Youth training requires a specific and different approach to design and implementation of physical preparation. As famously stated by Bompa, young people cannot merely be considered “mini adults” (5). The physiologic makeup of children and adolescents is markedly different from that of mature adults (38); it follows that the parameters applied to training design should reflect these differences.
Neural, hormonal, and cardiovascular systems develop with advances in biologic age, leading to corresponding changes in neuromuscular performance (42). Rates of development of a number of physiologic and physical performance parameters measured in young team-sports athletes are shown to peak at approximately the same time as they attain peak height velocity (41). The age at which this occurs is highly individual; typical ages are approximately 11.5 years for girls (3) and 13.8 to 14.2 years for boys (41). However, this age varies considerably; levels of biologic and physiologic maturation can be markedly different between young athletes of the same chronologic age (5,27).
What constitutes appropriate strength training and metabolic conditioning for a young player is therefore determined by, and is specific to, the individual player’s stage of physical development. The stage of physical maturity also influences the mechanism of training effects, such as whether improvements are predominantly mediated by neural factors or whether morphologic and physiologic adaptation plays the greater role (43). The emotional and psychological maturity of the individual is another important factor to be considered when designing and implementing strength training for a youth sports player (27,43).
Another area of training for young athletes that has received less attention is neuromuscular training, including specific instruction and practice of fundamental movement mechanics. Neuromuscular and postural control and movement biomechanics for jumping, landing, running, and changing direction all can be developed in the young team-sports player as a means to improve athleticism. Such development of fundamental movement skills may also help reduce injury risk by equipping the young player to be better at reacting to challenges in the game environment.
NEED FOR PHYSICAL PREPARATION WITH YOUNG TEAM-SPORTS PLAYERS
A major public health concern is the sedentary behaviors and decreasing levels of physical inactivity of youth worldwide (23). Regular physical activity and proper nutrition exert a major influence on growth and development in children and adolescents. From this perspective, appropriate physical preparation assumes increased importance to a young player’s athletic development given the apparent lack of habitual physical activity elsewhere in his or her lifestyle. The absence of such a program of physical preparation to help achieve a threshold level of physical activity may otherwise hinder young players’ development during critical periods in their growth and maturation to the extent that they may not fulfill their genetic potential (23).
As a result of modern sedentary lifestyles, young people are also often not physically prepared for the rigors of youth sports (12,27). Accordingly, the increase in participation in organized youth sports in North America has been accompanied by a dramatic increase in sports-related injuries (17,27). It has not been documented whether the increase in the number of injuries has been proportional to the increased numbers of participants or whether there has been a relative increase in the rate of injury among these young players.
Whatever the case, approximately one-third of young athletes participating in organized sports in the United States sustain injuries requiring medical attention (2). Incidence of medical treatment for sports injuries peaks between the ages of 5 and 14 years and progressively decreases thereafter (1). The ankle and knee are the most common sites of injury reported in these young athletes (1,2). Youth sports players also appear to be at greater risk of low back pain and acute lumbar spine injury, particularly during adolescence (28).
Inadequate physical preparation is believed to play a role in most sports-related injuries in young athletes (27). Conditions of muscle fatigue place athletes at greater risk of injury; tired players in the latter stages of a game are more likely to sustain injury than when they are fresh. Likewise, players are more likely to be injured early in the season when their fitness levels are not up to standard (45).
Physical preparation, which includes strength training and training to develop cardiovascular and respiratory fitness, is therefore an established part of strategy for prevention of sports injuries, including those in children and youth sports (32). Inadequate motor skills are another factor identified as increasing the risk of youth sports injury (1). Again, these abilities may be developed through appropriate athletic preparation.
Injuries incurred during youth sports are a frequently cited reason for ceasing to participate in sports as an adult (32). This cessation has negative health implications given the established links among physical inactivity, obesity, and chronic disease in adulthood. From this perspective, prevention of injury in youth sports assumes increased importance, beyond enhancing youth sports performance (32).
OVERUSE INJURY INCIDENCE IN YOUTH SPORTS
When organizing participation of adolescents in physical training and organized sports, it is important to recognize that young people are still growing (5,27). Coaches must consider that the bones, muscles, and connective tissues of the young athlete are not yet fully developed. As such, high volumes of repetitive practice may make the young player susceptible to overuse injury. This dictates not only that there is a need for age-appropriate practice and competition schedules, but also that young players’ physical preparation is designed specifically to reflect their stage of growth and maturation.
Biomechanical factors seem to play a role in the incidence of overuse injuries with youth sports participation. The rapid changes in the size and length of limbs during growth spurts alter the mechanics of athletic movements (20). As young athletes grow, these changes actually increase the forces and mechanical stresses involved in sports movements.
When the young player is undergoing a growth spurt, particular care should be taken, in view of the combined strain associated with rapid growth and physical stresses during competition and practices (38). During this time, the immature skeleton may be more susceptible to injury than at later stages in the athlete’s development; lumbar spine injuries particularly appear to increase in adolescent athletes (28). Growing cartilage is similarly more prone to injury in comparison to when the player reaches physical maturity, which can also be a factor in some overuse injuries (1).
Given time, muscles and connective tissues respond to accommodate these growth-related changes; however, there is a time lag before this adaptation takes place (20). Under normal circumstances, connective tissues remain within their failure limits during this lag phase. However, during puberty in boys particularly, there is a rapid increase in body mass and strength; tendon and ligament strengths respond relatively more slowly than muscle, meaning these structures are closer to their failure limits in young players during this phase of maturation (20). Repeatedly performing a given sports movement during this sensitive period in the young player’s development can then lead to overuse injury.
