Doctoral Degrees (Mechanical and Mechatronic Engineering)

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Browsing Doctoral Degrees (Mechanical and Mechatronic Engineering) by Author "Cockroft, Stephen John"

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    Novel motion capture methods for sports analysis : case studies of cycling and rugby goal kicking
    (Stellenbosch : Stellenbosch University, 2015-12) Cockroft, Stephen John; Muller, Jacobus Hendrik; Van den Heever, David Jacobus; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: The quest to understand and optimize human movement performance has advanced rapidly in recent years through innovations in movement science and technology. Motion capture technologies have become significantly more mobile, powerful and unobtrusive, enabling new research opportunities. This has resulted in the continuous development of novel quantitative methods for observing and interpreting expert performance in professional sports. A contribution is presented towards this ongoing endeavor via original methodologies for measurements of cycling kinematics using wireless inertial and magnetic measurement systems (IMMSs) and technique analysis of expert rugby union goal kicking using stereophotogrammetry. Three studies are presented detailing the design and validation of sensor fusion algorithms for IMMS tracking of cycling kinematics. The algorithms utilize a nonlinear complementary filtering structure together with domain constraints related to pendulum and planar motion. Using stereophotogrammetry to validate the tracking performance, it is shown that these filter adaptations eliminate typical measurement errors caused by continuous and time-varying dynamic accelerations and magnetic field disturbances. The first of the IMMS studies illustrated the use of a functional calibration technique to estimate the radius of rotation of an IMMS attached to the thigh. This technique was shown to reduce IMMS tracking errors per axis to 1°. A detailed assessment of the effect of soft tissue artifact on hip angle measurements is also given, and estimates of hip kinematics in the sagittal plane were accurate to within 1-2°. The following two studies focus on IMMS tracking of crank angles in the presence of severe magnetic interference, which precludes the use of traditional static pose calibrations. Two magnetometer-free algorithms are presented, one not requiring a sensor-to-segment calibration and another utilizing a functional calibration technique. Both methods were found to perform with accuracies of 2-3°. A novel optical motion capture method for tracking the crank angle was also developed using a two-segment definition. Three more studies present a novel technique analysis of fifteen professional goal kickers using stereophotogrammetry. The first study investigated the distance and angulation of the individual steps of the run-up as well as foot positioning relative to the tee and found that anthropometry did not play a major role in determining run-up geometry. The second study assessed phase timing, speed and acceleration during the approach and found that this only had a moderate to small association with foot speed at ball contact. The third study reports on rotational alignment of the thorax, pelvis and feet relative to the tee and target and discusses evidence for a tension arc movement strategy in the spine rotation angle. The most important finding in all three studies was high inter-individual variability and low intra-individual variability, which highlights the nonlinear, athlete-specific dynamics of motor control in sports. In short, this work contributes towards understanding and overcoming challenges to cycling analysis using IMMSs. The tracking algorithms are resistant to errors caused by magnetic interference, centripetal accelerations and sensor-to-segment calibration. Similarly, the technique analysis of rugby goal kicking contributes towards evidence-based coaching by providing novel methodologies and data for understanding performance.