Integrative Neuromuscular Training in Young Athletes, Injury Prevention, and Performance Optimization: A Systematic Review

applied sciences Article Integrative Neuromuscular Training in Young Athletes, Injury Prevention, and Performance Optimization: A Systematic Review Borja Sañudo 1,* , Juan Sánchez-Hernández 1, Mario Bernardo-Filho 2, Ellie Abdi 3, Redha Taiar 4and Javier Núñez 5 1Departamento de Educación Física y Deporte, Universidad de Sevilla, E-41013 Seville, Spain; [email protected] 2 Departamento de Biof í sica e Biometria, Universidade do Estado do Rio de Janeiro, 22050-032 Rio de Janeiro, Brazil; [email protected] 3Center of Pedagogy, Montclair State University, Upper Montclair, NJ 07043, USA; [email protected] 4Département d’Education Physique et sportive (EPS), Universitéde Reims, 51687 Reims, France; [email protected] 5Departamento de Deporte e Informática, Universidad Pablo de Olavide, 41704 Seville, Spain; [email protected] *Correspondence: [email protected]; Tel.: +34-652-387090 Received: 6 August 2019; Accepted: 10 September 2019; Published: 12 September 2019   Abstract: The aim of this systematic review was to evaluate the current evidence by assessing the effectiveness of integrative neuromuscular training programs in injury prevention and sports performance in young athletes. Different data sources were analyzed up to January 2018. Eligible studies contained information on population (young athletes), intervention (neuromuscular training), comparator (control group or another exercise intervention), outcomes (injury prevention or sport performance), and study design (randomized trials or prospective studies). The trials were restricted based on the language (English) and for publication date (after 1 January 2007). Fourteen randomized controlled trials were included: Seven included dynamic stability-related outcomes. Three assessed the coordination performing fundamental movements and sport-specific skills, while other five studies analyzed muscle strength and two assessed plyometric tests. Agility was evaluated in three studies and speed tests were also considered by four studies. Finally, fatigue resistance in three studies and injury risk in four were assessed. This review provides evidence that integrative neuromuscular training programs can enhance performance and injury prevention in young athletes, taken into account that adherence to the training program is adequate. Collectively, well-designed, randomized studies are necessary to collaborate with the present findings. Keywords: neuromuscular training; strength; injury prevention; young athletes 1. Introduction Integrative neuromuscular training (INT) is defined as a training program aimed to enhance physical fitness and prevent the aggregation of the neuromuscular deficits, along with the improvements of the motor competence, especially in youth with a lower level of motor skills [ 1 ]. This type of training in sync with the improved sports-related movement skills could also have an impact on injury prevention [ 2 ]. Further, recent literature has displayed that the absence of this training’s type before or during the adolescence can lead to imbalances and incorrect movement patterns. This fact might be associated with a greater injury risk [ 3 – 5 ], especially in young female athletes [ 6 ] and in those sports involving a high number of landings or change of direction maneuvers, such as football or Appl. Sci. 2019,9, 3839; doi:10.3390/app9183839 www.mdpi.com/journal/applsci Appl. Sci. 2019,9, 3839 2 of 18 basketball [ 7 ]. Although young athletes present a lower injury ratio in comparison with adults, these injuries entail a longer recovery period. The injuries could even lead to a cessation of the practice due to fear of recurrence [8], with consequent effect on their careers as athletes [9,10]. Recently, Fort-Vanmeerhaeghe et al. [ 11 ] developed a classification of the INT components that included two blocks: (a) The first block focused on the development of fundamental movement skills, which includes coordination, strength, plyometric, agility, dynamic stability, speed, and fatigue resistance training, and (b) in the second block sport-specific movement skills were included [ 11 ]. It is suggested that all these contents must be exercised in order to improve performance and reduce injury risks. By coordination, within the fundamental movement skills, authors usually refer to locomotor, manipulation, and stability skills (variety of movements and multitasking, including unanticipated reactions). It