Apples to Oranges: Inconsistencies in Defining and Classifying Youth Sport Populations.

In surveying the literature, the term “youth” is used to describe participants ranging from age 8 to 23 years with no consistent age definition provided. In addition, age terminologies such as “preadolescent” and “adolescent” are widely used in sports medicine literature without clear evidence that researchers completed a biological age assessment in addition to one's chronological age. This may lead to the false assumption that an assessment of maturation was completed. The inconsistent age terminology used in scientific literature brings confusion among readers and researchers alike, leading to significant challenges in synthesizing findings. General terms such as “youth” and “pediatric” are too broad to describe participant characteristics in a way that allows for accurate research comparisons. Consistency in age parameters associated with purposeful labeling and clearly defined groups is imperative for the future of research reports and clinical practices. The development and implementation of specific age terminology will likely improve clarity in sharing evidence and knowledge associated with studies of the youth population. This article aims to improve consistency of terminology used within the pediatric sports medicine literature based on evidence-based models of youth sport, both in a general sense (youth and pediatrics) and through 3 proposed age-based subcategories (childhood, early teen, and late teen).Three Key Points General terms (ie, youth and pediatric) are appropriate when referencing the broad age range comprising the youth sport spectrum (age 5-18 years); however, more specific terminology should be used to clearly identify participant characteristics whenever possible. More precise language must be used to define pediatric athletic populations to enable valid identification of participant characteristics and accurate synthesis of research findings. Aligned with evidence-based models of youth sport, we propose 3 subcategories for defining participants in sports medicine research based on their chronological age ranges: childhood (age 5-12 years), early teen (age 13-15 years), and late teen (age 16-18 years). INTRODUCTION The literature evaluating pediatric sports medicine widely varies and is inconsistent in age descriptors. Based on a search of injury prevention, biomechanical analysis, and psychosocial research reports, the terms “pediatrics,” “children,” “adolescents,” and “young adults” are interchangeable.1–3 In addition, major international health agencies such as the World Health Organization (WHO), Center for Disease Control and Prevention (CDC), and American Academy of Child and Adolescent Psychiatry (AACAP) have definitions of youth.1–3 Chronological age is commonly used in daily communications. However, “age” is delineated from various contextual and maturational phases (skeletal, somatic, and sexual ages).2,4–6 Interpreting data proves difficult because the terms are vague. Inconsistent terms bring confusion and perplexity in synthesizing findings. Consistency in age parameters associated with purposeful labeling and clearly defined groups is imperative. The overarching terms “youth” and “pediatric” require a consensus of definition. If a specific term is used to describe a specific group of the youth population consistently in scientific literature, it will likely improve clarity in sharing evidence and knowledge associated with studies of youth populations. It was our purpose to propose age-based terminology to improve consistency within pediatric sports medicine literature. Terms such as “youth”, “young,” “adolescent,” and “preadolescent” are used in sports medicine literature to describe the age, but what do these terms truly mean? Considerable variability exists in pediatric sports medicine in defining, labeling, and categorizing age. The inconsistent use of age terminology makes it difficult to compare findings. In a search of knee injury prevention literature, no consensus was found for the term “youth” nor “adolescent.” From 2011 to 2021, 48 articles included “youth” in the title, and 35 articles' titles included “adolescent.” Of these articles, “youth” was defined as anything from 8 to 26 years while “adolescent” definitions ranged from 9 to 21 years. No consistent age definition was used in these publications, although 12 to 17 years was the most common age range for defining “adolescent,” appearing in 4 articles.7–10 Focusing on youth sport and injury prevention, we chose to examine baseball, soccer, and softball for the most prevalent sports where youth are sustaining both upper and lower extremity injury. Although not considered a systematic literature review, data of the past 20 years showed no standardized age definition or reporting system. Forty-four articles were identified when “youth” was used in the title. Age ranged from 6 to 19 years, with no consensus distinction for the ranges. The most commonly used age for youth was 9 to 14 (n = 4) and 10 to 12 (n = 4) years.11–18 