# Morphological and Mechanical Symmetry of the Quadriceps and Hamstrings in Athletes
## 4 Discussion
The discrepancies could be attributed to two points: (I) Our protocol was based on torque-velocity concentric contractions, whereas [PERSON] et al. [47] used isometric contractions of knee extension and flexion. (II) The isometric contraction of knee extension and flexion torque at 45\({}^{\circ}\) knee flexion used by [PERSON] et al. [47] may not represent the maximum force capacity of both muscle groups, as the muscle is not at its optimal length for maximum force production [52]. In contrast to our results, the authors related that the relation between quadriceps and hannstrings did not seem to be determinate by muscle groups CSA relation. This discrepancy might be explained by differences in the population studied, as only nine soccer professional players participated in their study, and they only collected five maximum voluntary contractions of the knee extensors and flexors at 60\({}^{\circ}\)/s to assess Hcon:Qcon [42]. These disagreements in the literature might stem from a misunderstanding of the contribution of each muscle within the CSA of the involved muscle groups, as each muscle plays a distinct role in force production and performance [19; 47; 53]. In line with this, [PERSON] et al. [19] demonstrated the correlations between quadriceps CSA (e.g., rectus femoris) and isometric and isokinetic strength; regression analysis showed that CSA of the rectus femoris measured during the relaxed knee at 60\({}^{\circ}\) of flexion (r2 = 0.53-0.61) was a significant predictor of both isometric and concentric strength of knee extension. Similarly, [PERSON] et al. [47] found a substantial contribution of vastus medialis and long head of the biceps femoris CSA to maximum isometric voluntary contraction (r2 = 0.72; r2 = 0.27, respectively). Additionally, CSA of vastus medialis has been linked to performance in specific tasks, such as squat variations (r2 = 0.50-0.57) [53]. When CSA, EI, and torque variables are analyzed together, opposite Q:H correlations emerge. These differences between the extensors and flexors of the knee may be attributed to the influence of the pennation angle of fascicles, which, when reduced, affects how the ultrasound wave is reflected, producing a clear image. However, the quadriceps muscles have a greater pennation angle when compared to the hannstrings, thus the transmission of force from each fascicle to the line of action of the force is impaired [32; 54]. Additionally, EI can vary depending on the predominant muscle fiber type [55]. The opposite relation between EI and hannstrings compared to EI and quadriceps may be influenced by the greater amount of adipose, non-contractile tissue in flexor CSA [31]. Other measures of muscle size may provide a more comprehensive understanding of sports performance and injury prevention, for example, [PERSON] et al. [56] demonstrated that quadriceps muscle volume and knee extensor joint moment arm are significantly correlated with knee extensor isometric torque (r = 0.78 and 0.79, respectively). [PERSON] et al. [9] aimedto investigate the magnitude of volume asymmetries of the quadriceps and hamstrings muscles in healthy individuals and found volume asymmetries of the entire muscle group of quadriceps (8.8%) and hamstrings (11.4%) with a 95% limit of agreement. The present data have some limitations, such as the small sample size. Despite this, the results of the backward multiple linear regression model demonstrate high power and effect size. However, we suggest that future studies carry out our experiments with a larger sample size, thus ensuring greater reliability of the statistical model used in this study. Furthermore, assessment of CSA in the same femur length does not represents the complete muscle size. Otherwise, this technique correlates with the golden standard, magnetic resonance [57]. Furthermore, the lack of evidence about each muscle in function and muscle strength may be necessary in future research to understand the true role of CSA and muscle quality in sports performance.