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John H. Challis

This study examined the influence of force plate targeting, via stride length adjustments, on the magnitude and consistency of ground reaction force and segment angle profiles of the stance phase of human running. Seven male subjects (height, 1.77 m ± 0.081; mass, 72.4 kg ± 7.52; age range, 23 to 32 years) were asked to run at a mean velocity of 3.2 m · s–1 under three conditions (normal, short, and long strides). Four trials were completed for each condition. For each trial, the ground reaction forces were measured and the orientations of the foot, shank, and thigh computed. There were no statistically significant differences (p > .05) between the coefficients of variation of ground reaction force and segment angle profiles under the three conditions, so these profiles were produced consistently. Peak active vertical ground reaction forces, their timings, and segment angles at foot off were not significantly different across conditions. In contrast, significant differences between conditions were found for peak vertical impact forces and their timings, and for the three lower limb segment angles at the start of force plate contact. These results have implications for human gait studies, which require subjects to target the force plate. Targeting may be acceptable depending on the variables to be analyzed.

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John H. Challis

In three-dimensional image-based motion analysis, the direct linear transformation (DLT) is commonly used to measure locations of significant body landmarks. The major drawback of the DLT is that the control points used for calibration must encompass the volume in which the activity occurs. A new procedure is presented where the calibration frame is moved sequentially, permitting calibration of a volume much larger than that encompassed by the calibration frame. A calibration frame with a volume of 0.6 m3 was used to calibrate a volume six times greater, by placing the frame in eight different positions. Reconstruction accuracy was comparable with that for the original frame position. This new multiphase calibration procedure presents the opportunity for calibrating large volumes using a small calibration frame; this may be advantageous, for example, in sporting arenas, where the transportation or manufacture of a sufficiently large calibration frame may be problematic.

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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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John H. Challis

This article presents and evaluates a new procedure that automatically determines the cutoff frequency for the low-pass filtering of biomechanical data. The cutoff frequency was estimated by exploiting the properties of the autocorrelation function of white noise. The new procedure systematically varies the cutoff frequency of a Butterworth filter until the signal representing the difference between the filtered and unfiltered data is the best approximation to white noise as assessed using the autocorrelation function. The procedure was evaluated using signals generated from mathematical functions. Noise was added to these signals so mat they approximated signals arising from me analysis of human movement. The optimal cutoff frequency was computed by finding the cutoff frequency that gave me smallest difference between the estimated and true signal values. The new procedure produced similar cutoff frequencies and root mean square differences to me optimal values, for me zeroth, first and second derivatives of the signals. On the data sets investigated, this new procedure performed very similarly to the generalized cross-validated quintic spline.

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John H. Challis

Repeat measurements were made by 2 operators on a group of 50 physically active subjects (age, 20.7 years ± 1.8; males: height 1.780 m ± 0.043. mass, 78.09 kg ± 9.30; females: height. 1.680 m ± 0.064. mass. 66.67 kg ± 6.67) to determine the precision with which the subjects' limb segment inertial parameters could be estimated. Segmental inertial parameters were determined using 3 techniques. 2 of which involved modeling segments as geometric solids, and a 3rd which used the equations of Zatsiorsky et al. (1990). Precisions were high for all 3 techniques, with little difference between inter- and intra-operator precisions. The lowest precisions were obtained for the hands and feet. For these segments the use of repeat measures to improve precision is recommended. These results imply that with similarly trained measurers, comparison of inertial parameters determined using the same protocol but obtained by different operators is appropriate, and that it is viable to have 2 measurers taking measurements on the same subject to accelerate data collection.

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John H. Challis

Humans of different sizes move in very similar ways despite the size difference. The principles of geometric scaling provide insight into the reasons for the similar movement patterns observed. In human locomotion, body size influences endurance running performance, with shorter body sizes being an advantage due to better heat exchange compared with their taller counterparts. Scaling can also show the equivalence of child gait with that of adults in terms of stride length and walking velocity. In humans, maximum jump height is independent of standing height, a scaling result which has been validated by examining jumps with mass added to the body. Finally, strength scales in proportion to body mass to the two-thirds power, which explains why shorter people have greater relative body strength compared with taller individuals. Geometric scaling reveals the underlying principles of many human movement forms.

