Kinematic comparisons of 1996 Olympic baseball pitchers     

Rafael F. Escamilla  Glenn S. Fleisig  Nigel Zheng  Steven W. Barrentine  James R. Andrews 《Journal of sports sciences》2013,31(9):665-676

The aim of this study was to compare and evaluate the kinematics of baseball pitchers who participated in the 1996 XXVI Centennial Olympic Games. Two synchronized video cameras operating at 120 Hz were used to video 48 pitchers from Australia, Japan, the Netherlands, Cuba, Italy, Korea, Nicaragua and the USA. All pitchers were analysed while throwing the fastball pitch. Twenty-one kinematic parameters were measured at lead foot contact, during the arm cocking and arm acceleration phases, and at the instant of ball release. These parameters included stride length, foot angle and foot placement; shoulder abduction, shoulder horizontal adduction and shoulder external rotation; knee and elbow flexion; upper torso, shoulder internal rotation and elbow extension angular velocities; forward and lateral trunk tilt; and ball speed. A one-way analysis of variance (P ? 0.01) was used to assess kinematic differences. Shoulder horizontal adduction and shoulder external rotation at lead foot contact and ball speed at the instant of ball release were significantly different among countries. The greater shoulder horizontal abduction observed in Cuban pitchers at lead foot contact is thought to be an important factor in the generation of force throughout the arm cocking and arm acceleration phases, and may in part explain why Cuban pitchers generated the greatest ball release speed. We conclude that pitching kinematics are similar among baseball pitchers from different countries.  相似文献   

Degradability of building stone: Influence of the porous network on the rate of dissolution of carbonate and evaporitic rocks     

Víctor Cárdenes Van den Eynde  Felix Javier Mateos  Remigio Paradelo 《Journal of Cultural Heritage》2013,14(2):89-96

The susceptibility to weathering of eleven carbonate and evaporitic Spanish building rocks was studied by means of dissolution experiments. Mineralogy, petrology, geochemistry and porosity of the rocks were also characterized in order to determine the potential relationships between these properties of the rocks and their dissolution rates. Rock slabs were submerged in a 0.1 M HCl acid solution for 72 h and the amounts of Ca, Mg and S released were measured, as well as the weight loss of the samples during the acid attack. Alabasters, which presented very low porosity, were dissolved to a lesser extent than limestones and dolostones in the time of the experiment. A significant positive correlation was found for connected with the weight loss of the rocks during dissolution and with the kinetic rate of Ca dissolution. Overall, the results highlight the influence of the porous network in the degradability of building stones by controlling their rate of dissolution. There is a positive correlation for all the rocks between weight loss along acidic attack and connected porosity, but no relationship between mineralogical and petrographical composition and susceptibility to dissolution.  相似文献   

The development of mental rotation abilities through robotics-based instruction: An experience mediated by gender     

José Antonio González-Calero  Ramón Cózar  Rafael Villena  José Miguel Merino 《British journal of educational technology : journal of the Council for Educational Technology》2019,50(6):3198-3213

The research literature on the topic of “spatial ability” reveals that it has a major influence on achievement in the fields of science, technology, engineering and mathematics. Additionally, previous studies indicate the existence of a gender gap in spatial ability in favour of males. Mental rotation, one subskill of spatial ability, is an area in which gender differences are greater. At the same time, educational robotics is seen as one of the more promising technologies in educational contexts for the future. We conducted a quasi-experimental study with 142 seven- and eight-year-old students aimed at analysing whether robotics-based instruction may produce greater learning gains on mental rotation abilities compared to traditional instruction. The intervention was developed in the context of map-reading tasks, which are part of the curricular content in Mathematics and Social Sciences at this age. The results showed that for males, the robotics-based instruction promoted a significantly greater improvement in students’ mental rotation abilities compared to the control group. By contrast, no significant differences were observed for females. Although this study provides empirical evidence of the potential of robotics-based instruction, future studies need to deeply analyse the existence of gender differences in the learning gains obtained through educational robotics.  相似文献   

