## 4 Discussion
### Characteristics of the Intervention Programs
Nonetheless, the lack of these types of exercises can be clearly detected in the training programs described in the reviewed articles [19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32]. Plyometric training was the other content of INT widely included in the intervention programs [19; 20; 21; 22; 24; 26; 28; 30; 31; 32], which has been shown to improve performance and to decrease injury risk in young athletes [13]. In the current review, studies which included pyometric training in their INT programs improved dynamic stability [19; 20; 21; 28; 32], functional movement skills [19; 20], strength [21; 24], agility [21], speed [21; 24], fatigue resistance [24] tests, and reduced injury risk [22; 32]. Only three [26; 30; 31] of the 10 studies [19; 20; 21; 22; 24; 26; 28; 30; 31; 32] that used pyometric training did not report significant changes between-groups. One of the particularities of this INT content is the need for an adequate progression. This progression should start with low-intensity exercises performed at a slow velocity with a proper technique and develop at later stages to higher velocity and intensity drills [13]. In various studies [19; 20; 27], coordination of fundamental movement skills' training was included in their intervention programs. From these studies, we found two reported significant improvements in fundamental movement skills [19; 20] and dynamic stability tests [19]. One study [27] could not find significant changes probably due to the low compliance with the training program (mean player attendance at the training sessions was \(59.6\pm 14.3\%\)). These skills are particularly important in pre PHV athletes during sport-specific skills development in circum-PHV and post-PHV athletes [42]. Coordination should be trained at early stages due to the greater neural plasticity that children have at this age [43]. Moreover, children are encouraged to be involved in a variety of sports at this age in order to develop neural adaptations for many skills before sport specialization [11]. Likewise, a proper movement competency not only enhances physical ability but, in addition, is suggested to decrease injury risk [44]. However, none of the studies that included coordination in the program subsequently analyzed the injury rates. Furthermore, it should also be highlighted that the protocol of sport-specific skills was not clearly defined in the analyzed studies. Throughout the same line, four articles [22; 24; 28; 32] that included agility training in their protocols were found. Literature has defined agility as the combination of decision-making process and change of direction [45]. Decision-making will improve overall during adolescence based on the experience acquired by the athletes. Nevertheless, the change of direction technique can be enhanced in pre-PHV [15], probably as the result of the great neural plasticity of these youth athletes [43]. From the reviewed articles that used agility training in their protocols, we found improvements in dynamic stability [28; 32], strength [24], speed [24], fatigue resistance [24] tests, and injury prevention [22; 32]. Hence, it would be interesting to include these outcomes in the youth training programs to enhance performance and to reduce injury risk. Regarding speed training, evidence suggests developing this INT content across the different stages of growth [15]. From the reviewed literature, we found two studies [20; 24] that included speed training in their programs. These studies reported significant improvements in strength [24], speed [24], fatigue resistance [24], and fundamental movement skills [20]. Athletes should start training with the high-velocity capacity and the proper running biomechanics, with pylemetric, coordination, and sprint technique exercises. Prior to puberty and before maturation, these parameters are easier to change [46]. However, post-PHV athletes may improve more with a combination of neural basis training mainly focused on strength training [15].