The point of attachment of tendon to bone (i.e., apophysis) is an area particularly prone to overuse injury in the growing player (1). Microtrauma injury, or apophysitis, commonly occurs at the heel (Sever disease) and the elbow (i.e., “little league elbow”) in younger children (i.e., 7–10 years). A similar condition, Osgood–Schlatter disease, occurs at the insertion of the patella tendon and is often seen between the ages of 11 and 15 years (1).
In certain youth sports, there is a risk of overuse injuries simply because of the strains involved in repetitive performance of a particular sports skill movement during practices and games, such as in throwing sports. In the United States, it has been estimated that these overuse injuries make up approximately one-half of all sports-related injuries requiring medical treatment (20). To combat this problem, some governing bodies suggest limits for the number of repetitions of movements (e.g., number of throws) that can be performed by a young player during a practice session (20).
TRAINABILITY OF YOUNG ATHLETES
The scope for improvements in different aspects of fitness and motor performance varies as the young athlete passes through the different stages of physical maturation. Rates of development of a number of physiologic parameters appear to peak near the same time as peak height velocity (i.e., stage of maximal growth in height) in young team-sports players (41). The age at which peak height velocity is attained varies considerably but is reported to occur near 11.5 years for girls (3) and between 13.8 and 14.2 years for boys (41).
Preadolescents have considerable potential for motor learning. Many authors have stated that complex motor skills are not mastered until 10 to 12 years (1,3). It is suggested that there is a prime window of opportunity for motor development before puberty. Teaching basic movement mechanics for running, decelerating, and changing direction should form a fundamental part of training for all young players. Performing complex whole-body training exercises is advocated to enhance coordination and athleticism. Such training also develops kineesthetic awareness and proprioception and makes the young player better able to retain his or her balance under pressure from opponents and adjust to uneven terrain. Improving these functional abilities therefore may have a protective effect by helping to guard against injury (12).
Previously, the presumption had been that strength training before puberty was not viable or effective. However, it now appears that prepubescent children show significant scope for strength gains, far beyond those attributable to normal growth and maturation (11). Relative gains in strength documented with resistance training in prepubescent children are in fact of similar magnitude to those shown by adolescents (11).
However, there are trends for greater absolute strength gains in adolescents. Puberty triggers major physiologic and hormonal changes (38). Increases in circulating anabolic hormones during puberty considerably affect responses to strength training, particularly the scope for tissue hypertrophy. This is the case especially among adolescent boys. However, the power per kilogram (i.e., body mass) that adolescents are capable of generating is still less than that of adults (38).
During puberty, spontaneous growth-related improvements in motor performance and physiologic parameters occur. A longitudinal study of youth soccer players showed that these natural gains may plateau during the interval before the young athlete reaches peak height velocity; performance may even decrease during this period, as it occurs with 30 m speed scores (41). Then, at the age at which peak height velocity is attained, several of these natural gains in physiologic and motor performance scores appear to reach their peak rate of development. In the 12 to 18 months after peak height velocity, decreasing rates of growth-related improvements in many of these parameters are then observed (41). Hence, scores in motor performance, in the absence of training interventions, appear to plateau at the end of this phase of development in youth sports players.
Changes in musculoskeletal, cardiovascular, and respiratory systems during and after puberty have major implications for metabolic conditioning (38). There are marked differences between prepubescent and adolescent players in terms of responsiveness to anaerobic and aerobic training. During puberty, the responsiveness of young players for anaerobic exercise progressively increases (38). Before puberty, young athletes have very limited capacity for this type of training. The rate of maturation-related improvements in anaerobic capacity during puberty peaks near peak height velocity, but natural gains continue thereafter (41).
Children and adolescents show gains in cardiovascular and respiratory fitness with aerobic training (38). However, significant gains can be made particularly during puberty in young players as they reach their peak height velocity (i.e., near 14 years for boys and 12 years for girls), partly because of the aforementioned maturation effects (41).
TRAINING TO BUILD A FOUNDATION OF ATHLETICISM IN YOUNG TEAM-SPORTS PLAYERS
Because of the growing awareness of sport-specific training methods among coaches, parents, and young players themselves, there is often pressure to solely prescribe training that mimics the chosen sport in which the young player participates. One of the most important recommendations from authorities on youth training is that during all stages of development, the young player should perform a range of sports and training activities to facilitate overall athletic development (5). It is advocated that the young player should specialize only in terms of sport and playing position as he or she advances into late adolescence; much the same applies in terms of physical preparation.
Cook (7), a noted physiotherapist and coach of strength and conditioning, described a pyramid model for the abilities that constitute athleticism. The base layer of the athleticism pyramid consists of mobility and stability. Mobility is the active range of motion for functional movements, and stability is the ability to maintain posture and balance during athletic movement. The next layer up in the athleticism pyramid could be described as functional movement. All sports and athletic events feature fundamental movements in some combination, which include squatting and lifting, pushing and pulling, lunging, locomotion (e.g., running), and twisting (35). The top layer of the athleticism pyramid is functional skill, which can be viewed in terms of sport-specific strength and movement skill training.
Training to build young athletes therefore must start at the foundation of the athleticism pyramid and build upward. Development of mobility and stability is therefore the first priority when training young players. In turn, these qualities underpin the player’s ability to perform fundamental movements that are common to all sports. As Cook states, “Fundamental movement supports specific movement” (7). That is, players’ fundamental movement abilities will determine their ability to perform sport-specific movements. There is no point in trying to impose sport-specific training on flawed fundamental movement patterns. It follows that training activities at this stage of the athlete’s physical preparation should predominantly feature fundamental athletic movements. Exercise selection can then progressively shift to sport-specific movements with advances in the athlete’s physical development.
NEUROMUSCULAR AND MOVEMENT SKILL TRAINING
As discussed earlier, mobility and stability are major training goals to build the foundation of athleticism in young players. Neuromuscular training interventions often comprise dynamic stability and balance training exercises (37). These forms of neuromuscular training offer a means for development of whole-body balance and postural control (46), which underpin stability. Functional movement abilities, the next tier up in the athleticism pyramid, likewise can be developed through appropriate movement skill instruction and training (37).
