The Politics of School Desegregation in Los Angeles     

Nand E. Hart‐Nibbrig 《Equity & Excellence in Education》2013,46(5-6):17-26

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Foot strike patterns and ground contact times during high-calibre middle-distance races     

Phil Hayes  Nicholas Caplan 《Journal of sports sciences》2013,31(12):1275-1283

Abstract

The aims of this study were to examine ground contact characteristics, their relationship with race performance, and the time course of any changes in ground contact time during competitive 800 m and 1500 m races. Twenty-two seeded, single-sex middle-distance races totalling 181 runners were filmed at a competitive athletics meeting. Races were filmed at 100 Hz. Ground contact time was recorded one step for each athlete, on each lap of their race. Forefoot and midfoot strikers had significantly shorter ground contact times than heel strikers. Forefoot and midfoot strikers had significantly faster average race speed than heel strikers. There were strong large correlations between ground contact time and average race speed for the women's events and men’s 1500 m (r = ?0.521 to ?0.623; P < 0.05), whereas the men's 800 m displayed only a moderate relationship (r = ?0.361; P = 0.002). For each event, ground contact time for the first lap was significantly shorter than for the last lap, which might reflect runners becoming fatigued.  相似文献   

Effect of a Marathon Run on Serum Lipoproteins,Creatine Kinase,and Lactate Dehydrogenase in Recreational Runners     

Philip K. Schot  Barbara A. Hart  Michael Mueller 《Research quarterly for exercise and sport》2013,84(4):450-456

Abstract

The objective of this study was to determine the effect of a marathon run on serum lipid and lipoprotein concentrations and serum muscle enzyme activities and follow their recovery after the run. These blood concentrations were measured before, immediately after, and serially after a marathon run in 15 male recreational runners. The triglyceride level was significantly elevated postrace, then fell 30% below baseline 1 day after the run, and returned to baseline after 1 week. Total cholesterol responded less dramatically but with a similar pattern. High-density lipoprotein cholesterol remained significantly elevated and low-density lipoprotein cholesterol was transiently reduced for 3 days after the run. The total cholesterol/high-density cholesterol ratio was significantly lowered for 3 days. Serum lactate dehydrogenase activity significantly doubled postrace and then declined but remained elevated for 2 weeks. Serum creatine kinase activity peaked 24 hr after the run, with a 15-fold rise, and returned to baseline after 1 week. The rise of these enzymes reflects mechanically damaged muscle cells leaking contents into the interstitial fluid. It is concluded that a prolonged strenuous exercise bout in recreational runners, such as a marathon, produces beneficial changes in lipid blood profiles that are significant for only 3 days. However, muscle damage is also evident for 1 week or more from the dramatic and longlasting effect on enzyme levels. Laboratory values for these runners were outside normal ranges for some days after the race.  相似文献   

A mathematical model of the oar blade – water interaction in rowing     

Nicholas Caplan  Trevor Gardner 《Journal of sports sciences》2013,31(9):1025-1034

Abstract

Our aim was to present a mathematical model of rowing and sculling that allowed for a comparison of oar blade designs. The relative movement between the oar blades and water during the drive phase of the stroke was modelled, and the lift and drag forces generated by this complex interaction were determined. The model was driven by the oar shaft angular velocity about the oarlock in the horizontal plane, and was shown to be valid against measured on-water mean steady-state shell velocity for both a heavyweight men's eight and a lightweight men's single scull. Measured lift and drag force coefficients previously presented by the authors were used as inputs to the model, whichs allowed for the influence of oar blade design on rowing performance to be determined. The commonly used Big Blade, which is curved, and it's flat equivalent were compared, and blade curvature was shown to generate a 1.14% improvement in mean boat velocity, or a 17.1-m lead over 1500 m. With races being won and lost by much smaller margins than this, blade curvature would appear to play a significant role in propulsion.  相似文献   

SCHOOL DELINQUENCY AND SOCIAL BOND FACTORS: EXPLORING GENDERED DIFFERENCES AMONG A NATIONAL SAMPLE OF 10TH GRADERS     

Caroline O. Hart  Christian E. Mueller 《Psychology in the schools》2013,50(2):116-133

School delinquency has been linked to an array of negative educational outcomes, and if left unchecked, may lead to more serious problems in adulthood. Identifying the risk and protective factors that influence school delinquency is therefore crucial to develop effective intervention programs. Utilizing Hirschi's social bond theory as a framework, the authors investigated the relationships between social bonds (i.e., parental involvement, bond to school, beliefs, commitment to sport activities, commitment to non‐sport activities, and involvement) and school delinquency among a nationally representative sample of 10th graders. Special attention was given to gender differences. Results indicate that social bond measures account for a significant variance in school delinquency (11.2%, p < .001) above and beyond that predicted by socioeconomic status (1.5%, p < .001), and that the relationships between social bonds and school delinquency vary across gender. Practical and theoretical implications are for educators and researchers alike.  相似文献   

