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61.
Comparing the pre- and post-impact ball and racquet kinematics of elite tennis players' first and second serves: a preliminary study 总被引:1,自引:0,他引:1
Chow JW Carlton LG Lim YT Chae WS Shim JH Kuenster AF Kokubun K 《Journal of sports sciences》2003,21(7):529-537
The aim of this study was to compare the pre- and post-impact three-dimensional kinematics of the ball and racquet during first and second serves performed by elite tennis players. Data were collected from four male and four female right-handed professional players during competition using two high-speed cameras (200 Hz). For each player, one first serve and one second serve from the 'deuce' or right service court that landed within the specified target area were analysed. To test for significant differences between the first and second serves, Wilcoxon tests (P < or = 0.05) were performed on selected parameters. The results indicate that the ball travelled forward and to the left during the flight phase of the toss in all but one trial. The average pre-impact ball forward location for the first serve was significantly more in front and had a higher associated forward ball velocity than the corresponding values for the second serve. On average, the decrease in post-impact ball speed from the first to the second serve was 24.1%. No significant differences between the first and second serves were found in the pre-impact racquet head speed and orientation, which was represented as a unit vector perpendicular to the racquet face. The major adjustments made by the players when going from the first to second serve were a decrease in pre-impact ball forward location (P < or = 0.01) and an increase in the pre-impact racquet vertical and lateral velocities (both P < or = 0.05). This implies that the players tossed the ball closer to the body and imparted topspin and sidespin on the ball by changing the racquet vertical and lateral velocities when going from the first to the second serve. 相似文献
62.
The aim of this study was to assess the effect of to isometic contraction durations during propiceptive neuromuscular facilitation stretching on gains inflexion at the hip. Forty-three women (M age = 20.0 years, SD = 1.3) were assigned to one of three groups: 5-s isometric contraction (5-IC), 10-IC, and control. Flexibility was assessed at baseline and Weeks 3 and 6. Analysis of covariance, controlling for pretest differences, showed a significant interaction, F(2, 33) = 44.1, p < .001. Flexibility was significant lower in the control group relative to the 5-IC and 10-IC groups and in the 5-IC group relative to the 10-IC group at 3 and 6 weeks (3 weeks = 101.2 +/- 1.4 degrees, 114.3 +/- 1.5 degrees, 120.5 +/- 1.3 degrees; 6 weeks = 103.0 +/- 1.4 degrees, 126.1 +/- 1.6 degrees, 133.3 +/- 1.4 degrees for control, 5-IC and 10-IC groups, respectively). A longer contraction time led to greater increases in flexibility. 相似文献
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Raymond W. Kulhavy Mary T. White Bruce W. Topp Ann L. Chan James Adams 《Contemporary educational psychology》1985,10(3):285-291
College undergraduates read a 2400-word passage, responded to 16 multiple-choice questions, and received one of four types of feedback following their responses. Complexity of feedback was inversely related to both error correctability and criterion efficiency taken as a ratio of feedback study time to post-test corrects. The results were discussed in terms of depth of processing and instructional comprehension. 相似文献
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This study investigated development of the Big Five personality traits from early childhood into adulthood. An initial group of 137 Swedish children were assessed eight times between ages 2 and 29 years. Initial decreases in extraversion leveled off in early adulthood; agreeableness and conscientiousness increased from ages 2 to 29; neuroticism initially increased, leveled off in later childhood and adolescence, and decreased throughout early adulthood; while openness to experience showed an initial increase, then decreased and leveled off in early adulthood. Individual developmental trajectories varied significantly, particularly in relation to gender. Personality traits became increasingly stable, and the fact that childhood scores predicted scores in adulthood indicated that personalities are fairly stable across this portion of the life span. 相似文献
68.
The authors investigated whether some advantages of tutoring over other instructional methods are due to microadaptation, or, tutors basing their actions on assessments of tutees they develop during tutoring. In a 2 × 2 between-subjects experiment, independent variables were shared experience (tutors either worked with the same or a different tutee in each of four segment of the tutoring session) and communication context (face-to-face or computer-mediated). Although there were no overall learning differences across experience conditions, tutees who worked with the same tutor demonstrated better learning of concepts initially discussed during the final tutoring segment. Shared experience led to accurate competence assessments only in the computer-mediated context, suggesting that cognitive load influences assessment development. However, there was no evidence of micro-adaptation. 相似文献
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