Spatio-temporal metrics that distinguish plays in field hockey : a pilot study

In team invasion sports, tactical behaviour can be examined using spatio-temporal data, i.e. the position of the players at a given time. A review of the spatio-temporal metrics used in team invasion sports performance analysis indicated that thousands of variations of metrics being used. Information about the distribution of metrics' individual effects can inform us of the best variable-selection method. The aim of this pilot study was to estimate the distribution of strong marginal effects of spatio-temporal metrics of field hockey plays. With institutional ethical approval, the Womens’ and Mens’ gold medal games from the EuroHockey 2015 field hockey tournament were recorded. Best, acceptable and worst-case outcomes for plays were described by 1,837 spatio-temporal metrics. Each metric's marginal effects were estimated using Cramér's V, Mutual Information and the I-score. Values for Cramér's V of 0.2 and 0.4 to mark the boundaries of small, moderate and large effects. Less than 1% of metrics show large effects with > 87% of all metrics showing small effects as per the Cramér's V thresholds. These large effect metrics where all within the 98th percentile of Mutual Information values and within the 96th percentile of the I-score values, which supports the Cramér's V distribution of marginal effects. Therefore, according to the recommendations of Tibshirani (1996), univariate variable-selection methods will be the most appropriate for selecting important metrics

Markerless tracking of tennis racket motion using a camera

This research is concerned with tracking tennis racket movements. Previously, stereo camera systems have been used to track markers attached to rackets, which allows for racket movements to be obtained in three-dimensions. Typically, markers are manually selected on the image plane but this can be time consuming and inaccurate. This paper discusses a markerless method to measure three-dimensional racket movements using a camera. The method relies on a silhouette of a racket captured with a camera whose relative pose (rotation and translation) is unknown. A candidate relative pose is used to measure the inconsistency between the silhouette and a set of racket silhouettes captured with a fully calibrated camera. The measure of inconsistency can be formulated as a cost function associated with the candidate relative pose. By adjusting parameters of the pose to minimise the cost, an accurate estimation for the true pose of the racket can be made. A validation scheme was developed to compare pose estimates with data obtained using camera calibration software. Rotation about the axis of x, y, z' were accurate to within 2.5° for 88, 90 and 86 % of estimates respectively and resultant translation to within 5 mm for 72% of estimates. This research is the first step in a process to fully validate a novel method for measuring tennis racket movements in real play conditions

Analysing splash in competitive diving

Splash size is an important factor in competitive diving and is considered to have a large effect in final judged scores. The aim of this research was to develop a system to determine the splash size during different stages of the dive entry. One diver was recorded during a training session and their dives analysed using the developed system. Three splash metrics were calculated at time of complete entry and maximum splash: (1) width of splash, (2) height of splash and (3) size of splash. Results indicated that there was no relationship between the splash metrics at complete entry and time of maximum splash. This may have implications for the scoring of a dive if the judge is unable to distinguish between these two splash events

The dynamics of tennis ball impacts on tennis rackets.

A model of a tennis ball impact on a tennis racket has been developed in this study. An experimental investigation was conducted to detennine the dynamic properties of several different tennis balls. The balls were propelled at a piezoelectric force platform and the force acting on the ball was sampled, along with the ball rebound velocity. A visco-elastic model of this impact was developed and a set of model parameters were determined empirically for each ball type. The values of these parameters were independent of the ball impact velocity. The next stage of the study involved an experimental investigation of a ball impact on a head clamped tennis racket. In this experiment, tennis balls were propelled at the geometric string centre of a tennis racket. High speed cinematography was used to determine the ball and stringbed deformation during impact, and speed gates were used to measure the ball rebound velocity. A visco-elastic model of this impact was developed. The ball component of this model was identical to that for a model of a ball impact on a rigid force platform. The model parameter for the stringbed component was obtained from a simple quasi-static compression of the stringbed in which the applied force and resulting deformation were measured. The final stage of this study involved an investigation of the impact between a tennis ball and a freely supported tennis racket (this support method has been shown to be equivalent to a player gripping the tennis racket). In these experiments, the ball, stringbed and racket deformation were measured during impact, along with the velocity of the ball and racket after impact. A model was developed to simulate this impact in which it was assumed that the racket acted as a onedimensional flexible beam. The models which have been developed in this study are advancements of those which have been used in previously published literature. Experimental data was used to assess the accuracy of the results which were calculated by the models. An excellent correlation was found between the data calculated by the model and that measured experimentally

