The benefits of physical training (i.e., resistance training) for youth physical development are becoming widely recognized and more accepted (17,50). Muscular strength, muscular endurance, and power are terms that are commonly used in resistance training literature, and all these terms fall under the broad umbrella of musculoskeletal fitness (MSF) (29). However, these terms may not be appropriately delineated and discussed when addressing practical education, application (29,35,36), and health-related benefits of resistance training (23,26,29,38,48,63), as well as the possible acute and long-term consequences that may result from inappropriate applications of resistance training in youth (1,26,44).
As defined in a recent report by the Institutes of Medicine (29), “Musculoskeletal fitness is a multidimensional construct comprising the integrated function of muscle strength, muscle endurance, and muscle power to enable the performance of work against one's own body weight or an external resistance” (p. 155). The individual definitions of these 3 separate yet integrated dimensions of MSF provide a framework for addressing their respective performance and health-related benefits. Muscular strength is generally defined as the ability to produce measurable force within a muscle or torque about a single or multiple joints during maximal voluntary contraction. Muscular power generally reflects the rate at which muscles perform work (i.e., speed of contraction against a resistance). Last, muscular endurance is generally defined as the ability of muscles to perform repeated voluntary contractions against a constant load. Although these general descriptions are rather simplistic, they serve our purpose for delineating their respective performance and health-related benefits related to training adaptations.
Controversy remains regarding the type and intensity of training that should be promoted across youth (5–18 years old) (1,29,50). The lack of a united and unequivocal message indicating all types of MSF training are beneficial for youth is related to various issues that have not been adequately addressed in the literature. These issues include the types of MSF training methods implemented, specificity of subpopulations of youth (e.g., athletes versus normal populations) trained using these methods, and differences in ages at which these various types and intensities of training are promoted (e.g., children versus adolescents).
Two additional yet related issues stem from the actual research, or lack thereof, that incorporates MSF training in youth, specifically in prepubescent children. First, although emerging evidence addressing health and performance benefits of MSF in youth populations is positive (48), more longitudinal and long-term experimental studies need to be conducted to confirm this notion (29). Compared with adult literature, data relating to long-term performance and health-related benefits of the different aspects of MSF in prepubescent children is lacking in both sheer volume and in definitive findings (29). Second, an understanding of how to promote developmentally appropriate progressions of MSF across childhood and adolescence in both general populations as well as from an athletic performance standpoint needs to be addressed from a developmental perspective. Unfortunately, it is relatively commonplace practice for individuals who lack an adequate background in youth MSF training principles, to simply water-down adult models of training.
PERFORMANCE ENHANCEMENT AND INJURY PREVENTION
Modern competitive sport has resulted in dramatic improvements in both knowledge and technique over the past century with respect to fostering optimal development of all athletes, with some measure of that enhanced knowledge base germane to the young athlete. At the same time, youth athletic development has become an expanding niche in the strength and conditioning and fitness industries, with countless independent facilities and several franchise-based youth-centric fitness chains operating in the United States and around the globe. Additionally, programming for general youth fitness (i.e., not related to sports participation) also has become increasingly popular because of an expanding focus on health and wellness. Strength and power training is often a cornerstone of this youth programming, despite the fact that relatively little empirical data regarding appropriate prescription for youth currently exists. Last, the rationale for participation in youth programs (i.e., “having fun” and “making friends”) may be quite different than for adult participation. As such, these psychosocial aspects of youth training also critically influence measurable outcomes.
STRENGTH, POWER, AND ENDURANCE
Despite early concerns that resistance training and other forms of athletic conditioning should be avoided in children and adolescents because of the increased risk of injury, a great deal of data now exist that point to both the effectiveness (i.e., performance) and concurrent safety of such training when appropriately implemented and supervised (1,8,9,16–19,25,26,33,34,41,50,52,53,58,62,65). Furthermore, coupled with the fact that young athletes are well primed for motor skill acquisition, there may be no better time than childhood or early adolescence to introduce technical skills and complex multi-joint movements associated with power movements, provided that such skills are taught with a keen eye on appropriate loading and skeletal health (17–20). Additionally, other developmental factors such as endocrine response, biological age, training status, and anthropometrics (i.e., lever arm length, etc.) in subpopulations of youth should fundamentally alter the nature of proposed training programs. This is critically important as the risk of overtraining is relatively higher in youth, and the potential for positive training adaptations such as increased lean muscle mass and/or increased V[Combining Dot Above]O2max is significantly lower than in adults (1,3,7,10,15–18,28,37,39,50,63,64).
