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Making a Strong Case for Prioritizing Muscular Fitness in Youth Physical Activity Guidelines

Faigenbaum, Avery D. EdD, FACSM1; MacDonald, James P. MD, FACSM2; Stracciolini, Andrea MD, FAAP, FACSM3; Rebullido, Tamara Rial PhD

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Current Sports Medicine Reports: December 2020 - Volume 19 - Issue 12 - p 530-536
doi: 10.1249/JSR.0000000000000784
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Introduction

Since the publication of the landmark report on physical activity and health by the United States Surgeon General in 1996, regular participation in physical activity has increasingly been recognized as a passport to good health and well-being (1). It is widely recognized that all types of physical activity — from active transportation to competitive sport — can improve the physical and mental health of participants (2). Regular physical activity can reduce the risk of noncommunicable diseases, including heart disease, diabetes, and some types of cancer, and can help to prevent hypertension, overweight, and obesity (2). Physical activity also offers social and economic benefits. It has been estimated that even modest increases in youth physical activity could yield billions of dollars in medical cost savings and have far-reaching positive effects on adulthood earnings (3,4).

Despite the well-established benefits of physical activity, global levels of youth physical inactivity remain high (5,6). Data from 1.6 million students aged 11 to 17 years from 146 countries revealed that more than 80% were insufficiently active (5). These findings are consistent with the 2018 Global Matrix 3.0 Physical Activity Report Card confirming very high levels of physical inactivity among youth in the 5- to 17-year age range (6). Despite ongoing efforts over the past few decades to address this phenomenon, participation in physical activity among children and adolescents remains persistently low in the context of school, home, and community settings. Since the decline in physical activity appears to emerge very early in life (7), and physical activity behavior tends to track from childhood to adolescence (8), the current and future health of many contemporary youth is compromised. Investment at all levels is needed to combat physical inactivity in children and adolescents and encourage uptake and maintenance of physical activity throughout the life course.

Public health guidelines constitute a key information resource for parents, policymakers, and health care professionals. It is important to regularly assess the efficacy and practicality of these guidelines to guide policy development and goal setting for physical activity promotion. The most common recommendation in these documents is that all youth accumulate at least 60 min of moderate to vigorous physical activity daily (MVPA), with the majority of the daily discretionary activity classified as aerobic (9,10). Although evidence indicates that muscular fitness activities are foundational for ongoing participation in active play, exercise, and sport (11–13), current national and international youth physical activity guidelines underplay the critical importance of strength-building activities throughout childhood and adolescence (9,10). To this point, global declines in youth MVPA have coincided with temporal declines in measures of muscular fitness in children and adolescents (14–17).

It is time to reconsider the complexity and multifaceted nature of physical inactivity in youth and upgrade generic physical activity guidelines that fail to address the specific needs and abilities of this population. While it is unlikely that one strategy will be the answer, the current approach for promoting physical activity in children and adolescents is suboptimal, demonstrably lacking in effect. The global rise of noncommunicable diseases in youth further supports the critical need to review and revise youth physical activity guidelines that are consistent with the unique needs, abilities, and interests of children and adolescents. We argue for prioritizing muscular fitness in youth physical activity guidelines and propose a new conceptual model for promoting MVPA in children and adolescents. For ease of discussion, the term youth refers to children and adolescents collectively, and muscular fitness is defined as a global term that represents muscular strength, muscular power, and local muscular endurance.

What Are Physical Activity Guidelines for Youth?

Alhough it is unclear whether public health recommendations, in isolation, increase participation in MVPA, global efforts to establish national and international guidelines to increase physical activity at all levels across the life span are supported by key World Health Organization (WHO) publications (2,9,18). Common youth physical activity recommendations in these documents include accumulating at least 60 min of MVPA daily, with the majority of the MVPA being aerobic (9). The WHO also recommends muscle- and bone-strengthening activities at least 3 d·wk−1 (9). Other international physical activity guidelines from the European Union (19) and the Nordic countries (Iceland, Norway, and Sweden) (20) are consistent with WHO guidelines that call for the accumulation of at least 60 min of MVPA daily in forms that are developmentally appropriate, enjoyable, and involve a variety of activities. The updated WHO Global Recommendations on Physical Activity in Youth, Adults, and Older Adults are expected to be published in 2020 (21).

