Whole-body Center Of Mass Research Articles

Laboratory Balance Assessment

Laboratory balance assessments are distinct from clinical balance assessments because of their focus on research and reliance on instrumentation and quantification. Laboratory assessments have contributed broadly to the current understanding of both normal and impaired balance through analysis of the interactions of 1) the distal segment and the ground and 2) the motion of the body. A substantial number of studies have focused on the two-dimensional displacement of the center of pressure, such that more than 50 parameters have been developed based only on those paired coordinates. Other techniques more directly assess the motion of the whole body center of mass and the individual segments that compose it. A hybrid instrument used for both clinical and research purposes is computerized dynamic posturograhy, a system that enables independent assessment of the multiple body systems that contribute to balance. Because ankle-foot orthoses alter the mechanical and sensory links between the body and the ground, research in this area may benefit from examination of both the motion of the center of mass and the resultant deviation of the center of pressure.

Effects of Anti-Fatigue Flooring on Gait Parameters

Falls among industrial workers are the leading cause of work related injuries. Nowadays, many industries are opting for anti-fatigue cushioned flooring to reduce fall-related socio-economic cost. The goal of this study is to test SATECH's anti-fatigue flooring for stable gait. A pilot test with five healthy subjects (25–35 years old) has been conducted with six motion capture cameras and two force platforms embedded in a walkway to obtain kinematics and kinetics of whole body gait on two different floors. In conditions where the same floor material was used on the whole walkway, the anti-fatigue floor had smaller values of step length (p<0.01) associated with walking and lower values of required coefficient of friction (RCOF) (p<0.05). Toe clearance, heel contact velocity, whole body center of mass (COM) velocity, single foot stance, and double foot stance did not show significant differences between conditions. The results of this study suggest that subjects were to follow a more stable gait on the SATECH anti-fatigue flooring.

The effect of cane use method on center of mass displacement during stair ascent

The aim of the study was to examine the effect of forward and lateral cane placement on displacement of whole body center of mass (COM) and spatial relationship between COM and the cane during stair ascent (SA) in healthy adults. The data were obtained using three-dimensional motion analysis while ascending stairs non-reciprocally with following methods: (1) dominant foot stepped up first, then the opposite foot without a cane (NC); (2) forward placement of a quadricane followed by the ipsilateral foot, then contralateral foot (FCI); (3) forward cane placement followed by the contralateral foot, then ipsilateral foot (FCC); (4) ipsilateral foot stepping up first, followed by the contralateral foot, then the cane (LCI); (5) contralateral foot stepping up, followed by the ipsilateral foot, then the cane (LCC). The results indicated that the cane placement had significant effect on the medial–lateral (ML) COM displacement and the kinematics of the trunk. Lateral cane placement requires greater trunk extension and side-flexion. The major differences between ipsilateral and contralateral cane use are the motions at frontal and transverse planes in which the COM displacement in ML direction is larger in ipsilateral cane use. The COM displacement in ML direction is larger in ipsilateral use of cane (LCI and FCI). The results of this study may help clinicians better understand the possible nature of balance control in patients who ascend stairs with a cane, and suggest that the method of cane placement should be taken into consideration by clinicians when teaching the cane user to ascend stairs.

Aging does not affect generalized postural motor learning in response to variable amplitude oscillations of the support surface

Postural motor learning for dynamic balance tasks has been demonstrated in healthy older adults (Van Ooteghem et al. in Exp Brain Res 199(2):185-193, 2009). The purpose of this study was to investigate the type of knowledge (general or specific) obtained with balance training in this age group and to examine whether embedding perturbation regularities within a balance task masks specific learning. Two groups of older adults maintained balance on a translating platform that oscillated with variable amplitude and constant frequency. One group was trained using an embedded-sequence (ES) protocol which contained the same 15-s sequence of variable amplitude oscillations in the middle of each trial. A second group was trained using a looped-sequence (LS) protocol which contained a 15-s sequence repeated three times to form each trial. All trials were 45 s. Participants were not informed of any repetition. To examine learning, participants performed a retention test following a 24-h delay. LS participants also completed a transfer task. Specificity of learning was examined by comparing performance for repeated versus random sequences (ES) and training versus transfer sequences (LS). Performance was measured by deriving spatial and temporal measures of whole body center of mass (COM) and trunk orientation. Both groups improved performance with practice as characterized by reduced COM displacement, improved COM-platform phase relationships, and decreased angular trunk motion. Furthermore, improvements reflected general rather than specific postural motor learning regardless of training protocol (ES or LS). This finding is similar to young adults (Van Ooteghem et al. in Exp Brain Res 187(4):603-611, 2008) and indicates that age does not influence the type of learning which occurs for balance control.

