## 4 Discussion
### Characteristics of the Intervention Programs  
In the literature, strength training is considered as one of the best paradigms to enhance physical performance in youth athletes [35; 36]. Together with dynamic stability training, the INT content was used more in the training programs analyzed in the current study [19; 20; 21; 22; 23; 25; 26; 27; 28; 29; 30; 31; 32]. Furthermore, strength deficits have been associated with less neuromuscular control, which increases the occurrence of injuries mainly in the lower limbs [12]. In this systematic review, those studies which included strength training in their INT programs reduced the rates of injury [22; 29; 32]. However, these changes were also accompanied by improvements in dynamic stability [19; 21; 25; 28; 32], functional movement skills [19; 20], strength [21; 23], agility [21], speed [21], or fatigue resistance [23; 29]. Nonetheless, four [26; 27; 30; 31] of the 13 articles [19; 20; 21; 22; 23; 25; 26; 27; 28; 29; 30; 31] that used strength training did not find positive changes in any of the outcomes measured. This generates a controversy due to the program characteristics in these articles, which were very similar to those studies that did find significant improvements. Numerous explanations might be suggested for these discrepancies: (a) Application of exercise, (b) age of participants, and (c) contents included in the program. It could be speculated that it is the combination of training contents, rather than the sum of them individually, that confers the benefits on these young people. In fact, the four studies that did not find positive changes employed only three contents: Strength, dyometrics, and dynamic stability in three studies [26; 30; 31] and strength, dynamic stability and coordination in another one [27]. The age should be another key factor when programing this content. While pre-PHV athletes will have a neuromuscular type of adaptation, exercises must focus on techniques, postural control, and circum-PHV. Post-PHV athletes will not have just neuromuscular but also structural adaptations. The proposed task should additionally focus on improving technique in more advanced and sport-specific exercises and increasing execution speed [11]. Among the aforementioned studies with limited results, all were post-PHV (>14 years old)and one [31] presented the highest mean age of all analyzed studies. This suggests the need for an adjustment of adequate training loads and progression in each content. Dynamic stability was the other main content used in the intervention programs [19; 20; 21; 22; 23; 25; 26; 27; 28; 29; 30; 31; 32]. Evidence supports the need to train this content due to improvements in the sensorimotor system, which can improve neuromuscular control. This leads to a better joint dynamic stability and, therefore, a decrement in the injury risk [37]. In fact, the youth athletes' inability to maintain postural balance in static and dynamic actions has been associated with an increased likelihood of injury [38]. In the reviewed articles, those which included dynamic stability training in their INT programs also included strength training [19; 20; 21; 22; 23; 25; 26; 27; 28; 29; 30; 31; 32]. Thus, similar improvements were found for both contents, which allowed us to suggest that dynamic stability along with strength training might be a good strategy to improve performance and prevent injuries in young athletes. The literature shows that dynamic stability should be trained through exercises focused on the lower limbs and core [39], which challenges the feed-forward mechanism. This is described as anticipated actions that occur before the sensorimotor system detects changes in the environment [40]. This mechanism is the most important factor of training in order to maintain balance in landing, deceleration and cut off maneuvers, which helps to decrease injury risk [41].