Similarly, when the term adolescent was used in the title, 20 articles showed a lack of consistency in defining the age. The most prevalent age range for adolescents was 14 to 17 years.19–21 In the psychosocial sports literature, inconsistencies defining athletes' age comes from various interpretations of the term “adolescence.” Although this term implies a physical maturational component,22 studies have been linked to chronological age markers associated with broad developmental periods without an actual assessment of physical maturation. For example, “early adolescence” has been used to classify athletes who are age 13 to 15 years while “late adolescence” categorizes those age 16 to 18 years.23–25 Exceptions exist because these chronological age classifications align with terminology used in a pre-eminent model of youth sport, the Developmental Model of Sport Participation (DMSP).23,26 This stage-based model and associated literature tend to classify those who are age 6 to 12 years (the “sampling years”) as “children” or “in childhood,” those age 13 to 15 years (the “specializing years) as “early adolescence,” and those age 16 to 18 years (the “investment years”) as “late adolescence.” 23 The framework of the DMSP seems to have facilitated a degree of internal consistency within the psychosocial literature in referring to athletes based on their chronological age. However, implications of these states in terms of their explicit associations with physical maturation and development remain unclear; thus, it is important to continue refining the language used for accuracy and consistency. Looking broadly at the language used by guiding organizations, the lack of consensus in describing today's youth athletes becomes more apparent. The United Nations has defined a “child” as every human being below the age 18 years.27 However, the AACAP uses the term “child” for those who have not yet reached high school, at which point they become classified as “adolescents.”3 The WHO uses similar terminology but a different interpretation of age ranges, defining those 10 to 24 years as “young people,” with “adolescents” comprising a subgroup of this age range from 10 to 19 years and those who are age 15 to 24 years classified as “youth.”2 The American Academy of Pediatrics (AAP) has defined the upper age limit of pediatrics as 21 years, with adolescence identified as age 11 to 21 years. Furthermore, the AAP divides the adolescent group into early (11-14 years), middle (15-17 years) and late (18-21 years).28 The CDC uses a different classification system entirely, terming “middle childhood” as individuals who are 6 to 11 years, “young teens” as they progress to 12 to 14 years, and “teenagers” during the age range 15 to 17 years.1 The effect of these definitional inconsistencies among guiding organizations manifests in the various terminology and interpretations described across these multiple domains of pediatric sports medicine. When choosing terminology to describe the age range of participants being studied, it is important to consider the differences between chronological age and biological age. Chronological age is calculated as a single time point away from the date of birth.5 This type of age reporting is typically used in team sports to group athletes by age and grade level. However, chronological age does not account for biological maturation and individuals of the same chronological age can differ significantly in biological maturation.4,5,29,30 Biological age tells us where the individual falls in the spectrum of physical growth and maturation.5,31 Sexual age, skeletal age, and somatic age are the primary ways to determine an individual's biological age.4,5 Skeletal age is the degree of biological maturation according to the skeletal tissue development and is typically assessed using a left hand wrist radiograph and comparing it with a set of predetermined reference criteria.6,32 The most commonly cited assessment tools are the Greulich-Pyle method, Fels method, and Tanner Whitehouse method.33–37 Sexual age is the degree of biological maturation toward a fully functional reproductive capacity.6 The most commonly cited assessment tool for determining sexual age is the Tanner Staging Scale, which involves visual observation of secondary sexual characteristics.37,38 Finally, somatic age is the degree of growth in overall stature or specific body dimensions.39 Despite the added depth and precision in detailing these maturational aspects of a sport participant, assessing biological age is not always practical in sports medicine research. Age terminologies such as “preadolescent” and “adolescent” may lead to the false assumption that a measure of biological age was completed. Terms such as these are widely used in sports medicine literature, often without clear evidence that the researchers completed a biological age assessment. Of the 35 injury prevention articles reviewed that used the term “adolescent” in the title, only 17% (N = 6) reported using a measure of biological age.40–45 Two articles assessed maturation