Solutions for representing the whole-body centre of mass in side cutting manoeuvres based on data that is typically available for lower limb kinematics

While studying detailed lower limb mechanics of dynamic sports manoeuvres like side cutting it is often desirable but practically difficult to directly measure velocity profiles of the whole-body centre of mass (CoM). In the current study, representations of CoM, either based on a single marker placed on the pelvis or thorax, or based on segment kinematics of lower limbs with or without inclusion of trunk, were evaluated against whole-body CoM representation. Using the 95% limits of agreement method for comparison of two methods, strongest agreement was found between velocity of whole-body CoM and CoM representation based on lower limbs with the addition of the trunk. The CoM representation based on lower limbs only showed weaker agreement, but this representation was still markedly superior to single marker representations.

Effects of 8 weeks of balance or weight training for the independently living elderly on the outcomes of induced slips

The study was conducted to evaluate whether the balance or weight training could alter gait characteristics of elderly contributing to a reduction in the likelihood of slip-induced falls. A total of 18 elderly were evaluated for the study. The results indicated decreases in heel contact velocities and the friction demand characteristics after 8 weeks of training, although fundamental gait characteristics, such as walking velocity and step length, were not changed. The results also indicated an increase in transitional acceleration of the whole body center of mass. The number of falls after 8 weeks was reduced in training groups. These findings were found in conjunction with the improvements in knee flexor muscle and plantarflexor muscle strength. In conclusion, after training, elderly were less likely to initiate slips and more likely to recover from slips.

Identifying the control of physically and perceptually evoked sway responses with coincident visual scene velocities and tilt of the base of support

In this study, we have explored whether the impact of visual information on postural reactions is due to the same perceptual mechanisms that produce vection. Pitch motion of the visual field was presented at varying velocities to eight healthy subjects (29.9+/-2.8 years) standing quietly on a stationary base of support or receiving a 3 degrees toes-up tilt of the base of support. An infrared motion system recorded markers placed on body segments to record angular displacement of head and ankle and calculate whole body center of mass. Onset of the visual field motion and base of support movement were synchronized in all trials. We found that in the first 2 s following onset of visual field motion, both direction and amplitude of the linear displacement of whole body center of mass and angular displacement of the head, hip, and ankle were modulated by the velocity of visual scene motion. When the visual scene rotated in upward pitch, subjects overshot their initial vertical position with amplitudes that increased as velocity of the visual field increased. This behavior was even more evident when the base of support was tilted. These responses were much shorter than those observed in studies of vection. The dependence of the postural response amplitudes on the velocity of the visual field suggests, however, that there might be well-shared control pathways for visual influences on postural reactions and postural sway elicited by an illusion of self-motion.

Practice-related improvements in posture control differ between young and older adults exposed to continuous, variable amplitude oscillations of the support surface

Healthy older adults were repeatedly exposed to continuous, variable amplitude oscillations of the support surface to determine (1) whether age affects the capacity for postural motor learning under continuous perturbation conditions with limited predictability and (2) whether practice leads to modifications in the control strategy used to maintain balance in older adults. During training, a translating platform underwent 45-s trials of constant frequency (0.5 Hz) and seemingly random amplitude oscillations (range +\- 2 to 15 cm). The middle 15 s of each trial contained the same sequence of oscillation amplitudes. This repeated middle segment was the same as the repeated segment used in Van Ooteghem et al. (Exp Brain Res 187(4): 603-611, 2008) and was therefore used for analyses. To examine learning, participants performed a retention test following a 24-h delay. Kinematic data were used to derive spatial and temporal measures of whole body centre of mass (COM), trunk, thigh, and shank segment orientation, and ankle and knee angle from performance during the repeated middle segment. Results showed that with training, older adults maintained the capacity to learn adaptive postural responses in the form of improved temporal control of the COM and minimization of trunk instability a a rate comparable to young adults. With practice, however older adults maintained a more rigid, 'platform-fixed' control strategy which differed from young adults who shifted towards 'gravity-fixed' control and decreased COM motion. This study provides important insight into the ability of older adults to demonstrate longer-term improvements in postural regulation.

Different gait tasks distinguish immediate vs. long-term effects of concussion on balance control.

