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John H. Challis

Segmental moment of inertia values, which are often required to perform mechanical analyses of human movement, are commonly computed using statistical models based on cadaver data. Two sets of equations for estimating human limb moments of inertia were evaluated: linear multivariable equations and nonlinear equations. Equation coefficients for both sets of equations were determined using the cadaver data of Chandler et al. (1975). A cross-validation procedure was used to circumvent the problem of model evaluation when there is limited data with which to develop and evaluate the model. Moment of inertia values for the longitudinal axes were predicted with similar degrees of accuracy with either set of equations, while for the transverse axes the nonlinear equations were superior. An evaluation of the influence of the accuracy of moment of inertia estimates on resultant joint moments for three activities showed that the influence of these errors was generally small.

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Samantha L. Winter, Sarah M. Forrest, Joanne Wallace and John H. Challis

:10.1115/1.1894367 10.1115/1.1894367 16060352 23. Challis JH . Accuracy of human limb moment of inertia estimations and their influence on resultant joint moments . J Appl Biomech . 1996 ; 12 : 517 – 530 . doi:10.1123/jab.12.4.517 10.1123/jab.12.4.517 24. Challis JH , Kerwin DG

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Raymond C.Z. Cohen, Paul W. Cleary, Simon M. Harrison, Bruce R. Mason and David L. Pease

The purpose of this study was to determine the pitching effects of buoyancy during all competitive swimming strokes—freestyle, backstroke, butterfly, and breaststroke. Laser body scans of national-level athletes and synchronized multiangle swimming footage were used in a novel markerless motion capture process to produce three-dimensional biomechanical models of the swimming athletes. The deforming surface meshes were then used to calculate swimmer center-of-mass (CoM) positions, center-of-buoyancy (CoB) positions, pitch buoyancy torques, and sagittal plane moments of inertia (MoI) throughout each stroke cycle. In all cases the mean buoyancy torque tended to raise the legs and lower the head; however, during part of the butterfly stroke the instantaneous buoyancy torque had the opposite effect. The swimming strokes that use opposing arm and leg strokes (freestyle and backstroke) had smaller variations in CoM positions, CoB positions, and buoyancy torques. Strokes with synchronized left-right arm and leg movement (butterfly and breaststroke) had larger variations in buoyancy torques, which impacts the swimmer’s ability to maintain a horizontal body pitch for these strokes. The methodology outlined in this paper enables the rotational effects of buoyancy to be better understood by swimmers, allowing better control of streamlined horizontal body positioning during swimming to improve performance.

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Joseph J. Crisco, Michael J. Rainbow, Joel B. Schwartz and Bethany J. Wilcox

The purpose of this study was to examine the batting cage performance of wood and nonwood baseball bats used at the youth level. Three wood and ten nonwood bats were swung by 22 male players (13 to 18 years old) in a batting cage equipped with a 3-dimensional motion capture (300 Hz) system. Batted ball speeds were compared using a one-way ANOVA and bat swing speeds were analyzed as a function of bat moment of inertia by linear regression. Batted ball speeds were significantly faster for three nonwood bat models (P < .001), significantly slower for one nonwood model, and not different for six nonwood bats when compared with wood bats. Bat impact speed significantly (P < .05) decreased with increasing bat moment of inertia for the 13-, 14-, and 15-year-old groups, but not for the other age groups. Ball-bat coefficients of restitution (BBCOR) for all nonwood were greater than for wood, but this factor alone did not correlate with bat performance. Our findings indicate that increases in BBCOR and swing speed were not associated with faster batted ball speeds for the bats studied whose moment of inertia was substantially less than that of a wood bat of similar length.

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Deborah L. King, Allison S. Arnold and Sarah L. Smith

To be competitive internationally, figure skaters today must perform complex athletic skills such as triple axels. However, few skaters are executing such jumps consistently. In this study, a 3D kinematic analysis of five elite male skaters was undertaken to compare characteristics of single, double, and triple axels and to determine which parameters are most critical to completion of the triple axel. Results indicate that skaters increase their number of revolutions by increasing their rotational velocity, not by increasing their time in the air. The study also shows that skaters' triple axels travel horizontally only 70% as far as their single axels, an observation attributable to skaters' greater skid distances, greater takeoff angles, and consequently lower horizontal velocities in their triple axels. It appears that achieving a high rotational velocity by generating angular momentum at takeoff and by minimizing moment of inertia about the spin axis is a key to completing the triple axel.

