*4.3. Neuromuscular Properties*

Our intervention improved core muscle functionality, which was assessed by the TMG parameters. TC decreased (−1.5 ms) for ES, (−7 ms) LD, and (−1.9 ms) EO, and DM increased (+3.3 mm) for ES, (+0.5 mm) LD, and (+2.2 mm) EO compared with the baseline value. The control group was (5.8 ms) TC-ES, (3.7 ms) LD, (2.5 ms) EO, (0.2 mm) DM-ES, (−4.1 mm) LD, and (−1.0 mm) EO compared with the baseline value. The beneficial effect was large for TC-ES and TC-LD and moderate for TC-EO, DM-ES, DM-LD, and DM-EO. The group comparison (95% CI) was statistically significant *p* ≤ 0.05 for TC-ES, TC-LD, TC-EO, DM-LD, and DM-EO and *p* ≤ 0.12 for DM-ES. We assumed that the increase in TC and the decrease in DM for the control group were due to the slow process of recruiting motor units [16]. Our findings are consistent with some previous studies [16,22,52].

Rusu et al. [16] showed that a six-week isometric–concentric training could improve muscle functionality associated with a decrease in TC and an increase in DM values, leading to a high rate of Type II muscle fibres and a faster process of motor unit recruiting for the experimental group. Conversely, the control group showed an increase in TC and a decrease in DM values, indicating a slow motor unit recruiting process. Valverde et al. [22] found that a decrease in TC and increased DM values indicated a good response following muscle training. Monteiro and Massuca [52] showed that a decrease in muscle TC and an increase in DM had a beneficial effect on the muscle following training. Rusu et al. [16] suggested that concentric training could lead to an increased TC and a reduced DM, which is the opposite effect of isometric–concentric training and could occur due to the enlargement of the muscle as a result of concentric training. Muscle fatigue could change the TMG parameters, as it could increase TC and decrease DM as a result of high-intensity resistance, interval training, or endurance training over short periods [53]. However, our findings support that a decrease in TC and an increase in DM following isometric–concentric training could improve muscle functionality.

#### *4.4. Practical Application*

The results showed that a core-training intervention can enhance swimmers' performance and improve muscular functionality. The findings from this study can be generalised to any swimmer in the age group studied. Therefore, our findings can be helpful as evidence to assist coaches, trainers, and therapists in applying a core-training program along with regular training to improve the swimming performance of young swimmers. Nevertheless, this study has several limitations. First, puberty, maturation, and testosterone hormones could have influenced the results. They vary from child to child, cannot be controlled, and may mask training effects [54]. Further research should include or control for the effect of growth and maturation, which is a major hurdle when studying young athletes. Second, only male swimmers were recruited for the study, and thus, our findings cannot be generalised to female swimmers. Third, we investigated only the effect of a core-training intervention on the 50 m freestyle. We recommend that other swimming styles and longdistance swimming be included. Further studies are needed to investigate larger groups and athletes at different levels and those from different places or clubs. Fourth, some of the exercises used in the core-training program (e.g., shoulder press) might have caused an improvement in the upper and lower limb strength, which may influence the swimming performance. Fifth, we were unable to demonstrate the effect of the core-training intervention on stroke depth. Further research is needed to demonstrate the effect of core training on stroke depth. Nevertheless, we found a significant improvement in our performance measures after considering the effects of body mass and age.
