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In Men's Artistic Gymnastics the current trend in elite high bar dismounts is to perform two somersaults in an extended body shape with a number of twists. Two techniques have been identified in the backward giant circles leading up to release for these dismounts (J. Biomech. 32 (1999) 811). At the Sydney 2000 Olympic Games 95% of gymnasts used the "scooped" backward giant circle technique rather than the "traditional" technique. It was speculated that the advantage gained from the scooped technique was an increased margin for error when releasing the high bar. A four segment planar simulation model of the gymnast and high bar was used to determine the margin for error when releasing the bar in performances at the Sydney 2000 Olympic Games. The eight high bar finalists and the three gymnasts who used the traditional backward giant circle technique were chosen for analysis. Model parameters were optimised to obtain a close match between simulated and actual performances in terms of rotation angle (1.2 degrees ), bar displacements (0.014 m) and release velocities (2%). Each matching simulation was used to determine the time window around the actual point of release for which the model had appropriate release parameters to complete the dismount successfully. The scooped backward giant circle technique resulted in a greater margin for error (release window 88-157 ms) when releasing the bar compared to the traditional technique (release window 73-84 ms). 相似文献
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Sodium channel gating: no margin for error 总被引:2,自引:0,他引:2
Voltage-gated Na(+) channels are the workhorses of spike generation and propagation in excitable cells. Mutations in Na(+) channel genes have been identified in disorders causing episodic dysfunction of heart, skeletal muscle, and brain. Lossin and colleagues from Al George's lab report in this issue of Neuron that three missense mutations of SCN1A found in a dominant epilepsy syndrome disrupt inactivation, thereby producing small persistent inward Na(+) currents that may result in hyperexcitability and seizures. 相似文献
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The release window for a given dismount from the asymmetric bars is the period of time within which release results in a successful dismount. Larger release windows are likely to be associated with more consistent performance because they allow a greater margin for error in timing the release. A computer simulation model was used to investigate optimum technique for maximizing release windows in asymmetric bars dismounts. The model comprised four rigid segments with the elastic properties of the gymnast and bar modeled using damped linear springs. Model parameters were optimized to obtain a close match between simulated and actual performances of three gymnasts in terms of rotation angle (1.5 degrees ), bar displacement (0.014 m), and release velocities (<1%). Three optimizations to maximize the release window were carried out for each gymnast involving no perturbations, 10-ms perturbations, and 20-ms perturbations in the timing of the shoulder and hip joint movements preceding release. It was found that the optimizations robust to 20-ms perturbations produced release windows similar to those of the actual performances whereas the windows for the unperturbed optimizations were up to twice as large. It is concluded that robustness considerations must be included in optimization studies in order to obtain realistic results and that elite performances are likely to be robust to timing perturbations of the order of 20 ms. 相似文献
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Background
Cross-validation (CV) is an effective method for estimating the prediction error of a classifier. Some recent articles have proposed methods for optimizing classifiers by choosing classifier parameter values that minimize the CV error estimate. We have evaluated the validity of using the CV error estimate of the optimized classifier as an estimate of the true error expected on independent data. 相似文献5.
Osteocytes are released from the osteocytic lacunae when osteoclasts resorb the bone matrix during bone modeling and remodeling. It remains unknown how osteoclasts react when releasing osteocytes during bone modeling, and the fate of these released osteocytes is also unclear. Femoral mid-shafts of 2-day-old kittens were sectioned into serial 0.5 microm-thick semithin or 0.1 microm-thick ultrathin sections, and examined by light microscopy (LM) and transmission electron microscopy (TEM). The sections showed many osteoclasts at the endosteum but there were no osteoblasts. There were many half-released, fully released, half-exposed, and fully exposed osteocytes on the bone surfaces. Many cell-like structures were seen in the cell bodies of osteoclasts by LM, and some semithin sections were re-sectioned into ultrathin sections for re-observation by TEM. By TEM, these were determinated to be mononuclear cells. The serial ultrathin sections showed that the mononuclear cells appeared to be engulfed in osteoclasts on one section but that the cell was connected with the bone surface of the osteocytic lacuna on another section. These results show that the mononuclear cells in the osteoclasts were osteocytes. The present study suggests that osteoclasts engulf some osteocytes but do not engulf others when releasing osteocytes during bone modeling. 相似文献
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Zhou Y Atkins JB Rompani SB Bancescu DL Petersen PH Tang H Zou K Stewart SB Zhong W 《Cell》2007,129(1):163-178
