The Ideal Underwater Trajectory

In this thesis the underwater trajectory of the swimming start is defined as the path the swimmer travelled underwater from head entry to breakout. Using the results from Sections 1,2 and 3 a number of theoretical guidelines were able to be established for the ideal underwater trajectory, which could be used to elicit reductions in drag. These guidelines can used by coaches and sport scientist and be specifically applied to elite freestyle swimmers, similar to those tested throughout this thesis. While it can be argued that each swimmer’s underwater trajectory is individual and based on a number of individual factors such as underwater kicking ability and anthropometric characteristics; it is still an optimisation between not travelling too deep where there is an advantage of wave drag reduction and minimal distance that would allow for an efficient breakout. Thus, establishing evidence based guidelines is useful for coaches for a more targeted approach to start improvement.

The depth at which the swimmer is travelling during the glide and undulatory kick phase can have a positive effect on reducing hydrodynamic drag, and hence the trajectory adopted by the swimmer is important to reduce deceleration for good start performances. The outcomes of Chapter 4-7 suggest that because the swimmer spends much longer in the underwater phase the trajectory used is paramount to the overall success of start performances. This is due to the fact the swimmer would experience much more resistance travelling through water, than air. Hence any increases in magnitude of the first two phases would be negated immediately upon entry

154 due to increases in deceleration if the swimmer uses an incorrect trajectory. Given this, these recommendations have been able to refine earlier work by Houel et al. (2012) and can now be applied practically using the kick-start technique, which is currently being used by most swimmers in competition. The guidelines are displayed visually in Figure 18 and listed below:

 Hold their glide for 2 seconds prior to the first kick.

 Travel at least 0.5 m below the surface for as long as possible.

 Have a maximum depth of between 0.9 – 1.0 m.

 Start their first kick after the centre of their head reaches 6.5 m from the block.

 Aim to breakout around 10.5 m.

Hold glide for 2 s

Travel at least 0.5 m below the

surface

Max depth between 0.9 – 1.0 m

Start first kick after 6.5 m

Breakout at 10.5 m

155 It is understood that actions during the on-block and flight phases would affect the speed achieved during the underwater phase (Lyttle & Benjanuvatra, 2005; Vantorre et al., 2014). The above-water parameters would provide the initial conditions for the underwater phase (speed, angle of entry) but how these parameters are subsequently managed after the swimmer enters the water can differ and rely heavily on the trajectory used in the underwater phase. Consequently, the actions indicated in the guidelines from the ideal underwater trajectory would reduce the amount of resistance acting on the swimmer, leading to the maintenance of a higher velocity during the underwater phase without unnecessary energy loss; ultimately resulting in faster start performances. Minimising the drag experienced by a swimmer during the underwater phase would enable the propulsive efforts in the forward direction to be maximised. In this case it would allow for more speed to be generated as the swimmer commences undulatory kick following the glide phase.

Furthermore, Chapter 4 also found an elite swimmer would enter the water at approximately 6.0 m.s-1 then decelerate to approximately 2.38 m.s-1. As such, if the swimmer selects the wrong trajectory, depth or kicks at the wrong time they would increase the resistance acting on them and decelerate to a greater extent, negating any increase in take-off horizontal velocity. Initiating a premature action during the glide phase can result in increasing physiological costs as well as unnecessary loss of speed compared to the speed maintained in a passive glide position (Naemi & Sanders, 2008). This further highlights the importance of the underwater phase and choosing the correct trajectory. Combining these results with knowledge of the swimmer’s underwater kicking ability and individual anthropometric characteristics in future studies would allow for more individualised approaches to achieving the ideal underwater trajectory.

156 As has already been stated in Chapter 4, the swimming start is a complex movement comprised of a number of different phases. There are a number of parameters that are closely interlinked and contribute differently to overall start performance. The use of performance analysis in Chapter 4 and 5 the underwater phase was identified as the most important aspect of overall start performance. Within the underwater phase, the trajectory, maximum depth and timing of first kick were also identified as key parameters for performance. Following on from these findings, Chapter 7 was able to determine which trajectory and combination of key parameters would be fastest for performance. Chapter 8 was then able to provide the theoretical basis for the results in Chapter 7 through the use of hydrodynamic analysis. By combining the findings from Section 1, 2 and 3, a multi-disciplinary approach was used to establish theoretical recommendations for the ideal underwater trajectory for the elite sample of swimmers used for the studies in this thesis.