# Profiling of Physical Qualities of Highly Trained Portuguese Youth Soccer Players
## 4 Discussion  
To the best of our knowledge, this is the first study in soccer that compares physical assessment test results across different age categories and between playing positions, and correlates the physical tests. The main findings of this study were (i) higher age categories outperformed lower age categories in all tests; (ii) no differences were found between playing positions; and (iii) moderate correlations were seen between the IMTP and all the other tests while a strong correlation was observed between the SJ and CMJ. In accordance with our initial hypothesis, players from higher age categories achieved generally better results. Indeed, significant differences were found on the IMTP between U23 and U17 and U16, and U19 and U16. These findings could be explained by size-dependent factors, such as muscle mass, as well as size-independent factors like genetics and hormonal influences [32]. Players from higher age categories are also more experienced in strength training, benefiting from the morphological and neural adaptations associated with the recurring strength training stimulus, resulting in a greater maximum force capacity. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**IMTP** & **Squat** & **Countermovement** & **Broad** & **Sprint** \\  & **Jump** & **Jump** & **Jump** & **Sprint** \\ \hline
**IMTP** & \(r\) = 0.30 (\(p\) = 0.003) * & \(r\) = 0.43 (\(p\) = 0.001) * & \(r\) = 0.38 (\(p\) \(<\) 0.001) * & \(r\) = 0.33 (\(p\) = 0.002) * \\ \hline
**Squat Jump** & & \(r\) = 0.86 (\(p\) \(<\) 0.001) * & \(r\) = 0.59 (\(p\) \(<\) 0.001) * & \(r\) = 0.37 (\(p\) \(<\) 0.001) * \\ \hline
**Countermovement Jump** & & & \(r\) = 0.60 (\(p\) \(<\) 0.001) * & \(r\) = 0.42 (\(p\) \(<\) 0.001) * \\ \hline
**Broad Jump** & & & & \(r\) = 0.39 (\(p\) \(<\) 0.001) * \\ \hline
**Sprint** & & & & \(r\) = 0.39 (\(p\) \(<\) 0.001) * \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Pearson correlation analysis between the different physical performance tests. * denotes a significant correlation between tests (\(p\) \(<\) 0.05). This is corroborated by studies showing an increase in peak force, measured via IMTP testing, with age throughout different age categories [15; 33]. Regarding explosive force, statistically significant differences were found across all tests. The CMJ and SJ tests revealed differences between every age category compared to U16. Moreover, in the BJ, the U23 and U19 age categories performed better compared to the U16 age category. Jumping test performance provides shorter than necessary time-frames for players to achieve their maximal force capacity, and stronger players perform better in jump performance [34]. Stronger players present greater overall force generation capacity independent of the time constraints associated with force production. In fact, late rate of force development and maximum voluntary contraction torque were found to be correlated with muscle mass having a significant influence on the late rate of force development capacity [35]. However previous studies have reported divergent findings. For instance, [PERSON] and [PERSON] [19] found that vertical jump was not a suitable metric for distinguishing young Italian national players across different age categories (U17, U20, and U21). The results of the present study are consistent with this observation as the U23, U19, and U17 groups did not exhibit statistically significant differences in vertical jump performance. This lack of differentiation can be attributed to the average ages of the groups examined: 19.7 years for U23 and 18.1 years for U19, which approximately correspond to the U20 category in [PERSON] and [PERSON]'s study [19], and 16.6 years for U17, aligning closely with their U17 category. This age similarity could explain the similar performance, particularly between the U23 and U19 age categories. Regarding sprint performance, significant differences were observed in all age categories outperforming the U16.