Although the efficacy of strength, power, and endurance training for young athletes is relatively well established, what is lacking are specific data regarding precisely how youth-centric programming should be different from adult programming based on the biological, psychological, and emotional complexities of youth development. Furthermore, exercise mode and specific movement indications and contraindications also must be adequately addressed from an empirical standpoint. As a result, programming designed for youth many times is based around the idea of relatively light or easy adult programming. In contrast, the most effective stimulus to trigger optimal development in the youth should instead be based on the developmental level of the individual and appropriateness of the prescribed skill or drill, whether it is a resistance training technique or more related to global coordination development.
As with other aspects of MSF related to children and adolescents compared with adult-related data, a relative paucity of work regarding injury prevention has been published, with the vast majority of such work focused on knee injury prevention in late adolescence (22,40,43,56,57). The majority of such work indicates a positive relationship between improved strength and neuromuscular coordination and control and a reduction in severe knee injuries (namely, the anterior cruciate ligament) (22,40,56,57). Such investigations are of value and have certainly influenced practice, with a number of both formal and informal programs developed to foster improved neuromuscular coordination and control now relatively commonplace in an athletic environment. Most frequently, such programming has taken the shape of stand-alone “camp-based” programming such as the Cincinnati Sports Medicine Foundation's “Sports Metrics” program (4–6) or in plyometric-based structured warm-ups that are implemented within a traditional sports practice (e.g., Santa Monica Sports Medicine Performance Enhancement Program) (2,22,40,57). The findings of such investigations, although limited in scope, have shown considerable promise in reducing the risk of a potentially devastating knee injury.
The thrust of recent work regarding injury prevention has been to identify and address compensatory movement patterns and resultant overuse injuries. To date, the focus of most research efforts has been to assess late adolescent/early adult athletes (i.e., “college age” and above) and implement corrective exercise to minimize or eliminate compensatory movement patterns that can inhibit performance and ultimately lead to injury (14,21,31,47,49,54). The logical next step in this line of research would be to investigate the potential to prevent the onset of such patterns in a younger population, implement preventative and/or corrective exercise prescriptions throughout developmentally appropriate programming, and track the incidence rate of overuse conditions and acute injuries across time. Although such work certainly holds promise to significantly reduce both the onset and severity of injury related to improper movement patterns as well as performance, heretofore no such investigations have been published.
Although increasing numbers of young athletes, as well as youth in general, are being exposed to the potential benefits of resistance training at earlier ages, mere exposure to such information has revealed glaring inadequacies. Personalized instruction promoting appropriate developmental progressions and optimal technique, coupled with a persistent and growing need for qualified and competent instruction, are missing links in many youth physical development efforts. As a result, well intentioned yet ill-equipped athletes, parents, and/or coaches often attempt to water-down or otherwise regress elite-level adult programming with little to no consideration for the developmental appropriateness of such prescriptions. This process can be detrimental not only to developing peak performance but also to protecting youth from inadequate physical development and long-term injury risk (13).
MUSCULOSKELETAL FITNESS ROLE IN HEALTH PROMOTION
Recent literature reviews have demonstrated positive relationships between aspects of MSF and health markers and/or outcomes in youth (29,48,65). These reviews cite multiple health-related benefits associated with MSF training including adaptations in body weight status, cardiovascular/cardiorespiratory risk factors, metabolic risk factors, and bone health. The recent Institute of Medicine report (29) indicated that currently, there is adequate evidence supporting the relationship between MSF and health in humans. However, this conclusion “is based mainly on increasing evidence for the importance of musculoskeletal fitness, especially muscle strength and power, to health outcomes in adults” (p. 179).
Overall, data on youth populations are not as compelling when compared with adult data. Suggested rationale behind the relatively weak linkage of MSF to health in youth includes: (a) nonspecific methods designed to specifically test aspects of MSF to health markers/outcomes, (b) insufficiently designed interventions (e.g., short study durations and limited emphasis on resistance training dosage), (c) inadequate statistical power necessary to effectively detect significant relationships, (d) utilization of unique subpopulations and/or limited age ranges, and (e) lack of consistency regarding specific prescription of MSF tests related to muscle strength, power, or endurance (29).
Overall, the strongest (i.e., experimental and longitudinal studies) and most prevalent data supporting MSF linkages to health in youth are related to strength and power (29). Specifically, hand grip strength, bench press, and squat/leg press are the most common strength measures that relate to various health markers (10,28,30,32,42,45,46,55,63). The standing long jump and vertical jump were cited as the most commonly used tests for measuring “explosive strength” or power (29). Relationships between muscular endurance and health markers/outcomes are relatively weak compared with strength and power data (29). In addition, data-linking MSF to psychological health (e.g., self-efficacy, perceived competence, and self-worth) needs to be addressed (17). Overall, emerging data indicate that appropriate MSF training may (a) reduce joint injury risk, (b) enhance bone health, (c) improve body composition status, (d) improve cardiovascular and cardiorespiratory function, and (e) improve metabolic health in children and youth. Additional appropriately designed experimental studies need to be conducted to definitively establish these effects in all ages across youth.
