43 Interim performance progression of talented swimmers 3 age on the 31st of December of the swimming season (KNZB, 2018). Therefore, all ages mentioned in the present study refer to the age category in which a swimmer participated during the swimming season and not the calendar age of the swimmer. Defining swim performance and performance groups The present study includes swim performances of multiple generations, necessitating the correction of evolution in a given sport (Stoter et al., 2019; Post et al., 2020). The method we used to correct for the evolution in competitive swimming was introduced by Stoter et al. (2019) in the sport of speed skating and later successfully used by Post et al. (2020) in the sport of competitive swimming. Swim performances were related to the prevailing world record (WR) or the fastest time in textile of the corresponding sex. The prevailing WR is the official WR at the date the swimmer performed the swim time. WRs from 2008 or 2009 were replaced by the prevailing fastest time in textile. The corrected swim time will be referred to as relative Swim Time (rST) and is presented as a percentage of the world record or fastest time in textile. In this study, rST defines swim performance (see equation 1). Two performance levels were defined: elite and high-competitive. Each performance level was characterized by sex-specific limits to account for differences in competition level between males and females (see Table 1). The limits were calculated as the mean of 5 season best rST’s for the 50th swimmer from either the 100m freestyle performance FINA World Ranking Lists of 2015-2019 (FINA, 2019) or the 100m freestyle performance National Ranking Lists of the Netherlands 2015-2019 (Swimrankings, 2019). The limits of the elite performance level were equal to the average of the season best rST’s of the 50th male and female swimmer of the FINA World Ranking List 2015-2019. The limits of the highcompetitive level were defined so that they represented the 50th male and female swimmer of the National Ranking List of the Netherlands. We determined each swimmer’s best performance group by allocating the best rST ever to one of the two performance levels, meaning that a swimmer either once or multiple times has reached this performance level at any age. For example, if a male swimmer has a best rST of 109.0%, his best performance level corresponds with the limits of the highcompetitive performance group. Swimmers with a best rST ever outside the limits of the high competitive level (best rST >114.0% for males and best rST > 115.1% for females) were excluded from further analysis.
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