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Yoshiaki Takei

This study compared the techniques used by elite male gymnasts in performing blocking or pushoff against the horse and postflight in the handspring and salto forward tucked vault. Forty-one American gymnasts were compared with 51 Olympic gymnasts on the mechanical factors governing the blocking and body control for salto forward and kickout in postflight. A 16-mm high-speed camera recorded the performance of the gymnasts during the 1986 USA Gymnastics Championships and the 1988 Olympic Games. The results indicated that Olympic gymnasts assumed the tightest tuck position significantly nearer the peak of the parabolic path of CG and thus achieved significantly greater height of CG at the tightest tuck position during the somersault than did the U.S. gymnasts. The superiority of body control by the former after the tightest tuck to landing was evidenced by significantly longer time, larger horizontal and angular distances, greater average moment of inertia, and smaller angular velocity.

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Robert Tibold, Gabor Fazekas and Jozsef Laczko

A three-dimensional (3-D) arm movement model is presented to simulate kinematic properties and muscle forces in reaching arm movements. Healthy subjects performed reaching movements repetitively either with or without a load in the hand. Joint coordinates were measured. Muscle moment arms, 3-D angular acceleration, and moment of inertias of arm segments were calculated to determine 3-D joint torques. Variances of hand position, arm configuration, and muscle activities were calculated. Ratios of movement variances observed in the two conditions (load versus without load) showed no differences for hand position and arm configuration variances. Virtual muscle force variances for all muscles except deltoid posterior and EMG variances for four muscles increased significantly by moving with the load. The greatly increased variances in muscle activity did not imply equally high increments in kinematic variances. We conclude that enhanced muscle cooperation through synergies helps to stabilize movement at the kinematic level when a load is added.

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Yoshiaki Takei

The purpose of this study was to identify the mechanical factors that are crucial to successful performance of blocking and body control for salto forward and kickout in the handspring and salto forward tucked vault. A 16-mm camera operating at 100 Hz was used to record the performances. The subjects were 51 male gymnasts participating in the 1988 Olympic Games. It was hypothesized that some mechanical factors identified in the model were significantly correlated with successful performance of blocking and body control for salto forward and kickout as rated by the judges. Significant correlations indicated that a large horizontal velocity at touchdown on horse is an important prerequisite for effective blocking and subsequent performance of postflight. The results also indicated that the body’s moment of inertia should be minimized by assuming the full tuck position just before or near the peak of flight, which maximizes the time and height available for executing the kickout in midair. The small angular speed of body rotation due to early kickout and the long-held extended body position as it travels a large horizontal distance are not only effective in the expression of artistic amplitude for virtuosity points but are also crucial for control in landing.

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Rochelle L. Nicholls, Bruce C. Elliott, Karol Miller and Michael Koh

Ball exit velocity (BEV) was measured from 17 experienced baseball hitters using wood and metal bats of similar length and mass but different moments of inertia. This research was conducted in response to safety issues for defensive players related to high BEV from metal baseball bats reported in the literature. Our purpose was to determine whether metal bats, with their lower swing moment of inertia, produce a higher linear bat tip velocity than wooden bats swung by the same players. Analysis using high-speed videography indicated significant differences in the x-component of velocity for both the proximal (metal = 5.4 m s−1; wood = 3.9 m s−1) and distal ends of the bats (metal = 37.2 m s−1; wood = 35.2 m s−1), p < 0.01. The orientation of the bats with respect to the horizontal plane was also significantly more “square” 0.005 s prior to impact (270°) for the metal (264.3°) compared with the wood bat (251.5°), p < 0.01. Mean BEV from metal bats (44.3 m s−1) was higher than the 41 m s−1 velocity which corresponds to the minimum movement time for a pitcher to avoid a ball hit in his direction (Cassidy & Burton, 1989).

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Jason Wicke and Genevieve A. Dumas

Body segment inertial parameters are required as input parameters when the kinetics of human motion is to be analyzed. However, owing to interindividual differences in body composition, noninvasive inertial estimates are problematic. Dual-energy x-ray absorptiometry (DXA) is a relatively new imaging approach that can provide cost- and time-effective means for estimating these parameters with minimal exposure to radiation. With the introduction of a new generation of DXA machines, utilizing a fan-beam configuration, this study examined their accuracy as well as a new interpolative data-reduction process for estimating inertial parameters. Specifically, the inertial estimates of two objects (an ultra-high molecular density plastic rod and an animal specimen) and 50 participants were obtained. Results showed that the fan-beam DXA, along with the new interpolative data-reduction process, provided highly accurate estimates (0.10–0.39%). A greater variance was observed in the center of mass location and moment of inertia estimates, likely as a result of the course end-point location (1.31 cm). However, using a midpoint interpolation of the end-point locations, errors in the estimates were greatly reduced for the center of mass location (0.64–0.92%) and moments of inertia (–0.23 to –0.48%).