1,721,007 research outputs found
Statistically modelling tennis racket impacts with six degrees of freedom
Get PDFThe International Tennis Federation (ITF) is responsible for protecting the nature of tennis. The ITF uses computational models to predict how trends in equipment parameters could affect the games future. The current ball-racket impact model is limited to non-spinning, on-axis, normal ball impact simulations. The aim of this project was to develop a model of oblique, spinning, on- and off-axis ball-racket impacts.
Large scale test data (n > 1000) was collected using an impact rig and calibrated high-speed cameras. Impacts for a range of realistic velocities, spin rates and impact locations were collected, measured using automated image processing algorithms to digitise ball centroids. An established spin measurement method was improved to correct for perspective errors associated with the proximity of the cameras to the test volume. The automated algorithms were validated with experimental data and manual methods.
Multi-variate polynomial models to predict the lateral and vertical components of rebound velocities and rebound spin rate were trained and validated using a curve fitting toolbox and ‘n-fold and leave one out cross-validation’ method. Second order models best fit the training data, with the low predictive errors. Root-mean-squared errors were calculated using a test dataset, independent of the training data. These were 0.57 m·s-1 for the lateral rebound velocity model, 0.48 m·s-1 for the vertical rebound velocity model and 30.5 rad·s-1 for the rebound spin rate model. Variance was partially explained by experimentally established inherent variability of the ball and stringbed. Model output confidence was established by simulating small changes in model inputs. The simulated lateral and vertical components of rebound velocity, but not the simulated spin rate, were an order of magnitude greater than the measurement precision.
The new models were combined with ball aerodynamics and ball-to-surface impact models to simulate tennis court trajectories for oblique, spinning, on- and off-axis ball-racket impacts. Increasing stringbed stiffness or the lateral offset of impact location were found to decrease rebound velocity and increase rebound angle – markedly so for a 60 mm lateral offset. Increasing lateral offset also increased the rebound spin rate
Determining spatio-temporal metrics that distinguish play outcomes in field hockey
Get PDFTactical behaviour in field sports can be examined using spatio-temporal
metrics, which are descriptions of player behaviour derived from data of player
positions over time. Many metrics can be computed that describe the
cooperative and adversarial interactions between players. The methods
typically used by sports performance analysts cannot appropriately analyse the
many possible spatio-temporal metrics and their interactions. Tantalisingly, the
interactions between these descriptions of player behaviour could potentially
describe tactical differences in performance.
This thesis describes a programme of research that determined some spatiotemporal metrics that distinguish play outcomes in field hockey. Methods
inspired by genetic analysts were used to estimate the influence of
combinations of spatio-temporal metrics on the outcome of field hockey plays.
The novel application of the genetic methods to sports performance data raised
some practical difficulties. Adjustments to the method facilitated the selection of
distinguishing metric combinations from an initially large list of over 3,600
metrics.
The adjustments made to the genetic methods represent one of several
contributions to knowledge made by this programme of research. These
contributions will help performance analysts with the increasingly common task
of analysing high-dimensional data. Other contributions to knowledge are a
suite of metric combinations that distinguish play outcomes in field hockey and
empirical support for some tactical preconceptions.
The key finding of interest for players and coaches is that play outcomes in field
hockey are distinguished by proximity to the goal and passing execution. The
metrics that distinguish the several outcomes differ depending on the outcomes
being compared. Coaches and athletes should therefore recognise the variety
of tactics required to minimise negative outcomes and maximise positive ones
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Real-time analysis and feedback of performance indicators in elite diving
Get PDFThe difficulty and quality of dives required to win medals in Olympic springboard diving has increased throughout the modern era. A greater understanding of optimal diving technique, increased training opportunity and support from disciplines of sports science and medicine have influenced this trend.
Progress towards world class standards is enhanced by objective measurement of performance in a training and competition context from which an assessment of the effect of training interventions can be made, leading to a programme individualised for each diver.
A description of kinematic parameters representing high quality performance of the world’s hardest dives did not exist. Standards were therefore defined following analysis of dives performed over five years of springboard competition. This new knowledge contributes to a model called ‘What It Takes To Win’ (WITTW).
