Not too long ago, countless hours working on the farm and playing outside prepared young bodies for the physical demands of challenging life events. However, contemporary childhood is different, with global reports indicating that children and adolescents are not as active as they used to be, and this decline in daily physical activity appears to begin early in life (1,2). Technological advances have decreased the need to move, physical education is considered expendable, and many children have less exposure to “free play” because of safety concerns and issues of access. Based on these observations, many question whether or not young athletes are strong enough for the physical demands of sport practice and competition.
Much research supports the contention that muscular strength underpins many of the biomotor qualities influencing athleticism and sports performance (3–5). Yet too few young athletes participate in year-round strength and conditioning and achieve a level of physical performance that optimizes their potential to run faster, jump higher, and be stronger (5). Prerequisite levels of muscular strength are needed to break through a “strength barrier” to allow weaker young athletes to catch up to their stronger teammates. If young athletes are not exposed to an environment with regular opportunities to enhance their muscular strength, they will be less likely to develop the prerequisite skills and abilities that are needed for ongoing participation in sport activities. It is naive to believe that young athletes will simply “outgrow” muscle weakness; rather, there is evidence that low levels of muscular strength during the growing years tend to track into adulthood (6).
A Growing Deficit
Secular trends in measures of muscular strength indicate that modern day youth are weaker than previous generations (7–9). For example, in the late 1980s, the 50th percentile for modified pull-ups for 6- to 9-year-olds in the United States ranged from 6 to 10 repetitions, but the present 50th percentile for this same group is now 2 to 4 repetitions (10). This is just one example of a disturbing trend in muscular strength deficits which, consequently, predispose children and adolescents to yet higher levels of physical inactivity and sedentary behaviors. Recently, the term pediatric dynapenia has been used to characterize this phenomenon (11). Pediatric dynapenia, an acquired and treatable condition in youth characterized by muscle weakness and dysfunction not caused by neurologic or muscular disease, has broad consequences for the children in which it is found (12). Children with greater adiposity, for instance, have been found to have impairments in muscle composition and velocity-related strength which may influence their overall functionality and health-related quality of life (13). Dynapenic obesity is a recognized risk factor for functional decline in older adults (14), and it appears that the synergistic relationship between dynapenia and obesity also may limit performance in youth.
Participation in youth sport alone does not ensure that young athletes will attain a level of muscular strength that will optimize performance, reduce injury risk, and improve health. Nor does youth sport participation ensure adequate levels of physical activity: reports indicate that youth sport practices do not provide a sufficient amount of moderate-to-vigorous physical activity (MVPA) to meet daily recommendations because too much time is spent in sedentary or light physical activities (15,16). Weaker youth are less likely to perform as well as their stronger peers (17,18) and more likely to suffer a sports-related injury (19,20). It is likely that this gap between weaker and stronger youth will widen over time without purposefully targeting the strength deficits in the former. From a public health perspective, contemporary trends in youth physical inactivity and sport-related injuries will continue unabated.
New insights into the design of long-term athletic development programs underscore the importance of general physical preparation for sport, with a particular focus on strength development, since a certain amount of force production and force reduction is needed to perform athletic movements proficiently (4,21). These findings are supported by recent meta-analytic data that indicate that youth should perform resistance training, building a foundation of strength, prior to power-type training (22). From coaches to clinicians, those caring for young athletes will eventually need to address strength deficits as part of prehabilitation or rehabilitation programs throughout the individual’s career since muscle weakness at any age predisposes athletes to functional limitations and adverse health outcomes.
Building Strength Reserves
While enhancing muscle strength to improve fitness and performance is not a novel concept (23), a new model that targets strength deficits and builds strength reserves is needed to provide a logical and evidence-based approach to strength development in children and adolescents. Greater strength reserves can enhance neuromuscular fitness, improve athletic performance, and reduce sport injury risk (3,24,25). From an epidemiological perspective, population-based handgrip muscle strength measures have been found to be a significant determinant of summer Olympic games medal success (26). The importance of acquiring and maintaining high strength reserves to enhance athletic performance and reduce injury risk should not be overshadowed by generic recommendations to simply increase MVPA. Leading professional organizations within the fields of physical education, sport science, and pediatric medicine support participation in youth resistance training programs that are well-designed and supervised by qualified fitness professionals (27).
Without developmentally appropriate interventions that target muscle weakness, relatively weaker boys and girls may be unwilling and perhaps unable to catch up with their peers who possess higher levels of strength and motor skill prowess. Since prerequisite levels of muscular strength are needed to move proficiently, there is a critical need during childhood and adolescence to increase muscular strength and build strength reserves to develop healthy, resilient, and capable young athletes who are prepared for ongoing participation in sport (5,28). Strength reserves can be used (or not) to meet the demands of sport participation and overcome unexpected stressors. From this perspective, strength reserves have a protective function against the adverse consequences of sport, such as musculoskeletal injury. In support of these observations, weaker lower-extremity muscle strength has been found to predict traumatic knee injury in young female athletes (19).
The theoretical construct of strength reserves illustrated in the Figure may serve as a conceptual framework that explains interindividual differences in health and performance trajectories throughout childhood and adolescence:
The DREAM model highlights five phases of strength development: 1. Dynapenic, 2. Reduced, 3. Emerging, 4. Adapting, and 5. Mighty. The Dynapenic phase is characterized by abnormally low levels of muscular strength that limit a child’s ability to perform activities of daily life, such as climbing stairs. Youth in this phase do not move efficiently and often struggle to perform basic movement skills. When weak children get stronger by performing general physical activities they enter the Reduced phase as they begin to move and play. Youth in the Emerging phase make remarkable gains in muscular strength as they gain confidence and competence in their abilities to activate targeted musculature and lift heavier loads by regularly participating in structured resistance training. As youth continually build their strength reserve, they enter the Adapting phase whereby increases in muscular strength translate to observable improvements in power performance. The final Mighty phase is achieved by youth with high muscular strength reserves, enhanced athletic abilities, and a reduced risk of sports-related injuries.
The starting point and amount of time in each phase will depend on various factors including each participant’s health history, initial strength level, and resistance training skill competency. As children learn basic movements and build their strength reserves, their ability to perform more complex exercises will be influenced by their exposure to resistance training programs that address neuromuscular deficiencies, reinforce desired movement patterns, and target specific outcomes. While specific strength standards for all young athletes are lacking, previous work in elite youth soccer players with years of resistance training experience suggest relative strength levels on the back squat exercise should range from 0.7× body mass for 11- to 12-year-olds, 1.5× body mass for 13-to 15-year-olds, and 2.0× body mass for 16- to 19-year-olds (29).
Focusing only on the technical and tactical aspects of a specific sport falls far short of preparing young athletes for the demands of training and competition and protecting them from injury. There is no substitute for year-round participation in well-designed resistance training programs since stronger young athletes are better prepared to perform complex movements, master sport tactics and withstand the demands of sports practice and competition. Without ongoing participation in strength-building activities, strength reserves will begin to wane, and young athletes will become more vulnerable to sport stressors. A systematic approach to physical conditioning that optimizes gains in muscular strength, builds strength reserves, and progresses individuals through the stages of the DREAM model is vital for facilitating the continued development of athletic prowess in modern day youth.
The authors thank Devon Mulrine for graphic art assistance with the figure.
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