###### Abstract
Previous studies have shown that QF neuromuscular function remains altered in the short and long term after knee arthroscopy for meniscal surgery and/or ACL reconstruction. Compared with patients who had undergone meniscus surgery, patients with ACL reconstruction showed greater asymmetries in maximal voluntary contraction torque (15% vs. 5%, \(p=0.049\)) and VL thickness (6% vs. 0%, \(p=0.021\)). Patients demonstrated persistent quadriceps muscle weakness 6 months after ACL reconstruction [19]. Morphometric correlations have been established between QF architecture and injury, sarcopenia, frailty, and alterations in muscle strength and activation [20; 21; 22; 23]. Moreover, it is known that muscle performance is related to muscle architecture, strength and muscle activation capacity, which are directly related to a better quality of life, functionality and, in the case of injury, with a better prognosis for recovery [24; 25; 26; 27]. For this reason, it is considered essential to have tools that allow a valid and reliable evaluation of the variables that could condition muscular behavior, as well as range of normal values. Currently, there are several tools that allow the evaluation of activity, activation, functionality and muscle architecture, such as electromyography (EMG), magnetic resonance imaging (MRI) and ultrasonography (US) [28; 29; 30]. The characteristics of US, such as its low associated cost and speed of use, have led to an increase in its implementation in the clinical and research area in physiotherapy and rehabilitation, where it is called Rehabilitative Ultrasound Imaging (RUSI) [31; 32; 33]. RUSI has proven to be a valid and reliable tool when compared to gold standards such as MRI in the morphological evaluation of lower limb muscles [28; 34; 35; 36; 37; 38]. RUSI in B-mode provides information related to muscle morphometry (thickness, cross-sectional area (CSA), pennation angle, fascicle length) while RUSI in M-mode shows temporal parameters related to activation [39; 40]. RUSI M-mode shows by means of a time-depth graph the moment of activation and relaxation of the musculature from changes in the movement of the connective tissue within the muscle [41; 42]. It also makes it possible to determine the velocity at which these changes are generated, both at the beginning and at the end of muscle activation [43]. Currently, RUSI M-mode is used to assess the muscle activity of the knee extensor complex [44; 45; 46]. In a study conducted in subjects who underwent ACL reconstruction with a five-year follow-up, a decrease in the thickness of the RF muscle was found when compared to a control group. However, no changes in contraction/relaxation velocity were found. This study suggests the evaluation of the relationship between the muscular behavior of the RF and VL, VM and VIM in future investigations [47]. The close anatomical relationship between the RF and the VIM could condition the muscular behavior of one with respect to the other, both in the changes in thickness that occur during muscle contraction, as well as in the contraction and relaxation velocity. Based on the current scientific evidence where QF muscle behavior is evaluated by RUSI, the authors considered it of interest to determine if the muscle behavior of RF and VIM are related to each other, as well as to describe the anthropometric variables that could be correlated. To the best of the author's knowledge this is the first study aimed at determining the relationship between RF and VIM behavior measured by RUSI M-mode under maximal voluntary isometric contraction (MVIC) and anthropometric measurements.