Changes in angular momentum during the tennis serve     

Rafael E. Bahamonde 《Journal of sports sciences》2013,31(8):579-592

Three-dimensional cinematography and the direct linear transformation method were used to obtain the coordinates of the landmarks of five right-handed collegiate tennis players. A 15-segment model was used to calculate the total body angular momentum about three orthogonal axes (X, parallel to the baseline; Y, normal to baseline and pointing towards the net; and Z, pointing upwards) passing through the centre of mass and to obtain the segmental contribution of the trunk, arms and legs. Most of the clockwise angular momentum about the X-axis was concentrated in the trunk and the racket-arm. Between the events of maximum external rotation and ball impact, the clockwise angular momentum about the X-axis of rotation of most body segments was reduced and the racket-arm gained clockwise angular momentum. The body angular momentum about the Y-axis of rotation had two distinct patterns and was the result of the lateral rotation of the trunk as the racket shoulder was elevated in preparation for impact. This body angular momentum was clockwise from the event of maximum external rotation to impact for the players with the greatest ball speed, whereas it was counterclockwise for the other players. The angular momentum about the Z-axis of rotation was small and lacked a consistent pattern. The largest source of angular momentum in the tennis serve derives from the remote angular momentum about the X- and Y-axes of rotation, which are then transferred from the trunk to the racket-arm and finally to the racket. Near impact, most of the angular momentum (75.1%) was concentrated in the racket-arm. Of the angular momentum of the racket-arm, the largest percentages were concentrated in the racket (35.9%) and the forearm segment (25.7%).  相似文献   

Determinants of possession of the ball in soccer     

Carlos Lago  Rafael Martín 《Journal of sports sciences》2013,31(9):969-974

Abstract

In research on the importance of the possession of the ball in soccer, little attention has been paid to its determinants. Using data from 170 matches of the 2003 – 2004 Spanish Soccer League, we explain why differences in the possession of the ball among teams are so great. In particular, four variables are examined: evolving match status (i.e. whether the team is winning, losing or drawing), venue (i.e. playing at home or away), and the identities of the team and the opponent in each match. Results of linear regression analysis show that these four variables are statistically significant and together explain most of the variance in possession. In short, home teams have more possession than away teams, teams have more possession when they are losing matches than when winning or drawing, and the identity of the opponent matters – the worse the opponent, the greater the possession of the ball. Combinations of these variables could be used to develop a model that predicts possession in soccer.  相似文献   

Fluid balance of elite Brazilian youth soccer players during consecutive days of training     

Rafael P. Silva  Toby Mündel  Antônio J. Natali  Maurício G. Bara Filho  Jorge R. P. Lima  Rita C. G. Alfenas 《Journal of sports sciences》2013,31(7):725-732

Abstract

In this study we investigated pre-training hydration status, fluid intake, and sweat loss in 20 elite male Brazilian adolescent soccer players (mean ± s: age 17.2 ± 0.5 years; height 1.76 ± 0.05 m; body mass 69.9 ± 6.0 kg) on three consecutive days of typical training during the qualifying phase of the national soccer league. Urine specific gravity (USG) and body mass changes were evaluated before and after training sessions to estimate hydration status. Players began the days of training mildly hypohydrated (USG > 1.020) and fluid intake did not match fluid losses. It was warmer on Day 1 (33.1 ± 2.4°C and43.4 ± 3.2% relative humidity; P < 0.05) and total estimated sweat losses (2822 ± 530 mL) and fluid intake (1607 ± 460 mL) were significantly higher (P < 0.001) compared with Days 2 and 3. Data also indicate a significant correlation between the extent of sweat loss and the volume of fluid consumed (Day 1: r = 0.560, P = 0.010; Day 2: r = 0.445, P = 0.049; Day 3: r = 0.743, P = 0.0001). We conclude that young, native tropical soccer players arrive hypohydrated to training and that they exhibit voluntary dehydration; therefore, enhancing athletes' self-knowledge of sweat loss during training might help them to consume sufficient fluid to match the sweat losses.  相似文献   

Investigating grit variables and their relations with practice and skill groups in developing sport experts     

