## 4 Discussion
### Progression of Performance and Physiological Parameters during the Training Sessions  
The performance time of the 4 \(\times\) 50 m front crawl sprints during the 6-week period was not changed. This contrasts with studies that reported athletes' performance improved after 2- [33] and 4-week SIT periods [34] in sports other than swimming. Healthy non-athletes participated in previous studies rather than the well-trained swimmers in the present study [34] and sprint duration may be a critical factor affecting the final outcome (i.e., 4 \(\times\) 30 s with 4 min rest or 8 \(\times\) 15 s with 2 min rest) [33]. Swimming studies highlighted performance improvements after 7- to 14-week periods [10; 12; 35]. However, swimmers performed different training protocols with the characteristics of ultra-short race pace (i.e., eight repetitions \(\times\) 20 s) or high-intensity interval training (i.e., four repetitions \(\times\) 4 min) [12; 14; 35] compared to the current study (four repetitions\(\times\) 50 m starting every 2 min). Moreover, swimmers applied 2 \(\times\) 4 \(\times\) 50 m training sets three times per week. Despite the fact that this type of training is related to swimmers' performance improvement in short-distance events [6; 7; 10], we found no performance improvement in the current study in the 4 \(\times\) 50 m front crawl training set or in the 100 m test.  
Despite the unaltered performance time in 4 \(\times\) 50 m, this type of SIT (2 \(\times\) 4 \(\times\) 50 m) may be suitable for swimmers' physiological adjustments. Specifically, swimmers managed to increase blood lactate concentration after week 3 and week 5 compared to week 1. This finding is in accordance with that reported in other studies [12; 32]. Increments in swimmers' blood lactate after 4 \(\times\) 50 m may indicate a higher energy contribution from anaerobic metabolism [35]. It is likely that SIT leads to physiological adjustments related to anaerobic metabolism and possible increments in the activity of anaerobic enzymes such as phosphocreatine and lactate dehydrogenase [16; 26; 27; 34].  
However, there are some limitations in the current study that need to be mentioned. Swimmers performed the main training set (2 \(\times\) 4 \(\times\) 50 m) during the 6-week training competing in pairs, while in Pre and Post testing, the 4 \(\times\) 50 m performance was evaluated for each swimmer separately. Therefore, a comparison of the variables recorded during the training set with those obtained Pre and Post the 6-week period is not appropriate. Moreover, both male and female swimmers participated in the current study and no control group was included. The sample size of the present study was moderate, which may reduce the statistical power and increase the margin of error [36]. As such, a higher sample size, a more homogenous group of participants, and a longer period of training may be required to induce changes in swimmers' performance. Finally, we have only tested BL to examine possible changes in anaerobic metabolism; however, any contribution from phosphocreatine was not possible to examine. In addition, future studies may focus on the evolution of physiological changes by comparing different testing protocols.