International youth physical activity guidelines sparked the development of national physical activity recommendations. Information presented in the Table outlines 24 national youth physical activity guidelines published since 2010 (the year the WHO global recommendations were released). Some national reports stem from evidence-based frameworks regarding the dose-response to physical activity and are high-profile documents, whereas others focus on the public health promotion of physical activity and exist only on national language web sites. National guidelines provide specific guidance on physical activity amounts and modalities for different age groups and may serve to inform key stakeholders involved with primary prevention and health promotion.

Table - National Physical Activity Guidelines for children and adolescents since 2010*.
Year F I T Primary Mode Other Modes
Argentina (22) 2013 Daily MV ≥60 min Aerobic Strength
Australia (23) 2019 Daily MV ≥60 min Aerobic Strength
Austria (24) 2012 Daily MV ≥60 min Aerobic Strength, flexibility, skills
Canada (25) 2016 Daily MV ≥60 min Aerobic Strength
Chile (26) 2017 Daily MV ≥60–90 min Aerobic Strength, flexibility
China (27) 2017 Daily MV ≥60 min Aerobic Strength
France (28) 2018 Daily MV ≥60 min Aerobic Strength, flexibility
Germany (29) 2016 Daily MV ≥90 min Aerobic Strength
Italy (30) 2018 Daily MV ≥60 min Aerobic Strength
Malaysia (31) 2017 Daily MV ≥60 min Aerobic Strength
Mexico (32) 2015 Daily MV ≥60 min Aerobic Strength
Netherlands (33) 2017 Daily MV ≥60 min Aerobic Strength
New Zealand (34) 2012 Daily MV ≥60 min Aerobic Strength, flexibility
Paraguay (35) 2014 Daily MV ≥60 min Aerobic Strength
Philippines (36) 2010 Daily MV ≥60 min Aerobic Strength, flexibility
Qatar (37) 2014 Daily MV 60 min Aerobic Strength
Singapore (38) 2013 Daily MV ≥60 min Aerobic Strength
South Africa (39) 2013 Daily MV ≥60 min Aerobic Strength
Spain (40) 2015 Daily MV ≥60 min Aerobic Strength
Switzerland (41) 2012 Daily MV ≥60 min Aerobic Strength, flexibility, agility
Turkey (42) 2014 Daily MV ≥60 min Aerobic Strength
UK (43) 2019 Daily MV ≥60 min† All forms‡
USA (44) 2018 Daily MV ≥60 min Aerobic Strength
Uruguay (45) 2017 Daily MV ≥60 min Aerobic Strength
*Adapted from Parrish et al (10).
†An average of at least 60 min MVPA per day across the week.
‡A variety of physical activities including those that develop movement skills and muscle strength.
F, frequency; I, intensity; T, time; MV, moderate-vigorous.

National youth physical activity guidelines recommend at least 60 min of MVPA daily with a primary focus on aerobic exercise and a secondary focus on muscle- and bone-strengthening activities (10). Austria, France, Philippines, and Uruguay advise that youth older than 12 years perform strength exercises (24,28,36,45), and guidelines from South Africa and Switzerland suggest that strength-building activities should be limited to body weight exercises until adolescence (39,41).

Youth physical activity guidelines from New Zealand and the United States encourage children to participate in unstructured strength-building activities, such as jumping and climbing, but state that children do not need “formal muscle strengthening” programs (34,44). Although the vast majority of national PA guidelines understate the critical importance enhancing muscular fitness early in life, recent youth physical activity guidelines from the United Kingdom recommend MVPA in all forms to develop muscular fitness, movement skills, and bone strength (43).

Given the impact of national physical activity guidelines on policy development and program implementation, it is important to monitor and iteratively evaluate the influence and relevance of these recommendations for increasing youth MVPA. Although efforts have been made to refine and clarify national guidelines with evidence of improved quality, basic public health recommendations from 2005 for “school age” youth to accumulate 60 min or more of MVPA daily remain much the same today (46). The increasing prevalence of youth physical inactivity calls for a paradigm change that both recognizes and emphasizes the appropriate physical needs of modern-day children and adolescents to effectively impact youth physical activity worldwide.

Why Prioritize Muscular Fitness Development?

Recent epidemiological reports indicate that today's youth are not as active as they should be and the decline and disinterest in MVPA increases steadily after 6 years of age (5–7). No child is immune from contemporary lifestyles that often consist of noneducational screen time, sedentary recreational pursuits, and limited exposure to physical activities that enhance muscular fitness. There is an urgent need to address physical inactivity during childhood and adolescence before youth become resistant to exercise interventions later in life. This approach emphasizes the importance of recognizing the multidimensional development of youth physical inactivity and promoting strength-building activities for all youth regardless of physical ability or body size.

