"Hydro-kinematic" method for quantifying glide efficiency of swimmers

Abstract

The purpose of this thesis was to introduce and test methods of quantifying the glide efficiency and the hydrodynamic parameters related to an underwater glide of a human body in a streamlined position and to investigate their relationship with the size and shape characteristics of a body with consideration of actual anthropometry, morphology and posture of body in a streamlined glide position.The thesis comprises three studies. The aim of the first study was to develop and test a method of quantifying glide efficiency in a way that accounts for both the inertial and resistive characteristics of the gliding body as well as the differences in the instantaneous velocities. To achieve this, a displacement function was derived from the equation of motion of the body during a horizontal rectilinear glide. By fitting this function to the position-time data of a body during a rectilinear horizontal glide, a glide factor that indicates the glide efficiency was quantified. This factor represented a combined kinematic and hydrodynamic measure of a glide. As the glide efficiency of a body is influenced by the body shape as well as by the body size, the size-related and shape-related glide efficiencies were determined as separate entities. The validity and applicability of the method was established. Also the glide factor enabled the exact prediction of deceleration during a glide which was not possible knowing the resistive factor alone. It was found that the glide factor increased with decreasing velocity. The method was shown to be able to detect differences in the glide efficiency between subjects and across trials within subjects.The aim of the second study was to develop and test a method of quantifying the hydrodynamic properties of a human body in a streamlined position during an underwater glide so that the values of the resistive factor and the virtual mass can be determined separately. To achieve this aim a displacement function was derived from the equation of motion of the body during an inclined rectilinear glide. By fitting this function to the position-time data of a body during a rectilinear inclined glide, and taking iii advantage of the component of net buoyancy as a constant parameter in the equation of motion, the resistive and inertial parameters were quantified. As the resistive and inertial parameters of a body are influenced by the body shape as well as by the body size, the drag and added mass coefficients were determined to investigate resistive and added inertial properties of a body independent of its size. The validity and applicability of the method was established. The method was able to quantify the hydrodynamic resistance and added inertia parameters considering the glide under realistic conditions. Also added mass of a body during deceleration was quantified with this method. It was found that the added mass decreased with increasing velocity while the resistive force increased.The aim of the third study was to determine the true relationship between the size and the shape characteristics of the body and its hydrodynamic and glide efficiency parameters. In the third study the actual anthropometric measures, morphological indices and postural angles of the body in a streamlined position were determined, in order to quantify the size and shape characteristics of a body in a streamlined position. The correlations between these parameters and the glide efficiency and the resistive and added inertia parameters were obtained. It was found that the gliding ability is more dependent on having a good shape than on having a large body mass with a low crosssectional area. Also the difference of hydrodynamic parameters including the resistive factor and the added mass between two bodies is the result of the differences in the shape characteristics including morphological indices and postural angles rather than due to the differences in size. The results indicated that some of the morphological indices and joint angles investigated in this study were correlated to the glide efficiency and hydrodynamic parameters. The belief that more streamlined objects possess a lower added mass coefficient seems not to be applicable to the human body.The method developed has practical applications in testing swimming suits designed to improve performance. Using the 'Hydro-kinematic' method the suit effect on the ability of a body to entrain added masses of water together with its ability to reduce drag as well as the combined effect on the glide efficiency may be quantified. The results of this study indicate that in talent identification the evaluation should be based on the shape of the body rather than its size. The existence of relationship between some of the morphological indices and the hydrodynamic and glide efficiency parameters would also allow identifying the streamlining degree of a body without the requirement for the direct drag force measurements.