Prepubescent athletes have less mechanical efficiency than adolescent athletes do. Although mechanical efficiency improves as the young athlete progresses through puberty, adolescent athletes still have less mechanical efficiency than adults do (38). It follows that there is considerable scope for this aspect of performance to be improved through specific instruction and practice. Exercise economy has been identified as an area for development in young athletes (38), allowing the young player to sustain a higher relative work rate throughout the course of a match.
Some evidence supports the potential of neuromuscular training to improve athletic performance in young players. Jump training incorporating specific instruction and training of proper movement mechanics has been shown to improve vertical jump and movement biomechanics in high school female athletes (36). A neuromuscular training intervention significantly improved lower-limb alignment and reduced knee valgus angles in young female athletes (39). Similarly, balance training improved shuttle run agility performance in a mixed-gender recreationally active training group (46). Dynamic balance training has also been shown to significantly reduce impact forces on landing in adolescent female team-sports players (37). The neuromuscular control capacities that allow an athlete to dissipate impact forces and maintain proper lower-limb alignment have been identified as key factors in reducing players’ relative risk of injury (42). Both forms of neuromuscular training described earlier may thereby help guard against injuries through different mechanisms.
Prepubescent athletes have a tendency to have neuromuscular control deficits, particularly, valgus hip, knee, and ankle alignment during jump-landing tasks (2). This is indicative of impaired ability to control joint motion particularly at the knee and, as such, is associated with increased injury risk (14). Training to improve lower-limb neuromuscular control therefore appears important to correct the potentially injurious lower-limb alignment when it is observed in prepubescent team-sports players.
Young female athletes in particular showed these traits (2). Neuromuscular control issues may contribute to making female athletes ligament-dominant. Specifically, as a result of inadequate active muscular stabilization, girls can rely more on ligamentous support to assist in stabilizing lower-limb joints, subjecting these ligaments to greater strain (14). In combination with anatomic factors, including hypermobility and joint laxity of lower-limb joints, this strain can make girls more prone to lower-limb ligament injury than boys. Girls appear to show valgus knee motion to a greater extent on their dominant leg (14). Such side-to-side imbalances in neuromuscular control and coordination represent another risk factor for injury.
As boys pass through puberty, they undergo a neuromuscular spurt, which accompanied by limb growth and favorable changes in body composition (i.e., increased muscle mass relative to fat mass), improves their biomechanics (42). One observed aspect of this improvement is an enhanced ability to dissipate ground reaction forces on landing. These landing-impact forces, in turn, directly influence the loading absorbed through lower-limb joints (22). This neuromuscular spurt phenomenon does not occur in girls. The lack of any marked improvement in neuromuscular power and control, in combination with limb growth and body mass gains, can make the lower limbs even more unstable in adolescent girls (42).
Certainly, female players continue to have a tendency to have potentially injurious lower-limb alignment and movement mechanics as adolescents (3,42). In the absence of neuromuscular training, female players also have a tendency to preferentially recruit the quadriceps over the hamstring muscles during activity, known as quadriceps dominance (14). Such biomechanical factors and aberrant recruitment patterns are implicated in the gender differences in rates of anterior cruciate ligament injury after puberty, which is not seen before this stage of development. Various studies report adolescent female players to have between 2 and 10 times greater incidence of anterior cruciate ligament injury compared to male players, depending on the sport (17).
Neuromuscular training to address these issues therefore remains a priority for adolescent female players (2,42). Numerous studies support the capacity of neuromuscular training to offset this increased knee injury risk. After a neuromuscular training intervention, the knee injury incidence rates of high school female team-sports players were reduced to a level similar to those of untrained male athletes studied (22). The rates of knee injury in these adolescent female players after intervention were nearly 4 times lower than those in the untrained female players in the study.
Recent studies have shown that postpubescent male athletes may also continue to have valgus lower-limb alignment during drop-landing tasks, despite markedly increased lower-limb strength levels (3,39). It follows that appropriate screening and neuromuscular training therefore should not be neglected with adolescent male team-sports players either. The continuing need for neuromuscular training in boys during adolescence is also apparent in view of the rapid gains in body mass and strength in boys particularly, which are characteristic of this stage of their physical development. In prepubescent players, the neuromuscular control issues and injurious lower-limb alignment are offset by their lower body mass and movement velocity (3). In contrast, adolescent players are much heavier and generate greater forces and movement speeds. Both of these factors combine to markedly increase imposed stresses as a result of adolescent players’ greater inertia and momentum in relation to prepubescent players (3). The consequences of any deficits in neuromuscular control in adolescent players are therefore greatly magnified.
Prepubescent and adolescent players can benefit from conditioning to increase aerobic endurance (38). The training modes used should vary between stages of maturation; in the same way, the intensity, duration, and volumes prescribed should also alter as the player grows and matures. A wider variety of activities and cross-training modes is suggested for prepubescent players (5,27). As the young player matures, training guidelines change to reflect corresponding changes in physical capabilities. Training modes will become more specific to the sport, and the intensity of conditioning activities will likewise increase.
It is recommended that endurance training activities used with prepubescent players avoid monotony and aim to incorporate an element of fun (5,27). Before puberty, it is likewise suggested that intensity be limited to moderate levels and that training volumes (i.e., duration or distances covered) be gradually increased for training progression (5). Both these perspectives point to a less regimented approach in relation to that used with older groups of players. In this way, not only does conditioning remain enjoyable, but the young player also can self-regulate work intensity. Skill and sport-related movement drills can be adapted for conditioning (5,24). Ball games with simplified rules are another good choice for conditioning activities with these young players (5).
