首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2346篇
  免费   175篇
  国内免费   2篇
  2023年   6篇
  2022年   28篇
  2021年   46篇
  2020年   41篇
  2019年   61篇
  2018年   58篇
  2017年   56篇
  2016年   84篇
  2015年   116篇
  2014年   157篇
  2013年   189篇
  2012年   211篇
  2011年   203篇
  2010年   118篇
  2009年   97篇
  2008年   140篇
  2007年   160篇
  2006年   120篇
  2005年   97篇
  2004年   101篇
  2003年   124篇
  2002年   79篇
  2001年   14篇
  2000年   17篇
  1999年   16篇
  1998年   16篇
  1997年   12篇
  1996年   20篇
  1995年   13篇
  1994年   14篇
  1993年   16篇
  1992年   7篇
  1991年   7篇
  1990年   6篇
  1989年   7篇
  1988年   3篇
  1987年   2篇
  1986年   9篇
  1985年   5篇
  1984年   6篇
  1983年   7篇
  1982年   4篇
  1981年   5篇
  1980年   2篇
  1979年   2篇
  1977年   2篇
  1976年   4篇
  1975年   4篇
  1969年   3篇
  1962年   2篇
排序方式: 共有2523条查询结果,搜索用时 15 毫秒
71.
In populations that have a high degree of admixture, such as in Brazil, the sole use of ethnicity self-declaration information is not a good method for classifying individuals regarding their ethnicity. Here, we evaluate the relationship of self-declared ethnicities with genomic ancestry and mitochondrial haplogroups in 492 individuals from southeastern Brazil. Mitochondrial haplogroups were obtained by analyzing the hypervariable regions of the mitochondrial DNA (mtDNA), and the genomic ancestry was obtained using 48 autosomal insertion-deletion ancestry informative markers (AIM). Of the 492 individuals, 74.6% self-declared as White, 13.8% as Brown and 10.4% as Black. Classification of the mtDNA haplogroups showed that 46.3% had African mtDNA, and the genomic ancestry analysis showed that the main contribution was European (57.4%). When we looked at the distribution of mtDNA and genomic ancestry according to the self-declared ethnicities from 367 individuals who self-declared as White, 37.6% showed African mtDNA, and they had a high contribution of European genomic ancestry (63.3%) but also a significant contribution of African ancestry (22.2%). Of the 68 individuals who self-declared as Brown, 25% showed Amerindian mtDNA and similar contribution of European and African genomic ancestries. Of the 51 subjects who self-declared as black, 80.4% had African mtDNA, and the main contribution of genomic ancestry was African (55.6%), but they also had a significant proportion of European ancestry (32.1%). The Brazilian population had a uniform degree of Amerindian genomic ancestry, and it was only with the use of genetic markers (autosomal or mitochondrial) that we were able to capture Amerindian ancestry information. Additionally, it was possible to observe a high degree of heterogeneity in the ancestry for both types of genetic markers, which shows the high genetic admixture that is present in the Brazilian population. We suggest that in epidemiological studies, the use of these methods could provide complementary information.  相似文献   
72.
73.
Immunity to malaria is believed to wane with time in the absence of exposure to Plasmodium falciparum infection, but immunoepidemiological data on longevity of immunity remain controversial. We quantified serum cytokines and chemokines by suspension array technology as potential biomarkers for durability of immunity in immigrants with clinical malaria after years without parasite exposure. These were compared to serum/plasma profiles in naïve adults (travelers) and semi-immune adults under continuous exposure, with malaria, along with immigrant and traveler patients without malaria. Immigrants had higher levels of IL-2, IL-5 and IL-8 compared to semi-immune adults with malaria (P≤0.0200). Time since immigration correlated with increased IL-2 (rho=0.2738P=0.0495) and IFN-γ (rho=0.3044P=0.0282). However, immigrants did not show as high IFN-γ concentrations as travelers during a first malaria episode (P<0.0001). Immigrants and travelers with malaria had higher levels of IFN-γ, IL-6, and IL-10 (P<0.0100) than patients with other diseases, and IL-8 and IL-1β were elevated in immigrants with malaria (P<0.0500). Therefore, malaria patients had a characteristic strong pro-inflammatory/Th1 signature. Upon loss of exposure, control of pro-inflammatory responses and tolerance to P. falciparum appeared to be reduced. Understanding the mechanisms to maintain non-pathogenic effector responses is important to develop new malaria control strategies.  相似文献   
74.
Coordination between arm-trunk and trunk-leg is important for effective longswing performance. This research describes inter-segmental coordination changes after a practice period of longswing on high bar in a novice cohort. Novices were divided by initial skill level (talent) into two groups: spontaneously-talented, (ST, n = 10, closer to expert performance) and non-spontaneously-talented (NST, n = 15). Additionally, post-practice longswing coordination was compared to expert gymnasts (n = 9). Longswing amplitude and coordination (inter-joint reversal points and continuous relative phase, CRP) were assessed for pre- and post-practice sessions. ANOVAs showed similar practice effects in swing enlargements for the ST (11%) and NST (18%), but inter-joint reversal points and positive area in CRP during the downswing were different. Due to practice, the ST group paired shoulder and hip reversal points (events) during the downswing closer and with larger velocity of the arm in relation to the trunk than the NST group. The NST failed to modify coordination probably due to a large variability at the beginning of the downswing. Given a similar amount of practice, talent could help to achieve the right temporal events’ sequence during downswing, which would allow the exploration of different segmental coordination. However, upswing coordination of the novice groups (ST and NST) requires more focused practice to achieve expert levels than downswing, especially the arm-trunk coordination.  相似文献   
75.
76.
This study was focused on species of Peziza belonging to the “P. ammophila” complex, using both morphological and molecular approaches. Molecular and morphological analyses showed that several taxa are hidden under the name “ammophila” and, as a consequence, two additional species and one form are established: P. hellenica sp. nov., P. oceanica sp. nov. and P. ammophila f. megaspora f. nov. On the basis of the data that we have, these taxa seem to be related to some geographical areas; P. ammophila appears to be prevalent in northern, central and southern Europe, its f. megaspora is distributed in France and in the Netherlands, P. hellenica comes from Greece and P. oceanica from New Zealand. A cryptic species, not yet circumscribed satisfactorily, is described as P. deceptiva ad int. Moreover, the problem concerning the identification of P. ammophila is raised, the species is epitypified with a sequenced collection from Sicily (Italy) and a key to the species of the complex is provided.  相似文献   
77.

