## 1 Introduction  
Performance in soccer depends on psychological, physiological and biomechanical factors [1, 2]. The study of these factors not only helps improve performance but also works towards injury prevention. Performance and injury prevention are not isolated fields, and the presence of an injury can affect on-field performance. Measurement of the muscular capacity of players is an important factor in the evaluation and prediction of the functional capacity of the player [3]. Muscular capacity has been traditionally measured using isokinetic machines [4, 5]. These machines measure the torque generated in isolated joint rotations (for example, in knee flexion and extension), and the torque generated is associated with the primary muscle group responsible for that rotation. This could possibly disregard the contribution of individual muscles [4]. Moreover, one must consider that there could be multiple muscle groups responsible for a joint rotation, and that not all muscles of a muscle group participate in specific muscular function. Another drawback of the isokinetic machines is that they produce high muscular fatigue, restricting their regular use during the season [6]. Electromyography (EMG) is a diagnostic tool that could possibly overcome the shortcomings of the isokinetic machines [7; 8; 9], and in some cases, even complement the use of isokinetic machines [10]. EMG sensors permit the measurement of muscle activation of individual muscles and determine both the quantification of muscle activation and the timing of the same. In sports, surface EMG sensors are commonly used given their non-invasive nature permitting the measurement of surface muscles. Specifically in soccer, previous studies using EMG have been used to determine asymmetry in soccer players, which can affect both performance [11; 12] and injury [13]. Such comparative studies have been carried out in youth soccer players as well [14; 15] finding that there may be a neuromuscular pattern and that strength work could be related to a reduction in the injury rate in soccer players. Asymmetry in soccer players could be offset with proper exercise prescription, which facilitates improvements in musculoskeletal function by addressing the specific needs of the subject as an integral part of any rehabilitative, preventive or maintenance program [16]. Functional weight-bearing exercises have received a significant amount of attention as the preferred mode of exercise for lower extremity strengthening [17]. In studies with EMG, basic movements with additional weight (forward lunge, Bulgarian squat, lateral step-ups, squat) are frequently used so that they are easily replicable [18; 19], and such exercises do not cause an excessive muscular fatigue in players, permitting their use throughout the preseason and in-season. These are simple exercises used to strengthen the quadriceps and hamstrings, and their study could provide information about the activation of different muscles of the quadriceps and hamstrings and to differentiate the activation differences in the exercise phases (concentric, isometric and eccentric) [20]. Considering the muscle group that they target, these exercises can be crucial for soccer players given that recent research has shown that monitoring the electrical activity of individual muscles could help in the injury rehabilitation and prevention process [20; 21]. Specifically, in the case of soccer players, quadriceps and hamstrings account for 19% and 16% of all types of injuries, respectively [22]. The total number of hamstring injuries increased by 4% on a yearly basis in European soccer from 2001 to 2014, while the total number of injuries increased by 2.3% [23]. Muscular imbalances have been cited as an important risk factor for hamstring strain injury [24; 25]. This has traditionally been evaluated through isokinetic machines [26; 27], although recent studies have begun using surface electromyography (sEMG) in different strength exercises.