As illustrated in Figure 1, low levels of muscular fitness negatively impact physical, psychosocial, emotional, and behavioral factors that drive physical inactivity in youth. Since a certain amount of force-production and force-attenuation is needed to move proficiently on the playground as well as the sports field, weaker youth who lack confidence and competence in their physical abilities (i.e., physical illiteracy) may avoid nonessential MVPA to guard against embarrassment, humiliation, and failure (47). Moreover, inactive youth may be less willing to experience and tolerate sensations associated with MVPA due to a feeling of vulnerability or discomfort (i.e., kinesiophobia) (48). These MVPA fear-avoidance factors may lead to an increase in unhealthy behaviors and consequent risk of injury and illness.

Figure 1
Figure 1:
The multifactorial development of youth physical inactivity.

While low levels of muscular strength and power are well-recognized risk factors for functional disability and mortality in older adults (49), it appears that youth also are vulnerable to the inevitable consequences of dynapenia (48,50). The term dynapenia is defined as an identifiable and treatable condition in youth characterized by low levels of muscular strength and power not caused by neurologic or muscular disease (48,51). Pediatric dynapenia has been found to be associated with cardiometabolic risk (52), all-cause disability later in life (53), and major cause of premature death (54). Since muscular fitness phenotypes appear to track from childhood to young adulthood (55), youth who are not exposed to progressively challenging strength-building activities early in life may be less likely to acquire the desired skills and behaviors that support ongoing participation in MVPA (48,56,57).

Research findings show that regular participation in youth resistance training has a positive impact on fundamental movement skills (e.g., jumping, throwing, and running) (57), and a positive association between muscular fitness and physical activity (particularly vigorous physical activity) in children and adolescents has been reported (56). Notably, researchers identified a motor proficiency barrier for meeting physical activity guidelines in children (58). Almost 90% of children with low actual motor competence did not meet the public health recommendations of at least 60 min of MVPA daily (58). Meta-analytic findings provide additional support for a moderate to large positive relationship between motor competence and physical fitness from early childhood to early adulthood (59). Collectively, these findings underscore a potential synergistic adaptation whereby integrative physical activity interventions can complement naturally occurring adaptations throughout childhood and adolescence (60,61). Since fundamental movement skills are the basic building blocks for more advanced, complex movements, there is an opportunity to develop a basal level of muscular fitness early in life in order to provide a strong foundation for ongoing participation in a variety of physical activities. Indeed, meta-analytic findings underscore the importance of developing a foundation of muscular strength before performing more advanced power training activities (62).

Despite outdated concerns and misperceptions associated with youth resistance training, a compelling body of evidence indicates that regular participation in a well-designed resistance training program can offer observable health and fitness value for children and adolescents (12,13,63). A high level of muscular fitness in children and adolescents is associated with future health benefits (e.g., lower insulin resistance, higher bone mineral density) (63) and the most effective youth injury-reducing and performance-enhancing programs include strength-building activities (64–66). Notwithstanding the potential benefits of aerobic exercise, without integrative exercise interventions that target neuromuscular deficits early in life, contemporary youth may be unable to overcome an acquired strength deficit. Absent the necessary intervention, they may never catch up to their more active peers who possess average or better levels of muscular fitness.

Rethinking the Pediatric Physical Activity Pyramid

The current global focus on accumulating at least 60 min of MVPA daily has shifted the focus away from specific critical components of health- and skill-related physical fitness that drive ongoing participation in MVPA in youth. Health-related components of physical fitness include cardiorespiratory endurance, muscular strength, muscular endurance, and flexibility; skill-related components of physical fitness include agility, balance, coordination, speed, power, and reaction time (67). Persistent levels of insufficient physical activity in youth over many years call for intervention strategies that move past the question of “how much MVPA?” to “what type of MVPA?” to better understand and develop interventions that produce desired outcomes (68). Because of the timing of brain development and associated neuroplasticity for learning new skills, childhood and adolescence provide a unique opportunity to enhance both health- and skill-related components of physical fitness in supportive settings (68). Pediatric researchers and clinicians are beginning to consider both the quantity and the quality of the MVPA experience to reinforce desired movement patterns which underlie sustainable participation in a range of physical activities (68,69).