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Samantha L. Winter and John H. Challis

For a physiologically realistic joint range of motion and therefore range of muscle fiber lengths, only part of the whole muscle force-length curve can be used in vivo; that is, only a section of the force-length curve is expressed. Previous work has determined that the expressed section of the force-length curve for individual muscles can vary between subjects; however, the degree of intersubject variability is different for different muscles. This study determined the expressed section of both the rectus femoris and gastrocnemius—muscles with very different ratios of tendon slack length to muscle fiber optimum length—for 28 nonspecifically trained subjects to test the hypothesis that the value of this ratio affects the amount of variability in the expressed section. The force-length curves of the two muscles were reconstructed from moment-angle data using the method of Herzog & ter Keurs (1988). There was no relationship between the expressed sections of the force-length curve for the two muscles. Less variability was found in the expressed section of the gastrocnemius compared with the rectus femoris, supporting the hypothesis. The lack of relationship between the expressed sections of the two muscles has implications for motor control and for training muscle for rehabilitation.

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Benjamin W. Infantolino and John H. Challis

The pennated arrangement of muscle fibers has important implications for muscle function in vivo, but complex arrangement of muscle fascicles in whole muscle raises the question whether the arrangement of fascicles produce variations in pennation angle throughout muscle. The purpose of this study was to describe the variability in pennation angle observed throughout the first dorsal interosseous (FDI) muscle using magnetic resonance imaging (MRI). Two cadaveric muscles were scanned in a 14.1 tesla MRI unit. Muscles were divided into slices and pennation angle was measured in the same representative location throughout the muscle in each slice for the medial-lateral and anterior posterior-image planes. Data showed large nonuniform variation in pennation angles throughout the muscles. For example, for cadaver 2, pennation angle in 287 planes along the medial-lateral axis ranged from 3.2° to 22.6°, while for the anterior-posterior axis, in 237 planes it ranged from 3.1° to 24.5°. The nonnormal distribution of pennation angles along each axis suggests a more complex distribution of fascicles than is assumed when a single pennation angle is used to represent an entire muscle. This distribution indicates that a single pennation angle may not accurately describe the arrangement of muscle fascicles in whole muscle.

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Matthew T.G. Pain and John H. Challis

This study had two purposes: to evaluate a new method for measuring segmental dimensions for determining body segment inertial parameters (BSIP), and to evaluate the changes in mass distribution within a limb as a consequence of muscular contraction. BSIP were calculated by obtaining surface data points of the body under investigation using a sonic digitizer, interpolating them into a regular grid, and then using Green’s theorem which relates surface to volume integrals. Four skilled operators measured a test object; the error was approximately 2.5% and repeatability was 1.4% (coefficient of variation) in the determination of BSIP. Six operators took repeat measures on human lower legs; coefficients of variation were typically around 5%, and 3% for the more skilled operators. Location of the center of mass of the lower leg was found to move up 1.7 cm proximally when the triceps surae muscles went from a relaxed state to causing plantar flexion. The force during an impact associated with such motion of the soft tissue of the lower leg was estimated to be up to 300 N. In summary, a new repeatable and accurate method for determining BSIP has been developed, and has been used to evaluate body segment mass redistribution due to muscular contraction.

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Jason G. Gallucci and John H. Challis

This study examined the moment-producing capabilities of the gastrocnemius during isokinetic knee flexion tasks. Nine healthy men were tested using a Biodex isokinetic dynamometer. Each one completed 3 maximum repetitions at 3 angular velocities, 30, 75, and 150º/s, with his ankle braced in either full dorsiflexion or full plantar flexion. A computer model was used to simulate the experimental tasks. Experimentally, the moment produced at the knee joint with the ankle dorsiflexed was significantly higher than the moment with the ankle plantar-flexed at all 3 angular velocities, p < 0.05. This suggests that lengthening the gastrocnemius allowed for greater contribution of the gastrocnemius to the total moment produced at the knee during isokinetic knee flexions. The simulations supported the experimental data and suggested that, with the ankle dorsiflexed, the gastrocnemius acts on a more favorable part of the muscle’s force-length curve compared with the plantar-flexed condition. The results of the experimental work, along with the simulations, demonstrated that lengthening the gastrocnemius significantly increased the moment produced at the knee joint during isokinetic knee flexion tasks. These results have implications for instructions given to persons who perform leg curls for muscle strengthening, and for the design of knee flexion exercise machines.