## Materials and Methods
### Determination of Kinematic Variables  
Both during stages 1 and 2, the Kinovea program (version 0.8.15, [PERSON], Bordeaux, France) was used to determine kinematic variables. The time to cover each recorded 10-m section (T\({}_{10}\)) was determined on the basis of the line joining the markings on the edges of the pool. The beginning and end of the 10-m swim was identified as the moment when the head crossed the designated lines (Figure 1). Based on the recordings, the duration of five cycles in the middle zone (T\({}_{\text{SC}}\)) was also determined. The entry of the hand into the water was assumed as the beginning of the cycle. The same hand was dipped in the water again to complete the cycle. Based on the activities described above, for each covered section in the SAST and during the competition, the following were determined:  
* mean speed: v = 10/T\({}_{10}\) [m/s];
* stroke rate: SR = 60 \(\times\) 5/T\({}_{\text{SC}}\) [cycle/min];
* stroke length: SL = v \(\times\) 5/T\({}_{\text{SC}}\) [m];
* stroke index: SI = v \(\times\) SL [m\({}^{2}\)/s].  
The values of these variables from the SAST sample were used to later calculate the arithmetic mean (\(\overline{\text{x}^{\prime}}\)), characterizing swimming during the initial (v\({}_{\text{S}}\), SR\({}_{\text{S}}\), SL\({}_{\text{S}}\), SI\({}_{\text{S}}\)) as well as in the final parts (v\({}_{\text{F}}\), SR\({}_{\text{F}}\), SL\({}_{\text{F}}\), SI\({}_{\text{F}}\)) of the test. The first group of indices was calculated on the basis of the results obtained in the 1 st and 2 nd measurement sections (10 m). The second type was calculated on the basis of the results obtained while swimming the 5 th and 6 th measurement sections of the SAST.  
Additionally, the values of the fatigue indices were calculated in the SAST:  
* absolute: IFABS = SIS-SIF [dimensionless]relative: IFREL = IFABS \(\times\) 100/SIS [%]  
Regarding the races, analogous variables were determined, except for the fatigue indices. In the 100-m freestyle swimming competition, the values of \(\mathrm{v_{S}}\), \(\mathrm{SR_{S}}\), \(\mathrm{SL_{S}}\), \(\mathrm{SI_{S}}\) were obtained on the basis of the recording the first 50-m section, while \(\mathrm{v_{F}}\), \(\mathrm{SR_{F}}\), \(\mathrm{SL_{F}}\), \(\mathrm{SI_{F}}\) were determined thanks to video recording analysis carried out swimming the second (final) 50-m section.  
In a similar manner, the unified variables in the 200-m freestyle swimming competition were also determined. The values of \(\mathrm{v_{S}}\), \(\mathrm{SR_{S}}\), \(\mathrm{SL_{S}}\), \(\mathrm{SI_{S}}\) were determined on the basis of the record of covering the first 50 m of the distance, while to determine \(\mathrm{v_{F}}\), \(\mathrm{SR_{F}}\), \(\mathrm{SL_{F}}\), \(\mathrm{SI_{F}}\), the record of covering the 4 th length of the pool was used, i.e., the section between 150 m and 200 m of the distance. An additional source of data describing the performance of the tested swimmers at competitions were the official results available on the website: [[https://swimrankings.net](https://swimrankings.net)]([https://swimrankings.net](https://swimrankings.net)) (accessed on 6 October 2021). Further analyses included:  
time for the 100-m race: T100 [s]  
time for the 200-m race: T200 [s]