Perform kicking with or without jumping: joint coordination and kinetic differences between Taekwondo back kicks and jumping back kicks

We investigated joint coordination differences between Taekwondo back kicks and jumping back kicks, and how jumping (in performing the latter) would alter engaging ground reaction forces (GRF) in executing kicking. Ten skilful athletes volunteered to perform both kinds of kicking within the shortest time for three successful trials. Three high-speed cameras and two force platforms were used for data collection, and the trial with the shortest execution time was selected for analysis. Movements were divided into the rotation and attack phases. With comparable execution time and maximum joint linear/angular speeds, back kicks and jumping back kicks differ mainly in larger GRF in the latter, and in greater target acceleration in the former probably because the support leg prevented athletes’ rebounding after impact. In addition, more prominent antiphase and in-phase coordination between the shoulder segment and knee joint, and elongated rotation phase were found in jumping back kicks. Larger GRF values in jumping back kicks were generated for jump take-off rather than for a more powerful attack. In back kicks although the support leg remained ground contact, greatly decreased GRF in the attack phase suggested that the support leg mainly served as a rotation axis.     

Validation of two accelerometers to determine mechanical loading of physical activities in children

The purpose of this study was to assess the validity of accelerometers using force plates (i.e., ground reaction force (GRF)) during the performance of different tasks of daily physical activity in children. Thirteen children (10.1 (range 5.4–15.7) years, 3 girls) wore two accelerometers (ActiGraph GT3X+ (ACT), GENEA (GEN)) at the hip that provide raw acceleration signals at 100 Hz. Participants completed different tasks (walking, jogging, running, landings from boxes of different height, rope skipping, dancing) on a force plate. GRF was collected for one step per trial (10 trials) for ambulatory movements and for all landings (10 trials), rope skips and dance procedures. Accelerometer outputs as peak loading (g) per activity were averaged. ANOVA, correlation analyses and Bland–Altman plots were computed to determine validity of accelerometers using GRF. There was a main effect of task with increasing acceleration values in tasks with increasing locomotion speed and landing height (P < 0.001). Data from ACT and GEN correlated with GRF (r = 0.90 and 0.89, respectively) and between each other (r = 0.98), but both accelerometers consistently overestimated GRF. The new generation of accelerometer models that allow raw signal detection are reasonably accurate to measure impact loading of bone in children, although they systematically overestimate GRF.     

论辅导员群体习性与职业环境的冲突——辅导员职业动力不足原因分析

年轻、专职、高学历的群体特征及特殊的求职体验造就了辅导员特殊的群体习性,即理想化、高成就动机和强烈的自我证明意识。而辅导员职业场域在高校场域中的边缘位置和场域的不成熟性,决定了多数辅导员既无法凭借本场域的资源向更高权力场域流通,又很难凭借本场域满足自身的习性要求,辅导员职业动力不足恰是上述矛盾关系的反映。     

契约文书中的宗族参与——以江西地区为中心

清代至民国时期,宗族势力参与契约文书最主要、最直接的表现,就是在个人签订各种契约文书的过程中,往往需要得到宗族的认可或批准。在土地等财产的流转过程中,宗族势力或直接成为交易行为的主导者,利用说合中人和亲邻的角色施加影响,参与并直接影响交易过程;或是作为宗族财产的支配者对众存产业进行管理,参与家产的处分;或是作为交易行为的见证者,以见卖人、在场人的身份行使知情权和监督权,对交易过程施以间接影响。     

0Cr18Ni9不锈钢车削力试验研究

切削力是切削加工中的一种伴随现象,切削力的大小直接影响到刀具寿命、加工质量和加工效率。在分析车削力时域特征的基础上,通过0Cr18Ni9不锈钢的正交车削试验,研究了切削参数对切削力波动系数的影响规律。结果表明：动态性是车削力的基本属性,切向分力和进给分力是车削力的主要成分;切削用量三要素中,切削深度ap对进给分力波动系数Qf和切向分力波动系数Qc都有着显著的影响,两者随ap的变化趋势基本一致。     

浅谈平衡力法在物理学中的应用

平衡力是物理学中很重要而且应用又十分广泛的一个知识点,掌握好平衡力,灵活运用平衡力法讲解教学相关内容,解决相关物理题,将会使解题思路清晰,一些不易懂的原理、不好解的题目,会有“柳暗花明又一村”的效果。本文先说明什么是平衡力法,然后解读平衡力这个知识点,最后从教学角度和解题角度说明平衡力法应用的魅力所在,本文开阔视野,拓展思路,将是打开学好物理一扇窗,呈现学好物理一片天地。     

“二力平衡”教学设计

二力平衡是初中学生学习物理时第一次将运动和力联系起来的关键知识点,苏教版中学物理课程标准针对该知识点的要求是院巩固力和力的图示的知识,加深对受力问题的分析的能力,通过课堂互动以及实验探究的方式来掌握平衡概念、二力平衡条件及其应用。     

青少年足球运动员灵敏性结构模型的理论与实证研究简

近十余年来,我国足球运动员灵敏性方面与其他亚洲强队相比差距明显,如何提高灵敏性已经成为足球训练界关注的问题,提高足球运动员的敏捷性对提高足球整体竞技水平具有重要意义。本文运用了文献检索法、数理统计法、实验法等方法。指出足球运动员灵敏性结构模型在动作结构层面表现为“急停一变向一启动”,在力量结构层面表现为“制动力量一反应力量一启动力量”。通过对实验对象进行实验前后的对照,结果证明本研究所提出的足球运动员灵敏性结构模型是准确的,通过发展青少年运动员的制动力量、反应力量、启动力量能够很好的提高其灵敏性。     

中国群众体育参与的影响因素及驱动力研究简

采用主成分分析与因子分析法,对影响群众体育参与的10个因素进行数据矩阵分析。旨在准确把握中国群众体育参与影响因素和特征。分析确定主成分：驱动因子2个。包括：政府支持和体育发展因子、生活水平与文化素质因子。结果表明,政府支持和体育发展是群众体育参与的基础和保障,生活水平与文化素质影响群众对体育参与的规模与主动性。     

上肢超等长肌力练习对男子铅球教学的影响

采用实验对比法,探讨了上肢超等长肌肉力量练习对男子铅球教学的影响。结果发现,上肢超等长肌力练习对提高练习者上肢爆发力,掌握正确的推铅球技术,以及提高运动成绩,有积极的作用。