Chronological Age vs. Biological Maturation: Implications for Exercise Programming in Youth : The Journal of Strength & Conditioning Research

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Brief Review

Chronological Age vs. Biological Maturation

Implications for Exercise Programming in Youth

Lloyd, Rhodri S.1; Oliver, Jon L.1; Faigenbaum, Avery D.2; Myer, Gregory D.3,4,5; De Ste Croix, Mark B. A.7

Author Information
Journal of Strength and Conditioning Research: May 2014 - Volume 28 - Issue 5 - p 1454-1464
doi: 10.1519/JSC.0000000000000391
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Abstract

Introduction

The physical and physiological assessment of athletes is often performed at various stages during a yearly cycle of training to establish the effectiveness of individual blocks of training or the overall success of the annual training plan. However, when working with either children or adolescents such changes in performance can be significantly affected by growth and maturational factors. Although researchers have examined the influence of growth and maturation on physical performance in youth, practical information on different classifications of maturational assessments and their proper use by practitioners is limited. Consequently, this article will critically analyze existing literature to (a) define the terminology associated with the different age classifications, (b) review existing methods of assessing maturation, and (c) demonstrate how the assessment of maturation can be of multiple benefits to practitioners. For the purpose of this article, the term “practitioners” will refer globally to strength and conditioning coaches, youth sport coaches, physical education teachers, athletic trainers, physiotherapists, and health care providers.

Chronological age, which is calculated as a single time point away from the date of birth, has traditionally been used in sports to group age grade teams, identify talented performers, and set limits for exercise prescription. However, literature has clearly demonstrated that individuals of the same chronological age can differ markedly with respect to biological maturity (3,26,54). Biological maturation refers to progress toward a mature state and varies in timing and tempo and between different bodily systems (4). Significant interindividual variance exists for the level (magnitude of change), timing (onset of change), and tempo (rate of change) of biological maturation. Dependent on these variables, children will be viewed as either biologically ahead of their chronological age (early-maturing individual), “on-time” with their chronological age (average maturer), or behind their chronological age (late-maturing individual) (35). Based on theoretical data, Figure 1 shows the difference in linear development of chronological age vs. the nonlinear development of sexual maturation in both males and females. The figure also demonstrates the theoretical differential maturation rates for early- and late-maturing individuals. The relative mismatch and wide variation in biological maturation between children of the same chronological age emphasizes the limitations in using chronological age as a determinant in global exercise prescription for school-age youth.

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Figure 1:
Differences in developmental trends of chronological age and biological maturation of early- and late-maturing boys (left) and girls (right), based on theoretical data.

Pediatric researchers and clinicians have noted the importance of considering biological maturation to develop appropriate training programs to optimize training adaptation and minimize activity-related injury risk (20,31,32). In combination with training age, which is defined as the number of years an athlete has been participating in formalized training (31,47), regular monitoring of biological maturity is recommended to enable training prescription to be designed with an appreciation of the unique physical and physiological processes that are taking place as a result of maturation (31). It is important to appreciate and understand biological maturation in youth to be able to distinguish whether maturation or exposure to regular exercise training is responsible for observed changes in physical performance and injury risk. Previous research within the pediatric exercise science literature has made reference to the use of various “biological indicators” (31). However, to the best of our knowledge, a comprehensive review of available methods for assessing biological maturation in children and adolescents and an explanation of how these methods and principles can be used by practitioners to design and implement safe and effective exercise programs for youth does not exist in the literature.

Assessment of Biological Maturation

Although chronological age is easily determined, the assessment of biological maturity is more challenging to evaluate owing to the large interindividual variation in magnitude, timing, and tempo of the adolescent growth spurt (57). Numerous methods have been described within the literature and are commonly categorized into skeletal, sexual, or somatic maturity indicators (3). Despite these methods varying considerably, correlations among the different indicators are typically moderate to high (3,35), suggesting they possess a noteworthy degree of validity in reflecting estimated biological maturation (54).