The pediatric activity pyramid (PAP) illustrates the integrative relationship among three interrelated activity components and the foundational importance of strength activities (designed to enhance muscular fitness) and skill activities (designed to enhance fundamental movement skills) in the promotion of physical activity for all youth (Fig. 2). Unlike the hierarchical design of traditional youth physical activity pyramids with moderate intensity aerobic physical activities at the base (presumed to be more important) and muscular fitness activities toward the top (presumed to be less important), PAP recognizes the shared, coequal importance of strength, skill, and aerobic activities for the successful implementation of sustainable youth physical activity interventions.

Figure 2
Figure 2:
The PAP consists of three interrelated components that influence participation in MVPA in youth.

The concept of integrating both health- and skill-related fitness components into youth programs is not new. Throughout the early 20th century, physical education classes incorporated strength-building calisthenics into the physical conditioning lesson plan and circuit training developed in the 1950s included different stations of aerobic and strength activities arranged consecutively. More recently, training that includes strength- and skill-building activities has been found to improve health, enhance performance, and reduce the risk of youth sports-related injuries (61,70). Although there is not one optimal combination of exercises, sets, and repetitions that will promote favorable adaptations in all children and adolescents, technique-driven progression and performance of more complex movement skills over time will keep the training stimulus effective and enjoyable (70). Integrative training does not involve expensive exercise equipment or heavy external loads. A circuit of exercises with elastic bands, fitness ropes, medicine balls, and one's own body weight may be used to enhance the ability to express muscular force safely within a spectrum of movement competencies (71,72). Notably, integrative training has been found to pose a moderate to vigorous cardiometabolic stimulus in children (73).

Although there is no minimum starting age, in general most 6- and 7-year olds are ready from some type of integrative training that includes variety, progression, and game-like activities. For example, animal-like movements, such as “bunny jumps” and “frog squats,” can capture the imagination of younger children. Youth with training experience could advance to more complex exercises that target key movement patterns that involve pulling, pushing, bracing, accelerating, and decelerating (70,74). As such, training experience is an important consideration since it impacts the ability of youth to perform simple and complex movements with competence and confidence.

Interindividual differences in physical development and fitness abilities between youth of the same age are important considerations when designing exercise programs for children and adolescents.

At a time when physical education is considered expendable in some communities and too many aspiring young athletes appear ill-prepared for the physical demands of sports practice and competition, efforts are needed to inform relevant stakeholders that, rather than having an isolated focus on aerobic activities, youth physical activity interventions should consider an integrative approach that includes strength- and skill-building activities (13,61,68). Integrative exercise interventions that enhance both health- and skill-related components of physical fitness are needed to prevent the accumulation of risk factors, functional limitations, and interrelated processes that drive physical inactivity throughout childhood and adolescence (61,70). Although most youth will develop a minimal level of physical fitness with free play, the acquisition of necessary strength is not automatic: structured movement experiences with qualified instruction are needed to optimize gains in both health- and skill-related components of physical fitness (75). Notably, quality based physical education programs have been found to improve physical fitness outcomes and fundamental movement skills in children and adolescents (76,77).

Conclusions

An increasing number of children and adolescents worldwide are not accumulating the recommended amount of MVPA. There is a dire need to update policies and actions at all levels in order to shift the paradigm in support of successful daily physical activity in modern-day youth. The mounting evidence of the detrimental effects of physical inactivity on youth warrant a review of existing national physical activity recommendations. Generic recommendations to simply move more are not enough to activate this generation of girls and boys who are weaker and slower than previous generations. An understanding of the multidimensional development of youth physical inactivity and the importance of muscular fitness is needed to have the most significant impact on youth physical activity at the population level. The activity-promoting and health-enhancing benefits of muscular fitness warrant a stronger emphasis on this issue and a new conceptual PAP model for promoting MVPA in all youth.

In view of troubling trends in measures of muscular fitness in modern-day youth over the past few decades, specific recommendations to address neuromuscular deficiencies early in life are an important first step to support ongoing participation in MVPA. About 60 years ago Hans Krauss and Wilhelm Raab coauthored a seminal text entitled Hypokinetic Disease in which they stated it was incongruous for underexercised youth to spend countless hours performing therapeutic exercise to restore the loss of muscular fitness that never should have been lost in the first place (78). These words are still relevant today. It is, at last, time for a change.

The authors declare no conflict of interest and do not have any financial disclosures.

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