## 4 Discussion
### Pre- vs. Post-Training Changes  
Swimmers in all groups managed to improve swimming performance after the 6-week training period. Previous studies have reported that the concurrent training applied two to four times per week improved performance by 2% to 4% in distances of 25 to 400 m after 6 to 12 weeks of training [10; 28; 29]. However, in previous studies, the swimmers did not perform concurrent training on the same training session. In particular, they followed a concurrent application under different periodization models; for example, they applied dryland and swimming sessions separated by 7 h or on different days [10; 15; 16]. Moreover, SIT training only may mask swimming performance improvement during the concurrent dryland and SIT training, since such a type of training may by itself improve performance [13; 15; 30]. Probably, the concurrent dryland and SIT, or SIT alone, improved the aerobic and anaerobic metabolism during the 6 weeks of training period [31; 32; 33]. Furthermore, irrespective of concurrent resistance and SIT, or SIT only, the expected increments in glycolytic, oxidative enzyme activity, muscle buffering capacity, and ionic regulation is apparent [31; 32; 33]. Notwithstanding, swimmers in the current study managed to improve the performance after 18 sessions compared with a previous study, where 27 sessions were applied [13]. On the contrary, the SR and SL remained unaltered after the 6-week training period and this is in agreement with previous findings [10; 15]. However, the swimmers increased their SI after 6 weeks of training in all groups. Irrespective of the training group, all swimmers were more efficient (higher SI) during the 4 \(\times\) 50 m sprints after the 6-week mesocycle [14]. Possibly, neuromuscular and mechanical adaptations occur after the application of the concurrent maximum strength or muscular endurance dryland training and SIT, or SIT only. It is well known that the SI is a biomechanical parameter that relates with swimming speed [14] and it is possible that SI increments may be more closely connected to speed improvements. The correlation of performance changes with SI changes indicates that the swimmers in the G-ME and G-CON who improved their SI were also able to improve performance in the 4 \(\times\) 50 m training set. The increased SI with unchanged SL during the 4 \(\times\) 50 m sprints may indicate that the swimmers applied more propulsive force during this set, but without affecting their swimming economy. The last was also observed in a previous study where the increased SR without a decreased SL helped in adjusting stroke mechanics \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Variables** & **G-MS** & **G-ME** & **G-CON** \\ \hline performance time (\%) & \(-4.7\pm 2.8\) & \(-4.3\pm 4.7\) & \(-0.5\pm 2.5\) \# \\ SR (\%) & \(4.0\pm 4.3\) & \(1.9\pm 7.1\) & \(-3.1\pm 6.4\) \\ SL (\%) & \(1.2\pm 4.3\) & \(3.2\pm 10.9\) & \(4.0\pm 7.2\) \\ SI (\%) & \(6.3\pm 6.6\) & \(8.5\pm 17.4\) & \(4.6\pm 8.6\) \\ \hline \hline \end{tabular} \#_p_ < 0.05, between G-MS and G-CON.  
\end{table} 
Table 7: Performance time, stroke rate (SR), stroke length (SL), and stroke index (SI) percentage differences (%\(\Delta\)) in the 4 \(\times\) 50 m training set in week 6 vs. week 1 for the three groups of swimmers participating in the study. G-MS: group of maximum strength, G-ME: group of muscular endurance, and G-CON: control group. and maintain economy [33]. The performance change in G-CON was correlated with SR change, indicating that swimmers in this group were able to improve SR concomitantly with their performance. It is possible that SIT during the 6-week period improved specific fitness aspects (i.e., buffering capacity, aerobic power) concomitantly with swimming efficiency [34]. However, we did not measure any physiological variables, and further studies need to be conducted for safer conclusions.