The purpose of this study was to longitudinally compare the sensitivity of previously documented paradigms for measuring balance control during gait following a concussion. We hypothesized that gait with a concurrent cognitive task would be most sensitive to the effects of concussion on dynamic balance control. Individuals with concussion (n = 30) and matched controls (n = 30) performed a single task of level walking, attention divided walking, and an obstacle-crossing task at two heights. Testing occurred four times post-injury. Balance control during gait was assessed with whole-body center of mass and center of pressure motion. The single-task level walking task did not result in any significant differences in balance control between individuals with concussion and control subjects. Within 48 hours post-injury, individuals with concussion walked slower and allowed less motion of their center of mass in the sagittal plane when attention was divided during walking, but there were no group differences by day 6 for this task. Group differences in balance control during obstacle crossing was unremarkable during the first two testing sessions, but by day 14 individuals with concussion displayed less mediolateral motion of their center of mass. Attention divided gait is able to better distinguish gait adaptations immediately following a concussion, but obstacle crossing can be used further along in the recovery process to detect new gait adaptations.

Measurements of vertical displacement in running, a methodological comparison

The aim was (1) to evaluate measurements of vertical displacements (Vdisp) of a single point on sacrum as an estimate of the whole body centre of mass (CoM) Vdisp during treadmill running and (2) to compare three methods for measuring this single point. These methods were based on a position transducer (PT), accelerometers (AMs) and an optoelectronic motion capture system. Criterion method was Vdisp of the whole body CoM measured with the motion capture system. Thirteen subjects ran at 10, 12, 14, 16, 18, 20 and 22kmh−1 with synchronous recordings with the three methods. Four measurements of the (Vdisp) were derived: (1) Vdisp of CoM calculated from a segment model consisting of 13 segments tracked with 36 reflective markers, (2) Vdisp of the sacrum recorded with the PT, (3) Vdisp of the sacrum calculated from the AM, and (4) Vdisp of the sacrum calculated as the mid point of two reflective markers (sacrum marker, SM) attached at the level of the sacral bone. The systematic discrepancy between the measurements of sacrum Vdisp and CoM Vdisp varied between 0 and 1.5mm and decreased with increasing running velocity and decreasing step duration. PT and SM measurements showed strong correlation, whereas the AM showed a variability increasing with velocity. The random discrepancy within each subject was 7mm for all three methods. In conclusion single-point recordings of the sacrum Vdisp may be used to monitor changes in Vdisp of CoM during treadmill running.

The accuracy of kinematic whole body centre of mass location estimation

Introduction: Measurements of location of the human whole body centre of mass (COM) are becoming more important in many fields, but show particular value in clinical gait analysis and various sport science applications. To assess the accuracy of the commonly used kinematic (segmental) method of COM location modelling we compared two such optically-based models against direct suspension-based COM location measurements. Methodology: The two popular conventional models to estimate COM location were derived from either direct measurements made on cadavers or indirect gamma scanning methods in live humans.

A method for determining minimal sets of markers for the estimation of center of mass, linear and angular momentum

A new method is proposed for finding small sets of points on the body giving sufficient information for estimating the whole body center of mass (CoM), as well as the linear momenta (LM) and angular momenta (AM). In the underlying model each point (whose trajectory is tracked by a marker) is a point mass: Hence the body is represented by a simple system of point masses. The first step is to determine the appropriate set of points and the mass of each point, which is assumed to be specific for the movement performed. The distribution of the mass to each marker is determined from training data for which the true (or reference) trajectories of the CoM, LM or AM are known. This leads to a quadratic optimization problem with inequality constraints. The use of the method is demonstrated on data from discus throw. Results indicate reasonably small errors, considering the magnitude of other error sources, in CoM position (average magnitude of estimation error 1–2 cm), and moderate errors in AM (13–20% of peak value).

Hip extensor EMG and forelimb/hind limb weight support asymmetry in primate quadrupeds

Higher weight support on the hind limb than forelimb is among the distinctive characteristics of primate quadrupeds. Although often assumed to be due to a more posteriorly positioned whole body center of mass, there are little data to support such a difference. Reynolds (1985. Am J Phys Anthropol 67:335-349) notes that the distribution of forces on the limbs can also be influenced by average limb posture, but suggests that this effect is too small to account for the asymmetry in weight support observed in primates. Instead, he proposes that high hind limb forces are brought about by an active process of shifting weight off the forelimbs and onto the hind limbs through use of hind limb retractors. In this study, we use video records of walking animals to explore the degree to which average limb posture in primates and other quadrupedal mammals deviates from vertical, and use electromyography to test Reynolds' model of hind limb retractor activity and posterior weight shift. The limb posture results indicate that primate forelimbs oscillate about a vertical or slightly retracted axis, and though the hind limbs are slightly protracted, the magnitude of deviation from vertical is too small to have a major effect on weight support distribution. The electromyographic results reveal higher levels of hip extensor activity in antipronograde primates that bear a higher proportion of weight on their hind limbs. This lends support to Reynolds' suggestion that some primates use muscles to actively shift weight onto hind limbs to relieve stresses on forelimbs less well structured for weight support.

