Muscle fatigue induced by exercise simulating the work rate of competitive soccer

Fatigue represents a reduction in the capability of muscle to generate force. The aim of the present study was to establish the effects of exercise that simulates the work rate of competitive soccer players on the strength of the knee extensors and knee flexors. Thirteen amateur soccer players (age 23.3+/-3.9 years, height 1.78+/-0.05 m, body mass 74.8+/-3.6 kg; mean+/-s) were tested during the 2000-2001 soccer season. Muscle strength of the quadriceps and hamstrings was measured on an isokinetic dynamometer. A 90 min soccer-specific intermittent exercise protocol, incorporating a 15 min half-time intermission, was developed to provide fatiguing exercise corresponding in work rate to a game of soccer. The exercise protocol, performed on a programmable motorized treadmill, consisted of the different intensities observed during soccer match-play (e.g. walking, jogging, running, sprinting). Muscle strength was assessed before exercise, at half-time and immediately after exercise. A repeated-measures analysis of variance showed significant reductions (P < 0.001) in peak torque for both the quadriceps and hamstrings at all angular velocities (concentric: 1.05, 2.09, 5.23 rad x s(-1); eccentric: 2.09 rad x s(-1)). The peak torque of the knee extensors (KE) and knee flexors (KF) was greater before exercise [KE: 232+/-37, 182+/-34, 129+/-27, 219+/-41 N x m at 1.05, 2.09 and 5.23 rad x s(-1) (concentric) and 2.09 rad x s(-1) (eccentric), respectively; KF: 126+/-20, 112+/-19, 101+/-16, 137+/-23 N x m] than at half-time (KE: 209+/-45, 177+/-35, 125+/-36, 214+/-43 N x m; KF: 114+/-31, 102+/-20, 92+/-15, 125+/-25 N x m) and greater at half-time than after exercise (KE: 196+/-43, 167+/-35, 118+/-24, 204+/-43 N x m; KF: 104+/-25, 95+/-21, 87+/-13, 114+/-27 N x m). For the hamstrings:quadriceps ratio, significant changes were found (P < 0.05) for both legs, the ratio being greater before than after exercise. For fast:slow speed and left:right ratios, no significant changes were found. We conclude that there is a progressive reduction in muscle strength that applies across a range of functional characteristics during exercise that mimics the work rate in soccer.     

美国数学教学问题一束

众所周知,当前美国数学教学的主要趋势是“问题解决”(Problem Solving)。这里的一些“问题”在美国是比较常见的。有些在国内也经常碰到,但课堂上用得不多,从培养学生的创造性数学思维来看,这些问题会具有一定的价值。——编者     

摄影和摄象技术三维直接线性变换(DLT)再现精度的比较

记录人体运动的准确性在很大程度上取决于记录方法的精确度。计算出的物体运动学的数值,主要依赖对物体运动记录的准确性。人体运动的生物力学分析经常需要对16mm摄影或摄象技术记录的内容进行分析.由于摄象具有成本低,使用方便,处理时间短的特点,使摄象技术比摄影更具吸引力.然而必须考虑摄象精确度是否与摄影标准相同.要使摄象技术合法地进入人体运动分析领域,摄象记录的精度必须达到摄影记录的     

Red pigment production by Penicillium purpurogenum GH2 is influenced by pH and temperature

The combined effects of pH and temperature on red pigment production and fungal morphology were evaluated in a submerged culture of Penicillium purpurogenum GH2, using Czapek-Dox media with d-xylose as a carbon source. An experimental design with a factorial fix was used: three pH values (5, 7, and 9) and two temperature levels (24 and 34 °C) were evaluated. The highest production of red pigment (2.46 g/L) was reached with a pH value of 5 and a temperature of 24 °C. Biomass and red pigment production were not directly associated. This study demonstrates that P. purpurogenum GH2 produces a pigment of potential interest to the food industry. It also shows the feasibility of producing and obtaining natural water-soluble pigments for potential use in food industries. A strong combined effect (p<0.05) of pH and temperature was associated with maximal red pigment production (2.46 g/L).     

Ds-echinoside A, a new triterpene glycoside derived from sea cucumber, exhibits antimetastatic activity via the inhibition of NF-κB-dependent MMP-9 and VEGF expressions

Ds-echinoside A (DSEA), a non-sulfated triterpene glycoside, was isolated from the sea cucumber Pearsonothuria graeffei. In vitro and in vivo investigations were conducted on the effects of DSEA on tumor cell adhesion, migration, invasion, and angiogenesis. In this study, we found that DSEA inhibited the proliferation of human hepatocellular liver carcinoma cells Hep G2, with a half-maximal inhibitory concentration (IC₅₀) of 2.65 μmol/L, and suppressed Hep G2 cell adhesion, migration, and invasion in a dose-dependent manner. DSEA also reduced tube formation of human endothelial cells ECV-304 on matrigel in vitro and attenuated neovascularization in the chick embryo chorioallantoic membrane (CAM) assay in vivo. Immunocytochemical analysis revealed that DSEA significantly decreased the expression of matrix metalloproteinase-9 (MMP-9), which plays an important role in the degradation of basement membrane in tumor metastasis and angiogenesis. DSEA also increased the protein expression level of tissue inhibitor of metalloproteinase-1 (TIMP-1), an important regulator of MMP-9 activation. From the results of Western blotting, the expressions of nuclear factor-kappa B (NF-κB) and vascular endothelial growth factor (VEGF) were found to be remarkably reduced by DSEA. These findings suggest that DSEA exhibits a significant antimetastatic activity through the specific inhibition of NF-κB-dependent MMP-9 and VEGF expressions.     

Catch a falling apple: Isaac Newton and myths of genius

Newton has become a legendary figure belonging to the distant past rather than a historical person who lived at a specific time. Historians and scientists have constantly reinterpreted many anecdotal tales describing Newton's achievements and behaviour, but the most famous concerns the falling apple in his country garden. Newton's apple conjures up multiple allegorical resonances, and examining its historical accuracy is less important than uncovering the mythical truths embedded within this symbol. Because interest groups fashion different collective versions of the past, analysing mythical tales can reveal fundamental yet conflicting attitudes towards science and its practices.     

把握当下:为学术图书馆的未来而定义北大核心CSSCI

图书馆员如果想依然守护和传承一代又一代人的文化成就,就应当肩负起未来信息收藏和提供获取途径的责任。本文探讨学术图书馆作为学术机构教、学、研重要组成部分所面临的种种挑战,并重点谈及图书馆正在经历的急速变化,以及面对这些变化,大型学术图书馆将如何与院系合作共同促进信息的获取,从而激发学术的创新和发展。