Throughout puberty, growth-related gains in endurance occur naturally as a result of development of cardiovascular and respiratory systems, and these can be harnessed to augment training responses. During this stage, young athletes’ capacity for anaerobic exercise also increases (41). However, individual players’ tolerance for training will differ according to their stage of development. This can vary widely in a group of players of the same chronologic age (5,27). Consideration must be given to this fact when training players of an age at which they may be undergoing puberty. A continuing emphasis on appropriate metabolic conditioning would appear to be vital for female players, particularly to counter the decrease in aerobic endurance that is otherwise observed in girls after the onset of puberty (38).
Adolescence is identified as the time for specialization in young players’ physical preparation (5,38). Physiologic changes during puberty increase young players’ capacity for, and responsiveness to, anaerobic training (38). This form of conditioning is a requirement of most team sports; therefore, it follows that anaerobic training should feature increasingly in adolescent players’ physical preparation. Various modes of interval training are shown to be effective in improving measures of endurance fitness and performance indices with team-sports players (21,25). High-intensity interval hill running has been shown to elicit significant endurance improvement, including lactate threshold, in young soccer players, which importantly also carried over to measures of soccer performance during matches (21). A soccer-specific protocol running through a set course involving dribbling a ball alternated with backward and forward shuttle sprints through cones is also reported to elicit sufficiently high intensities (i.e., 93% HRmax or 91% Vo2max) for anaerobic conditioning in older players (25).
Skill-based conditioning games also show potential for use in developing anaerobic capacity in different team sports, notably rugby union and soccer (16,25,31). By manipulating the number of players on each side, playing area, and rules, exercise intensities in the range of 87% to 91% HRmax (31) and 91% HRmax (25) have been reported in different groups of first-division male soccer players in England and Norway, respectively.
Aerobic endurance remains a key requirement for young players in most sports. There is a need for the training modes used to develop aerobic endurance with adolescent players to become increasingly specific to the sport. Therefore, cross-training modes should be emphasized only during the off-season. By selecting appropriate work bouts and rest and recovery intervals, it is possible for repeated high-intensity training to elicit aerobic and anaerobic endurance gains (44). With time, it follows that the maturing player’s aerobic endurance development may be predominantly achieved through this form of training (31), particularly during the playing season. Again, doing so may be achieved through appropriate skill-based conditioning drills or conditioning games (21,25).
SAFETY AND EFFECTIVENESS OF YOUTH RESISTANCE TRAINING
The benefits of youth resistance training are well documented and are becoming universally accepted among health professionals, particularly in the United States (11,12) and increasingly in the United Kingdom (43). Fitness professional associations and health organizations are now in agreement that age-appropriate youth resistance training is safe and beneficial when performed under qualified supervision (12,27,43). However, public recognition of these benefits continues to lag behind, and misunderstanding and misconceptions remain.
Historically, the concerns about youth resistance training stem from a perceived risk of damaging growth plates. This could potentially interfere with normal growth. In fact, damage to growth plates has never been documented with strength training programs for children that were administered and supervised by qualified personnel. Studies using appropriate youth resistance training, in fact, report low incidence of injuries of any type (11). Far from stunting growth, the contemporary evidence is that resistance training, in combination with proper nutrition, has the potential to enhance growth within genetic bounds at all stages of development (11).
The most common causes of injury when undertaking resistance training include incorrect lifting technique, attempting to lift excessive loads, inappropriate use of equipment, and absence of qualified supervision (11). All these factors can be reduced or eliminated with properly administered and supervised training (43).
Naturally, young players, as with any inexperienced lifters, should engage in only strength training programs prepared by qualified coaches, with safe equipment, and supervised by qualified instructors. However, if these conditions are met, there are no safety grounds to stop young players from undertaking supervised strength training (27).
The reality is that children are exposed to far greater forces and of longer duration during sports and recreational physical activity than those encountered during strength training, even if they were to perform a maximum lift (11). Of all resistance training exercises, the strength training exercises possibly impose the greatest forces on the young musculoskeletal system. Even so, injury data suggest that strength training and competition conducted under the supervision of qualified coaching is one of the safer athletic activities engaged in by young athletes (19).
MECHANISMS FOR STRENGTH GAINS IN PREPUBESCENT AND ADOLESCENT ATHLETES
For prepubescent athletes, lower levels of circulating anabolic hormones limit the contribution of hypertrophy (i.e., lean tissue growth) to strength gains (11), and the changes to muscles that do occur appear to be more qualitative rather than quantitative. Neural effects thus appear to underpin much of the gains from resistance training in these younger athletes.
Such neural adaptations are suggested to include improved recruitment and activation of the muscles mobilized during the training movement. Enhanced motor coordination within and between muscle groups is also thought to contribute to strength gains after training. By the nature of these training adaptations, such strength gains would seem to be less permanent; prepubescent players will show marked detraining effects once regular resistance training is discontinued (11). However, modest (1 or 2 days per week) maintenance programs do appear to be sufficient to sustain strength gains.
The greater hormonal response to resistance training in adolescence than at earlier stages of development leads to structural changes to the muscles and associated connective tissues (11). As a result, marked changes in muscle hypertrophy and gains in fat-free mass are seen in this older age group. Such increases in muscle cross-sectional area and changes in muscle proteins therefore augment the gains in strength of neural origin that occur.
STRENGTH TRAINING FOR PERFORMANCE ENHANCEMENT IN YOUTH SPORTS
It is becoming recognized that young players can experience benefits from strength training similar to those observed with adults (11). All youth sports demand, to varying degrees, strength and power to overcome the player’s own body weight when moving and the resistance of opponents, particularly in contact sports. It follows that developing strength through resistance training should positively affect performance in the young player’s sport (43).
The effects of strength training in young players, which include increased strength and improved motor skills and coordination, have the potential to improve athleticism. Improvements in scores on motor performance measures are often observed after resistance training in children (43). Positive changes have been noted in vertical jump, standing long jump, sprint times, and agility run times (11).
