## 1 Introduction  
Quadriceps femoris consists of four muscle heads: Vastus Lateralis (VL), Rectus Femoris (RF), Vastus Medialis (VM) and Vastus Intermedious (VI). VL is a surface head positioned at the external side of the thigh. RF is placed in the middle of the front side of the thigh, while VM is located at the anterior side. VI is placed at the interior compartment of the thigh as it is located under the three surface heads and above femur [1]. Each head has unique architectural properties such as volume, physiological cross-sectional area (PCSA; the largest cross-sectional area point of a pennate muscle, perpendicular to its muscle fibers), muscle thickness, muscle length, fascicle length, and fascicle pennation angle [2]. Due to its' position, VL is the most studied head of the quadriceps and it has been used as a surrogate for the whole quadriceps muscle for the evaluation of muscle size, electromyographic activity, and metabolic properties and muscle fiber composition, either for clinical or athletic purposes [3; 4; 5; 6; 7; 8; 9; 10; 11].  
Inactivity, aging, chronic diseases and systematic training (mostly resistance training), induce several well-known changes on quadriceps femoris size and morphology. These changes can be evaluated by several different methods, either invasive (muscle biopsies)or non-invasive (magnetic resonance imaging, ultrasonography, etc.). More specifically, the ultrasonography technique is a non-invasive, easy to use, valid and reliable method to evaluate quadriceps muscle cross-sectional areas (CSA), volumes and architecture characteristics [12; 13; 14; 15; 16; 17]. Thus, muscle ultrasonography has attracted the interest of many clinical and sports scientists, for the evaluation of quadriceps muscle CSA and architecture characteristics, both in cross-sectional and longitudinal studies. For example, the ultrasonography technique has effectively been used for the evaluation of training-induced changes in the whole quadriceps femoris CSA [18; 19; 20; 16]. Nevertheless, the majority of the researchers seem to evaluate only one muscle head of quadriceps femoris per time, mostly the VL's CSA and/or architecture characteristics before and after different training programs, for the determination of quadriceps femoris' training-induced muscle hypertrophy [21; 22; 23; 24; 25; 26; 27; 28; 29]. The decision for the evaluation only of VL for the training-induced hypertrophy is mainly based on reports, indicating that VL may be a good surrogate for the entire quadriceps muscle size, metabolic properties and muscle fiber composition as previously presented in cross-sectional studies [8; 11; 30; 31]. However, after systematic resistance training, the training-induced hypertrophy in the four muscle heads of quadriceps femoris is inhomogeneous, with significant differences between the four heads of quadriceps femoris, but also within the regions of each muscle head separately [19; 32]. Thus, considering the inhomogeneous training-induced hypertrophy between the four muscle heads of quadriceps femoris [19; 32], it is questionable if the evaluation of only VL's CSA through ultrasonography could provide representative, reliable and valid information about the training-induced hypertrophy of the whole quadriceps femoris. The identification of which of the four muscle heads of quadriceps femoris is/are a good surrogate/s of quadriceps hypertrophy, is of high scientific and practical importance, especially in long-termed training studies, with many participants, where the evaluation and the analysis of the whole quadriceps femoris CSA is time-consuming. Therefore, the purpose of the present study was to investigate which of the four muscle heads of quadriceps femoris is the best surrogate of quadriceps hypertrophy, following resistance training, evaluated by ultrasonography.