Effects of external loads on balance control during upright stance: Experimental results and model-based predictions

The purpose of this study was to identify the effects of external loads on balance control during upright stance, and to examine the ability of a new balance control model to predict these effects. External loads were applied to 12 young, healthy participants, and effects on balance control were characterized by center-of-pressure (COP) based measures. Several loading conditions were studied, involving combinations of load mass (10% and 20% of individual body mass) and height (at or 15% of stature above the whole-body COM). A balance control model based on an optimal control strategy was used to predict COP time series. It was assumed that a given individual would adopt the same neural optimal control mechanisms, identified in a no-load condition, under diverse external loading conditions. With the application of external loads, COP mean velocity in the anterior–posterior direction and RMS distance in the medial–lateral direction increased 8.1% and 10.4%, respectively. Predicted COP mean velocity and RMS distance in the anterior–posterior direction also increased with external loading, by 11.1% and 2.9%, respectively. Both experimental COP data and model-based predictions provided the same general conclusion, that application of larger external loads and loads more superior to the whole body center of mass lead to less effective postural control and perhaps a greater risk of loss of balance or falls. Thus, it can be concluded that the assumption about consistency in control mechanisms was partially supported, and it is the mechanical changes induced by external loads that primarily affect balance control.

Effects of quadriceps fatigue on the biomechanics of gait and slip propensity

This study examines how lower extremity fatigue of the quadriceps alters gait variables related to slip propensity. Sixteen healthy young adults were recruited to walk across vinyl floor surfaces in states of fatigue and no fatigue. Kinematic and kinetic data were collected using a three-dimensional motion analysis system and force plates. The results indicated a significant increase in both the heel contact velocity and required coefficient of friction and a decrease in the transitional acceleration of the whole body center of mass and peak knee joint moment in the fatigue trials. Thus, suggesting that slip propensity could increase with fatigue. Additionally, there was increased knee flexion and reduced ankle dorsiflexion at the heel contact phase of the gait cycle during fatigue trials. These findings provide new insights into the biomechanical relationship between localized muscle fatigue and gait parameters associated with slip propensity. The present study concluded that localized muscle fatigue affects gait parameters and hence can be considered as a potential risk factor for slip-induced falls.

Slalom Performance in Élite Alpine Ski Racing can be Predicted by Fore/Aft Movement Dynamics

Slalom racers attempt to minimize the loss of speed by taking straighter lines between gates and by reducing ski/snow friction. Relative motion between the skier and skis along the outside ski's longitudinal axis, often referred to as "fore/aft movement" in coaching terminology, may be an important aspect of turning technique to reduce friction. Characteristics of fore/aft movement may therefore be important in differentiating fast from slow skiers. PURPOSE: To determine if a quantitative method may be established to identify a fore/aft strategy that faster skiers employ. It is predicted that the fore/aft movement of faster skiers will be equally distributed about the middle of the ski while that of slower skiers will be skewed in either the forward or aft direction. METHODS: The performances of 6 members of the Norwegian men's Europa Cup team were analyzed through 2 complete turns during a slalom race simulation. Skier 3-D positions were determined from 4 panning cameras and calibration control points distributed near the course (Nachbauer et al., 1996). Utilizing the method of Schiefermüller et al. (2005), a local coordinate system (LCS) originating at the outside ankle joint center was defined at each point in time. The LCS axes were defined as being parallel and perpendicular to the longitudinal axis of the outside ski and normal to the plane of the snow surface (x, y, and z axes, respectively). Motion of the whole body center of mass in the LCS x-dimension was time-normalized to the turn cycle and integrated over normalized time for the fore and aft portions of the turn cycle, separately. The fore and aft integrals were then divided by the % of turn cycle which they represented. Finally, the forward values were divided by the aft values to create a fore/aft ratio. A ratio of 1 would indicate a symmetrical distribution of fore/aft movement. These ratios were then correlated with performance time. RESULTS: The means (SD) for the fore/aft ratio and performance time were 2.44 (1.41) and 2.082s (.055s), respectively. Controlling for each skier's entrance velocity, the partial correlation coefficient (r = 0.98, p =.004 two-tailed) was significant. CONCLUSION: The strong, positive correlation coefficient indicates that faster skiers demonstrated a greater symmetry between fore and aft movement than slower skiers.