Skeletal Age

Skeletal age, or “bone age,” refers to the degree of biological maturation according to the development of skeletal tissue (34,35). It should be noted that trained radiographers should perform this method of assessment, and in most cases only qualified medical personnel would be expected to analyze the radiographs. Maturation of the skeletal system involves a long-term transition from cartilaginous structures (prenatal stage of growth) to a fully developed skeleton of bones by early adulthood (Figure 2). With the use of x-rays or radiographs, and an understanding of the developmental process from early ossification to fully mature skeletal tissue, skeletal age verification is arguably the gold standard method of assessing biological maturation. Accordingly, a number of methods of assessment exist, which rely on the use of matching a left hand–wrist radiograph to a set of predetermined reference criteria (34). The most commonly cited assessment tools are the Greulich-Pyle method (24), Fels method (52), and Tanner-Whitehouse (TW) method (60–62).

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Figure 2:
Reference image radiographs of the left wrist for males (top) and females (bottom) at 12 months, 6 years, 12 years, and 18 years of age. Reprinted with permission from Gilsanz and Ratib (22). Adaptations are themselves works protected by copyright. So in order to publish this adaptation, authorization must be obtained both from the owner of the copyright in the original work and from the owner of copyright in the translation or adaptation.

Greulich-Pyle Method

The Greulich-Pyle method, which is also viewed as an atlas-based technique, was first validated using a collection of reference radiographs from white children of high socioeconomic status (24). This method of assessment involves the radiograph of a child's left wrist being compared against the reference x-ray plates of varying levels of skeletal maturation. Skeletal age is subsequently determined as that which most closely reflects one of the standard radiographs of the atlas system. For example, if the x-ray of a 10-year-old girl matches closely with the reference plate x-ray of a 12 year old, then her skeletal age would be determined as 12 and she would be deemed an early maturer. This method is based on the premise that bone tissue matures in a uniform manner; however, such an approach fails to account for individual rates of development of different bones, and therefore concerns exist with the Greulich-Pyle method for accurate determination of biological maturation. Specifically, the Greulich-Pyle method is susceptible to both over- and underestimates of maturation based on the mismatched bone growth development and may not be applicable across diverse ethnicities (35).

Tanner-Whitehouse Method

The TW method, which has been revised twice since the original TW1 method (62), requires the practitioner to use a combination criterion that includes multiple bones of the hand and wrist (13 bone assessment including the radius, ulna, and phalanges; or the 20 bone assessment incorporating the radius, ulna, phalanges, and carpals). Irrespective of the assessment method (13 or 20 bone analysis), the radiographs of these bones are analyzed and compared with a series of statements and detailed shape analyses of each bone. Individual bones are given an independent maturation score, and then the cumulative score is converted to a skeletal age value. Although the TW1 (62) and TW2 (61) versions were validated using British children, the most recent TW3 method (60) is based on samples derived from European, South American, North American, and Japanese youth. One potential drawback of the TW3 method is that it is a fairly complex and time-consuming process and requires a reasonable degree of subjective decision making. The difficulty of assessment combined with the need for radiographic evaluation limits the utility of the TW method to be used in youth training environments.

Fels Method

Based on data collected between the 1930s and 1970s within the Fels Longitudinal Study (52), the Fels method provides maturity assessments for the radius and ulna, carpals and metacarpals, and phalanges. Each bone is graded according to age and sex; ratios between length and width of the epiphysis and metaphysis of the long bones are measured, and the degree of ossification for the pisiform and adductor sesamoid is recorded. Using specific software, skeletal age and standard error of estimate are then calculated. Although the Fels method is again a complex and time-consuming process, the ability of the assessment tool to provide an estimate of the standard error of measurement is obviously of benefit for the long-term tracking of biological maturation in children and adolescents.

Although the aforementioned methods of assessing skeletal age are often considered as the optimal method of determining biological age, issues surrounding the cost, availability, time, need for specialist equipment, and requisite assessor expertise mean that these techniques may not be accessible or indeed appropriate for most practitioners. Previous concerns surround the deliberate exposure of children to radiation that the technique warrants; however, research suggests that with modern technology the exposure is less than normal background radiation associated with everyday life (34). More recently, dual-energy x-ray absorptiometry has been introduced to assess skeletal development. Dual-energy x-ray absorptiometry assessments are typically faster, are more precise, can provide higher resolution images, and can be used to analyze a greater number of body regions than traditional x-ray techniques (21,25). Although radiographic assessment of maturation is considered the “gold standard,” the concerns with access and complexity of measurement limit its usage among practitioners. Accordingly, more noninvasive assessments are typically used to determine biological maturation in youth.

