22 Chapter 2 Inclusion criteria For the purpose of this study, it is important to outline the individual performance development from a young age on towards the adult age of peak performance (or beyond). Therefore only those swimmers who; 1) were between 12 and 24 years old (female) or between 12 and 26 years (male) old; 2) were in competition for at least 5 seasons; 3) had at least one SBT within the age category of 16 years or younger; and 4) had at least one SBT within the age category of 20 years (female) or 22 years (male) or older were included (Allen et al., 2014; 2015). This resulted in 5,636 individual swimmers (3,259 female, 2,377 male) with 40,063 SBT’s (22,239 female, 17,824 male) with an average of 7.6 ± 2.1 observations per swimmer. Defining swim performance and performance development The present study includes swim performances of multiple generations, necessitating the correction of evolution in a given sport (Stoter et al., 2019). The continuous increase in world-class performances at Olympic Games and World Championships clearly reflects the evolution in a sport, as well as the improvement of world records (Stanula et al., 2012; König et al., 2014). For example, at the 100m freestyle event, the world record for females has been improved from 54.48 seconds to 51.71 seconds with 2.9 seconds (~5.3%) from 1994 to 2017 (FINA) and for males from 48.42 seconds to 47.04 seconds (fastest time in textile) with 1.38 seconds (~2.9%). To correct for evolution in competitive swimming, we use a method to compare performance over multiple generations, introduced and validated by Stoter et al. (2019). First, each swimmer’s SBT per swimming season between 2018 and their earliest available competitive performance was determined. Second, SBT’s 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 athlete swam the SBT. WRs from 2008 or 2009 were replaced by the prevailing fastest time in textile. The corrected SBT will be referred to as relative Season Best Time (rSBT) and is presented as a percentage of the world record or fastest time in textile. In this study, rSBT defines swim performance (see equation 1).
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