A practical method to calculate kinematic metrics from dives in training also did not exist, limiting comparison between training and WITTW standards. To bridge this gap, a flexible method for analysing dives in training and competition was developed and a bespoke tool created to calculate and feedback performance data with a greater level of sensitivity than in related studies in the sport. Automatic tracking was designed and implemented to facilitate ‘real-time’ measurement of kinematic data, providing a new training process where objective data added to subjective interpretation of quality throughout training.
Four World Class Programme divers were tracked through a season’s preparatory phase. Change in performance was measured and an analysis conducted to compare progress towards WITTW standards and assess the influence of strength and conditioning training in performance outcomes.
Statistical analysis of longitudinal training data showed that independent variables relating to ‘best’ performances were not common to all divers and that an individualised set of critical variables could be identified for each diver as strengths around which to focus training
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Measuring relay exchange kinematics in short-track speed skating using a multi-camera network
Get PDFTo support their targeted improvement of the relay exchange, Great Britain Speed
Skating required a tool that could be used to advance knowledge on ‘how to execute the
relay exchange effectively’. A tool that measures relay exchange kinematics in
representative race scenarios, over its entirety, and with an acceptable level of
measurement error (± 0.19 m·s-1). A review of existing measurement solutions found
that the Olympic Oval (CAN) multi-camera network was the only tool that came close
to meeting this criterion. However, while this multi-camera network satisfied the
metrics, scenarios, and scope of relay exchange measurement, its ± 1.53 m·s-1 error
exceeded the target measurement error. For these reasons, this thesis developed a multicamera network to measure accurate, two-dimensional, relay exchange kinematics.
The literature review identified that the accuracy of the National Ice Centre (GBR)
multi-camera network was dependent on five sources of measurement error.
Accordingly, a series of investigations quantified how these errors propagated,
independently, to errors in relay exchange kinematics. In the case where these errors
exceeded the target measurement error, additional studies investigated minimising each
error. Using this empirically informed measurement workflow, Monte Carlo
simulations showed that the multi-camera network’s total error was ± 0.17 m·s-1. This
error was within the target measurement error and significantly less than the
benchmark Olympic Oval (CAN) multi-camera network. Investigations into the
execution of the relay exchange demonstrated how this reduction in error allowed Great
Britain Short-Track Speed Skating to advance knowledge on ‘how to execute the relay
exchange effectively’. In turn, supporting the team’s targeted improvement of the relay
exchange, and ultimately, their aim of delivering medal-winning performances at the
Winter Olympic Games
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Finite element model of a tennis ball impact with a racket
No full textPrevious authors have produced analytical models which accurately simulate tennis impacts. However, currently there are few published studies on the simulation of tennis impacts using finite-element (FE) technique. The purpose of this study was to produce accurate FE models of tennis impacts, which will serve as design tools as well as aid in furthering the understanding of how the ball, string-bed and racket behave during play.
An FE model of a pressurised tennis ball was produced in Ansys/LS-DYNA 10.0 and validated against experimental data. The ball model was updated to simulate the extreme playing temperatures of 10 and 400C and validated against experimental data, obtained inside a climate chamber. Following validation of the ball model, an FE model of a head-clamped racket was produced and validated against experimental data. The validation included a range of inbound velocities, angles and spin rates, for impacts at a number of nominal locations on the string-bed. Finally, an FE model of a freely suspended racket was constructed and validated against experimental data. Impacts were simulated at a number of nominal impact locations on the string-bed, with a range of ball inbound velocities, angles and spin rates. The impacts were recorded using two Phantom 4.2 high-speed cameras and analysed in 3D. The FE models were all in good agreement with the experimental data, for the individual stages of the validation.
A parametric modelling program was produced to be used in conjunction with the model. This program enables the user to adjust a variety of parameters, such as the inbound velocity of the ball, impact location and mass of the racket, and run simulations without any specialist knowledge of the FE model. This program was used to analyse the model against ball to racket impact data obtained during player testing. There was relatively good agreement between the model and player testing data.
Finally, the model was used to determine the influence of racket structural stiffness, mass and the position of the balance point, when performing a typical topspin forehand. It was found that using a head-heavy racket, with high structural stiffness and mass, will increase the rebound velocity and topspin of the ball, for a shot of this type at the centre of the string-bed.
Keywords: tennis ball, tennis racket, high speed cinematograph, finite-element modelling.</p
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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