Sexual Age

Tanner Criteria

Sexual age refers to the degree of biological maturation toward fully functional reproductive capability (35). It must be emphasized that only trained clinicians are qualified to assess sexual maturation, and at all times this should be completed with the consent and assent of the parent and child, respectively. Additionally, owing to the sensitive nature of the assessment process, up-to-date criminal background checks of the assessor, and the anonymity and confidentiality of those being assessed should be maintained at all times.

Traditionally, sexual maturation has been assessed through observations of secondary sexual characteristics (breast, genitalia, and pubic hair development), which are then compared against 5 distinct reference “stages,” referred to as Tanner stages 1 (TS1), TS2, TS3, TS4, and TS5 (58). Specifically, observation of female characteristics would include age at menarche and breast and pubic hair development, whereas male characteristics would include genital and pubic hair development (59). Table 1 provides an overview of breast, genitalia, and pubic hair development according to each Tanner stage (58).

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Table 1:
Distinct stages of sexual development for pubic hair, male genitalia, and breasts (adapted from Roche and Sun (53)). Adaptations are themselves works protected by copyright. So in order to publish this adaptation, authorization must be obtained both from the owner of the copyright in the original work and from the owner of copyright in the translation or adaptation.

In addition to the Tanner criteria, testicular volume can also be used as a means to estimate genital maturity in young males (3,50). Qualified and experienced medical doctors can perform the Tanner assessment within a clinical setting. However, because of the invasive nature of direct visual observation of Tanner staging, and the concerns that this brings to both youth and their parents, self-assessment techniques have been used which require the child to compare their own sexual characteristics to those of reference drawings or photographs (58). Research suggests that such techniques produce reproducible and reliable data (38); however, practitioners should be cognizant of the fact that boys typically overestimate, whereas girls generally underestimate their own sexual development (29,65).

Despite the fact that the Tanner criteria are most often used to assess sexual maturation, certain limitations associated with the method exist. First, the method has an inability to differentiate children or adolescents within stages (i.e., 2 children could be rated within the same stage, but 1 is only just at the start of the stage, whereas the other child is nearing the end of the stage). Second, the assessment tool is unable to provide an insight into the tempo of maturation (i.e., a child may pass through the stages at a faster rate than another child of the same chronological age), thus making comparison between individuals more problematic. Finally, the Tanner criteria are confined only to the period of pubertal phase of growth and maturation and therefore cannot be used for children or adults outside of those timeframes. This may be a limitation to those teaching or coaching prepubertal children or young adults because they will be unable to ascertain where the child is in relation to the onset of puberty. Additionally, practitioners should be aware that Tanner stages are not interchangeable between different indicators (i.e., breast, genitalia, and pubic hair development can occur at different tempos), and that if reporting biological age from sexual maturation, then specific stages for each indicator should be provided (35). It must be reiterated that coaches or exercise practitioners should never perform observations of secondary sexual characteristics to determine Tanner staging; and cumulatively, these limitations inhibit the utility of Tanner staging to support exercise prescription for youth.

Age at Menarche

Specific to females, age at menarche is often used to determine at which age a female experiences their first menstrual period (35,54). Despite the notion that females can recall the time point at which they started their first menstrual cycle to within approximately 3 months (28), this method of assessing biological maturation will typically be a retrospective measure (i.e., through simply asking the individual to recall the onset of their first period). Owing to the temporal delay between the onset of puberty and the subsequent onset of menarche, identifying a girl who is premenarcheal does not necessarily suggest that she is also prepubertal, and therefore this method of assessing sexual maturation has limitations that preclude its use in exercise- and sport-related related settings.

Before the onset of adolescence, boys and girls will follow similar rates of maturation even though boys will consistently outperform girls in a range of biomotor skills (e.g., speed, strength, and endurance) (4). However, maturational differences become more evident upon the onset of the adolescent growth spurt, with females experiencing clear sex-specific physiological changes. Such processes include increased fat mass, differential rates of development in stature and muscular strength, commencement of menstruation, increases in joint laxity and valgus knee angle, and an increased quadriceps: hamstring utilization ratio (45). All of these factors have been identified as potentially increasing the risk of injury in female athletes (45). Consequently, practitioners need to possess an understanding of growth-related changes in female athletes and training considerations that account for sex-specific differences in biological maturation.