Rafael A. B. Tedesqui  Bradley W. Young 《High Ability Studies》2017,28(2):167-180

Grit, the tendency to work hard toward long-term goals, maintaining effort and interest over years despite failure and adversity, has predicted several achievement outcomes. However, minimal work has examined grit within a sport expertise development framework, and specifically its association with deliberate practice (DP) in sport. Participants, 250 athletes (109 female; M_age = 23.40, SD = 10.14), completed the 12-item Grit Scale, practice measures, and reported their skill level (local to international). Factor analyses confirmed a 10-item two-factor model for use in our sport sample: consistency of interests (CI; 6 items; α = .81) and perseverance of effort (PE; 4 items; α = .75). A full latent variable model controlling for age showed PE significantly predicted weekly amounts of DP and engagement in mandatory and optional practice contexts; CI was inversely associated with threats to athletes’ sport commitment. Among senior athletes, both Expert and Advanced skill groups had higher PE levels than a Basic/Intermediate group, ps < .05. The tendency to work hard toward long-term goals seemingly enables athletes to persist with practice activities that are pivotal to expert sport development.  相似文献   

The role of cued speech in the development of Spanish prepositions     

Santana Hernández R  Torres Monreal S  García Orza J 《American annals of the deaf》2003,148(4):323-332

The aim of the present study was to advance the knowledge of the linguistic development of students with prelingual profound deafness, especially the acquisition and use of prepositions in Spanish, a lexical category with an important role in the verbal comprehension. The researchers sought to learn the level of mastery students with prelingual profound deafness can achieve in the command of prepositions, depending on the system of communication they have been exposed to: classic oralism, Cued Speech, or signed language. The results show that the different systems of communication contribute, to different degrees, to the acquisition of Spanish prepositions, with the best results being obtained with Cued Speech.  相似文献   

Discontinuities in university student experiences of learning through discussions     

Robert A. Ellis  Rafael A. Calvo 《British journal of educational technology : journal of the Council for Educational Technology》2006,37(1):55-68

This article reports on research into the student experience of learning through discussions in a third‐year undergraduate engineering subject. Information engineering students studying e‐commerce were required to engage in face‐to‐face and online discussions as a key aspect of their learning experience. This study investigates the quality of the experience of learning through those discussions using quantitative methodologies from Student Learning Research. Analyses of student ratings on the closed‐ended questionnaires identify qualitatively different approaches to learning through discussions, both in face‐to‐face contexts and online. Relationships are found among the variation in the quality of these approaches and how students perceive issues such as workload, what they thought they were learning through discussions and performance. Implications arising from these results identify different suggestions for teaching students to discuss successfully in different contexts.  相似文献   

Embodied cognition as grounding for situatedness and context in mathematics education   总被引：1，自引：0，他引：1  

Rafael E. Núñez  Laurie D. Edwards  João Filipe Matos 《Educational Studies in Mathematics》1999,39(1-3):45-65

In this paper we analyze, from the perspective of 'Embodied Cognition', why learning and cognition are situated and context-dependent. We argue that the nature of situated learning and cognition cannot be fully understood by focusing only on social, cultural and contextual factors. These factors are themselves further situated and made comprehensible by the shared biology and fundamental bodily experiences of human beings. Thus cognition itself is embodied, and the bodily-grounded nature of cognition provides a foundation for social situatedness, entails a reconceptualization of cognition and mathematics itself, and has important consequences for mathematics education. After framing some theoretical notions of embodied cognition in the perspective of modern cognitive science, we analyze a case study – continuity of functions. We use conceptual metaphor theory to show how embodied cognition, while providing grounding for situatedness, also gives fruitful results in analyzing the cognitive difficulties underlying the understanding of continuity. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Correction to: Kindergarten and First-Grade Students’ Understandings and Representations of Arithmetic Properties     

Ramirez Uclés  Rafael  Brizuela  Bárbara M.  Blanton  Maria 《Early Childhood Education Journal》2022,50(2):357-357

Early Childhood Education Journal - The original version of this article unfortunately contained a typo in co-author name.  相似文献