The available data from the limited number of studies that have been published indicate that increases in flexibility can be made, particularly if the resistance training incorporates specific stretching and warm-up and cool-down (43). This appears to refute concerns in some youth sports that resistance training will lead to the young athlete becoming muscle-bound and consequently decrease their flexibility and range of motion. Warm-up before training and team practices should comprise dynamic flexibility exercises; this form of stretching appears to offer the most effective preparation for dynamic activity (30). There is also some evidence of adverse effects on athletic performance associated with performing static stretching immediately before dynamic activity (6). Static and partner-assisted stretching still has a role to play as a means to develop flexibility. However, based on the available evidence, it seems sensible to restrict their use to the cool-down after sessions or to standalone flexibility sessions to avoid any detrimental effects on performance.
STRENGTH TRAINING FOR INJURY PREVENTION
Participation in team sports involves some inherent risk of injury. Although these injuries can never be eliminated entirely, appropriate training can assist in minimizing injury incidence and help reduce the severity of those injuries that do occur. Young players are subject to additional risk because of physiologic and developmental factors. The strains on connective tissues during growth and the changing properties of the growing tissues render these structures more prone to injury in the young player than in the adult (1). Strengthening muscles and connective tissues with strength training offers a means to increase the forces they are capable of sustaining and helps to make the young player more resistant to soft-tissue injury. In adolescents particularly, it is important to strengthen these connective tissues to accommodate the rapid gains in strength and body mass that occur during puberty (1).
Strengthening muscles around upper-limb and lower-limb joints through appropriate training similarly offers a means to increase the active stability provided to these joints, which can serve a protective function (43). Strength training has been shown to improve neuromuscular control indices during jumping and landing in female adolescent athletes (29). Such development of motor control and coordination improves balance and active joint stability, both of which are important to help reduce the incidence of lower-limb injury.
In young female players, lower-limb strength development, in general, and hamstring strengthening, in particular, should be major areas of emphasis (3). Measures of hamstring strength are reported to plateau early in female athletes’ physical development, with older age groups (i.e., 13–17 years) showing no significant gains on this measure compared to 11-year-old girls (3). The hamstrings compress the knee joint and oppose anterior shear forces during weight-bearing closed-chain movements; as a result of these functions, the hamstrings are described as an anterior cruciate ligament agonist (22). The relative weakness of the hamstrings of female players is of clinical relevance given the 2 to 10 times greater rates of noncontact knee ligament injury in adolescent female athletes compared with male athletes (17).
Prepubescent athletes are shown to have a greater tendency than older athletes to show asymmetric lower-limb performance, based on scores with single-leg hopping functional tests (2). In the absence of intervention, such imbalances may persist after puberty in boys and girls (3). Appropriate strength training offers a means to help correct such right–left imbalances in lower-limb function, particularly in combination with plyometric or dynamic balance training (37). This role of strength training in correcting side-to-side strength imbalances is crucial for young players at all stages of development. Strength and flexibility imbalances are identified as major risk factors for injury (26). Strength imbalances can have negative consequences for both limbs. Overreliance may place excessive strain on the stronger limb, whereas the weaker limb is less able to actively counter injurious forces (14).
Studies show that young players who have strength training experience tend to sustain fewer injuries (12). Incidence of injury in strength-trained youngsters is approximately one-third that of young athletes without any strength training experience (5). In addition to reducing overall incidence of injury, strength training can also help to reduce the severity of injuries. After an injury, strength-trained young players also respond better to rehabilitation, resulting in a more rapid return to training and competition (12,27).
For these reasons, strength training is recommended in a preconditioning role for young people before they start to compete in organized youth sports (5). Young players who are better conditioned and less prone to injury because of appropriate physical preparation, including strength training, are more likely to continue to participate in youth sports. In this way, strength training can help reduce dropout rates and help to keep youngsters healthy later in life (12).
Aside from the benefits of general strength training, targeted strength training involving particular exercises may also be used to guard against certain injuries that commonly occur in sports. This targeted injury prevention role for strength training is often overlooked, particularly in young athletes. Too often exercises to strengthen areas that are prone to injury are prescribed only once an injury has already occurred. Unfortunately, there are currently an insufficient number of prospective studies in the literature involving youth sports players to provide evidence-based training guidelines regarding effective training for injury prevention (32).
TRAINING TO DEVELOP BONE HEALTH AND CONNECTIVE TISSUE
In much the same way as for adults, positive links are established between physical activity and bone mineral density and connective tissue integrity in young people (18,43). Although genetics is a determining factor, the major stimulus for accumulation of bone mass and mineral content is mechanical loading (18). The cross-sectional area and architecture of connective tissues are also trainable; appropriate strength training can develop the strength and size of tendons and ligaments (9).
Mechanical loads must exceed a threshold to trigger adaptive responses (9). As a result, high-force weight-bearing and strength-type activities appear most suitable for use to elicit bone and connective tissue adaptations. Dynamic skeletal loading (i.e., loading during movement) appears to be relatively more osteogenic than the same loads applied under static conditions (18). It follows that relatively high mechanical loading occurring during dynamic training activities should result in the greatest bone adaptation.
Recommended exercises generally involve weight bearing so that the young athlete’s own body weight provides additional loading (9). Sporting activities that involve high ground reaction forces are associated with increased bone mineral content and density (18). For this reason, sprinting, jumping, and other lower-body plyometric exercises are good training activities for developing bone strength because they offer high ground reaction forces and impact loading. Young athletes in all running-based sports and athletic events can benefit from these exercises. However, training volumes, such as total foot contacts for plyometric training, must be monitored to avoid excessive strains and potential overuse injury.