Techniques to Predict Biological Maturation in Youth

Somatic Assessments

Somatic age refers to the degree of growth in overall stature, or of specific dimensions of the body (such as recumbent length). From longitudinal data, it is apparent that somatic growth is nonlinear in its development, with periods of rapid growth interspersed with relative plateaus in development (57). Common measures of somatic growth include assessments of longitudinal growth curves, prediction of age at peak height velocity (PHV), and the use of percentages and predictions of adult stature (35).

Growth Rates

The processes of growth are difficult to study and consequently a number of indirect measures of body size and proportions have been established to assess overall development. The onset of sexual maturation is reflected in marked changes to the endocrine system that stimulate skeletal maturation. This can readily be observed in the rate of growth in stature and other body dimensions (such as limb lengths). The most basic level of assessment involves longitudinal anthropometric assessment of breadths, widths, and lengths of specific individual landmarks: overall stature and body mass; sum of skinfolds to determine levels of adiposity; and a combination of the above to provide an estimate of somatotype. From a practical perspective, the repeated collection of height over a period of time would enable the analysis of growth curves. For example, the longitudinal assessment of stature would reveal the magnitude and rate of change of height over time (centimeter per year). From such growth curves, age at PHV can be identified, which reflects the age at maximum rate of growth during the adolescent growth spurt (Figure 3; modified from Stratton and Oliver (57)). The growth spurt is determined by initiation of sexual maturation and subsequent changes in endocrine function promoting rapid growth, which can be observed as changes in body size.

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Figure 3:
Change in growth rates (centimeter per year) with chronological age (years) for early-, average-, and late-maturing individuals based on theoretical data (adapted from Stratton and Oliver (57)). Adaptations are themselves works protected by copyright. So in order to publish this adaptation, authorization must be obtained both from the owner of the copyright in the original work and from the owner of copyright in the translation or adaptation.

Research demonstrates that PHV typically occurs at around the age of 12 years in females and 14 years in males (35). Early-maturing youth display PHV approximately 1 year (or more) before the mean age at PHV, average-maturers experience mean age at PHV, whereas late-maturing youth have an age at PHV which is at least 1 year later than the mean age at PHV (3). One issue for youth training practitioners is that the identification of age at PHV requires regular longitudinal tracking of growth from middle childhood until late adolescence. Additionally, PHV can only be identified retrospectively after a peak has been observed. The collection of such data over an extended period of time may not be common practice within some sports or training settings, especially at an amateur level, which may hamper longitudinal analysis. For example, a youth sport coach may start working with a young athlete at the age of 14, but potentially have no data on growth changes experienced by the child in previous years. Consequently, it would be challenging to ascertain at which point in time the child started their growth spurt.

Predicting Peak Height Velocity

In situations where longitudinal tracking of stature and body mass is not possible (e.g., cross-sectional analysis or a short-term training block), age from PHV can be calculated using the equations proposed by Mirwald et al. (42). It is acknowledged that differential growth rates exist between the legs and the trunk, with the long bones of the legs experiencing peak growth ahead of the shorter bones of the trunk. Mirwald et al. (42) created the predictive equations to incorporate this growth pattern to estimate years from PHV to within a standard error of approximately 6 months. Essentially, the equations require the attainment of chronological age (years and months), body mass, standing height, and seated height, which can be used to determine years from PHV for a male (equation 1) or female (equation 2) at any given single point in time.

Adult Stature Predictions

Another method of assessing somatic maturity is using percentages of predicted adult stature. Practitioners could use the percentage of adult stature to determine the maturational state of a group of young athletes. For example, if the final adult stature is predicted, then a practitioner could distinguish between a group of players who are currently the same height, but may be relatively closer to their final adult height. Consequently, a youth coach could differentiate between athletes who are genetically predisposed to be tall vs. an athlete who is simply maturing at an earlier stage. Such an approach would obviously have some application for training prescription, but also for talent identification programs to determine which children were genetically predisposed for certain sports or positions where stature is advantageous.

