ix Flexibility has traditionally been considered an important component of human physical fitness but this conjecture lacks supporting empirical evidence. While there is extensive published research examining the relative importance of flexibility and the impact of various methods of stretching on levels of flexibility, performance and injury risk, the quality of studies has varied considerably, reliability and validity of methodology has not always been proven, and rationale has at times been questionable. Additionally, much literature has focused on static flexibility which is not necessarily related to properties of the musculotendinous unit and thus dynamic flexibility. This thesis was designed to fill gaps in the existing literature by using accepted methods to establish relative and absolute reliability of hamstring flexibility tests, consider the comparability of static and dynamic components of the global concept of flexibility and explore how dynamic flexibility and performance are influenced by fatiguing exercise and subsequent static stretching. The first aim was realised by a repeated measures study designed to establish the intraday and interday, intrarater reliability and measurement error of static and dynamic measures of hamstring flexibility. Significant relative reliability for measures of static and dynamic hamstring flexibility was demonstrated via intraclass correlation coefficient (3,1) but limits of agreement analysis indicated there was a degree of absolute measurement error that must be interpreted in relation to analytical goals. The second aim required evaluation of relationships shared by static and dynamic measures of hamstring flexibility. Significant relationships between the different static flexibility tests were established but the extent of unexplained variance indicated that only measurements from the same tests should be directly compared to each other. Relationships between different measures of dynamic flexibility and static flexibility varied from non-significant to moderately strong, suggesting that measures of static and dynamic flexibility are not identical and results should not be interchanged between the two types of tests. Due to a lack of explanatory empirical evidence, the final chapter aimed via a prospective randomised repeated measures study to investigate the impact of fatigue and post-exercise static stretching on measures of dynamic flexibility and performance. Fatigue resulted in no significant changes to passive or active dynamic flexibility measures but a significant worsening of static flexibility levels and perceived stiffness. Post-exercise stretch resulted in significantly increased passive and active energy absorption immediately and 18 hours post-exercise and in significantly reduced joint position sense immediately post-exercise. Effect sizes were small so the clinical meaningfulness of performing post-exercise static stretching is questionable, particularly if performed in place of other, potentially more beneficial practices. / Thesis (DPhil)--University of Pretoria, 2013. / gm2014 / Biokinetics, Sport and Leisure Sciences / unrestricted

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Researchers have offered a variety of solutions for overcoming the old and challenging problem of undesired vibrations. The optimum vibration-control solution that can be a passive, semi-active or active solution, is chosen based on the desired level of vibration-control, the budget and the nature of the vibration source. Mechanical vibration-control systems, which work based on variable stiffness control, are categorized as semi-active solutions. They are advantageous for applications with multiple excitation frequencies, such as seismic applications. The available mechanical variable stiffness systems that are used for vibration-control, however, are slow and usually big, and their slowness and size have limited their application. A new semi-active variable stiffness solution is introduced and developed in this thesis to address these challenges by providing a faster vibration-control system with a feasible size. The new solution proposed in this thesis is a semi-active variable stiffness mount/isolator called the antagonistic Variable Stiffness Mount (VSM), which uses a variable stiffness spring called the Antagonistic Variable stiffness Spring (AVS). The AVS is a kinematically singular prestressable mechanism. Its stiffness can be changed by controlling the prestress of the mechanisms links. The AVS provides additional stiffness for a VSM when such stiffness is needed and remains inactive when it is not needed. The damping of the VSM is constant and an additional constant stiffness in the VSM supports the deadweight. Two cable-mechanisms - kinematically singular cable-driven mechanisms and Prism Tensegrities - are developed as AVSs in this thesis. Their optimal configurations are identified and a general formulation for their prestress stiffness is provided by using the notion of infinitesimal mechanism. The feasibility and practicality of the AVS and VSM are demonstrated through a case study of a typical engine mount by simulation of the mathematical models and by extensive experimental analysis. A VSM with an adjustable design, a piezo-actuation mechanism and a simple on-off controller is fabricated and tested for performance evaluation. The performance is measured based on four criteria: (1) how much the VSM controls the displacement near the resonance, (2) how well the VSM isolates the vibration at high frequencies, (3) how well the VSM controls the motion caused by shock, and (4) how fast the VSM reacts to control the vibration. For this evaluation, first the stiffness of the VSM was characterized through static and dynamic tests. Then performance of the VSM was evaluated and compared with an equivalent passive mount in two main areas of transmissibility and shock absorption. The response time of the VSM is also measured in a realistic scenario.

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