Applying resistance through strength training is another means to generate the mechanical stresses required for an osteogenic response (18). This particular role of strength training for young athletes has been termed “anatomical adaptation” (5). Such effects include increased strength of supporting connective tissues, passive joint stability, and increased bone density and tensile strength (12). In the same way as weight-bearing activities are recommended, structural multijoint strength training lifts, such as variations of the squat, lunge, and step-up, offer a means to elicit whole-body skeletal adaptations. Site-specific gains in strength and cross-sectional area of bone and connective tissues associated with the muscles recruited during a given strength training exercise can also be used (9). Specifically, strength training exercises can be used to strengthen bones and connective tissues at particular sites that tend to be exposed to strain in the particular sport, such as the shoulder girdle in contact sports.
Immediately before and during puberty appears to be a key phase that offers a window of opportunity for skeletal adaptations (18). It is suggested that osteogenic training activities therefore can be used to amplify the skeletal growth and growth-related gains in lean body mass that occur naturally during these stages. However, studies have shown that postpubescent females may be less responsive to skeletal adaptation, which suggests an earlier and narrower window of opportunity for female players (18).
Increases in bone density brought about by strength training are of relevance to female players from a longer-term health perspective. Women have a higher incidence of osteoporosis than men do during late adulthood. During adolescence, the growing skeleton seems to be particularly responsive to training (18). For this reason, young players, and females in particular, are recommended to perform dynamic weight-bearing exercise and appropriate strength training during childhood and adolescence (9,27). Increasing the player’s bone mineral content at this stage of development is likely to have a favorable impact on his or her risk profile for osteoporosis in later life.
YOUTH TRAINING AND BODY COMPOSITION
As mentioned earlier, it is suggested that there be a threshold level of physical activity that, in combination with proper nutrition, is required for young players to achieve their genetic potential in terms of growth and maturation (23). A structured program of physical preparation offers a means to guarantee that this goal is achieved, particularly at critical periods in the young player’s growth and maturation. Furthermore, dedicated training sessions for youth sports also appear vital to help players maintain lean body composition given the background of decreasing physical inactivity and increasing obesity among youth.
During and after puberty, some characteristic changes in body composition can be unfavorable to performance and potentially to health, specifically the gains in body fat mass in females particularly (38). Appropriate resistance training, in conjunction with aerobic exercise, has been proposed for losing body fat and for maintaining weight, in much the same way as is recommended for adults (12). In view of the increasing incidence of childhood obesity, the potential of physical preparation, including resistance training, to favorably alter body composition can be seen as advantageous from a health perspective (23).
Conversely, the potential for strength training to increase lean body mass is of relevance to players in collision sports. In sports such as rugby and American football, physical size is a determining factor for participation at higher levels. Young players are naturally predisposed to, and selected for, particular playing positions on the basis of their anthropometric characteristics (i.e., height and body mass) and strength capabilities (10). Without experience of systematic strength training, young players are unlikely not to have undergone the requisite physical development.
As in other collision sports, such as American football, rugby union players’ body mass and muscularity have increased at a disproportionate rate during the past 25 years, particularly since the advent of professionalism (40). This trend among certain playing positions in these collision sports places them increasingly further away from the general population norms in terms of their physical characteristics.
Players at higher levels of competition in rugby football have greater lean body mass than those participating in lower leagues. Body mass has also been shown to correlate with the respective performance of rugby union national teams in World Cup competition, with the heavier playing squads progressing further in the competition (40).
The need for different aspects of physical preparation, including strength training, metabolic conditioning, and neuromuscular training has been described for a range of sports and for young athletes at different stages of maturation. The efficacy of each of these different components of physical preparation for athletes, in general, and young team-sports players, in particular, is also becoming increasingly well established. The nature of responses to each of these forms of training at different stages of growth and maturation has also been examined, but further research is necessary to provide a clearer picture.
Any training program should be geared to the physical and emotional maturity of the individuals in the group. Because of the paucity of well-controlled studies in the literature, there is a shortage of conclusive recommendations regarding training design for young populations at different stages of maturation (38). Thus, guidelines have been published that differentiate between chronologic age and, more importantly, biologic age (5,11,27).
Fundamentally, the primary emphasis of training for young team-sports players is on balanced physical development and building a foundation of athleticism. Only once this has been undertaken and physical maturation has taken place should the focus then progressively shift to specialized preparation for the particular sport and playing position.
TRAINING RECOMMENDATIONS FOR YOUNG PLAYERS
As discussed, rates of growth and maturation within a group of players can vary widely. Phases of development are therefore difficult to define within a squad when training young players competing in team sports. For the purposes of this section, guidelines will be divided into before pubescence (i.e., before showing physical signs indicative of the onset of puberty), early puberty (i.e., the phase between the onset of puberty and attaining peak height velocity), and adolescence (i.e., the period after peak height velocity has been attained and advancing into adulthood).
The divisions between stages are necessarily vague. The average age at which peak height velocity is attained (i.e., marking the transition between early puberty and adolescence, as defined earlier) is near 12 years for girls and 14 years for boys (33), but there is considerable variability in this age. Observing changes in physical characteristics, assessing neuromuscular performance, and monitoring seated and standing heights at regular intervals will help in determining the progression between stages. Standing and seated heights are the most helpful objective measure to track with young players when used to plot velocity curves (i.e., gain in height per unit of time) for each player (4). Seated height is helpful because trunk length tends to lag behind leg growth. Ultimately, it depends on the coach to use his or her experience and observations of each player’s performance during training over time as the deciding factor that determines how and when to progress training for each individual player.
Within these guidelines, consideration must also be given to the training age of the young player entering a program of physical preparation. Previous training experience, strength training in particular, will influence individual decisions regarding training prescription. For example, a player who is in the early puberty category based on age and physical characteristics but enters the program with 2 years of previous strength training experience may be ready for more complex training exercises than another player who is significantly older but has no previous training experience. Regardless of the age or stage of growth and maturation of the player, initial training prescription will reflect the primary objective of developing competency performing fundamental movements and addressing any functional deficits. Only once this has been undertaken should the focus then shift to performance-related training goals and more advanced training.