Such an approach requires the prediction of final adult stature, which does not provide any information regarding the timing and tempo of maturation. The most basic method of predicting final adult stature involves calculating midparental height and using a correction of adding or subtracting 6.5 cm for boys and girls, respectively, based on an average difference in stature between both sexes (28). Calculations are shown below for males (equation 3) and females (equation 4), whereby 13 cm is added to or subtracted from the combined parental height in cm, and the result is then divided by 2.

In addition to midparental height, Khamis and Roche (27) developed a regression equation that also includes current stature and weight of the child, to estimate eventual adult height. Additionally, Beunen et al. (5) produced an alternative prediction model that uses chronological age, standing height, sitting height, and subscapular and triceps skinfolds. Whereas Cole and Wright (12) developed a chart that uses the current standing height of the child and adjusts for regression to the mean to predict eventual height. Methods used to predict adult stature typically possess standard errors of approximately 3–5 cm, with the error of prediction narrowing with increasing age (12). Because skeletal age assessments are not required, the aforementioned methods may serve as a viable assessment tool for practitioners.

Using the regression equations identified by Mirwald et al. (42), researchers have developed an alternative noninvasive method of predicting adult stature using cumulative height velocity curves (56). Using retrospective data from previously published research (2,33,41), the method of Sherar et al. (56) used regression equations to predict years from PHV (42), which in addition to the maturity-specific cumulative velocity curves, were used to estimate height left to grow. Height left to grow was then added to the standing height of individuals at the time of testing to provide a final predicted adult height. These predicted heights were then validated against eventual adult heights (56). Using 95% confidence intervals, the measurement error associated with predicting final adult height was ±5.35 cm for males and ±6.81 cm for females (56). Coaches and scientists would need to decide whether this provides suitable precision and accuracy dependent on the importance of adult stature in a given sport or position. Of interest to practitioners who are directly responsible for youth exercise programming, online tools (http://taurus.usask.ca/growthutility) are available which can calculate both predicted adult stature and years from PHV based on basic anthropometric data.

Comparison of Methods

Although slight variations in somatic, sexual, and skeletal maturation are evident throughout adolescence, research suggests that the different indicators (skeletal, sexual, and somatic) used to assess biological maturation possess moderate to high relationships (r > 0.70 (6)). Consequently, although skeletal age is still viewed as the gold standard method of assessing biological maturation, practitioners can use alternative assessment methods to aid in the design and progression of their training prescription. Factors such as the availability of equipment, requirement of expertise for analysis, degree of invasiveness of assessment methods, time and cost implications, and the degree of measurement error associated with the assessment method that is used to assess biological age need to be considered. Of note, legal and ethical issues surrounding any method of assessing biological maturation should be considered in addition to local guidelines from school boards and sports centers.

Frequency of Assessment

Minimal evidence exists to indicate whether there is an optimal frequency for which to assess the status of maturation in children. However, it is recommended that to monitor for the onset of the growth spurt, basic somatic measurements should be taken by the practitioner approximately every 3 months (13,35). Such a time frame should enable worthwhile changes in growth to take place and avoid attention being given to small insignificant natural fluctuations in standing or seated height that children may demonstrate on a daily basis. Additionally, because of the time constraints associated with practice, competition, travel, academic timetables, and other commitments, assessments every 3 months should avoid excessive time being devoted within the overall athletic development program, and therefore enable practitioners to assign more time and attention to technical competency, conditioning for injury prevention, and overall athletic performance enhancement.

Assessing Growth and Maturation: Applications for Practitioner

Training Prescription

The concept of long-term physical development of youth athletes has received increased attention within the literature (18,31). Although there has been much contention over the lack of supporting evidence for previous long-term athlete development models (18), certain training modalities would appear to be more appropriate at specific stages of maturation. Resistance training is now recognized as an essential component of a young athlete's development program for performance, health, and injury-reducing benefits (30). Modes of strength and conditioning inclusive of plyometrics and weightlifting exercises (clean and jerk, snatch, and their derivatives) can be promoted in youth training programs on the proviso that those designing and delivering the programs are suitably qualified to teach all forms of resistance training (30).

