preferred velocity (0.53 ± 0.082 m/s) and continued the length of the laboratory walkway (∼4 m). Joint mechanics were contrasted between gait cycles. Relative to the initiation cycle at the top of the staircase, the dissipative knee extensor (K3) and hip flexor (H2) moments and powers were independent of progression velocity and approximated steady-state (i.e., constant) values after the first cycle of the trail limb (Step 5 to Step 3). In contrast, a salient relationship was observed between progression velocity and ankle joint mechanics at initial-contact. The plantiflexor moment, power and work at initial-contact (A1) increased with centre of mass velocity. Our results demonstrate that while the knee extensor moment is the primary dissipater of mechanical energy in stair descent, the ankle plantiflexors are the primary dissipaters associated with increased progression velocity. In addition, the results show that steady-state stair descent may not be attained during the first gait cycle of the trail limb. These data shed light on locomotive strategies used in stair descent and can be applied in biomechanical models of human stair gait. Researchers and practitioners should take into consideration the influence of gait cycle and progression velocity when evaluating lower extremity function in stair descent.

Limb collapse, rather than instability, causes failure in sit-to-stand performance among patients with parkinson disease.

Previous studies focused on describing successful sit-to-stand (STS) performance in patients with Parkinson disease (PD). Little is known about why these patients fail to perform this transfer activity. Objective This study aimed to determine the role of dynamic stability and limb support in governing successful STS performance in patients with PD and to determine the limits of recovery for discriminating between successful and failed STS trials. This was a cross-sectional study. Twenty-eight patients with PD were instructed to perform the STS task. Kinematic data on 18 successful trials and 14 failed trials (when the patients fell backward) were collected with a motion analysis system. Dynamic stability was determined by the anteroposterior position of the body's center of mass (COM) relative to the base of support (BOS) and by the anteroposterior velocity of the COM relative to the BOS (Velocity(COM/BOS,AP)). Limb support was characterized by the hip height (Height(hip)). /b> The findings revealed no between-group ("risers" versus "fallers") differences in dynamic stability. The fallers shifted their COM in a significantly more anterior position to compensate for their lower Velocity(COM/BOS,AP) at seat-off. It was in the vertical direction that the fallers had significantly reduced peak COM velocity (Velocity(COM,vertical)) and lower corresponding Height(hip) than the risers. Results of a stepwise regression model showed that Velocity(COM/BOS,AP) and Height(hip) at the instant of peak Velocity(COM,vertical) could best predict the STS outcome (success versus failure), with an overall prediction accuracy of 87.5%. The limit differentiating successful from failed STS trials was: Height(hip)=-0.814 Velocity(COM/BOS,AP) + 0.463. Limitations All of the patients were community dwelling and had a moderate level of disease severity. The results cannot be generalized to those who are institutionalized or with advanced PD. Limb support and ill-timed peak forward COM velocity, rather than dynamic stability, play the dominant roles in determining successful STS performance in patients with PD.

Center of mass and base of support interaction during gait

During gait the body is in a continuous state of imbalance, with each subsequent step preventing a fall. Gait balance is maintained by regulating the interactions between the center of mass (CoM) and base of support (BoS). The purpose of this study was to investigate the interaction of the CoM position and velocity (CoMv) in relation to the dynamically changing BoS throughout gait. This was quantified using: (1) The shortest distance from the CoM to the boundary of the BoS; (2) The distance from the CoM to the centroid of the BoS; and (3) The distance from the CoM to the BoS along the direction of the CoMv. These interactions were investigated in healthy young adults, healthy older adults, and elderly fallers, who performed level walking at a self-selected speed. Elderly fallers demonstrated a conservative CoM–BoS separation at toe off and reduced balance control ability, specifically a decreased time to contact, when compared to healthy young adults at heel strike. Decreased time available in responding to perturbations might result in a greater number of falls. Understanding foot position and CoM trajectories might allow for appropriate rehabilitation practices.