NEUROMUSCULAR AND MOVEMENT SKILLS TRAINING
Neuromuscular training should be initiated early in young players’ physical preparation. Doing so is important to help correct the valgus lower-limb alignment during athletic movements that is common in prepubescent athletes of both genders (2). This form of training also has a role to play in improving the movement efficiency that has been identified as lacking among these young players (38).
The starting point for proven short-term neuromuscular training programs appropriate for young team-sports players is instruction of athletic position and safe movement mechanics (22). This fundamentals phase of established neuromuscular training protocols can be implemented with young players during this stage of development. This phase includes instruction and practice of jumping, landing, and change-of-direction movements as discrete skills.
Neuromuscular movement skill training with young athletes may be effectively augmented by dynamic balance and stabilization exercises (37). Dynamic balance and single-leg stabilization exercises in various postures, with an emphasis on correct lower-limb alignment, control, and balance therefore should be initiated with prepubescent players of both genders.
It follows that the lower capacity of prepubescent athletes for anaerobic exercise should be reflected in the training used with these players. Most training at this stage of development should be aerobic in nature. However, the training modes used to achieve this may be skill-based to reduce training monotony and include elements of fun and competition.
Skill or game-related movement drills can be adapted for use as conditioning activities. For example, an obstacle course can be constructed to involve different movements and ball skills, perhaps running relays between teams of athletes (5). Alternatively, ball games with simplified rules may be used with the numbers on each team and playing area being manipulated to alter exercise intensity. This less structured approach allows the young player to self-regulate work intensity according to his or her individual tolerance.
In general, if a child is ready for participation in organized sports, he or she is likely ready to undergo instruction and resistance training. However, for young players with known or suspected medical conditions, medical clearance should be sought before participation in resistance training, as with other sports (11). The specific design of training for young team-sports players should be reflective of their psychological and emotional development to engender motivation and facilitate compliance (43).
When coaching prepubescent players, it is important that training should be enjoyable and give the young player an immediate sense of fun and discovery-based learning (43). In practical terms, the choice of training exercises and loads used should be conducive to allow this approach to be safely implemented. For example, body-weight resistance exercises are more appropriate when training more complex whole-body movements.
Meta-analyses have identified repetition schemes from 6 to 15 reps and 50% t o100% RM to be effective for resistance training with young athletes (13). In general, resistance training volumes of 2 or 3 sets and frequency of training of 2 or 3 days per week appear to be most effective. When young players are introduced to strength training, light loads and high repetition schemes (i.e., 12–15 reps) are most appropriate (11). During this early stage of training, progression should be achieved by increasing the number of sets performed and the number of exercises in the workout. The relative loading and number of training days used can then be increased later.
Adequate rest and recovery are key components of successful youth resistance training. Younger players and those in the early stages of their physical preparation will require more recovery time between training days. Training on inconsecutive days therefore is recommended for prepubescent players to maximize the effectiveness of training and reduce the risk of injury (12).
Because many of the benefits of strength training before puberty stem from improved coordination, balance, and proprioception, it follows that exercise modes that favor the development of these aspects should be emphasized when training prepubescent players. Body-weight resistance exercises and free weights offer advantages from this point of view, in comparison to fixed-resistance machines, although these exercises may require closer supervision. Another consideration if choosing to use resistance machines with young players is that the apparatus must be fitted to the dimensions of the person. Some apparatuses cannot be adjusted sufficiently to be suitable for use (11).
Exercise selection should feature a combination of unilateral and bilateral exercises appropriate to the young player’s capabilities. The inclusion of unilateral exercises in prepubescent players’ training is important to promote balanced development between limbs (27). These exercises do not allow the young player to compensate with his or her stronger limb, as it is possible with bilateral exercises. Bilateral exercises should also feature in the young player’s program at this stage of maturation as a means to develop strength from a more stable base of support (Table 1a).
NEUROMUSCULAR AND MOVEMENT SKILLS TRAINING
Puberty is characterized in boys particularly by a progressive improvement in neuromuscular abilities. This change is known as the neuromuscular spurt (42). However, before attainment of peak height velocity, there may be short-term decrements in some aspects of neuromuscular performance. It follows that neuromuscular training should be progressed during this phase of the young player’s development in a way that is responsive to their individual rate of neuromuscular development and sensitive to any short-term changes.
Training to reduce the potentially injurious loading that occurs with poor neuromuscular control of lower-limb alignment assumes increased importance given the gains in body mass that occur during puberty, which in turn increase the loading and stresses imposed on joints and connective tissues. In female players, this form of training is advocated as a means to artificially create a neuromuscular spurt, similar to that which occurs naturally in boys during this phase of maturation (42). Neuromuscular training is a priority for female players to tackle the higher rates of lower-limb injury in comparison to male players, which become apparent from this stage of maturation onward (29).
Neuromuscular training during this stage will continue to feature dynamic balance and stabilization work. These exercises have numerous progressions, which can be implemented as appropriate with advances in maturation and neuromuscular performance.
During and after puberty, metabolic training responses become increasingly specific to the type of metabolic training used. It follows that the training modes used must account for this specificity and increasingly reflect the demands of the sport and playing position from this stage of maturation onwards. Cross-training activities will tend to feature less during the playing season, but they remain an important training tool, particularly for off-season training.
Depending on individual tolerance, the intensity of metabolic conditioning will likewise be progressively increased during this stage to reflect the higher intensities of exertion experienced during competitive games. Conditioning games can be manipulated (e.g., reducing the number of players on each side and modifying the size of the playing area) to become more demanding. The rest intervals used between conditioning drills may also be decreased again within individual tolerance.