Purpose

The aim of the current study is to describe the functionality of free software developed for energy system contributions and energy expenditure calculation during exercise, namely GEDAE-LaB.

Methods

Eleven participants performed the following tests: 1) a maximal cycling incremental test to measure the ventilatory threshold and maximal oxygen uptake (V˙O2max); 2) a cycling workload constant test at moderate domain (90% ventilatory threshold); 3) a cycling workload constant test at severe domain (110% V˙O2max). Oxygen uptake and plasma lactate were measured during the tests. The contributions of the aerobic (AMET), anaerobic lactic (LAMET), and anaerobic alactic (ALMET) systems were calculated based on the oxygen uptake during exercise, the oxygen energy equivalents provided by lactate accumulation, and the fast component of excess post-exercise oxygen consumption, respectively. In order to assess the intra-investigator variation, four different investigators performed the analyses independently using GEDAE-LaB. A direct comparison with commercial software was also provided.

Results

All subjects completed 10 min of exercise at moderate domain, while the time to exhaustion at severe domain was 144 ± 65 s. The AMET, LAMET, and ALMET contributions during moderate domain were about 93, 2, and 5%, respectively. The AMET, LAMET, and ALMET contributions during severe domain were about 66, 21, and 13%, respectively. No statistical differences were found between the energy system contributions and energy expenditure obtained by GEDAE-LaB and commercial software for both moderate and severe domains (P > 0.05). The ICC revealed that these estimates were highly reliable among the four investigators for both moderate and severe domains (all ICC ≥ 0.94).

Conclusion

These findings suggest that GEDAE-LaB is a free software easily comprehended by users minimally familiarized with adopted procedures for calculations of energetic profile using oxygen uptake and lactate accumulation during exercise. By providing availability of the software and its source code we hope to facilitate future related research.  相似文献   
78.
Plasmodium vivax is the most widely distributed human parasite and the main cause of human malaria outside the African continent. However, the knowledge about the genetic variability of P. vivax is limited when compared to the information available for P. falciparum. We present the results of a study aimed at characterizing the genetic structure of P. vivax populations obtained from pregnant women from different malaria endemic settings. Between June 2008 and October 2011 nearly 2000 pregnant women were recruited during routine antenatal care at each site and followed up until delivery. A capillary blood sample from the study participants was collected for genotyping at different time points. Seven P. vivax microsatellite markers were used for genotypic characterization on a total of 229 P. vivax isolates obtained from Brazil, Colombia, India and Papua New Guinea. In each population, the number of alleles per locus, the expected heterozygosity and the levels of multilocus linkage disequilibrium were assessed. The extent of genetic differentiation among populations was also estimated. Six microsatellite loci on 137 P. falciparum isolates from three countries were screened for comparison. The mean value of expected heterozygosity per country ranged from 0.839 to 0.874 for P. vivax and from 0.578 to 0.758 for P. falciparum. P. vivax populations were more diverse than those of P. falciparum. In some of the studied countries, the diversity of P. vivax population was very high compared to the respective level of endemicity. The level of inter-population differentiation was moderate to high in all P. vivax and P. falciparum populations studied.  相似文献   
79.
80.
Interest in the use of corncobs as feedstock for bioethanol production is growing. This study assesses the feasibility of sequential thermochemical diluted sulfuric acid pretreatment of corncobs at moderate temperature to hydrolyze the hemicellulosic fraction, followed by enzymatic hydrolysis of the whole slurry, and fermentation of the obtained syrup. The total sugar concentration after enzymatic hydrolysis was 85.21 g/l, i.e., 86 % of the sugars were liberated from the polymeric fractions, together with a low amount of furfural (0.26 g/l) and 4.01 g/l of acetic acid. The syrups, which contained 36.3, 40.9, 4.47, and 1.84 g/l of xylose, glucose, arabinose, and mannose, respectively, were fermented (pH 7, 37 °C, 150 rpm) to ethanol with the metabolically engineered acetate-tolerant Escherichia coli strain MS04 under non-aerated conditions, producing 35 g/l of ethanol in 18 h (1.94 gEtOH/l/h), i.e., a conversion yield greater than 80 % of the theoretical value based on total sugars was obtained. Hence, using the procedures developed in this study, 288 l of ethanol can be produced per metric ton of dry corncobs. Strain MS04 can ferment sugars in the presence of acetate, and the amount of furans generated during the sequential thermochemical and enzymatic hydrolysis was low; hence, the detoxification step was avoided. The residual salts, acetic acid, and solubilized lignin present in the syrup did not interfere with the production of ethanol by E. coli MS04 and the results show that this strain can metabolize mixtures of glucose and xylose simultaneously.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号