## 4 Discussion
### Progression of Performance and Biomechanical Variables during the 6-Week Training Period  
The swimmers in all groups improved their performance within the 4 th week of training (see Figure 4). Other studies have reported similar findings, either with the concurrent training [10] or with a sprint interval training only [38]. However, the present study is the first that examined the progression of swimming performance during a training period and including different dryland training content. Possibly, these types of training and the specific characteristics (intensity, rest, duration) facilitated cardiovascular and neural adaptations in a short period of 4 weeks of swimming training [38]. Despite the fact that we did not measure any physiological variables in the current study, we found that DS (indicating fatigability) was similar between groups throughout the 6 weeks of training period. In addition, the calculated DS was decreased in all groups in the 4 th compared with 1 st and 2 nd weeks. It is possible that physiological and neural adjustments occurred in the 4 th week in agreement with performance improvements at the same time point of intervention (see Figure 4). This finding in the current study may indicate that the swimmers perceived less effort (decreased DS) during the 4 \(\times\) 50 m training set, because of the training-induced adaptations (see Figure 4).  
It has been shown that the biomechanical variables may explain any performance progression within this short period of training [39]. We found that all groups increased their SL and SI during the 6-week training period. These SL and SI increments may be connected with strength gains and related variables such as motor unit recruitment [37]. In addition, the SL and SI increased (see Figures 6 and 7) while DS was decreased (see Figure 4) at week 4 in all groups. It is likely that swimmers in all groups adjusted their technique to be more efficient (decreased DS) during the 4 \(\times\) 50 m training set. However, the G-MS and G-ME groups increased their SR during the 4 \(\times\) 50 m training set. This may indicate that the concurrent application of MS or ME with the SIT session facilitated a progressive increase in muscle strength and adequate neural adaptations, increasing SR and SL. However, any fatigue induced by MS and ME sessions may have forced swimmers to adjust to a higher SR during the subsequent swimming training session [40]. Thus, the increase in SR in the MS and ME groups in week 6 may indicate accumulated fatigue.  
Whatever the case, swimmers in each group may have applied different adjustments in biomechanical variables that progressively increased the speed in the MS and ME groups.  
Possibly, the concurrent application of dryland training with the characteristics of maximum strength or muscular endurance and SIT is a promising type of training compared with SIT only, in improving the biomechanical variables in swimmers. Then, swimming coaches may construct a training session which includes the concurrent MS and SIT or ME and SIT. There are some limitations of the present study that should be mentioned. Both male and female swimmers participated in the study and the SIT session included short duration efforts and resting intervals (approximately 35 s and 90 s, respectively) between each 50 m sprint.