152 Chapter 7 velocity was defined as the clean swimming velocity (10-m distance divided by time for the 10-m distance, m/s) between the 10- and 20-m segment of the 25-m trial. Regardless of distance and stroke, this parameter is crucial for any swimmer aiming to touch the wall first (Barbosa et al., 2010), given that clean swimming predominates in (long course) swimming events (Gonjo & Olstad, 2020). Stroke rate (Hz) was calculated as the number of strokes completed by the swimmer during this 10-m segment (Poujade et al., 2002), one stroke rate cycle being defined as the time between the entry of one hand until the following entry of the same hand (Huot-Marchand et al., 2005). Stroke length (m) was calculated as the ratio between swimming velocity over the 10-m segment and the corresponding stroke rate (Poujade et al., 2002). Stroke index (SI), an indirect measure of swimming efficiency, was calculated by multiplying swimming velocity by stroke length. The SI measures the ability of the swimmer to complete a given distance with a particular speed in the fewest possible number of strokes (m2/s) (Costill et al., 1985). Maximal swimming velocity and SI were taken as outcome measures for further analyses. Data processing To enable meaningful comparisons among swimmers specialized in different strokes and distances, outcomes were related to relevant reference values and expressed as a percentage, rather than absolute values (see equation 1). This approach is essential because direct comparisons of absolute values in swim performance and test scores within our sample could potentially lead to misconceptions. For instance, it is widely acknowledged that the breaststroke is inherently slower to perform than the freestyle (Moser et al., 2020). Similarly, when considering distance, it is evident that the duration of an event increases with the length of the distance to travel (Moser et al., 2020). Taking these stroke-specific and distance-related nuances into account ensures a more accurate evaluation of swimmers' capabilities. Consequently, swim time was related to the prevailing world record (WR), a method initially introduced by Stoter et al. (2019) in speed skating and subsequently applied in competitive swimming (Post et al., 2020a, 2020b). Lower percentages on relative Swim Time (rST) indicated swim performances closer to the WR. Moreover, scores on swimming tests were related to the average start time, turn time, clean swimming velocity and SI of male and female finalists at the European Championships in 2021 (Born et al., 2022). Stroke-specific data of the 100- and 200-m events were used as reference values for sprinters (50-100-m) and middle-distance (200-400-m) swimmers in our sample respectively (see Appendix A). Higher percentages on relative maximal swimming velocity (rMSV) and stroke index (rSI) indicate scores more close to the European elite level (set to 100%). For example, the maximal swimming velocity of an early junior male freestyle sprinter (1.85 m/s) was related to the average clean swimming velocity of the 100-m freestyle European male finalists (1.98 m/s), resulting in a rMSV of 93.4% ((1.85/1.98)*100%).
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