is believed that these abilities allow control and optimize different sport-specific movements, thus decreasing the injury risks in youth athletes [ 11 ]. Additional aspects, such as strength, have been extensively analyzed in the literature. Resistance training might include core and lower limb positions and stability but also the application of upper-body and lower-body exercises and pushing/pulling strength exercises [ 11 ]. Strength deficits have been associated with less neuromuscular control and increasing the appearance of injuries in the young population [12]. Moreover, plyometric [ 13 ], considered as the development of stretch shortening cycle ability and agility training (development of skills at maximum speed integrating changes of direction actions) [ 11 ] also have been suggested in order to improve the performance and decrease injury risks in this population group. Literature also highlighted dynamic stability training (balance training that includes dynamic actions) as a means to manage neuromuscular control because of the improvements in the sensorimotor system, which enhances joint dynamic stability and may also reduce the risk of injury in youth athletes [ 14 ]. Furthermore, it seems that speed is the main determinant in the performance of many sports. The inability to produce speed in sprint, optimally during childhood, may decrease the possibility of achieving high competitive levels [ 15 ]. This term is closely related with agility and would include the development of skills at maximum speed [ 11 ]. On the other hand, fatigue resistance training could be defined as the development of skills under fatigue conditions [ 11 ]. It is known that neuromuscular fatigue is an important risk factor in numerous sports injuries [16]. Despite the importance of this type of training, especially in young athletes, the number of studies that include all INT contents in their intervention programs is scarce. The knowledge on its effect of injury prevention and performance has to be determined in this population. Therefore, this systematic review focuses on the analysis of the current evidence assessing the effectiveness of INT programs on injury prevention and sports performance in young athletes. 2. Methods 2.1. Eligibility Criteria For this systematic review, the Preferred Reporting Items for Systematic Reviews, Meta-Analyses (PRISMA) statement, and checklist were used. The inclusion criteria were studies performed in the young population that assessed the efficacy of INT training compared to a control group, with other types of training or with no training. The population considered was both young and athletes. Children up to 11 years old in girls and 13 years old in boys in addition to adolescents between ages of 12 and 18 years old for girls and 14–18 years old for boys, as previously defined by Lloyd et al. [ 17 ] were examined. Exclusively, the studies including two or more components of INT (i.e., fundamental and specific movement skills, strength, plyometrics, speed, agility, coordination, dynamic stability, and fatigue resistance) in the training program, which were compared to a control group (i.e., randomized controlled trial or prospective study) were considered. Further, only studies that clearly detailed parameters of exercise containing a description of at least training intensity or volume were included. Finally, only studies published in English after 1 January 2007 were assessed. Appl. Sci. 2019,9, 3839 3 of 18 2.2. Search Strategy A literature search was performed in the following electronic databases: PubMed, Cochrane Central Register of Controlled Trials, Web of Science, CINAHL, MEDLINE, and SPORTDiscus. The last search was conducted in January 2018. The search strategies varied according to the different databases and used the following systematic search terms: Neuromuscular control or neuromuscular training or integrative neuromuscular training and strength training, or plyometric training or speed training or agility training or fundamental movement skills or specific movements skills or coordination training or dynamic stability or fatigue resistance and youth athletes or young or adolescents and injury prevention or sport performance. Language was limited to English and participants were all human. 