Mechanics and energetics of step-to-step transitions isolated from human walking

We isolated step-to-step transitions from other contributors to walking mechanics using a cyclical rocking task and then examined the contribution of individual joints to the total work required to redirect the velocity of the center of mass (COM). Nine participants were instructed to rock backward and forward in the sagittal plane, eliminating the need to swing the legs and progress forward. To systematically increase the required work, we increased step length from 60 to 100% of leg length, keeping rocking frequency constant. The individual limbs method quantified the COM work and the joint power method apportioned the COM work among its various sources. As predicted by a physics-based model, we found that work in rocking was performed mainly during the step-to-step transitions and increased strongly with step length. We also found that increases in the average COM work rate exacted a proportional metabolic cost. The similar patterns of COM work and COM work rate during rocking and walking support the use of rocking to isolate the mechanics of step-to-step transitions. We found that the ankle was the main joint contributing to the positive work required to redirect the COM velocity during forward rocking. At the longest length, it accounted for 88% of the work performed by the trailing leg joints. Interestingly, the summed contribution of ankle, knee and hip joint work accounted for only 39% of the front leg negative COM work during the forward transition, suggesting that most of the collision work is performed by passive tissue.

Mechanical Work and Metabolic Cost of Walking after Weight Loss in Obese Adolescents

This study was performed to investigate whether changes in biomechanical parameters of walking explain the reduction in net metabolic cost after weight loss in obese adolescents. Body composition and metabolic and mechanical energy costs of walking at 1.25 m·s(-1) were assessed in 16 obese adolescents before and after a weight loss. Center of mass (COM) and foot accelerations were measured using two inertial sensors and integrated twice to determine COM and foot velocities and displacements. Potential and kinetic energy fluctuations of the COM and the external mechanical work were calculated. Lateral leg swing was calculated from foot displacements. As expected, the decrease in net metabolic cost was greater, which would have been expected on the basis of the amount of weight loss. The smaller lateral leg swing after weight loss did not explain part of the decrease in net metabolic cost. The reduced body mass required less leg muscle work to raise and accelerate the COM as well as to support body weight. The decrease in body mass seems also associated with a lesser leg muscle work required to raise the COM because of smaller vertical motions. As a result of the inverted pendulum mechanism, the decrease in vertical motions (hence in potential energy fluctuations) was probably related to the decrease in mediolateral kinetic energy fluctuations. Moreover, the lesser amount of fat mass in the gynoid region seems related to the decrease in net metabolic cost of walking. The reduction in net metabolic cost of walking after weight loss in weight-reduced adolescents is associated with changes in the biomechanical parameters of walking.

Centre of mass kinematics of fast bowling in cricket

Kinematic studies have shown that fast bowlers have run-up velocities, based on centre of mass velocity calculations, which are comparable to elite javelin throwers. In this study, 34 fast bowlers (22.3 ± 3.7 years) of premier grade level and above were tested using a three-dimensional (3-D) motion analysis system (240 Hz). Bowlers were divided into four speed groups: slow-medium, medium, medium-fast, and fast. The mean centre of mass velocity at back foot contact (run-up speed) was 5.3 ± 0.6 m/s. Centre of mass velocity at back foot contact was significantly faster in the fastest two bowling groups compared to the slow-medium bowling group. In addition, stepwise multiple regression analysis showed that the centre of mass deceleration over the delivery stride phase was the strongest predictor of ball speed in the faster bowling groups. In conclusion, centre of mass kinematics are an important determinant of ball speed generation in fast bowlers. In particular, bowlers able to coordinate their bowling action with periods of centre of mass deceleration may be more likely to generate high ball speed.

Postural control while dressing on two surfaces in the elderly

Falling is a worldwide problem faced by the elderly, and it has serious consequences. The elderly falls in the bathroom very often, and this raises the concern of the connection between the high incidence of falls in the bathroom and postural control of putting on shorts. Because little is known about postural control while putting on shorts of any age group, this study investigated the effects of age and surface on postural control while putting on shorts. Healthy young adults (n =15; age range: 21-27 years) and the elderly participants (n = 15; age range: 63-78 years) were compared in the temporal-spatial parameters, movement of center of mass (COM), and its interactions with center of pressure (COP) and kinematic measures under two conditions: anti-slipping mat and tile. The results demonstrated that the elderly had lengthened cycle time, decreased single-limb-support ratio, decreased medial-lateral velocity of COM, decreased relative displacement between COM and COP, decreased hip abduction-adduction angle, knee flexion angle, and the height of foot elevation. The elderly demonstrated a conservative strategy that may reduce the mechanical load on the supporting limb but preserve medial-lateral balance during the task. The adopted strategy may be relevant to the reduction of muscle strength and proprioception loss during natural aging process. Although surface did not affect the motion patterns of participants while putting on shorts, the slippery surface should be avoided. Other environmental modification such as a grab bar and bright illumination may have advantages in reducing the risk of falls.

