平面Bonnesen等周不等式的进一步加强

设欧氏平面R2中域D的面积为A,周长为L,r及R分别为D的最大内接圆半径及最小外接圆半径。利用参考文献中和分几何方法,给出了平面Bonnesen等周不等式的进一步加强,证明了L2-4πA≥π2(R-r)2(πR+πr-L)2.     

不同水平网球选手正手击球技术运动学研究

摘要：在网球的基本技术中,正手被视为发球后最重要的一项技术,本研究主要运用文献资料、测量等研究方法针对国内12名男子网球选手正手击球技术的特征展开研究,重点了解与击球技术动作相关的关节角度,线速度和角速度等运动学特征,旨在揭示不同水平选手技术动作的差异,并进一步认识影响正手击球速度的核心要素。结果表明与一般水平选手相比,优秀选手在正手击球瞬间表现出更快的拍头水平速度和更快的肩部水平速度,但在躯干转动的角速度上却不存在显著性差异。在不考虑组别的情况下,击球时过程中选手的肩、髋、球拍的转动角差异很大,说明考虑标准化的技术动作并没有实际意义。     

Magnitude of multiplanar breast kinematics differs depending upon run distance

Recommendations for breast support, dynamic breast pain assessment, and implications for sports performance have been made within breast biomechanics research; however, these studies have been based upon short exercise protocols (2–5 min). The aim of this study was to investigate the effect of breast support on multiplanar breast kinematics over a 5-kilometre run. Ten female participants (34D or 32DD) conducted two 5-kilometre runs, in a low and high breast support. Relative multiplanar breast kinematics were averaged over five gait cycles at six intervals of a 5-kilometre run. Increases in multiplanar breast kinematics were reported from the start to the end of the run, with the greatest rate of increase in breast kinematics reported within the first two kilometres of running. The greatest relative increases in breast range of motion (34%), velocity (33%), and acceleration (41%) were reported in the superioinferior direction at the fifth kilometre (33 min of running) in the high breast support. Key findings suggest that the run distance, and therefore run duration, employed for both fundamental research and product validation protocols should be carefully considered and it is suggested that running protocols for assessing breast biomechanics should exceed 7 min.