One critically important factor in promoting these types of studies is feasibility across all ages of youth, not only for appropriate assessment but also for establishing developmentally appropriate training programs. Not debatable is the support for supervision of MSF training by competent and knowledgeable professionals (17,18). However, preparing coaches, practitioners, and parents with a sufficient breadth and depth of knowledge and experience to effectively engage youth populations has not been adequately addressed. For example, although promoting 1 repetition maximum (1RM) or multiple RM bench press, leg press, and/or squats may be acceptable for a small subset of youth who have appropriate training and who are supervised by knowledgeable and experienced professionals, promoting these types of tests and training are, arguably, not the most appropriate means for promoting MSF in untrained youth and preadolescent children from a developmental perspective (11,24,34). Instead, practitioners who work with youth populations must take into consideration the developmental readiness of an individual, specifically with young children and with those who are not able to effectively coordinate and control their own body mass. So where do we start?
INTEGRATION OF POSITIVE MUSCULOSKELETAL FITNESS ATTRIBUTES
Perhaps most importantly, globally devised and improved efforts to enhance the implementation of MSF training should incorporate the aforementioned information and suggestions from previous literature. Adapting training programs with this information in mind will be beneficial not only for performance and injury prevention for the relatively low percentage of youth that continue their athletic pursuits into high school but also for all youth who would benefit from the health-enhancing outcomes of improved MSF. Even more optimistic is the thought that intensifying efforts to enhance MSF, specifically beginning in early childhood, will promote more opportunities for all children to be successful and participate in sports, physical education, and health-enhancing physical activity at an early age. Thus, promoting positive trajectories of physical and psychological development that will encourage more children to continue to pursue and achieve their individual goals (e.g., athletics and/or health), rather than drop out of activities because they are not successful and are not confident in their abilities, would seem to be a logical goal (59).
Recent theoretical approaches addressing youth physical development may provide a more holistic and integrated approach for promoting positive trajectories of MSF as well as other behavioral and psychological attributes (3,43,59). All these approaches are distinctive in their own right; however, they all have a common theme relating to appropriate developmental progressions that integrate general movement skills (i.e., fundamental motor skill development) and specific MSF training. Interestingly, motor skill development (e.g., complex, multi-joint ballistic skills—jumping, hopping, kicking, striking, and throwing) and MSF development (e.g., strength, endurance, and power) are integrated from a neuromuscular development perspective (51,61) and will promote not only athletic potential (3) but also more functional physically active and physically fit youth (16,43). Of greater importance, these effects also may track into early adulthood (60,61).
One critical concept to understand from a developmental perspective is that terminology used in “resistance training” and “motor development” literature, specifically related to the development of neuromuscular coordination, is fundamentally the same regarding how it can be applied to youth training progressions. For instance, fundamental locomotor and object control skills including different forms of jumping, hopping, skipping, throwing, and striking are also described as plyometric activities as they are complex multi-joint ballistic movements. Variations of these movements generally require high levels of effort, include eccentric and concentric muscle activity that promotes the stretch-shortening cycle, and promotes progressive overload of the musculoskeletal system. In addition, many foundational “resistance training” movements also require complex coordination patterns and/or an understanding of how to promote isometric contractions (i.e., stabilization) of specific joints while promoting concentric/eccentric phases of movement in other joints (e.g., push-ups and squats). In essence, all forms of resistive-type movements (i.e., manipulating body weight or external resistance) require the appropriate development and consistent execution of coordination and control to effectively produce the desired movement. When addressing motor development literature, the development of complex fundamental motor skills (e.g., running, jumping, hopping, skipping, throwing, striking) take years to develop (27). In essence, thousands of trials with initial high levels of variability and adaptational pathways within individual progressions of development are hallmarks of motor learning and development. These pathways also include high variability when advancing (i.e., changing) coordination, specifically with intersegmental movement coordination in complex multi-joint skills. This variability is because of a variety of reasons including differential instruction, changing task demands, growth, neural adaptations, and the complexity that is involved in the development of coordination and control of thousands of degrees of freedom that can be detailed at the joint, muscle group, individual muscle, and intramuscular levels. In motor development literature, this complexity initially addresses coordination and control of an individual attempting to manipulate the mass of his/her own body weight and/or extremities to achieve the goal of the task (i.e., rudimentary locomotor and striking movements). Adding external devices and/or implements (e.g., striking implements, balls, bars, weights) with varying amounts of mass and variations in speed of movements makes this arduous task even more complex and difficult to attain high levels of skill. See Table 1 for a comparison of developmental and resistance training language.
