In this study,the wave propagation properties of lattice metamaterials with Koch fractal structures are investigated in terms of band structures and directional wave propagation.The analytical models of lattice metama...In this study,the wave propagation properties of lattice metamaterials with Koch fractal structures are investigated in terms of band structures and directional wave propagation.The analytical models of lattice metamaterials are established using the finite element method,and the dispersion relation is solved using the Bloch’s theorem.The band structures of the lattice metamaterials with different numbers of iterations are studied,and the group velocities at a selected frequency are calculated to analyze the directional wave propagation characteristics.Furthermore,dynamic responses of the finite structures are calculated using commercial finite element software to verify the band gaps and directional wave propagation behaviors in the lattice metamaterials.The results show that multiple and low band gaps are present in the lattice materials with various geometric parameters of the Koch fractal,and the position of the lowest band gap decreases as the number of iterations increases.The results indicate the potential applications of lattice metamaterials with Koch fractals for vibration isolation and multi-functional design.展开更多
The propagation characteristics of flexural waves in two-dimensional thin-plate phononic crystals （PCs） are analysed with the plane wave expansion （PWE） method to yield phase constant surfaces, which predict high ...The propagation characteristics of flexural waves in two-dimensional thin-plate phononic crystals （PCs） are analysed with the plane wave expansion （PWE） method to yield phase constant surfaces, which predict high directivity of flexural wave propagation for certain frequencies outside the band gap. The prediction is validated through the computation of the harmonic responses of a finite structure with 9 × 9 unit cells. The results indicate that directional propagation of flexural waves is an while specific effects of the directional propagation in inherent characteristic of two-dimensional thin-plate PCs a finite structure vary with the positions of excitations.展开更多
目的系统评价弹力带抗阻运动改善冠心病患者心肺功能、运动能力的效果。方法计算机检索PubMed、Web of Science、Cochrane Library、中国知网、维普网、万方、中国生物医学数据库关于以弹力带进行抗阻运动对改善冠心病患者心肺功能、运...目的系统评价弹力带抗阻运动改善冠心病患者心肺功能、运动能力的效果。方法计算机检索PubMed、Web of Science、Cochrane Library、中国知网、维普网、万方、中国生物医学数据库关于以弹力带进行抗阻运动对改善冠心病患者心肺功能、运动能力的研究,检索日期自建库至2020年10月。由两位研究者独立进行质量评价与资料提取,使用RevMan 5.3软件进行meta分析。根据运动干预措施分为弹力带抗阻运动组和对照组。结果共纳入12篇随机对照试验。与对照组比较,弹力带抗阻运动组患者的峰值摄氧量[MD=4.01,95%CI(2.62,5.41),P<0.00001]、无氧阈[MD=2.35,95%CI(2.01,2.69),P<0.00001]、左心室射血分数[MD=8.35,95%CI(5.42,11.28),P<0.00001]明显提高,6分钟步行试验距离明显延长[MD=59.09,95%CI(32.52,85.66),P<0.0001]。结论有弹力带抗阻运动参与的心脏康复较其他运动方式在改善冠心病患者心肺功能、运动能力的康复治疗上具有更显著的效果。展开更多
目的:探讨4周可变阻力训练在复合式训练中对大学生篮球运动员深蹲最大力量和爆发性运动表现的干预效果。方法:招募23名身体健康的男性篮球专项大学生,随机分为实验组和对照组,训练干预4周,每周2次。训练内容包括深蹲、跳深、下蹲跳、立...目的:探讨4周可变阻力训练在复合式训练中对大学生篮球运动员深蹲最大力量和爆发性运动表现的干预效果。方法:招募23名身体健康的男性篮球专项大学生,随机分为实验组和对照组,训练干预4周,每周2次。训练内容包括深蹲、跳深、下蹲跳、立定跳远和20 m冲刺,深蹲强度为80%~85%1RM。实验组和对照组的训练内容和负荷量均相同。实验组采用占总强度40%的弹力带负荷取代部分自由重量。比较4周训练前后身体成分、深蹲最大力量、下蹲跳、蹲跳、立定跳远、20 m冲刺的变化。结果:4周训练后,①实验组和对照组的体重、去脂体重以及体脂率均未出现显著性差异(P>0.05);②深蹲相对力量存在组别和时间的交互效应(VRT vs CRT:+18.99%vs+14.38%,P=0.019,η^(2)=0.247),实验组和对照组的深蹲最大力量(P<0.001,效应量(ES)=0.82-0.88)和相对力量(P<0.001,ES=0.98-1.05)均出现了极其可能的中等程度提高;③蹲跳高度的组别和时间交互效应趋向显著(VRT vs CRT:+6.43%vs+0.58%,P=0.052,η^(2)=0.177),实验组的蹲跳高度相比对照组出现了非常可能的中等程度提高(P=0.013,ES=1.16);下蹲跳高度仅实验组出现了可能的小程度提高(P=0.007,ES=0.33);④实验组和对照组的立定跳远与20 m冲刺表现均未出现显著性差异(P>0.05)。结论:与恒定阻力训练相比,4周可变阻力训练对深蹲相对力量表现的提升效果更明显,且运用复合式训练方式能更有效地提高蹲跳高度。展开更多
基金Funding for this work has been provided by the National Natural Science Foundation of China(Nos.11872313 and 11502202)National Key R&D Program of China(2017YFB1102801)Fundamental Research Funds for the Central Universities and Seed Foundation of Innovation and Creation for Graduate Students in Northwestern Polytechnical University(CX2020107).