Training load and submaximal heart rate testing throughout a competitive period in a top-level male football team     

Vincenzo Rago  Peter Krustrup  Rafael Martín-Acero  António Rebelo 《Journal of sports sciences》2020,38(11-12):1408-1415

ABSTRACT

The aim of this study was to investigate training load and cardiorespiratory fitness in a top-level Spanish (LaLiga) football team (n = 17). The submaximal Yo-Yo intermittent recovery test level 1 (Yo-Yo IR1_SUB) was performed in four moments of the competitive period from early February (E1) to early May (E4). Training load was quantified using a 10-Hz global positioning system and heart rate (HR) recording (n = 837 individual training sessions), while match load was quantified using semi-automated cameras (n = 216 individual match observations). Cardiorespiratory fitness moderately improved as the season progressed (P < 0.05; effect sizes = 0.8 to 1.2). Cumulative total distance covered during training between E1 and E4 was negatively correlated with percentage of changes in mean HR during the last 30 s of Yo-Yo IR1_SUB (P = 0.049; r = ?0.47 [?0.71; ?0.14]; moderate). HR during the last 30 s of Yo-Yo IR1_SUB was negatively correlated to total distance covered during the match (P = 0.024; r = ?0.56 [?0.80; ?0.17]; moderate). Yo-Yo IR_SUB can be used to monitor seasonal changes in cardiorespiratory fitness without the need to have players work until exhaustion. Cardiorespiratory fitness given by mean HR during the last 30 s of the test seems meaningful in relation to match performance.  相似文献   

Using the Web as corpus for self-training text categorization     

Rafael Guzmán-Cabrera  Manuel Montes-y-Gómez  Paolo Rosso  Luis Villaseñor-Pineda 《Information Retrieval》2009,12(3):400-415

Most current methods for automatic text categorization are based on supervised learning techniques and, therefore, they face the problem of requiring a great number of training instances to construct an accurate classifier. In order to tackle this problem, this paper proposes a new semi-supervised method for text categorization, which considers the automatic extraction of unlabeled examples from the Web and the application of an enriched self-training approach for the construction of the classifier. This method, even though language independent, is more pertinent for scenarios where large sets of labeled resources do not exist. That, for instance, could be the case of several application domains in different non-English languages such as Spanish. The experimental evaluation of the method was carried out in three different tasks and in two different languages. The achieved results demonstrate the applicability and usefulness of the proposed method.  相似文献   

Pre-game hydration status, sweat loss, and fluid intake in elite Brazilian young male soccer players during competition     

Da Silva RP  Mündel T  Natali AJ  Bara Filho MG  Alfenas RC  Lima JR  Belfort FG  Lopes PR  Marins JC 《Journal of sports sciences》2012,30(1):37-42

In this study, we assessed the pre-game hydration status and fluid balance of elite young soccer players competing in a match played in the heat (temperature 31.0 ± 2.0 ° C, relative humidity 48.0 ± 5.0%) for an official Brazilian soccer competition. Fluid intake was measured during the match, as were urine specific gravity and body mass before and after the game to estimate hydration status. Data were obtained from 15 male players (age 17.0 ± 0.6 years, height 1.78 ± 0.06 m, mass 65.3 ± 3.8 kg); however, data are only analysed for 10 players who completed the full game. The mean (± s) sweat loss of players amounted to 2.24 ± 0.63 L, and mean fluid intake was 1.12 ± 0.39 L. Pre-game urine specific gravity was 1.021 ± 0.004, ranging from 1.010 to 1.025. There was no significant correlation between sweat loss and fluid intake (r = 0.504, P = 0.137) or between urine specific gravity and fluid intake (r = -0.276, P = 0.440). We conclude that young, native tropical soccer players started the match hypohydrated and replaced about 50% of the sweat lost. Thus, effective strategies to improve fluid replacement are needed for players competing in the heat.  相似文献   

Synthesising information from various texts: A study of procedures and products at different educational levels     

Mar Mateos  Isabel Solé 《European Journal of Psychology of Education - EJPE》2009,24(4):435-451