2.3. Data Collection Process and Quality Analysis Two authors (J.S. and B.S.) independently screened titles and abstracts to choose potential eligible studies. Full-text articles were obtained and independently evaluated for inclusion in the present review based on the inclusion criteria. Disagreements were resolved by consensus and, if necessary, the study’s authors were contacted for clarification guidance. The flow chart can be observed in Figure 1. Each study that met the inclusion criteria was abstracted for information regarding: Number of trained and untrained athletes, age and sex, sport, main and secondary outcomes, main results, INT contents included, frequency, duration, and intensity of the training program. Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 21 A literature search was performed in the following electronic databases: PubMed, Cochrane 92 Central Register of Controlled Trials, Web of Science, CINAHL, MEDLINE, and SPORTDiscus. The 93 last search was conducted in January 2018. The search strategies varied according to the different 94 databases and used the following systematic search terms: Neuromuscular control or neuromuscular 95 training or integrative neuromuscular training and strength training, or plyometric training or speed 96 training or agility training or fundamental movement skills or specific movements skills or 97 coordination training or dynamic stability or fatigue resistance and youth athletes or young or 98 adolescents and injury prevention or sport performance. Language was limited to English and 99 participants were all human. 100 2.3. Data Collection Process and Quality Analysis 101 Two authors (J.S. and B.S.) independently screened titles and abstracts to choose potential 102 eligible studies. Full-text articles were obtained and independently evaluated for inclusion in the 103 present review based on the inclusion criteria. Disagreements were resolved by consensus and, if 104 necessary, the study’s authors were contacted for clarification guidance. The flow chart can be 105 observed in Figure 1. Each study that met the inclusion criteria was abstracted for information 106 regarding: Number of trained and untrained athletes, age and sex, sport, main and secondary 107 outcomes, main results, INT contents included, frequency, duration, and intensity of the training 108 program. 109 110 Figure 1. Flow diagram of the search process of integrative neuromuscular training effects in injury 111 prevention and performance on youth athletes. 112 The methodological quality was individually assessed by using an adaptation of the Cochrane 113 methods [18], which included the generation of randomization sequence, the allocation concealment, 114 blinding of participants or assessors and description of withdrawals and dropouts (see Table 1). 115 Table 1. Quality assessment of studies. 116 Study (Author, Year) Generation of Randomized Sequence Allocation Concealment Blinding of Participants or Assessors Description of Withdrawals and Dropouts Barendrecht [19] No No No Yes Brown [20] Yes No No Yes Chaouachi [21] Yes No No No Figure 1. Flow diagram of the search process of integrative neuromuscular training effects in injury prevention and performance on youth athletes. The methodological quality was individually assessed by using an adaptation of the Cochrane methods [18], which included the generation of randomization sequence, the allocation concealment, blinding of participants or assessors and description of withdrawals and dropouts (see Table 1). Appl. Sci. 2019,9, 3839 4 of 18 Table 1. Quality assessment of studies. Study (Author, Year) Generation of Randomized Sequence Allocation Concealment Blinding of Participants or Assessors Description of Withdrawals and Dropouts Barendrecht [19] No No No Yes Brown [20] Yes No No Yes Chaouachi [21] Yes No No No Emery [22] Yes Yes No Yes Faigenbaum [23] Yes No No Yes Fernandez [24] No No No Yes Filipa [25] No No No Yes Klugman [26] No No No Yes Lindblom [27] Yes No No Yes Mcleod [28] No No Participants and assessors Yes Richmond [29] Yes Yes No Yes Steffen [30] Yes No No Yes Steffen [31] Yes No No Yes Steffen [32] Yes No No Yes 3. Results The literature search identified 471 potentially relevant studies (Figure 1). Following the screening of the titles, 305 studies were excluded in addition to 97 duplicates. Concerning the eligibility criteria, the remaining 69 studies were analyzed. Ultimately, 55 studies were excluded, 27 were due to the mean age which was higher than 18 years old, 13 included only one component of INT in their training protocols, seven were performed before 2007 and eight studies did not include a control group. This resulted in 14 included studies to be analyzed in the present review. 