文摘In this study,the wave propagation properties of lattice metamaterials with Koch fractal structures are investigated in terms of band structures and directional wave propagation.The analytical models of lattice metamaterials are established using the finite element method,and the dispersion relation is solved using the Bloch’s theorem.The band structures of the lattice metamaterials with different numbers of iterations are studied,and the group velocities at a selected frequency are calculated to analyze the directional wave propagation characteristics.Furthermore,dynamic responses of the finite structures are calculated using commercial finite element software to verify the band gaps and directional wave propagation behaviors in the lattice metamaterials.The results show that multiple and low band gaps are present in the lattice materials with various geometric parameters of the Koch fractal,and the position of the lowest band gap decreases as the number of iterations increases.The results indicate the potential applications of lattice metamaterials with Koch fractals for vibration isolation and multi-functional design.
文摘The propagation characteristics of flexural waves in two-dimensional thin-plate phononic crystals （PCs） are analysed with the plane wave expansion （PWE） method to yield phase constant surfaces, which predict high directivity of flexural wave propagation for certain frequencies outside the band gap. The prediction is validated through the computation of the harmonic responses of a finite structure with 9 × 9 unit cells. The results indicate that directional propagation of flexural waves is an while specific effects of the directional propagation in inherent characteristic of two-dimensional thin-plate PCs a finite structure vary with the positions of excitations.
文摘目的系统评价弹力带抗阻运动改善冠心病患者心肺功能、运动能力的效果。方法计算机检索PubMed、Web of Science、Cochrane Library、中国知网、维普网、万方、中国生物医学数据库关于以弹力带进行抗阻运动对改善冠心病患者心肺功能、运动能力的研究,检索日期自建库至2020年10月。由两位研究者独立进行质量评价与资料提取,使用RevMan 5.3软件进行meta分析。根据运动干预措施分为弹力带抗阻运动组和对照组。结果共纳入12篇随机对照试验。与对照组比较,弹力带抗阻运动组患者的峰值摄氧量[MD=4.01,95%CI(2.62,5.41),P<0.00001]、无氧阈[MD=2.35,95%CI(2.01,2.69),P<0.00001]、左心室射血分数[MD=8.35,95%CI(5.42,11.28),P<0.00001]明显提高,6分钟步行试验距离明显延长[MD=59.09,95%CI(32.52,85.66),P<0.0001]。结论有弹力带抗阻运动参与的心脏康复较其他运动方式在改善冠心病患者心肺功能、运动能力的康复治疗上具有更显著的效果。
文摘目的:探讨4周可变阻力训练在复合式训练中对大学生篮球运动员深蹲最大力量和爆发性运动表现的干预效果。方法:招募23名身体健康的男性篮球专项大学生,随机分为实验组和对照组,训练干预4周,每周2次。训练内容包括深蹲、跳深、下蹲跳、立定跳远和20 m冲刺,深蹲强度为80%~85%1RM。实验组和对照组的训练内容和负荷量均相同。实验组采用占总强度40%的弹力带负荷取代部分自由重量。比较4周训练前后身体成分、深蹲最大力量、下蹲跳、蹲跳、立定跳远、20 m冲刺的变化。结果:4周训练后,①实验组和对照组的体重、去脂体重以及体脂率均未出现显著性差异(P>0.05);②深蹲相对力量存在组别和时间的交互效应(VRT vs CRT:+18.99%vs+14.38%,P=0.019,η^(2)=0.247),实验组和对照组的深蹲最大力量(P<0.001,效应量(ES)=0.82-0.88)和相对力量(P<0.001,ES=0.98-1.05)均出现了极其可能的中等程度提高;③蹲跳高度的组别和时间交互效应趋向显著(VRT vs CRT:+6.43%vs+0.58%,P=0.052,η^(2)=0.177),实验组的蹲跳高度相比对照组出现了非常可能的中等程度提高(P=0.013,ES=1.16);下蹲跳高度仅实验组出现了可能的小程度提高(P=0.007,ES=0.33);④实验组和对照组的立定跳远与20 m冲刺表现均未出现显著性差异(P>0.05)。结论:与恒定阻力训练相比,4周可变阻力训练对深蹲相对力量表现的提升效果更明显,且运用复合式训练方式能更有效地提高蹲跳高度。