Mammalian neural progenitor cells divide asymmetrically to self-renew and produce a neuron by segregating cytosolic Numb proteins primarily to one daughter cell. Numb signaling specifies progenitor over neuronal fates but, paradoxically, also promotes neuronal differentiation. Here we report that ACBD3 is a Numb partner in cell-fate specification. ACBD3 and Numb proteins interact through an essential Numb domain, and the respective loss- and gain-of-function mutant mice share phenotypic similarities. Interestingly, ACBD3 associates with the Golgi apparatus in neurons and interphase progenitor cells but becomes cytosolic after Golgi fragmentation during mitosis, when Numb activity is needed to distinguish the two daughter cells. Accordingly, cytosolic ACBD3 can act synergistically with Numb to specify cell fates, and its continuing presence during the progenitor cell cycle inhibits neuron production. We propose that Golgi fragmentation and reconstitution during cell cycle differentially regulate Numb signaling through changes in ACBD3 subcellular distribution and represent a mechanism for coupling cell-fate specification and cell-cycle progression. 相似文献
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Small area estimation methods typically combine direct estimatesfrom a survey with predictions from a model in order to obtainestimates of population quantities with reduced mean squarederror. When the auxiliary information used in the model is measuredwith error, using a small area estimator such as the Fay–Herriotestimator while ignoring measurement error may be worse thansimply using the direct estimator. We propose a new small areaestimator that accounts for sampling variability in the auxiliaryinformation, and derive its properties, in particular showingthat it is approximately unbiased. The estimator is appliedto predict quantities measured in the U.S. National Health andNutrition Examination Survey, with auxiliary information fromthe U.S. National Health Interview Survey. 相似文献
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Food records, including 24-hour recalls and diet diaries, are considered to provide generally superior measures of long-term dietary intake relative to questionnaire-based methods. Despite the expense of processing food records, they are increasingly used as the main dietary measurement in nutritional epidemiology, in particular in sub-studies nested within prospective cohorts. Food records are, however, subject to excess reports of zero intake. Measurement error is a serious problem in nutritional epidemiology because of the lack of gold standard measurements and results in biased estimated diet-disease associations. In this paper, a 3-part measurement error model, which we call the never and episodic consumers (NEC) model, is outlined for food records. It allows for both real zeros, due to never consumers, and excess zeros, due to episodic consumers (EC). Repeated measurements are required for some study participants to fit the model. Simulation studies are used to compare the results from using the proposed model to correct for measurement error with the results from 3 alternative approaches: a crude approach using the mean of repeated food record measurements as the exposure, a linear regression calibration (RC) approach, and an EC model which does not allow real zeros. The crude approach results in badly attenuated odds ratio estimates, except in the unlikely situation in which a large number of repeat measurements is available for all participants. Where repeat measurements are available for all participants, the 3 correction methods perform equally well. However, when only a subset of the study population has repeat measurements, the NEC model appears to provide the best method for correcting for measurement error, with the 2 alternative correction methods, in particular the linear RC approach, resulting in greater bias and loss of coverage. The NEC model is extended to include adjustment for measurements from food frequency questionnaires, enabling better estimation of the proportion of never consumers when the number of repeat measurements is small. The methods are applied to 7-day diary measurements of alcohol intake in the EPIC-Norfolk study. 相似文献
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Synopsis Fish production estimates determined from weights back-calculated to the time of annulus formation were found to underestimate actual production by 12 to 44%. It is suggested that production should either be estimated from weights at the time of collection or a correction factor should be employed when back-calculating weights. 相似文献
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Integrated palaeoecological evidence for biodiversity at the floodplain-forest margin 总被引:2,自引:0,他引:2
This paper presents the results of a combined palynological and palaeoentomological investigation into the changing diversity of Holocene floodplain forest biota at Bole Ings in the lower reaches of the River Trent. The aim in combining the two techniques represents an attempt to overcome some of the inherent differences of scale and resolution which characterize different types of fossil species data. The two lines of ecofactual evidence, pollen and beetle remains indicate changes in the biodiversity of the floodplain, both in terms of the diversity and abundance of individual species, habitats and ecosystem structure. The results of the study demonstrate the potential of this approach in tracing the decline in plant and entomofaunal diversity at the floodplain forest margin. Although there are recognized limitations imposed by taphonomic uncertainties, taxonomic resolution and restricted ecological data, the combined results provide added details of variations in both species composition and structural diversity within the landscape. The results emphasize the need for multidisciplinary research design in palaeoecological investigations of biodiversity. 相似文献