The production of written syntheses, which requires reading various sources and integrating information from these sources into one’s own text, has been characterised as a potentially useful task for promoting constructive learning. This article describes research aimed at examining and characterising written syntheses and the processes involved in producing them. A case study was carried out of 45 students from four different educational levels (ranging from secondary school to university) performing synthesis tasks set by their teachers. An analysis was made of the synthesis tasks set and the syntheses produced, the prototypical procedures carried out at each educational level, and the quality of the written products.  相似文献   

Mixed order robust adaptive control for general linear time invariant systems     

Javier A. Gallegos  Manuel A. Duarte-Mermoud  Rafael Castro-Linares 《Journal of The Franklin Institute》2018,355(8):3399-3422

We provide a solution to the adaptive control problem of an unknown linear system of a given derivation order, using a reference model or desired poles defined in a possibly different derivation order and employing continuous adjustment of parameters ruled by possibly another different derivation order. To this purpose, we present an extension for the fractional settings of the Bezout’s lemma and gradient steepest descent adjustment. We analyze both the direct and indirect approaches to adaptive control. We discuss some robustness advantages/disadvantages of the fractional adjustment of parameters in comparison with the integer one and, through simulations, the possibility to define optimal derivation order controllers.  相似文献   

Technological dependence and the failure of educational institutions to support development     

Rafael L. Irizarry 《International Journal of Educational Development》1983,3(2):163-181

Academics, foundations and international agencies concerned with development have repeatedly pointed out the failure of educational institutions to support economic development in underdeveloped countries, particularly in providing the occupational skills and research knowledge in science and technology required by the productive sectors of the economy. The causal factors of this failure have been attributed to institutional deficiencies and traditionalism of backward societies where ‘desk’ jobs and independent professions enjoy higher esteem than technology-related occupations.This paper argues that such educational patterns are rooted in the structures of dependency of the underdeveloped countries. First, employment is concentrated in the non-productive services sector of the economy where desk jobs abound. Second, dependent industrialization entails the predominance of external linkages of industry to developed countries, and technological dependency both in its organic and intellectual components. Industry in underdeveloped countries does not carry out the more complex technological operations of the production cycle or research and development activities for new technology, both of which are done in developed countries. In the absence of this activity, educational institutions cannot ‘learn’ and keep abreast of the modern technological knowledge and processes required for jobs in industry.  相似文献   

Kinematics used by world class tennis players to produce high-velocity serves     

Fleisig G  Nicholls R  Elliott B  Escamilla R 《Sports biomechanics / International Society of Biomechanics in Sports》2003,2(1):51-64

The purpose of this study was to quantify ranges and speeds of movement, from shoulder external rotation to ball impact, in the tennis service actions of world class players. Two electronically synchronised 200 Hz video cameras were used to record 20 tennis players during singles competition at the Sydney 2000 Olympic games. Three-dimensional motion of 20 landmarks on each player and racquet were manually digitized. Based upon the mean values for this group, the elbow flexed to 104 degrees and the upper arm rotated into 172 degrees of shoulder external rotation as the front knee extended. From this cocked position, there was a rapid sequence of segment rotations. The order of maximum angular velocities was trunk tilt (280 degrees/s), upper torso rotation (870 degrees/s), pelvis rotation (440 degrees/s), elbow extension (1510 degrees/s), wrist flexion (1950 degrees/s), and shoulder internal rotation. Shoulder internal rotation was greater for males (2420 degrees/s) than females (1370 degrees/s), which may be related to the faster ball velocity produced by the males (50.8 m/s) than the females (41.5 m/s). Although both genders produced segment rotations in the same order, maximum upper torso velocity occurred earlier for females (0.075 s before impact) than for males (0.058 s). At impact, the trunk was tilted 48 degrees above horizontal, the arm was abducted 101 degrees and the elbow, wrist, and lead knee were slightly flexed. Male and female players should be trained to develop the kinematics measured in this study in order to produce effective high-velocity serves.  相似文献