3.1. Characteristics of the Included Studies Table 2shows that most studies were executed exclusively on young female population [ 20 , 25 – 28 , 30 – 32 ], four studies analyzed both male and female athletes [ 19 , 22 , 23 , 29 ], while two studies [ 21 , 24 ] included only male athletes. Seven studies analyzed soccer players [ 22 , 25 – 27 , 30 – 32 ] and other studies were performed in handball [ 19 ], tennis [ 24 ] and basketball players [ 28 ]. Moreover, we found one study in players of different sports (i.e., field hockey, volleyball, soccer, and basketball) [ 20 ], and three remaining studies were developed in unspecified sports [ 21 , 23 , 29 ]. Regarding the age of the athletes and the peak of height velocity (PHV), which normally occurs around the age 12 in females and 14 in males [ 33 ], in twelve studies [ 19 – 22 , 24 – 28 , 30 – 32 ] mean age was post-PHV (>14 years old), while only in two studies [23,29] the mean age of the athletes was pre-PHV (<14 years old). Appl. Sci. 2019,9, 3839 5 of 18 Table 2. Characteristics of the included studies. Author Participants Sport Age Main and Secondary Outcomes Main Results Brown [20] N=43 female athletes. NMT, n =10, CORE, n =7, PLYO, n =13, CG, n =13. Basketball, field hockey, soccer, and volleyball. NMT: 14.1 ±1.2 years, CORE: 15.0 ±0.6 years, PLYO: 14.8 ±0.6 years, CG: 14.7 ±2.6 years. FUNDAMENTAL MOVEMENT SKILLS: Sagittal and frontal planes hip and knee kinematics and kinetics. Greater peak knee flexion following training, during both bilateral (p=0.027) and unilateral landings (p=0.076, d =0.6) was reported in NMT. PLYO demonstrated reduced hip adduction (p= 0.010) and greater knee flexion (p=0.065, d =0.6) during bilateral landings following training. A significant time interaction was observed in CG showing a significant reduction in peak stance knee abduction moment (p=0.003). Chaouachi [21] N=42 male athletes. PLYO, n =14. Balance and plyometric group (Bal-PLYO), n =14. CG, n =14. Public school students with no after school activities or formalized strength and conditioning training programs. PLYO: 13.7 ±0.8 years, Bal-PLYO: 13.3 ± 0.8 years, CG: 13.5 ±0.8 years. STREGNTH, SPEED, DYNAMIC STABILITY, AND AGILITY: Lower-body strength test (one repetition maximum -1RM- leg press), power test (horizontal and vertical jumps, triple hop for distance, reactive strength, and leg stiffness), running speed (10-m and 30-m sprint), static and dynamic balance (standing Stork test and SEBT), and agility (shuttle run). Bal-PLYO training was considered likely to be superior to PLYO training in leg stiffness (d =0.7), 10-m sprint (d =0.6), and shuttle run (d =0.5). In 8 of the 11 dependent variables, the difference between Bal-PLYO vs. PLYO training was unclear. Bal-PLYO training was considered likely to be meaningfully better than CG in all dependent variables (d =0.6–1.4). PLYO training was considered likely to be meaningfully better than CG in all dependent variables (d =0.3–1.2) except leg stiffness and 10-m sprint. Emery [22] N=744 male and females. NMT, n =380. CG, n =364 Soccer NMT: U13-15 =46.6%, U16-U18 =53.4%, Female =42.4%, Male =57.6%, CG: U13-15 =48.9%, U16-U18 =51.1%, Female =69%, Male =31%. INJURY SURVEILLANCE: Injury definition was soccer injury resulting in medical attention and/or removal from a session and/or time loss. Significant differences were found in injury rates between NMT (2.08 injuries/1000 player-hours) and CG (3.35 injuries/1000 player-hours). Faigenbaum [23] N=40 males and females. NMT, n =10 boys and 11 girls. CG, n =6 boys and 13 girls. Children from two second-grade PE classes in a public school All groups: Boys =7.6 ± 0.3 years, Girls =7.5 ±0.3 years. Total =7.6 ±0.3 years. STRENGTH, DYNAMIC STABILITY, AGILITY, AND FATIGUE RESISTANCE: Push-up, curl-up, standing long jump, single-legged hop, single-legged balance, sit-and-reach flexibility test, shuttle run, and 0.8-km run. Intervention effects in the girls enhanced NMT-induced gains in performance relative to CG on the curl-up, long jump, single-legged hop, and 0.8-km run (P <0.05) after controlling for baseline. Boys did not demonstrate similar adaptations from the NMT program (P ≥0.05). Fernández [24]N=16 male athletes. NMT, n =8, CG, n =8. Tennis All groups: 16.9 ±0.5 years. STRENGTH, SPEED, AND FATIGUE RESISTANCE: Sprint test (10 and 30 meters) CMJ, RSA, and maximal aerobic performance using a 30-15 intermittent fitness test (30-15 IFT). After training, 10-m sprint, CMJ, and RSA were significantly improved in NMT (P ≤0.05). Appl. Sci. 2019,9, 3839 6 of 18 Table 2. Cont. Author Participants