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Hendrik Schmidt Tobias Alber Tim Wehner Robert Blakytny Hans-Joachim Wilke 《Journal of biomechanics》2009,42(12):1926-1934
Mesh convergence tests are often insufficiently performed in finite element analyses. There are many parameters which may have an effect on the mesh convergence behavior. The aim of this study was to identify the influence of different parameters on the mesh convergence behavior.For this purpose we used a simplified axis-symmetrical model of a single pedicle screw flank with surrounding bone to simulate a pull-out test. In parameter studies, the flank radii and the contact conditions at the bone–screw interface were varied. These parameter studies were carried out using an implicit and explicit solver. Thereby, the convergence criteria and the number of the substeps for the implicit nonlinear iteration process as well as the velocity and the material density for the explicit approach were considered.The mesh convergence behavior was influenced by varying the flank radii and the contact conditions. The implicit calculations led to a reaction force, which converged rapidly to a certain value with increasing mesh density, whereas the maximum von-Mises stress showed substantial convergence problems. The number of substeps and the convergence criteria of the iteration process strongly influenced the implicit solutions. In contrast, the maximum von-Mises stresses resulting from explicit calculations converged to a certain value after only a few refinement steps. Different pull-out velocities substantially affected the mesh convergence behavior, while the material density showed only a negligible influence.The results indicated the need to perform an appropriate mesh convergence test when using finite element methods. We were able to show that different parameters strongly influence the mesh convergence behavior and we demonstrated that convergence tests do not always lead to a satisfactory or acceptable solution. 相似文献
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A theoretical analysis is made of the evolution of behavioural strategies in contest situations. It is assumed that behaviour will evolve so as to maximize individual fitness. If so, a population will evolve an ‘evolutionarily stable strategy’, or ESS, which can be defined as a strategy such that, if all members of a population adopt it, no ‘mutant’ strategy can do better. A number of simple models of contest situations are analysed from this point of view. It is concluded that in ‘symmetric’ contests the ESS is likely to be a ‘mixed’ strategy; that is, either the population will be genetically polymorphic or individuals will be behaviourally variable. Most real contests are probably asymmetric, either in pay-off to the contestants, or in size or weapons, or in some ‘uncorrelated’ fashion; i.e. in a fashion which does not substantially bias either the pay-offs or the likely outcome of an escalated contest. An example of an uncorrelated asymmetry is that between the ‘discoverer’ of a resource and a ‘late-comer’. It is shown that the ESS in asymmetric contests will usually be to permit the asymmetric cue to settle the contest without escalation. Escalated contests will, however, occur if information to the contestants about the asymmetry is imperfect. 相似文献
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Background
Feature gene extraction is a fundamental issue in microarray-based biomarker discovery. It is normally treated as an optimization problem of finding the best predictive feature genes that can effectively and stably discriminate distinct types of disease conditions, e.g. tumors and normals. Since gene microarray data normally involves thousands of genes at, tens or hundreds of samples, the gene extraction process may fall into local optimums if the gene set is optimized according to the maximization of classification accuracy of the classifier built from it. 相似文献17.
Growth bands and fault bars, widespread features of feathers that form during regeneration, have largely been studied independently. Growth bands result from normal regeneration: each pair of dark/light bands forms every 24 h. Fault bars are a response to stress during regeneration, creating a translucent line that can break the feather. We studied the relative position and width of these two structures in feathers of nestling and adult white storks Ciconia ciconia. We first confirmed that one growth band represents 24 h of feather regeneration. Fault bars did not occur at random within growth bands: 65.7% (in nestlings) and 45.6% (in adults) of them occurred in one out of six defined segments within a growth band, namely that segment corresponding to the first one‐third of night time hours. The width of fault bars relative to growth bands suggested that fault bars were produced during a median (range) of 7.0 h (2.7–27.0) in nestlings and 3.7 h (1.8–7.9) in adults. Fault bars were concentrated at feather tips in nestlings, but at central locations in adult feathers. Our results suggest that, in general, fault bars are a discrete event of a finite duration occurring mainly during the night (particularly in nestlings). This, along with current knowledge, suggests that acute stressors, rather than chronic ones, are responsible for fault bar formation. Thus, such acute punctual stressors (a matter of minutes) can have long‐lasting (months?years) physiological effects due to the wing load increase from feather breakage caused by fault bars. 相似文献
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