Measuring the inertial properties of a tennis racket

Simple and bifilar pendulum were used to measure the mass moments of inertia of three tennis rackets. The pendulum setups were filmed using an off-the-shelf camcorder, with a stopwatch in view to provide timing data. The measurement accuracy was assessed using calibration rods of known mass moment of inertia. The simple pendulum method was found to be most accurate (<1.0 % difference to theoretical value) when a square profile rod was used as a pivot. The bifilar pendulum was found to be very accurate (0.0% difference to theoretical value) but sensitive to non-parallel support wires. A Babolat Racket Diagnostic Centre (RDC) was assessed using four calibration rods, of known mass moment of inertia. Measurement agreement was greater than 99.0% for mass moments of inertias within the range of 220 – 380 kg·cm2

Relay exchanges in elite short track speed skating

In short track speed skating, the relay exchange provides an additional strategic component to races by allowing a team to change the skater involved in the pack race. Typically executed every 1 ½ laps, it is the belief of skaters and coaches that during this period of the race, time can be gained or lost due to the execution of the relay exchange. As such, the aim of this study was to examine the influence of the relay exchange on a team's progression through a 5000 m relay race. Using data collected from three World Cup relay events during the 2012-13 season, the time taken to complete the straight for the scenarios with and without the relay exchange were compared at different skating speeds for the corner exit prior to the straight. Overall, the influence of the relay exchange was found to be dependent on this corner exit speed. At slower corner exit speeds (12.01 - 13.5 m/s), relay exchange straight times were significantly faster than the free skating scenario (P < 0.01). Whilst at faster corner exit speeds (14.01 - 15 m/s), straight times were significantly slower (P < 0.001). The findings of this study suggest that the current norm of executing relay exchanges every 1 ½ laps may not be optimal. Instead, varying the frequency of relay exchange execution throughout the race could allow: (1) time to be gained relative to other teams; and (2) facilitate other race strategies by providing an improved opportunity to overtake

Validation of a single camera, spatio-temporal gait analysis system

such as tennis. However during competition, it is impracticable to instrument players. A markerless, view-independent, footsurface contact identification (FSCi) system was developed and validated. The FSCi system analysed standard colour video sequences of walking and running (barefoot and shod) from four unique camera perspectives; output data were compared to three-dimensional motion analysis. Results demonstrated that data for 99.6% of foot contacts (all camera perspectives) were identified. The calculation of gait variables, i.e. step length etc., was performed automatically for 91.3% of foot contact data; 8.7% of data required manual intervention for analysis. Resultant direction root-mean square error (RMSE) for foot contact position was 52.1 and 52.2 mm for barefoot and shod walking respectively. Resultant direction RMSE for foot contact position during running was 91.4 and 103.4 mm for barefoot and shod conditions respectively. The FSCi system measured basic gait parameters of walking and running without interfering with the activity being observed. The system represents a flexible approach which could be used for in situ gait analysis. The FSCi system could be used for gait analysis in competitive tennis however performance of the system when applied to larger filming areas, e.g. tennis courts, must be evaluated

ASSESSING TENNIS PLAYER INTERACTIONS WITH TENNIS COURTS

Different types of tennis injury have been associated with play on different court surfaces and current knowledge of tennis player and court interactions is limited. This paper provides a brief overview of tennis injury incidence, player movements and the biomechanics of slips. The discussion proposes a new direction for assessing tennis player-surface interactions and outlines current work. It is envisaged that current work will contribute to the understanding of tennis player-surface interactions and be of practical use in the future regulation of tennis courts

A review of spatio-temporal metrics in invasion game sports.

INTRODUCTION: A team's tactical behaviour is often quantified using metrics that relate to player locations at a given time. This spatio-temporal data is easily translated into many metrics that describe the point, line and surface relations of players and the ball over time. This study reviews the spatio-temporal metrics used in invasion game sports. METHOD: A review of six online databases provided 355 publications from which spatio-temporal metrics were extracted. RESULTS: A generalised summary of extracted metrics and metric specifications was compiled. Combining metrics and metric specifications creates all extracted metrics and some new suggestions. The compilation is presented in a table that contains the associated literature from the review as further reading. DISCUSSION & CONCLUSION: Three conclusions were drawn from the review. Firstly, spatio-temporal metrics don’t suffer from the same lack of consensus on definitions as other sports performance analysis metrics. Secondly, some metrics might not be generalizable to all invasion game sports. Third, the structure of the location and time data constrains the metrics that could be used