Independent influence of gait speed and step length on stability and fall risk

With aging, individuals’ gaits become slower and their steps shorter; both are thought to improve stability against balance threats. Recent studies have shown that shorter step lengths, which bring the center of mass (COM) closer to the leading foot, improve stability against slip-related falls. However, a slower gait, hence lower COM velocity, does the opposite. Due to the inherent coupling of step length and speed in spontaneous gait, the extent to which the benefit of shorter steps can offset the slower speed is unknown. The purpose of this study was to investigate, through decoupling, the independent effects of gait speed and step length on gait stability and the likelihood of slip-induced falls. Fifty-seven young adults walked at one of three target gait patterns, two of equal speed and two of equal step length; at a later trial, they encountered an unannounced slip. The results supported our hypotheses that faster gait as well as shorter steps each ameliorates fall risk when a slip is encountered. This appeared to be attributable to the maintenance of stability from slip initiation to liftoff of the recovery foot during the slip. Successful decoupling of gait speed from step length reveals for the first time that, although slow gait in itself leads to instability and falls (a one-standard-deviation decrease in gait speed increases the odds of fall by 4-fold), this effect is offset by the related decrease in step length (the same one-standard-deviation decrease in step length lowers fall risk by 6 times).

Balance problems during obstacle crossing in children with Developmental Coordination Disorder

The present study investigated the visuomotor and balance limitations during obstacle crossing in typically developing (TD) children and those with Developmental Coordination Disorder (DCD) (7–9 years old; N = 12 per group). Spatiotemporal gait parameters as well as range and velocity of the centre of mass (COM) were determined in three conditions: overground walking at a self-selected speed, crossing a low obstacle and crossing a high obstacle (5% or 30% of the leg length, respectively). Both groups walked more slowly during obstacle crossing than walking over level ground. In addition, both groups exhibited a significant decrease in the spatial variability of their foot placements as they approached the obstacle, which was then negotiated with a similar strategy. There were no differences in approach distance, length of lead and trail step, or lead and trail foot elevation. Compared to walking over level ground, obstacle crossing led to a longer swing phase of the lead and trail foot and increased maximal medio-lateral COM velocity. In children with DCD, however, medio-lateral COM velocity was higher and accompanied by significantly greater medio-lateral COM amplitude. In conclusion, the results indicate that while TD-children and those with DCD exhibit satisfactory anticipatory control and adequate visual guidance, the latter group have a reduced ability to control the momentum of the COM when crossing obstacles that impose increased balance demands.

Measurement of the differential cross section of the photoinitiated reactive collision of O(D1)+D2 using only one molecular beam: A study by three dimensional velocity mapping