In terms of resistance training, a physical education teacher or strength and conditioning coach would be ill advised to prescribe hypertrophy-based resistance training to a child who has not yet experienced the pubertal growth spurt because of limited concentrations of anabolic hormones (30). Instead, prepubertal children should be prescribed a resistance training program that focuses on enhancing muscle strength, function, and control because of the high degree of plasticity in neuromuscular function during this developmental period (30,46). Because of their increased tissue pliability (15), children can typically recover more quickly from fatigue-inducing resistance training sessions than adults (16,66), and consequently children can often be prescribed shorter rest intervals than adolescents or adults. However, technical competency during resistance training sessions should be carefully monitored at all times, and practitioners should consider a child's cognitive development that can influence one's ability to follow instructions and perform simple and complex movements (46). Furthermore, the importance of having fun in a supportive environment should not be overlooked. Consequently, many of the popular commercial metabolic high-intensity training programs that entail high volumes of work with insufficient recovery are deemed inappropriate for youth. Such programs risk de-emphasizing the development and maintenance of proper technique and allow for poorly performed repetitions as clients attempt to reach a predetermined number of repetitions at any cost. Irrespective of the training mode used, being able to determine the degree of biological maturity of a young athlete would hopefully assist the decision-making processes of the strength and conditioning coach to accurately prescribe resistance training sessions appropriate for the individual's stage of development. Practically, a strength and conditioning coach could also make use of maturational assessments to divide a large cohort of youth players according to maturational status (i.e., pre-PHV, circa-PHV, and post-PHV), which would help ensure that players of similar physical maturity were training synonymously.

Monitoring Training Adaptation

Practitioners will routinely administer testing batteries to establish the effectiveness of a training intervention. However, uniquely children and adolescents can experience performance adaptations (e.g., increase in jump height and sprint speed) purely as a result of growth and maturation. Consequently, when working with a child or adolescent, a practitioner should monitor maturational status to help evaluate any changes in performance in response to a training intervention. Furthermore, pediatric research suggests that practitioners can use basic anthropometric measures commonly incorporated within noninvasive measures of maturity to assess training-induced changes in lower-limb volumes and composition in youth athletes (9). Adult data suggest that basic anthropometric measurements can also be used to determine skeletal mass properties (23); however, equivalent research has not been completed on youth populations.

Although it is important that practitioners determine the effectiveness of their program, and that they take into account the measurement error associated with any mode of testing, it is also important that maturational assessments are routinely taken to help explain fluctuating changes in performance throughout childhood and adolescence. For example, knowledge of biological maturation could be of use to practitioners to help explain fluctuations in motor skill competency. “Adolescent awkwardness” is a developmental phenomenon that refers to a temporary disruption in basic motor skill execution resulting from the early onset of the adolescent growth spurt (49,51). Adolescent awkwardness typically occurs approximately 6 months pre-PHV, and longitudinal monitoring of leg length and use of the regression equation of Mirwald et al. (42) can assist practitioners in identifying youth who are potentially at risk. Adolescent awkwardness will typically involve adolescents experiencing a temporary disruption in motor coordination simply as a result of growth and maturation as opposed to any training-induced performance decrement. For youth experiencing this developmental phenomenon, it is recommended that training volume loads are modified to avoid excessive loadings during the phase of rapid skeletal growth and to provide sufficient opportunity for individuals to relearn motor control patterns. This would also serve as an appropriate stage of development to refine key fundamental movement skills, to maintain levels of physical literacy, and to limit the chances of potentially injurious technical deficiencies being acquired, such as poor landing mechanics.

Talent Identification

Although some limitations exist with noninvasive indicators of maturity, if used appropriately, they can ensure that less mature players who are technically gifted within the same chronological age group are not routinely disadvantaged owing to maturity-associated differences in physical and functional variables (36). For example, maturity status has been identified as a significant contributor to aerobic fitness (10), anaerobic power (8), explosive power (17,37,63), sprinting (37,63), and change of direction speed (63) in young athletes (11–16 years). Furthermore, it has been shown that early-maturing youth are typically at an advantage for physical performance tests owing to their relatively greater levels of muscle strength, explosive power, and running speed (39). Additionally, through the use of simple noninvasive maturational assessments, it could be established that an early-maturing youth has been selected for a specific position despite the fact that a later-maturing individual is predicted as having a taller adult stature. Consequently, any talent identification process must acknowledge and account for maturity-related variation in performance. The relative age effect (RAE) is a phenomenon whereby youth who are born early in the selection year are routinely selected ahead of those born later in the year (11,44). Such a selection bias is often because of the older children possessing greater size, strength, and speed, and having gained more exposure to their chosen sport(s) (55). Consequently, less mature children might not be selected for competitions despite possessing good levels of skill or may volitionally drop out of a sport because of a lack of perceived competence or lack of success (14). The bias for the selection of earlier maturing individuals is exacerbated by the likelihood that these individuals will also receive greater opportunities for expert coaching and training (7).

