# Morphological and Mechanical Symmetry of the Quadriceps and Hamstrings in Athletes
## 4 Discussion  
The primary objective was to assess the morphological muscle symmetries (CSA and EI) and torque of quadriceps and hamstrings between the DO and ND limbs in competitive athletes practicing team sports. The secondary objective was to examine the relationship between the maximum concentric torque (60\({}^{\circ}\)/s and 300\({}^{\circ}\)/s) of quadriceps and hamstrings, based on CSA and EI, in the respective muscle groups. No significant differences were found between limbs for CSA, EI, or torque, indicating bilateral symmetries in quadriceps and hamstrings between the DO and ND sides. Additionally, CSA and EI of quadriceps and hamstrings were determinants of knee extensor and flexor maximal concentric torque at 60 and 300\({}^{\circ}\)/s. Our findings on symmetry between DO and ND limbs and their muscle morphological characteristics (CSA and EI) corroborated the findings of [PERSON] et al. [41], reporting no differences between sides for the CSA and EI of the quadriceps in cyclists. Furthermore, [PERSON] et al. [42] also found no difference in quadriceps and hamstring CSA between DO and ND limbs in soccer players. Combining these findings suggests that the extent of morphological asymmetries may influence muscle strength, as the absence of torque asymmetries between sides appears to be associated with a lack of morphological differences. The investigation of torque asymmetries has become a prominent topic in sports science, as it may impact in injury risk, performance and functional adaptation [5; 43; 44]. Regarding injury risk, limb strength asymmetry (equal or more than 15% of strength imbalance between sides) can lead to biomechanics alterations in sports movements, increasing the risk of injury [45]. Additionally, the muscle size characteristics (e.g., muscle volume or CSA) may have influence in strength as well as the extent of strength asymmetries. When strength asymmetry is identified, implementing a targeted strength training program is recommended to restore the balance in force production [46; 47; 48; 49; 50]. Our results also highlighted muscle quality by EI analysis as an important variable for understanding muscle force, reflecting the amount of contractile tissue in a given area [12]. Our results demonstrated that the assessment of EI can complement another small aspect of musculoskeletal performance alongside CSA. Corroborating our findings, Fukumoto  
Figure 3: Three-dimensionally illustrated linear regression between independent variables (cross-sectional area—CSA; echo intensity—EI) of hamstrings with dependents variables (knee flexor maximal voluntary concentric contraction at 60 and 300\({}^{\circ}\)/s) in dominant and non-dominant limb of team sports athletes. et al. [24] showed that muscle thickness and EI, as independent variables, significantly explained 29% of muscle strength. Additionally, they found a low negative correlation between EI and muscle force (r = \(-\)0.40), consistent with the findings of [PERSON] et al. [27], who reported low and moderate correlations of muscle EI with isometric and concentric knee extension torque (r = \(-\)0.48 to \(-\)0.67, respectively). However, the lack of evidence on the specific role of EI in each muscle remains a limitation, as this variable has been shown to be associated with performance-related tasks such as vertical jumps, sprints and agility [51]. Given these associations, incorporating EI in future studies could lead to more accurate prediction of muscle strength, as it provides insights into muscle quality that CSA alone may not capture. Our findings demonstrated that CSA and EI from quadriceps and hannstrings are determinants of knee extension and flexion concentric torque, corroborating [PERSON] et al. [47], who also found quadriceps CSA explain 79% of maximum isometric knee extension torque in healthy subjects. However, the authors reported that hannstrings CSA only represents 25% of maximum isometric knee flexion torque in health subjects, a value lower than our findings (more than 70%).