In order to measure the state selective double differential cross section of a reactive collision, the preparation of the reactants with defined initial velocities and quantum states in number densities high enough to achieve an acceptable count rate is most important. At the same time, secondary collisions have to be prevented in order to ensure that the nascent products are not thermalized. Usually, the best way to control the initial conditions is to use crossed molecular beams, but the number density decreases quadratically with the distance from the nozzle orifice which can be a problem, especially if a molecular product with a large number of populated states is to be analyzed state specifically by REMPI spectroscopy. In this contribution we would like to present a method for measuring the quantum state selective differential cross section of a photoinitiated reaction that combines the advantages of the PHOTOLOC technique (high reactant densities) and the parallel beams technique used by the groups of Kitsopoulos, Orr-Ewing, and Suits (defined relative velocity of the reactants). Moreover, an algorithm based on a Bayesian backward reconstruction developed by W. H. Richardson [J. Opt. Soc. Am. 62, 55 (1972)] has been derived. Both, one reactant and the precursor of the other reactant, are present in the same molecular beam and the center of mass velocity is selected by shifting the dissociation and the detection laser in time and space. Like in comparable methods, this produces a bias in the measured velocity distribution due to the fact that the reaction takes place in the whole volume surrounding the laser beams. This has been also reported by Toomes et al. in the case of the parallel beams technique and presents a general problem of probing reaction products by REMPI spectroscopy. To account for this, we develop a general approach that can be easily adapted to other conditions. The bias is removed in addition to deconvolution from the spread in reactant velocities. Using the benchmark system O((1)D)+D(2) with N(2)O as the precursor, we demonstrate that the technique is also applicable in a very general sense (i.e., also with a large spread in reactant velocities, products much faster than reactants) and therefore can be used also if such unfortunate conditions cannot be avoided. Since the resulting distribution of velocities in the laboratory frame is not cylindrically symmetric, three dimensional velocity mapping is the method of choice for the detection of the ionized products. For the reconstruction, the distance between the two laser beams is an important parameter. We have measured this distance using the photodissociation of HBr at 193 nm, detecting the H atoms near 243 nm. The collision energy resulting from the 193 nm photodissociation of N(2)O is 5.2+/-1.9 kcal/mol. Our results show a preference for backward scattered D atoms with the OH partner fragment in the high vibrational states (v=4-6), in accord with previously published results claiming the growing importance of a linear abstraction mechanism for collision energies higher than 2.4 kcal/mol.

Control of center of mass motion state through cuing and decoupling of spontaneous gait parameters in level walking

Can the center of mass (COM) motion state, i.e., its position and velocity relative to the base of support (BOS), which dictate gait stability, be predictably controlled by the global gait parameters of step length and gait speed, or by extension, cadence? The precise relationships among step length and gait speed, and the COM motion state are unknown, partially due to the interdependence between step length and gait speed and the difficulty in independent control of both parameters during spontaneous level walking. The purposes of this study were to utilize simultaneous audio-visual cuing to independently manipulate step length and gait speed, and to determine the extent to which the COM position and velocity can be subsequently controlled. Fifty-six young adults were trained at one of the three gait patterns in which both the step length and gait speed were targeted simultaneously. The results showed that the cuing could successfully “decouple” gait speed from step length. Although this approach did yield reliable control of the COM velocity through manipulation of gait speed ( R 2=0.97), the manipulation of step length yielded less precise control of COM position ( R 2=0.60). This latter control appears to require manipulation of an additional degree-of-freedom at the local segment level, such that the inclusion of trunk inclination with step length improved the prediction of COM position ( R 2=0.80).

EFFECTS OF KNEE OSTEOARTHRITIS ON BODY'S CENTER OF MASS MOTION IN OLDER ADULTS DURING LEVEL WALKING

Knowledge of the control of the body's dynamic stability in patients with knee osteoarthritis (OA) is helpful for the management of these patients and for the evaluation of treatment outcomes. The purpose of the current study was to investigate the dynamic stability of patients with knee OA during level walking using variables describing the motion of the body's center of mass (COM) and its relationship to the center of pressure (COP). Kinematic and kinetic data during level walking were obtained from 10 patients with bilateral knee OA and 10 normal controls using a motion analysis system and two forceplates. Compared to the normal controls, patients with knee OA exhibited normal COM positions and velocities at key instances of gait but with significant changes in COM accelerations. In the sagittal plane, adjustments to the anterioposterior acceleration of the COM throughout the complete gait cycle were needed for better control of the COM during the more challenging latter half of single leg stance. Diminished A/P COM–COP separation was also used to maintain body stability with reduced joint loadings. In the frontal plane, this was achieved by increasing the acceleration of the body's COM towards the stance leg. The more jerky motion of the body's COM observed may be a result of reduced ability associated with knee OA in the control of the motion of the COM. Strengthening of the muscles of the lower extremities, as well as training of the control of the COM through a dynamic balance training program, are equally important for the dynamic stability of patients with knee OA.