Through the use of maturation indicators, coaches can make informed judgments of performance by considering how mature the young athlete is in relation to their peers. Maturity-related differences in height and weight can start to emerge from 6 years of age, which continue to increase with adolescence (35), and thus early-maturing youth stand to possess a size advantage from early in life. Using predictive methods of maturation (most likely percentage of predicted adult stature), those early-maturing individuals could be identified who are demonstrating advanced growth trends because of early skeletal maturation. Additionally, the natural development of numerous performance measures has been shown to peak around the time of PHV or peak weight velocity (PWV), therefore, practitioners should not presume that rapid gains around this period are because of training interventions used.

Early Sport Specialization

There appears to be an increase in the number of preadolescent youth who specialize in a single sport (1,43). This may include regular exposure to sport-specific training, regularly participating in a high proportion of competitions, or competing in multiple teams within the same sport (43). Within the scope of early sport specialization, it is likely that the timing and tempo of maturation will lead to individuals being identified for specific positions within a team. For example, early-maturing individuals may be selected for size-related positions in those sports where additional height may be advantageous (e.g., basketball, volleyball, or netball). This situation can lead to youth not only being exposed to higher volumes of sport-specific training but also a narrow range of position-specific movement skills.

This notion has recently been identified within the literature, with Fransen et al. (19) showing that youth who spent many hours in a variety of sports performed better in gross motor coordination and standing broad jump performance than those who specialized early in a single sport. Additionally, research indicates that youth who specialize early in a sport are at an increased risk of overtraining and overuse injury as well as burnout (i.e., stress-induced withdrawal) (1,40,48,64). In the instance of a strength and conditioning coach working with a young athlete who has specialized early in a single sport, the training focus should be dedicated to enhancing global movement skills, improving muscle weakness or imbalances, and promoting self-esteem and perceptions of confidence.

Summary

It is essential that practitioners working with youth recognize the unique requirements of children and adolescents, and that they can appreciate the influence of biological maturation on athletic performance and development as it relates to exercise programming. This article reviewed available methods for assessing biological maturation and demonstrated how specific assessment tools are available that can be used in sport settings and youth training facilities. In summary, this review has addressed the following questions:

What is the relative importance of assessing biological maturation?

  • Biological maturation should be viewed as an additional variable to consider when designing exercise programs for youth; however, it should not drive training prescription on its own.

How do the different assessment strategies compare in terms of accuracy and applicability?

  • It is generally accepted that the gold standard method of determining biological maturation is through skeletal age assessment; however, for a number of ethical, financial, resource, and time constraints, practitioners are advised to assess maturation from basic somatic measurements.

Which assessment strategy should be used if testing individuals in a cross-sectional analysis, or an individual for the first time?

  • For cross-sectional and first-time assessments, where the current status of maturation is unknown and there is no previous information relating to longitudinal growth, then the use of age at PHV predictions are recommended.

Which assessment strategy should be used for long-term tracking of individuals?

  • Where practitioners will be working with youth for an extended period of time, then it is advised that longitudinal tracking of stature, limb length, and body mass is taken, allowing growth curves to be used.

How often should assessments be taken for the monitoring of biological maturation?

  • Basic somatic measurements should be taken by the practitioner approximately every 3 months to enable the identification of any worthwhile changes in growth.

How can the assessment of biological maturation be of use to practitioners?

  • Assessing maturation can inform talent identification, assist in avoiding the negative effects of the RAE, aid program design, and help practitioners determine if their programs are providing benefits beyond the level of expected natural development.

Acknowledgments

There are no conflicts of interest to report within this article. The study did not receive any external funding and is the result of independent research by the team of investigators.

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      Keywords:

      youth fitness; somatic age; skeletal age; sexual age; youth sports injury; early sport specialization

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