高水平400m自由泳比赛中“第1次加速”现象的思考

运用文献资料法、数理统计法和逻辑分析法，对近10年国内外优秀400m自由泳运动员共111人次的决赛成绩进行分析，探讨该项目配速方案的总体特征和阶段特征，尤其是“第1次加速”现象，旨在对今后我国该项目训练与配速研究提供理论参考。结论：大多世界级选手途中游段都存在“第1次加速”现象；世界优秀选手的竞技水平与其比赛中该现象的出现概率及提升幅度呈正相关；目前我国优秀选手大赛中暂无“第1次加速”；产生该现象的原因可能包括生理生化、比赛战术、心理状态等3个方面。     

A dynamic model of the head acceleration associated with heading a soccer ball

This study develops a dynamic model of head acceleration, which incorporates physiologically related neck muscle contributions, to further the understanding of the mechanical behaviour of the head-neck system during soccer heading. An inverted pendulum is combined with a linear visco-elastic element to model the head-neck system following a half-sine input force. Model parameter values were varied to obtain agreement with previously published experimental data (Naunheimet al., 2003), and were subsequently compared to literature values. The model predicted the same mechanical angular kinematics as observed experimentally both during and post impact. The greatest acceleration was in the anterior direction at the instant the ball left the head, attributed to the elastic stiffness of the neck musculature. The head-neck stiffness and damping coefficients determined from the model (350 N m rad^-1 and 4 N m s rad^-1, respectively) were similar to those reported elsewhere when subjects were asked to resist maximally. The model may be subsequently used to investigate differences in technique and ability with respect to the salient model parameters to further our biomechanical understanding of soccer heading.     

爆发力的力学分析和训练探讨

根据生物力学、生理学原理,认为爆发力是机体在克服一定阻力的条件下,在极短的时间内发挥出最大的肌张力,是一种力量的加速度,用力的时间梯度加以评定更为合理;并对影响爆发力主导因素进行分析,提出爆发力训练的注意事项,为促进爆发力的训练提供参考依据.     

我国现代职业教育发展的现状及对策

十八大报告明确提出要“加快发展现代职业教育”,本文阐述了什么是加快发展现代职业教育、加快发展现代职业教育的意义、我国职业教育的现状及加快发展的对策。     

用气轨系统与Spss测定重力加速度

使气轨处于水平方向的动态平衡状态下,通过设定系统质量,调整系统在竖直方向上所受的外力,用物理天平称量滑块质量,用智能计时系统测出滑块在水平方向的加速度。由实验原理、不确定度分析,用Spss的线性估计功能分析实验数据,得到系统质量一加速度综合量与竖直方向砝码质量的定标曲线,并验证得系统质量一加速度综合量与竖直方向的砝码质量存在线性关系,由此标定出重力加速度,并用置信概率为95％的不确定度对实验数据及其结果进行分析和评价,得到较为合理的实验结果。     

不确定度理论在重力加速度测量实验中的应用

本文首先对直接测量量和间接测量量的测量不确定度评定方法作了详细介绍,其次对单摆的周期公式作了严格推导,根据多年的不确定度教学实践,并结合实际测量数据,说明了如何应用不确定度理论对通过单摆实验测量重力加速度的实验数据处理方法,最后对实验教学中的测量结果进行了详细的不确定度分析,这不仅便于学生理解和掌握实验数据处理方法,而且更有利于学生提高对大学物理实验的主动性和热情.     

Facilitating Grade Acceleration: Revisiting the Wisdom of John Feldhusen

This article revisits the 1986 Feldhusen, Proctor, and Black recommendations on grade skipping. These recommendations originally appeared as 12 guidelines. In this article, the guidelines are grouped into three general categories: how to screen accelerant candidates, how to engage with the adults in the acceleration process (e.g., teachers, parents), and how to support the accelerated students. The authors then reviews the literature since the publication of Feldhusen et al.'s original article. This body of research includes grade skipping, early entrance to college, and early entrance to school and supports the three general categories. However, some findings provide nuanced changes to the guidelines. The past 25 years of research also presents some additional considerations about acceleration not considered by Feldhusen et al. These topics include gender issues, resistance to acceleration, and methodological concerns.     

Kinematics of table tennis topspin forehands: effects of performance level and ball spin

Abstract

The purpose of this study was to investigate whether performance level and ball spin affect arm and racket kinematics of the table tennis topspin forehand. Nine advanced and eight intermediate male table tennis players hit topspin forehands against light and heavy backspins. Five high-speed video cameras were used to record their strokes at 200 fps. Contributions of joint rotations to the racket speed, the racket kinematics at ball impact, the time required for racket acceleration and the maximum slope of the racket speed-time curve (s _max) were determined. The advanced players showed a significantly larger contribution of lower trunk axial rotation to the racket speed at impact and a significantly larger value of s_max, and tended to require a less time for racket acceleration than the intermediate players. The racket speed at impact was not significantly different between the two player groups. The players adjusted the racket face angle rather than the inclination of the racket path at impact to the different ball spins. The results suggest that the ability to accelerate the racket in less time in the topspin forehand against backspin balls may be an important factor that affects the performance level.     

Peak movement and collision demands of professional rugby league competition

ABSTRACT

To quantify the peak movement and contact demands of National Rugby League (NRL) and European Super League (ESL) competition players were tracked during 10 NRL (166 files) and 10 ESL (143 files) matches using microtechnology devices. The peak 1- to 5-min periods were then calculated for average match speed (m·min^?1), and acceleration (m·s^?2) when 0, 1, 2, and ≥3 collisions per min occurred. Linear mixed effect models and Cohen’s effect size statistic (± 90%CI) were used to determine the differences in movement profiles when collisions occurred. Compared to no collision periods, as frequency of collisions per minute increased, there were progressive reductions in running speed for most positional groups. The addition of 1 or more collisions per min resulted in average effect size reductions in match speed of ?0.14 for NRL forwards, ?0.89 for NRL backs, ?0.48 for ESL forwards, and ?2.41 for ESL backs. ESL forwards had the highest frequency of peak periods involving 3 or more collisions per min, 22% of all periods, followed by NRL forwards (14%), NRL backs (10%) and ESL backs (8%). This study highlights the peak movement and collision demands of professional rugby league competition and allows practitioners to develop training drills that reflect worst case scenarios.     

Comparison of accelerometer cut points to estimate physical activity in US adults

Abstract

The purpose of this study was (1) to describe physical activity prevalence, categorised according to the 2008 Physical Activity Guidelines for Americans (2008 Guidelines), using different accelerometer cut points and (2) to examine physical activity prevalence patterns by reported cut points across selected characteristics. Cut points from 9 studies were used to estimate physical activity prevalence in a national adult sample (n = 6547). Estimates were stratified by validation study activity protocols used to derive cut points – ambulatory (walking/running) and lifestyle activities (e.g. gardening, housework, walking). Results showed that the prevalence of meeting the 2008 Guidelines ranged from 6.3% to 98.3% overall and was lower for cut points derived from ambulatory (median = 11.5%, range = 6.3–27.4%) compared to lifestyle (median = 77.2%, range = 60.6–98.3%) protocols. Prevalence patterns across protocols differed for age, but were similar for other characteristics. In conclusion, prevalence of meeting the 2008 Guidelines varied widely, indicating that choice of cut point had an impact on prevalence. To generate future accelerometer cut points one may consider developing cut points for demographic subgroups using a variety of lifestyle physical activities.