## 4 Discussion  
The purpose of the study was to investigate which of the four muscle heads of quadriceps femoris is the best surrogate of quadriceps resistance training-induced hypertrophy, evaluated by ultrasonography. The main finding of the present study was that training-induced changes in VL and VI CSAs, may be good surrogates of the whole quadriceps muscle hypertrophy following strength training, at least when the CSAs are evaluated with ultrasonography and at the 40% of the distance between the center of the patella and medial aspect of the anterior superior iliac spine, proximal to knee. According to the results of the present study, it could be suggested that when the evaluation of the whole quadriceps training-induced hypertrophy could not be performed, clinical and sports scientists may evaluate first the training-induced changes in VL's CSA, and if this evaluation is also not possible then to choose VI for the estimation of the whole quadriceps training-induced hypertrophy. Although VM CSA was significantly increased following training while RF CSA remained unchanged, these muscle heads are suggested to be avoided as predictors of the whole quadriceps hypertrophy. The results of the present study are in line with previous reports, especially with cross-sectional studies, indicating that VL is a good surrogate for the whole quadriceps muscle size, electromyographic activity, and force/power production [8; 11; 30; 31]. However, according to the authors knowledge, this is the first study to present that the ultrasonographic evaluation of training-induced hypertrophy of VL and VI could be used for the representation of whole quadriceps muscle hypertrophy. The results of the present study are of high practical importance, especially for clinical and sports scientists who investigate the effect of resistance training on muscle mass and morphology. A strong aspect of the present study was that it employes all of the needed statistical analyses as well as an absolute agreement analysis between the percentage training-induced changes in CSA of T versus those of VL, RF, VM and VI. Until now, the majority of the studies investigating which of the four heads of quadriceps is the best surrogate for T quadriceps only the Pearson-r correlation coefficient and/or the ICC coefficient [8; 11; 30; 31]. However, using Pearson-r correlation coefficient and/or the ICC coefficient could not provide sufficient evidence about the accuracy and reliability in such type of studies [36; 37; 38; 39; 40; 41; 42]. Thus, further statistical analysis was needed, such as SEM, CV, mean of the differences, Bland-Altman plots, LOA, SEL and RC, to provide readers and professionals details regarding the accuracy as well as the extent of the existing error between the mesoaments [36; 37; 38; 39; 40; 41; 42]. According to the results of these analyses, in the present study, all of the established criteria for high accuracy, reliability, repeatability and reproducibility have been met for the ultrasonographic evaluation of VL's and VI's CSAs' training-induced increases, as potent surrogates of whole quadriceps muscle hypertrophy. Contrariwise, very low values were observed for VM and especially for RF. Specifically the training-induced change in RF hypertrophy were significantly different from those observed either in the other three heads of quadriceps, but mostly of the whole quadriceps hypertrophy. Significant inhomogeneous hypertrophy between the four heads of quadriceps has been previously reported, with significant hypertrophy to observed in whole quadriceps muscle, and specifically in VL, VI and VM, while no hypertrophy in RF was found after different types of multijoint lower body exercise [18; 19], such as those adopted in the present study. However, the highest hypertrophy in RF compared to the other three quadriceps heads has been reported only after resistance training with leg extension [32; 43; 44; 45; 33].