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The aim of this study was to investigate effects of speed and plane of motion on stability during locomotion in skilled football players. Ten male national-level football players participated in this study to run forward, backward and in lateral directions on a treadmill at 80%, 100% and 120% of their preferred running speeds. The coordinate data of passive reflective markers attached to body segments were recorded using motion capture systems. Time series data obtained from the ankle marker were used for further analyses. The largest finite-time Lyapunov exponent and maximum Floquet multiplier were adopted to quantify local and orbital dynamic stabilities, respectively. Results showed that speed did not significantly change local and orbital dynamic stabilities in any of running patterns. However, both local and orbital dynamic stability were significantly higher in the secondary plane of progression. Data revealed that in running, unlike walking, stability in the direction perpendicular to the direction of running is significantly higher, implying that less active control is required in the secondary plane of progression. The results of this study could be useful in sports training and rehabilitation programmes where development of fundamental exercise programmes that challenge both speed and the ability to maintain stability might produce a tangible enhancement of athletic skill level. 相似文献
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By far the most challenging task faced by schools in post-apartheid South Africa, has been the distance educational leaders were mandated to put between the educational institutions and the apartheid legacy of racial discrimination and exclusion. It is therefore not surprising that there are two dominant approaches to educational leadership, namely the transactional and transformational leadership paradigms that were considered as apposite to ensure the implementation of new educational policy and curriculum developments. Indeed, the theoretical debates about educational leadership have extended expositions of the concepts of transactional and transformative leadership that are often perceived as forms of educational leadership that can enhance the democratisation of educational institutions, such as schools. However, after the demise of apartheid all attempts to move beyond authoritative practices have been shown to be undesirable; and the transactional and transformative forms of educational leadership might have too hastily and prematurely abandoned authoritative forms of leadership. Therefore, in this article the authors argue in defence of authority as constitutive of transformative leadership in schools by reflecting on current leadership practices in schools. 相似文献
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The importance of effector visual feedback has previously been indicated using an opaque screen to prevent sight of the catching hand for the final 150-200 ms of a flight path of 850 ms in total. Attention was drawn to the finding that the ability to position the arm correctly in the line of flight of the ball deteriorated as a consequence of the number of trials without visual feedback. However, the use of only 20 test trials allows a possibility that the proprioceptive system may be able to re-establish predominant control under conditions of prolonged visual decrement. In order to verify this notion, the number of experimental trials was quadrupled for 24 subjects of mixed sex in a replication of the initial paradigm. The results provided preliminary support for the view that the positioning of the catching arm is more seriously affected by visual occlusion than the timing of the grasp phase. However, increments in accuracy of limb orientation were evidenced in all screen conditions, as the number of trials increased. The grasp component of one-handed catching appeared to remain unaffected by the increase in experimental trials. The relative influence of skill level on catching errors was next addressed by comparing the performance of the six best and worst catchers in the group. The results did not support previous research which found a relation between skill level and error type. The number of position, but not grasp, errors decreased as a function of task practice for both subgroups. Future investigations should, perhaps, focus on the developmental nature of movement control in one-handed catching to obtain a clearer picture of this relation. 相似文献
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Scott MA van der Kamp J Savelsbergh GJ Oudejans RR Davids K 《Research quarterly for exercise and sport》2004,75(2):130-137
We investigated how perturbing optical information affects the guidance of an unfolding hitting action. Using monocular and binocular vision, six participants were required to hit a rectangular foam object, released from two different heights, under four different approach conditions, two with object rotation (to perturb the viewed surface area) and two without. Participants made more misses under monocular conditions, and the presence of rotation had a significant effect on the duration between the arm's peak velocity and its contact with the object. We conclude that binocular information contributes to the timing of interceptive tasks. In addition, the guidance of interceptive actions does not appear to be based solely on object expansion information picked up from the viewed surface area. 相似文献
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Pedro T. Esteves Duarte Araújo Keith Davids Luís Vilar Bruno Travassos Carlos Esteves 《Journal of sports sciences》2013,31(12):1285-1293
Abstract In this study, we examined the effects of relative positioning of attacker–defender dyads to the basket on interpersonal coordination tendencies in basketball. To achieve this aim, four right-hand dominant basketball players performed in a 1 vs. 1 sub-phase, at nine different playing locations relative to the basket (from 0° to 180°, in 20° increments). Performers’ movement displacement trajectories were video-recorded and digitized in 162 trials. Results showed that interpersonal coordination tendencies changed according to the scaling of the relative position of performers to the basket. Stable in-phase modes of coordination were observed between performers’ longitudinal and lateral displacements (50.47% and 43.02%) on the left side of the court. On the right side of the court, a shift in the dominant mode of coordination was observed to a defender lead-lag of ?30°, both for longitudinal and lateral displacements (30.51% and 32.65%). These results suggest how information about dribbler hand dominance and relative position to the basket may have constrained attacker–defender coordination tendencies in 1 vs. 1 sub-phases of basketball. 相似文献
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Mark A. Scott John van der Kamp Geert J. P. Savelsbergh Raôul R.D. Oudejans Keith Davids 《Research quarterly for exercise and sport》2013,84(2):130-137
Abstract We investigated how perturbing optical information affects the guidance of an unfolding hitting action. Using monocular and binocular vision, six participants were required to hit a rectangular foam object, released from two different heights, under four different approach conditions, two with object rotation (to perturb the viewed surface area) and two without. Participants made more misses under monocular conditions, and the presence of rotation had a significant effect on the duration between the arm's peak velocity and its contact with the object. We conclude that binocular information contributes to the timing of interceptive tasks. In addition, the guidance of interceptive actions does not appear to be based solely on object expansion information picked up from the viewed surface area. 相似文献
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Pedro Silva Ricardo Duarte Jaime Sampaio Paulo Aguiar Keith Davids Duarte Araújo 《Journal of sports sciences》2014,32(20):1888-1896
AbstractThis study analysed the influence of field dimension and players’ skill level on collective tactical behaviours during small-sided and conditioned games (SSCGs). Positioning and displacement data were collected using global positioning systems (15 Hz) during SSCGs (Gk+4 v. 4+Gk) played by two groups of participants (NLP- national-level and RLP-regional-level players) on different field dimensions (small: 36.8 × 23.8 m; intermediate: 47.3 × 30.6 and large: 57.8 × 37.4 m). Team tactical performance was assessed through established dynamic team variables (effective playing space, playing length per width ratio and team separateness) and nonlinear signal processing techniques (sample entropy of distances to nearest opponents and the teams’ centroids’ mutual information). Results showed that the effective playing space and team separateness increased significantly with pitch size regardless of participant skill level (P < 0.001, η2 = 0.78 and P < 0.001, η2 = 0.65, respectively). Playing length per width ratio increased with pitch size for the NLP but was maintained at a relatively constant level by RLP across treatments indicating different playing shapes. There was significantly more irregularity in distances to nearest opponents for the NLP in small (P = 0.003) and intermediate fields (P = 0.01). Findings suggest that tactical behaviours in SSCGs are constrained by field size and skill level, which need to be considered by coaches when designing training practices. 相似文献