Adequate rest and recovery continue to be integral aspects when scheduling strength training during puberty. Given the concomitant strains of growth and maturation during this stage, recovery time is crucial between training days. Training on inconsecutive days therefore continues to be advocated during puberty (12).
Exercise selection will also reflect the need for balanced development and improving strength for fundamental movements. General strength exercises and unilateral exercises should feature prominently during puberty. With advances in training experience, structural multijoint lifts (e.g., variations of the squat and deadlift) can be introduced. In the case of experienced young lifters, resistance training exercises can also be integrated into the strength training program as appropriate; under qualified supervision, this form of training carries no greater risk for young players than other athletic activities (19). Resistance training exercises and their variations should be taught initially by using a light implement, such as a broom handle or empty barbell, with the emphasis on the quality of the lifting movement.
Whatever the exercise, the focus throughout should be on proper lifting form, with loading limited until the young player has mastered the lifting technique. The coach should also be vigilant for temporary reductions in coordination and performance that may occur as the young player approaches peak height velocity and be prepared to modify exercise selection and loading accordingly. Likewise, loading should be restricted from the point of view of attenuating the stresses on skeletal and connective tissue structures during phases of rapid growth (38). Loading of the lumbar spine particularly should be carefully monitored in recognition of the higher risk of lumbar spine injury at this phase of development (28).
In the absence of corrective training, significant asymmetries are observed before and after puberty (3). In terms of function and injury prevention, it is vital that any differences in performance between dominant and nondominant limbs are addressed during this stage through strength training. Practical recommendations include manipulating the number of reps and sets performed with each limb for single-limb strength exercises (8). For example, the young player may perform 3 reps on his or her weaker side for every 2 on the dominant side while keeping the load constant.
For female players, targeted hamstring strength training should begin during this stage, ideally in conjunction with neuromuscular training to help increase hamstring recruitment during dynamic activity (22). This is important to offset the quadriceps dominance that can increase strain on the anterior cruciate ligament in female athletes (Table 1b) (14).
NEUROMUSCULAR AND MOVEMENT SKILLS TRAINING
The fundamentals phase characteristic of earlier neuromuscular training will be progressed during this stage of development to more demanding jumping exercises and change-of-direction drills. For all exercises, the emphasis should remain on proper movement mechanics and correct lower-limb alignment, particularly during landing and change-of-direction movements (37).
As the player matures and his or her neuromuscular performance develops, further advances in training may include single-leg plyometric-type jumping and landing exercises in various directions and unanticipated change-of-direction movement drills. Dynamic balance and stabilization exercises can similarly be progressed during this stage by incorporating various training devices to increase the demand for balance and proprioception (37,46).
In many team sports, adolescent players will require repeated sprint conditioning to elicit appropriate anaerobic training effects (38). Accordingly, anaerobic interval training appropriate to the sport should be progressively introduced during this stage (5). Various modes of high-intensity interval conditioning, including hill running (21) and high-intensity sport skill conditioning drills (24,25), can be effective. By manipulating the numbers on each team, playing rules, and playing area, skill-based conditioning games can also elicit sufficiently high exercise intensities for repeated sprint conditioning (16,25,31).
If it is appropriate to the sport, speed–endurance work may be introduced into speed and agility work undertaken by players, providing that the adolescent player is technically proficient. Before this stage in development, any speed and agility training undertaken by players would exclusively be categorized as neuromuscular training, with emphasis on movement mechanics and complete recovery between reps.
Aerobic endurance remains a training priority for adolescent players. Because this is a stage of increasing specialization in training, the training modes used for individual training should be increasing mode-specific. It follows that cross-training should be largely restricted to off-season training. Furthermore, aerobic endurance development may be predominantly achieved by using interval training with appropriate work bouts and intervals of rest and recovery (44). Optimal combinations of work-to-rest ratios can elicit an almost maximal stimulus for anaerobic and aerobic effects (44). However, these repetition schemes, by definition, will be highly demanding and as such should be progressively introduced during late adolescence. Skill-based conditioning games aimed at aerobic endurance development used in squad training should likewise become more specific to the sport.
Adolescent athletes are more conducive to a more adult longer-term approach with regard to physical preparation undertaken in a more structured training setting (43). The starting point for any strength training will depend on the adolescent player’s training history. If appropriate strength development has occurred before or during puberty, strength training may be progressed to include a more advanced and sport-specific exercise selection. However, if significant deficits are noted, the starting point will be developing strength for fundamental movements and addressing imbalances. In this case, initial exercise selection will be more reflective of general training.
When assuming physical preparation has been undertaken before this stage of maturation, training design should be increasingly based on a comprehensive needs analysis of the sport and playing position. As with adults, exercise specificity influences young players’ responses to strength training. Exercise selection therefore should be progressively sport-specific, within the constraints of the skill level and training experience of the young player.
Sport-specific exercise selection will vary according to the team sport and playing position. Typically, explosive multijoint lifts (e.g., resistance training exercises and barbell jump squats) that incorporate triple extension of the hips, knees, and ankles will feature in the adolescent player’s program because this extension is the principle biomechanical action common to many dynamic movements in team sports (15). Likewise, unilateral support exercises should necessarily account for a significant portion of the young athlete’s training on the basis that most game-related movements are executed while supported partly or fully on 1 leg (34).
Exercise selection from an injury prevention viewpoint should be based on injury data for the sport and for the playing position if available. Specifically, targeted strength training exercises should be included in adolescent players’ training to address areas identified as being prone to injury in the sport and playing position.
In the case of contact sports, hypertrophy may be an important program goal for adolescent players to varying degrees depending on their playing position. The shoulders should be an area for specific strengthening and hypertrophy because they are the site for impact forces during collisions with other players (Table 1c) (15).