by estimating the time from peak height velocity (PHV) using the equation developed by Moore et al while Holden et al assessed landing kinematics of 184 adolescent male and female athletes (mean age 13 ± 0.3 years) during the growth spurt.41,46 Holden et al41 classified participants as pre-PHV or mid PHV into an early puberty group and those as post-PHV into a late puberty group. They found that male athletes demonstrated greater increases in knee flexion during the growth spurt than female athletes in a drop jump landing. Following the same methodology to estimate the time from PHV, Mann et al45 described injury and illness among competitive adolescent distance runners age 13 to 19 years. In addition, 2 articles reported using the Pubertal Maturation Observational Scale (PMOS).42,44 Specifically, Quatman-Yates et al44 investigated longitudinal trajectories of lower extremity strength across different states of maturation, including prepubertal or pubertal status in a group of 39 female athletes finding that from prepuberty to postpuberty, the athletes had an increase in knee extension strength and a decrease in hip abduction and hamstring to quadriceps strength ratio. In addition, DiCesare et al42 followed a similar methodology, including female athletes who were classified as prepubertal or pubertal on the PMOS in their investigation of the association of sports specialization with lower extremity biomechanical deficits in adolescent female athletes. They found that athletes who specialized early had a significantly larger post pubertal increase in peak knee abduction angle and knee abduction moment and a smaller increase in peak knee extensor moment during a drop vertical jump compared with multisport athletes. Another study, assessing the relationship between skeletal growth and increased knee abduction moments between men and women during a drop vertical jump, assessed maturation using an estimate of percent adult stature.43 The authors defined maturity as 92% adult stature or greater and found that maturation resulted in greater knee abduction moments at landing in women but not men. Finally, in a report using the Tanner criteria to determine the stage of pubertal development, the authors identified 29 male participants as early adolescents (Tanner stage 2.8 ± 0.6) in their investigation of the effects of an in-season integrative neuromuscular training program on performance.40 They found that an 8-week program resulted in positive performance adaptations in early adolescent soccer players. The findings of the aforementioned articles using an assessment of biological maturation suggest that classifying maturational status has important implications in injury prevention research. In addition, it is important to understand whether the intervention exposure or physical maturation is responsible for the observed changes in intervention-based research. For example, it is possible that changes in strength and power over time could result from growth and development rather than the intervention being studied.5 Thus, as researchers, we must use appropriate age terminology that accurately reflects the type of age assessment performed. PROPOSED TERMINOLOGY The AAP defines the term “pediatrics” as the specialty of medical science concerned with children's physical, mental, and social health from birth to young adulthood.3,47 The UN defines “youth” as those between the age of 15 and 24 years, which clashes with published literature and popular discourse to use this term for athletes with younger chronological ages.27 These age ranges are far too broad to describe participants in sports medicine research and require further breakdown. Instead, terms such as “youth” and “pediatrics” should be used as umbrella terms for athletes who are yet to be prescribed “adult” status in contemporary society (less than 18 chronological years), with subcategorization into smaller cohorts. In alignment with the DMSP, we are proposing 3 subcategories: childhood (5-12 years), early teen (13-15 years), and late teen (16-18 years). It should be noted that the recommended terminology is based on standard verbiage associated with the DMSP; however, terminologies such as “preadolescent” and “adolescent” have been replaced with “teen” to explicitly clarify that chronological age is the classifying mechanism, rather than suggesting the formal assessment of biological age as the measure of maturational status. Moreover, the tentative age ranges of these 3 subcategories generally align with the progression through the North American school system (ie, childhood = elementary school; early teen = middle school; and late teen = high school), providing tangible markers for transition points as athletes develop. Given the inconsistent use of age terminology in the current literature, it is important to incorporate this new terminology when describing previous research. This will allow for more precise comparisons between studies while also minimizing invalid generalizations and interpretations of accumulating research findings in this domain.