It is critical to understand that there are various “stages” in the development of multi-joint coordination patterns and that many youth will not demonstrate the highest developmental level of movement due to a lack of experience and/or appropriate training. Thus, knowledgeable practitioners who fully understand developmental progressions of movement will recognize that youth may or may not be able to demonstrate effective or highly advanced movement patterns of these movement skills. They also will understand that developing the most advanced fundamental movement patterns, critical for functional athletic performance, will take time to develop and that this development will not happen “overnight.” The breadth and depth of knowledge pertaining to multiple aspects of youth development is critical to be able to promote appropriate training progressions based on individuals' physical and cognitive developmental levels and training experience (27). Thus, the integrative nature of MSF and motor skill development should be addressed from a developmental perspective when prescribing developmentally appropriate progressions of physical development across all ages.
DEVELOPMENTAL AND PRACTICAL APPROACHES TO PROGRAM DESIGN
One approach for designing developmentally appropriate MSF programs for youth may include a theoretical model involving a 4-phase developmental system, including preparatory, discovery, exploration, and transformation phases (12) that are age related, but not age determined. Using such an approach will accommodate differences across the wide span of developmental levels that youth (i.e., 4–18 years) demonstrate, specifically when many children and adolescents have virtually no specific training or experiences with MSF training. A more specifically tailored and individualized modular approach to exercise selection allows for a program that consistently provides adequate yet varied exercise stimuli, depending on the needs (e.g., athletic performance or simply health based) and developmental level of the individual. Table 2 provides common exercise examples for each phase and component part.
PREPARATORY PHASE (2–5 YEARS)
The goals of the preparatory phase should be to provide ample movement experiences in an enriching and stimulating environment. A typical program prescription would consist of approximately 10 minutes of body awareness and movement creativity (squatting, bending, climbing, tumbling, etc.), 5 minutes of object manipulation (picking up/putting down and throwing balls, objects, etc.), 5 minutes of coordination (rhythm skills, hopping, jumping, etc.), 5 minutes of cooperation/rule following (Simon says, etc.), and 5–10 minutes of semi-structured free play. The neuromuscular adaptation attributes of this type of program design are similarly addressed by Myer et al. (43) who define this type of training as “integrative neuromuscular training.” This type of training has been shown to be successful with young children, even with short-term exposure (12).
DISCOVERY PHASE (6–9 YEARS)
The goals of the discovery phase should be to continue to provide movement experiences in a progressively more stimulating environment. This phase may be very similar in design to the preparatory phase (depending on the readiness of the individual), with increases in exercise duration and complexity within each category. Typical program prescription would consist of approximately 10 minutes of body awareness and creativity, 10 minutes of object manipulation, 10 minutes of coordination, 10 minutes of cooperation/rule following, and 5–10 minutes of semi-structured free play. Again, providing a more or less advanced training progression can be implemented depending on the experiences and training status of the individual.
EXPLORATION PHASE (10–13 YEARS)
The goals of the exploration phase should be to build on skills and capabilities acquired during previous phases in progressively more challenging ways. Participants at this stage can begin to explore more “formality” in movement. Such learning and movement should still be “fun” and not work-like. Typical program prescription may consist of approximately 10 minutes of active range of motion/mobility drills, 15 minutes of systemic general exercise preparation (jumping jacks, burpees, hops, skips, etc.), 10 minutes of coordination enhancement (rhythm skills, spatial awareness, etc.), 10 minutes of systemic strength (crawling, squatting, bar technique skills, etc.), and 10–15 minutes of structured or semi-structured game play (obstacle courses, tag, agility, etc.). Again, proper supervision and knowledge of an individual's training status and developmental capabilities is critical for appropriate progression to more advanced stages of training.
TRANSFORMATION PHASE (MORE THAN 14 YEARS)
The goals of the transformation phase should be to use previously acquired motor fluency to begin to apply movement patterns in smooth, controlled, and increasingly specialized ways. Participants at this stage will typically begin to incorporate more form-based skills and drills into programming with a steady emphasis on skill refinement and sports performance. Typical program prescription may consist of approximately 5–10 minutes of tissue quality work (foam roll, self-myofascial release, etc.), 10 minutes of range of motion/activation/torso mobility drills, 10 minutes of systemic general exercise preparation (movement-specific dynamic exercises, etc.), 20 minutes of strength and power technique (cleans, squats, lunges, etc.), 10 minutes of power execution (various jumps and explosive throws with med ball, etc.), and 10–15 minutes of static mobility/stretching exercise.
We hope that the information provided in this article provides practitioners with a more global perspective for addressing youth physical development. We also hope that this information will not only inspire practitioners to approach youth physical development using a developmental perspective but also promote additional research to further delineate the benefits of MSF training on performance, injury prevention, and health. Long-term data demonstrating decreased MSF, physical activity, and increasing obesity in youth demand that we address these issues with increased vigor and with an unequivocal message and direction.
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