Crawling forward on drag and propulsion in swimming
Authors:   , Schreven Sander 
Published: January 2023

A brief overview was provided in Chapter 1 of the three main approaches that have been pursued over the years in the biomechanical and hydrodynamic study of swimming, namely the power balance, (computational) fluid dynamics and kinematics. The main concepts, methods and accomplishments of these approaches were summarised, followed by a discussion of their strengths and limitations. This discussion provides the backdrop for the studies presented in the subsequent chapters of the thesis.

Chapter 2 reports a methodological study conducted to examine a potential methodological limitation of the Measuring Active Drag (or MAD) system. This study follows a power balance approach. The MAD system was developed in the late 1980s to determine active drag in swimming by measuring the push-off force exerted at fixed pads placed below the waterline. The aim of the study reported in Chapter 2 was to determine the effect of inter-pad distance on active drag at a given speed.

A prerequisite for understanding the relationship between swimming movements and the resulting propulsion is that the movements of the body are accurately measured in 3D and that the collected data are properly filtered to reduce remaining measurement noise. In the analysis of movement data this is often done by applying a low-pass filter. However, the choice of a cut-off frequency for this filter is typically rather arbitrary. The study reported in Chapter 3 aimed to evaluate a new method to find the optimal cut-off frequency for filtering kinematic data. The method in question involved the use of rigid marker clusters to determine the dynamic precision of a given 3D motion analysis system, and to subsequently use this precision as criterion to find the optimal cut-off frequency for filtering the data. We tested this method using a model-based approach in a situation in which measurement noise is a serious concern, namely the registration of the kinematics of swimming using a video-based motion analysis system.

Chapter 4 provides an encompassing literature review of the hydrodynamic effects of different hand and arm movements during swimming as found in (mostly) fluid dynamics studies published in the literature until the date of publication of the review in 2017. The review aimed to identify lacunae in current methods and knowledge, and to distil practical guidelines for coaches and swimmers seeking to increase swimming speed. Experimental and numerical studies were discussed, examining the effects of hand orientation, thumb position, finger spread, sculling movements, and hand accelerations during swimming, as well as unsteady properties of vortices due to changes in hand orientation.

The study reported in Chapter 5 is the pièce de résistance of this thesis. We compared the contributions of selected technique, power and anthropometric measures on sprint performance during arms-only front crawl swimming in the anticipation that measures from each domain would show up as significant contributors. The ratio between power output and drag was the only significant predictor of the maximal swimming speed (v = 0.86·power/drag). The variations in this ratio explained 65% of the variance in swimming performance. This indicates that sprint performance in arms-only front crawl swimming is strongly associated with the power-to-drag ratio and not with the isolated power variables and the anthropometric and technique variables selected in this study.