## 4 Discussion  
In this study, the surface muscular activation of two important muscle groups, the hamstrings and the quadriceps, were measured in strength exercises in elite youth soccer players. These exercises are frequently used and easily replicable. No significant inter-limb differences were observed, rejecting hypothesis 1. Muscle activation patterns showed differences across the intra-muscular group to which they belonged, confirming hypothesis 2. Muscle activation differed across the concentric, isometric and eccentric phases of each exercise, confirming hypothesis 3. This is one of the first investigations to study the muscle activation pattern in elite youth soccer players and has important applications where differences in the muscle activation pattern could indicate muscular deficiencies leading to other muscles assuming part of that deficiency. The results show that no significant differences were found between dominant and non-dominant leg in electrical activity. Other previous studies obtained similar results with professional or semi-professional samples [46; 47]. Although soccer is an asymmetric sport, the modern demands of the sport lay an emphasis on training both limbs equally. Another important factor to consider is that the participants in this study were elite youth players, playing in national and European competition. Previous research has shown inter-limb differences in strength decrease with an increase in level of play [48]. In fact, such inter-limb differences in strength could be a precursor to injuries [3], and hence the monitoring of inter limb differences could be important in injury prevention programs. Figure 7: Mean activation (\(\mathrm{V}\times 10^{-5}\)) of all the muscles in each phase in squat. The first phase corresponds to the descent, the second to the isometric phase and the third to the ascent. The vertical lines around the data represent the SD. In all exercises, the muscular activation patterns of the quadriceps muscles were repeated, with the vastus medialis being predominant, followed by the vastus lateralis and finally the rectus femoris (Figure 4). These results were similar to those obtained with amateur athletes previously for the lunge [18] and squat exercises [20; 49]. The similar pattern of activation of quadriceps and hamstrings could indicate a synergy of work in the proposed exercises [50]. The results showed that there is a very similar activation pattern in the three exercises and in all the muscles (Figure 4). The activation pattern could be used after an injury as a way to quantify when the player has recovered his initial form, so it is important that sEMG tests are performed at different times of the season. This can be applied in the case of the patellofemoral pain syndrome, where vastus medialis activation is the primary goal in the early stages of recovery [51]. In the hamstrings, the greater intra-muscular activation (Figure 4) of the semitendinosus muscle compared to that of the biceps femoris account for its predominance [52; 53]. A balance in the synergy of the hamstring muscles is important in the reduction of risk of a potential injury [53]. This synergy helps to reduce the tension on the biceps femoris [21; 54], which plays a fundamental role in the hamstring injury, since biceps femoris is affected in 80% of all hamstring lesions [55]. A low hamstring to quadriceps ratio is believed to be a risk factor for injuries [56], and many rehabilitation and prevention programs focus on strengthening the hamstrings. The heterogeneity of activation pattern of different exercises must be taken into consideration when developing hamstring strength [29], and these exercises help in the coactivation of the hamstring and quadriceps muscle groups. In the quadriceps, in all the muscles and exercises (Figures 5-7), greater activation was found in the ascent phase (concentric) than in the descent phase (eccentric), coinciding with previous studies [57; 58].