Sport Age Main and Secondary Outcomes Main Results Filipa [25]N=20 female athletes. NMT, n =13. CG, n =7. Soccer NMT: 15.4 ± 1.5 years, CG: 14.7 ±0.8 years. DYNAMIC STABILITY: SEBT Significant improvements in SEBT were observed in both limbs for NMT in the posterolateral direction and in the posteromedial direction of the left limb. Klugman [26]N=49 female athletes. NMT, n =15. CG, n =34. Soccer ETG: 14.1 ±0.4 years. CG: 14.7 ±1.0 years. FUNDAMENTAL MOVEMENT SKILLS: Tuck jump test. Both groups significantly reduced their landing and jumping deficits. There was not a significant interaction of training status between groups. Lindblom [27]N=41 female athletes. NMT, n =23, CG, n =18. Soccer NMT: 14.2 ±0.7 years. CG: 14.2 ±1.1 years. DYNAMIC STABILITY, STRENGTH, PLYOMETRICS, AGILITY, ADN SPEED: SEBT, CMJ, triple-hop for distance test, a modified Illinois agility test, and 10-m and 20-m sprint tests. Non-significant time effects were observed in NMT from baseline to follow-up. Changes were observed in CG compared to NMT for a sub-score of the SEBT and in the modified Illinois agility test (p<0.05). Mcleod [28]N=62 female athletes. NMT, n =37, CG, n =25. Basketball NMT: 15.6 ± 1.1 years, CG: 16.0 ±1.3 years. DYNAMIC STABILITY: BESS and SEBT. A significant decrease in total BESS errors in NMT compared with CON (P =0.003) was found. NMT subjects also reported significantly fewer BESS errors in the single-foam and tandem-foam conditions than CG and demonstrated improvements in the single-foam compared with their pre-test (P =0.033). Between-group changes at posttest were observed in the lateral, anteromedial, medial, and posterior directions. They also improved in NMT compared with CG (P <0.05) using the SEBT. Richmond [29] N=725. NMT, n =353 (female 57.2%, male 42.8%), CG, n =372 (female 57.0%, male 43%). Physical education students. NMT: 13 years, CG: 13 years. INJURY SURVEILLANCE AND FATIGUE RESISTANCE: Sports injuries (total, lower extremity injury, time loss, ankle, and knee sprain injury) secondary outcomes also included: waist circumference, and aerobic fitness (Leger 20-meter shuttle run test). The NMT program was efficacious in reducing sport-related injury rates and improving measures of adiposity and fitness in the intervention group. Steffen [30] N=2020 female athletes. NMT, n =1073. CG, n = 947. Soccer. All groups: Ages between 13 and 17 (with 43% in NMT and 45% in CG with 15 years of age). INJURY SURVEILLANCE: Overall injury rate (proportion of injured players, the incidence of ankle, knee, groin, hamstrings, and other injuries). Neither a difference was observed in the overall injury rate between NMT (3.6 injuries/1000 h) and CG (3.7/1000 h) nor in the incidence for any type of injury. Only 14 out of 58 intervention teams completed more than 20 prevention training sessions, therefore, the study reported low adherence to the program. Appl. Sci. 2019,9, 3839 7 of 18 Table 2. Cont. Author Participants Sport Age Main and Secondary Outcomes Main Results Steffen [31]N=34 female athletes. NMT, n =18. CG, n =16. Soccer. All players were aged 16–18 years (17.1 ±0.8). STRENGTH, SPEED, AND SPORT-SPECIFIC SKILLS: Isokinetic and isometric strength protocols for the quadriceps and hamstrings, isometric hip adduction, and abduction strength, vertical jump tests, sprint running and soccer skill tests. There was no difference between the NMT and CG for any of the outcome measured. Steffen [32] N=226 female athletes. CTG, n =78. RG, n =68. CG, n =80. Soccer. All players between 13 and 18 years old. NMT: U16 (57.7%)-U18 (42.3%), RG: U16 (26.5%)-U18 (73.5%), CG: U16 (22.5%)-U18 (77.5%). DYNAMIC STABILITY, PLYOMETRICS AND INJURY SURVEILLANCE: SEBT, single-leg balance, triple hop, and jumping over a bar test. Injury risk. Compared to CG, single-leg balance (OR =2.8, 95% CI 1.1 to 4.6) and the anterior direction of the SEBT improved significantly in NMT (OR =4.7, 95% CI 2.2 to 7.1), while two-leg jumping performance decreased (OR = − 5.1, 95% CI − 9.9 to −0.2). Significant improvements in five of six reach distances in the SEBT were found, favoring players who highly adhered to the program (FIFA 11+). Moreover, injury risk was lower for those players (IRR =0.28, 95% CI 0.10 to 0.79). NMT: Neuromuscular training group; PLYO: Plyometric group; CORE: Core group; CTG: Comprehensive group; RG: Regular group; CG: Control group; DJ: Drop jump; SEBT: Star excursion balance test; CMJ: Countermovement jump test; RSA: Repeated sprint ability; BESS: Balance error scoring system; IRR: Injury rate