Cascade optical chromatography for sample fractionation     

Alex Terray  Joseph D. Taylor    Sean J. Hart 《Biomicrofluidics》2009,3(4)

Optical chromatography involves the elegant combination of opposing optical and fluid drag forces on colloidal samples within microfluidic environments to both measure analytical differences and fractionate injected samples. Particles that encounter the focused laser beam are trapped axially along the beam and are pushed upstream from the laser focal point to rest at a point where the optical and fluid forces on the particle balance. In our recent devices particles are pushed into a region of lower microfluidic flow, where they can be retained and fractionated. Because optical and fluid forces on a particle are sensitive to differences in the physical and chemical properties of a sample, separations are possible. An optical chromatography beam focused to completely fill a fluid channel is operated as an optically tunable filter for the separation of inorganic, polymeric, and biological particle samples. We demonstrate this technique coupled with an advanced microfluidic platform and show how it can be used as an effective method to fractionate particles from an injected multicomponent sample. Our advanced three-stage microfluidic design accommodates three lasers simultaneously to effectively create a sequential cascade optical chromatographic separation system.  相似文献   

Single view silhouette fitting techniques for estimating tennis racket position     

Nathan Elliott  Simon Choppin  Simon Goodwill  Terry Senior  John Hart  Tom Allen 《Sports Engineering》2018,21(2):137-147

Stereo camera systems have been used to track markers attached to a racket, allowing its position to be obtained in three-dimensional (3D) space. Typically, markers are manually selected on the image plane, but this can be time-consuming. A markerless system based on one stationary camera estimating 3D racket position data is desirable for research and play. The markerless method presented in this paper relies on a set of racket silhouette views in a common reference frame captured with a calibrated camera and a silhouette of a racket captured with a camera whose relative pose is outside the common reference frame. The aim of this paper is to provide validation of these single view fitting techniques to estimate the pose of a tennis racket. This includes the development of a calibration method to provide the relative pose of a stationary camera with respect to a racket. Mean static racket position was reconstructed to within ±2 mm. Computer generated camera poses and silhouette views of a full size racket model were used to demonstrate the potential of the method to estimate 3D racket position during a simplified serve scenario. From a camera distance of 14 m, 3D racket position was estimated providing a spatial accuracy of 1.9 ± 0.14 mm, similar to recent 3D video marker tracking studies of tennis.  相似文献   

Lower extremity joint coupling variability during gait in young adults with and without chronic ankle instability     

Thomas Lilley  Joseph Hart  Jay Hertel 《Sports biomechanics / International Society of Biomechanics in Sports》2018,17(2):261-272

Chronic ankle instability (CAI) is a condition resulting from a lateral ankle sprain. Shank-rearfoot joint-coupling variability differences have been found in CAI patients; however, joint-coupling variability (VCV) of the ankle and proximal joints has not been explored. Our purpose was to analyse VCV in adults with and without CAI during gait. Four joint-coupling pairs were analysed: knee sagittal-ankle sagittal, knee sagittal-ankle frontal, hip frontal-ankle sagittal and hip frontal-ankle frontal. Twenty-seven adults participated (CAI:n = 13, Control:n = 14). Lower extremity kinematics were collected during walking (4.83 km/h) and jogging (9.66 km/h). Vector-coding was used to assess the stride-to-stride variability of four coupling pairs. During walking, CAI patients exhibited higher VCV than healthy controls for knee sagittal-ankle frontal in latter parts of stance thru mid-swing. When jogging, CAI patients demonstrated lower VCV with specific differences occurring across various intervals of gait. The increased knee sagittal-ankle frontal VCV in CAI patients during walking may indicate an adaptation to deal with the previously identified decrease in variability in transverse plane shank and frontal plane rearfoot coupling during walking; while the decreased ankle-knee and ankle-hip VCV identified in CAI patients during jogging may represent a more rigid, less adaptable sensorimotor system ambulating at a faster speed.  相似文献   

计算机基础教学的改革势在必行     

韩卫  张符志  徐玉辰 《教学研究(河北)》1998,(2)

通过分析我校94级、95级和96级三届学生参加河北省计算机等级考试(一级)的考试成绩,对学校计算机基础教学工作的效果和质量进行全面的总结和定位,在肯定已经取得成绩的基础上,指出目前教学工作中迫切需要解决的问题,以及解决这些问题应该采取的方法。  相似文献   

Survivors: How to keep your best people on board after downsizing     

Gayle Caplan  Mary Teese  Judith Christofferson 《Performance Improvement》2002,41(2):44-47

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