Modifiable performance domain risk-factors associated with slip-related falls

Falls are a major source of injury in older adults. Many falls occur after slipping. This study determined performance-related factors that both contribute to slip-related falls and that may be effectively and efficiently modified through targeted intervention. Thirty-five young adults and 21 healthy older adults (age: 70.9 ± 5.1 years) were slipped in a laboratory using a slippery surface. The biomechanics of the 18 older adults who fell and the 30 younger adults who recovered following slips were analyzed. A set of potentially modifiable variables, initially based on significant between-groups differences, was further analyzed using stepwise discriminant analysis and logistic regression. The discriminant analysis correctly categorized 93.8% of the falls and recoveries based on two variables; the velocity of the slipping foot relative to the velocity of the whole body center of mass (COM), and the lateral placement of the recovery foot relative to the COM. The logistic regression determined the expected change in the odds of a recovery following a slip given a hypothesized intervention-induced improvement of these variables. Decreased velocity of the slipping foot relative to the COM, or decreased lateral placement of the recovery foot relative to the COM to zero, increased the odds of recovery by 17% and 27%, respectively. This suggests that intervention targeted at improving these specific lower extremity control variables following the onset of a slip has the potential to significantly decrease slip-related fall risk.

A comparison of the reverse and power punches in oriental martial arts

In this study, we compared mechanical factors in the reverse and three-inch power punches. Twelve expert male martial artists stood on a force plate, and executed reverse and power punches against a padded target fixed to a wall-mounted force plate. The force plates measured horizontal forces, and subsequently impulses and body centre of mass velocity changes. The motions of four markers attached to the arm were also collected, and were used to compute the horizontal velocities of the knuckle and of the arm centre of mass. The power punch produced smaller velocities immediately before impact than the reverse punch for the whole-body centre of mass (0.14 vs. 0.31 m · s−1), for the arm centre of mass (2.86 vs. 4.68 m · s−1), and for the knuckle (4.09 vs. 6.43 m · s−1). The peak force exerted by the fist was much smaller in the power punch than in the reverse punch (790 vs. 1450 N). However, the linear impulse exerted by the fist during the first 0.20 s of contact was slightly larger in the power punch than in the reverse punch (43.2 vs. 37.7 N · s). The results indicate that the power punch is less potent than the reverse punch, but slightly more effective for throwing the opponent off balance.

The ‘extrapolated center of mass’ concept suggests a simple control of balance in walking

Next to position x and velocity v of the whole body center of mass (CoM) the ‘extrapolated center of mass’ (XcoM) can be introduced: ξ=x+v/ω0, where ω0 is a constant related to stature. Based on the inverted pendulum model of balance, the XcoM enables to formulate the requirements for stable walking in a relatively simple form. In a very simple walking model, with the effects of foot roll-over neglected, the trajectory of the XcoM is a succession of straight lines, directed in the line from center of pressure (CoP) to the XcoM at the time of foot contact. The CoM follows the XcoM in a more sinusoidal trajectory. A simple rule is sufficient for stable walking: at foot placement the CoP should be placed at a certain distance behind and outward of the XcoM at the time of foot contact. In practice this means that a disturbance which results in a CoM velocity change Δv can be compensated by a change in foot position (CoP) equal to Δv/ω0 in the same direction. Similar simple rules could be formulated for starting and stopping and for making a turn.

The Role of Center of Mass Kinematics in Predicting Peak Utilized Coefficient of Friction During Walking*

Slips frequently occur when the friction required between the foot and floor exceeds available surface slip resistance. To date, the ability to identify variables that predict an individual's friction needs during walking, or utilized coefficient of friction (COFU), remains limited. Understanding COFU in the context of pedestrian/walkway accidents is important as individuals who demonstrate higher COFU are at a greater risk of slipping. This study determined if whole body center of mass (CM) kinematics were predictive of peak COFU during walking. Ground reaction forces and kinematic data were recorded simultaneously as subjects walked. Stepwise regression analysis determined that the combination of the subject's CM-to-center-of-pressure angle and CM anterior (i.e., forward) velocity predicted 62% of the variance in peak COFU during weight acceptance (p < 0.001). The identified relationships between CM kinematics and peak COFU provide insight into how gait and individual anthropometric characteristics may increase risk for slip initiation.