ratio. Appl. Sci. 2019,9, 3839 8 of 18 3.2. Outcomes Measures Dynamic stability related outcomes were assessed in seven studies [ 19 , 21 , 23 , 25 , 27 , 28 , 32 ]. As defined by Fort-Vanmeerhaeghe et al. [ 11 ], the term “dynamic stability” was considered as the training of lower limb dynamic stabilization (with three categories of progression: Static balance, dynamic balance, and dynamic stabilization) and core dynamic stability. Further studies assessed coordination performing fundamental movements [ 19 , 20 , 26 ] and sport-specific skills [ 31 ], while other studies analyzed muscle strength [ 21 , 23 , 24 , 27 , 31 ] and plyometric [ 27 , 32 ] tests. Agility [ 21 , 23 , 27 ] and speed [ 21 , 24 , 27 , 31 ] tests were also considered by different studies. Finally, fatigue resistance [ 23 , 24 , 29 ] and injury risk [22,29,30,32] were also examined. Among the selected studies, five [ 19 , 21 , 25 , 28 , 32 ] found significant improvements in dynamic stability. Four of them reported significant increments in Star excursion balance test (SEBT) in the intervention group compared with control participants [ 21 , 25 , 28 , 32 ]. Furthermore, positive changes in single leg stability (one-leg hop test) were found [ 19 ]. Nevertheless, two studies [ 23 , 27 ] did not report changes between-groups in this outcome. Significant changes compared to the control group were also observed in coordination performing fundamental movement skills (i.e., jumping and landing technique) in two studies [ 19 , 20 ]. Barendrecth et al. [19] showed that the in-season training group had the greatest benefits on knee kinematics and single leg stability. Although, Brown et al. [ 20 ] showed a greater peak knee flexion of landings in the neuromuscular training group in both bilateral (p=0.027) and unilateral landings ( p=0.076 ). By contrast, non-significant between-group differences were observed by Klugman et al. [26] . With respect to the sport-specific skills, the only study that assessed this outcome [ 31 ] observed no differences between group performances of soccer players after a period of neuromuscular training. Among the five studies that assessed muscle strength, three reported significant improvements [ 21 , 23 , 24 ]. Fernandez et al. [ 24 ] showed a 2% increment in countermovement jump (CMJ) in the experimental group. Moreover, Chaouachi et al. [ 21 ] found improvements in 1RM leg press test, CMJ and maximal hopping test in the neuromuscular training group compared with the control group. This agrees with Faigenbaum et al. [ 23 ] who reported significant changes in the training group (girls in physical education classes) in the curl up, long jump and single leg hop tests. Notwithstanding, two studies [ 27 , 31 ] did not find any improvement in strength after the training protocol. Neither of the two studies which analyzed plyometric performance reported positive changes between training and control groups [27,32]. Once the agility tests were analyzed, again contradictory results were found. Uniquely, the study performed by Chaouachi et al. [ 21 ] reported increments in shuttle run performance (d=0.52). However, non-significant improvements were seen on agility tests in two additional studies [ 23 , 27 ]. This controversy persists in the speed tests results, while two studies found significant differences in 10-m sprint [ 21 , 24 ] and one in 30-m sprint [ 21 ], the remaining two studies did not show changes in this outcome [27,31]. With respect to the outcome of fatigue resistance, one study [ 23 ] conducted in girls, found positive changes. Moreover, a different study [ 24 ] observed significant changes in repeated sprint ability (RSA), with performance increments of ≈ 1.5%, but did not find changes in the 30-15 intermittent fitness test. Contrarily, Richmond et al. [ 29 ] showed that the experimental group, submitted to strength and dynamic stability training, had significant improvements in oxygen uptake (VO 2max ) in the Leger 20-meter shuttle run test. With respect to the injury risk, three studies [ 22 , 29 , 32 ] found significant improvements in the training group compared to the control group. In their study, Emery et al. [ 22 ] reported lower injury rates in the training group (2.08 injuries/1000 h) when compared to the control group ( 3.35 injuries/1000 h ). Moreover, Richmond et al. [ 29 ] presented an injury rate of 7.1 per 100 students in the experimental group and 14.5 per 100 students in the control group. In a study, Steffen et al. [ 32 ] included an experimental group that performed a protocol combining strength, plyometrics, dynamic stability, and Appl. Sci. 2019,9, 3839 9 of 18 agility during four months with two, three sessions per week. It was reported that the injury risk is significantly lower in the high adherence group in lower extremities (IRR =0.28, 95% CI 0.10 to 0.79) when compared with the control group. Uniquely, one of the analyzed studies [ 30 ] described no significant changes between groups in injury risk after a period of strength, plyometric, and dynamic stability training (training group =3.6 injuries/1000 h, control group =3.7/1000 h). 