## 4 Discussion  
The main purpose of this study was to determine the relationship between RF and VIM behavior measured in RUSI M-mode under MVIC and anthropometric measurements. The hypothesis of this study is that there could be a linear correlation between anthropometric variables and muscle behavior measured by ultrasound. The results obtained in this research showed a significant correlation in the comparison of some variables, especially those related to the VIM. In relation to the variables' QT length, distance between spines, RF and VIM contraction ratio, no significant correlations were found. There is a correlation between resting VIM thickness and three other variables: three-thirds thigh perimeters, RF length and lower limb length. VIM thickness contraction correlates with three other variables: proximal, medial and distal thigh perimeters, RF length and lower limb length. The only correlation found in relation to RF was resting thickness with RF length. In the correlations described above, the higher the RUSI measurement, the higher the anthropometric variables were. Figure 4: Significant correlations (\(p<0.05\)). Regarding the statistical analysis it must be noted that this test indicates that the best model is the one that assumes an exchangeable correlation structure versus independence, so the model that assumes a fixed correlation between each pair of RF and VIM measurements in each subject fits better than the one that assumes no correlation. This is a statistical argument that reinforces considering the data related, because the best model is the one that assumes this correlation structure, although biomechanically it is possible to consider two independent muscles. The \"exchangeable\" one is the one with the least amount of information not explained by the model and the one where the difference between the standard and robust errors is the smallest (Figure 3). In previous studies where QF has been evaluated, RUSI has proven to be a valid and reliable tool with excellent ICC in the determination of muscle thickness (ICC = 0.95), contraction and relaxation velocity (ICC 0.99 (CI 0.99-1.00)) [47; 51; 52]. The results for RF thickness at rest (2.11 \(\pm\) 0.44) and contraction (2.72 \(\pm\) 0.52), are comparable to those obtained in previous studies describing resting RF values of (2.1 \(\pm\) 0.22) [45] and contraction (2.72 \(\pm\) 0.17) [47]. The data obtained in relation to VIM thickness at rest (1.69 \(\pm\) 0.40) are like those obtained in a previous study in a similar population (1.71 \(\pm\) 0.44) [53]. Regarding the values of thickness in contraction of the VIM, the results obtained (2.11 \(\pm\) 0.38) are not comparable to others, since to the authors' knowledge there are no reference values described in the literature to date. A previous study demonstrated a loss of VIM thickness at rest after ACL reconstruction surgery [54]. It is considered that it could be of interest to determine the possible changes in thickness at contraction, since they could provide information regarding the functionality of the muscle. The results obtained in this investigation in relation to this variable could serve as a criterion for comparison in future studies. These results give more robustness to the existing normality values described for the thickness of RF and VIM in healthy subjects. Considering the clinical significance of the loss of RF and VIM thickness in both subjects with ACL injuries and in subjects who are in a state of frailty or sarcopenia, we consider that for future research these data could be of interest to determine global reference values. There is evidence for the validity and reliability of contraction and relaxation velocity measurements in different muscle groups, however, there is still not enough evidence to establish normative values [42; 55; 56]. The results obtained for RF contraction and relaxation velocities (4.42 \(\pm\) 2.51 and 2.39 \(\pm\) 0.77) differ considerably from those described in studies with similar methodologies (1.07 \(\pm\) 0.08 and 0.93 \(\pm\) 007) [47].