3.3. Characteristics of the Intervention Programs The strength and dynamic stability showed in Table 3were the contents of INT and were included in the analyzed studies [ 19 – 23 , 25 – 32 ]. Plyometric training was used in ten studies [ 19 – 22 , 24 , 26 , 28 , 30 – 32 ]. Three studies [ 19 , 20 , 27 ] included coordination performance fundamental movement skills training in their intervention programs, while four [ 22 , 24 , 28 , 32 ] used agility training and two-speed training [ 20 , 24 ]. The length of the exercise programs varied in the included studies from six weeks to eight months. Study duration was six weeks in two studies [ 20 , 28 ], eight weeks in four [ 21 , 23 – 25 ], and 10 weeks in other three studies [ 19 , 26 , 31 ]. The remaining studies had a duration of: 11 weeks [ 27 ], 12 weeks [ 29 ], 16 weeks [32], 20 weeks [22], and eight months [30]. The frequency of the sessions for most studies was two times per week [ 19 , 23 – 25 , 27 , 28 ]. The exceptions were four studies in which participants had three training sessions per week [ 20 , 21 , 26 , 31 ] and two studies that had a variable duration between two and three sessions per week [ 29 , 32 ]. Conjointly, in one study [ 30 ], the intervention group was trained 15 consecutive sessions and thereafter one session per week. Information regarding the number of training sessions per week was not provided in one study [22]. The duration of the sessions was between 15 and 20 min in most studies [ 19 , 22 , 23 , 27 , 29 – 32 ], throughout the time, in other three studies, the sessions lasted 60 min or more (20,25,28). Three studies [ 21 , 24 , 26 ] did not provide any information related to the duration of the sessions. In these studies, participants performed five to six exercises of strength, plyometric and dynamic stability training [ 21 , 26 ], or plyometric, agility and speed training after eight minutes of dynamic warm-up [ 24 ]. Appl. Sci. 2019,9, 3839 16 of 18 most INT contents used in the reviewed studies were strength, dynamic stability, and plyometrics. These contents were followed by coordination of fundamental movement skills, speed, and agility training. However, none of the studies included all the discussed contents in the same protocol. Finally, lack of protocols that included fatigue resistance and specific movement skills training was reported. This is contrary to the recommendations of the literature highlights, with respect to the benefits of discussed INT contents in young athletes. Future studies are recommended to analyze the effects of a protocol that include all INT contents in young athletes at different growth stages. Researchers need to pay specific attention to pre-PHV athletes and in boys, where a lack of evidence in the literature was found. Needless to say, new lines of research should include INT programs with more drills that challenge the feed-forward mechanisms. This has been described in the literature as one of the most important factors in order to decrease injury risk. Author Contributions: Conceptualization, B.S. and J.S.-H.; methodology, M.B.-F.; formal analysis, R.T.; investigation, B.S.; writing—original draft preparation, B.S. and J.N.; writing—review & editing, B.S. and J.N.; supervision, M.B.; review & editing, E.A. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest. References 1. Rogasch, N.C.; Dartnall, T.J.; Cirillo, J.; Nordstrom, M.A.; Semmler, J.G. Corticomotor plasticity and learning of a ballistic thumb training task are diminished in older adults. J. Appl. Physiol. 2009 ,107, 1874–1883. [CrossRef] [PubMed] 2. Bergeron, M.F.; Mountjoy, M.; Armstrong, N.; Chia, M.; C ô t é , J.; Emery, C.A.; Faigenbaum, A.; Hall, G.; Kriemler, S.; L é glise, M.; et al. International Olympic Committee consensus statement on youth athletic development. Br. J. Sports Med. 2015,49, 843–851. [CrossRef] [PubMed] 3. Hewett, T.E.; Myer, G.D.; Ford, K.R. 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