Human biology of physical activity in the growing child

It is now established, without a doubt, that physical activity is essential for the physical (Poitras et al. 2016) and mental (Biddle and Asare 2011) health of the growing child. Globally, however, many youth are not reaching the minimum physical activity guidelines required to achieve the associated health benefits (Guthold et al. 2019) and there is a well-observed steep decline in physical activity as children progress through adolescence (Kimm et al. 2002; Dumith et al. 2011; Corder et al. 2016). While the decline in unstructured activity (e.g. active play) with age is likely an inevitable consequence of progressing to the mature state (Rowland 1998), increasing the age of onset, and reducing the steepness of the decline is paramount for the current and future health of adolescents.

One of the most visible forms of moderate-to-vigorous physical activity in children and adolescents is organised sport, which has a host of benefits outside of physical health including social (e.g. sportsmanship), psychological (increased self-esteem, particularly with team sports (Evans et al. 2017)), and positive leadership (Fraser-Thomas et al. 2005). While youth attendance in organised sport has increased in some countries over the last two decades (e.g. Mathisen et al. 2019), particularly at the younger ages, disengagement during the adolescent years is still evident, with a common observation that girls are more susceptible to dropping out at a younger age than boys (Chalabaev et al. 2013).

Over the past 20 years, we have seen a surge of evidence on the correlates of physical activity (and to a lesser extent sport participation) in children and young people. However, perhaps less well understood is the impact that the processes of growth and biological maturation have upon children’s physical activity behaviour. Adolescence hosts stark changes in physiology, psychology, and behaviour, but the age at which children experience these changes differs widely (e.g. up to 5 years) leading to marked biological variation, both between and within genders of the same age. Many studies in public health consider biological maturity as a confounding influence/covariate in analyses, and within Sport Science, there has been ample observational research documenting maturation as an important predictor of performance and selection in young athletes. What is less clear, however, are the various mechanisms and processes through which these associations occur, how individual differences in maturation and development are best managed in a sporting context, and the extent to which these processes may also impact general engagement in physical activity. Thus, this special issue includes a diverse compilation of cross-sectional, longitudinal, qualitative, and quantitative studies that have a specific focus on the impact of biology on adolescents’ participation in physical activity and sport. We hope that it provides a fuller picture of the evidence base to date, the future directions of the field, and the challenges that researchers and practitioners need to overcome.

Utilising a prospective design, Baxter-Jones and colleagues (Baxter-Jones et al. 2020) investigated the role of anthropometric and maturational factors upon the selection and retention of over 800 adolescents across a range of sports. The majority of adolescents who tried out for sports were born within the first half of the competitive year, tall for their age and, in many sports, advanced in maturation. Anthropometric and maturational factors did not, however, predict future selection and deselection. This suggests that developmental differences may have greatest impact upon initial entry into the sport but have limited impact beyond this point. Findings support the need for strategies to counter selection biases against later born and/or later developing adolescents.

In an invited commentary, Eisenmann, Till, and Baker (Eisenmann et al. 2020), further highlight the need to monitor growth and maturation in youth athletes and discuss how this information can be utilised to optimise the identification and development of talented young athletes. Highlighting examples of good practice, the authors describe how information related to maturational status and timing can be used to inform coaching decisions and the design and implementation of training programmes. Most importantly, the authors identify the education of stakeholders (e.g. coaches, parents, athletes) as a key strategy in optimising the development of and opportunities for both early- and late-developing athletes.

Presenting a case study from UK football (soccer), Hill and colleagues (Hill et al. 2020) discuss how educational principles derived from Social Development Theory and research conducted in mixed-age classrooms can be used to support the learning and development of early- and late-maturing players in bio-banded (i.e. maturity matched) competitions and training. Drawing heavily from the work of Vygotsky and Bruner (Wood et al. 1976) the authors explain how coaches and sports science practitioners can “scaffold” learning environments to optimise challenge and opportunity for development. Preliminary findings from the case study suggest that these strategies can be used to better prepare athletes for the practice of bio-banding and optimise benefits of this strategy.

Three papers in the special issue focus specially on the impact of biological maturity on physical performance. A paper provided by Myburgh et al. 2020 illustrates how sex-specific developmental curves for sprint acceleration can be used in elite adolescent tennis players (n = 3120). Intuitively, they showed that early-maturing males and females had poorer performances when acceleration was considered relative to biological rather than chronological age, highlighting the bias inherent in traditional evaluation processes. The authors suggested that examining performance relative to developmental curves based on biological maturity (not chronological age) may be a practical and more accurate method of monitoring long-term athlete development. Second, Guimares et al. 2020 utilised 4 years of longitudinal data on 512 boys (Canadian, Portuguese, Brazilian; 8–17 years) to show that peaks in performance align to age at peak height velocity (APHV) (e.g. explosive muscular strength peaks prior to APHV whereas static muscular strength peaks after APHV). These are important observations and need to be built upon and translated to teachers and coaches so they can better understand the impact of timing of biological maturity/peak statural growth on performance. Lastly, Santos et al. 2020 examined the association between biological maturity, growth, fitness, and gross motor coordination in over 7000 children and adolescents residing in Amazon and Peru. Younger age, being female, lower height for age (i.e. stunting), lower overall physical fitness, and overweight status was related to lower gross motor coordination. Timing of biological maturity was, however, unrelated. Lastly, a number of school environment variables, such as playground facilities and frequency of physical education, were significant predictors of gross motor coordination. These findings should be used to inform intervention and policies within these specific populations.

In a timely contribution, Moore et al. 2020 provide a much-needed systematic review and narrative synthesis of research exploring the relationship between timing of biological maturation and physical activity (including active transport), sport, and sedentary behaviour. Adhering to PRISMA guidelines, the authors concluded there was modest evidence of early maturity as a potential risk factor for disengagement from physical activity and increase in sedentary behaviours in both boys and girls. The review also highlights the wide heterogeneity in measures of both physical activity and biological maturity suggesting a need for measurement consistency and/or data harmonisation across studies.

A study by Cumming et al. 2020 used a biocultural approach to interrogate a large (n = 1062) dataset of multi-ethnic girls (11–14 years) living in the Midlands of the UK. Results showed that early maturity (assessed via predicted APHV) in adolescent girls was associated with less accelerometer assessed moderate-to-vigorous physical activity and this association was partly explained (mediated by) lower perceptions of body attractiveness and physical self-worth. These findings suggest that perceptions of pubertal change may be as, if not more, important than change itself and should be considered within interventions to enhance physical activity.

In a unique paper using data from the Avon Longitudinal Study of Parents and Children (ALSPAC) and UK Biobank, Elhakeem and colleagues (Elhakeem et al. 2020) consider the impact of pubertal timing upon physical activity during adolescence and in middle-aged and older-aged adults (40–70 years). Adjusting for age, education, and lifestyle factors, later pubertal onset (i.e. late maturity) was associated with greater accelerometer assessed total physical activity in girls at 14, but not 16 years of age. Pubertal timing was unrelated to both total and moderate-to-vigorous physical activity in boys at both 14 and 16 years of age. Pubertal timing had no significant association with physical activity in mid-older age suggesting that other factors may be more important to consider at these stages of life.

An ongoing challenge in practice and in applied research is the assessment of biological maturity. Two studies in the supplement, one by Parr et al. 2020 and the other by Teunissen et al. 2020, investigate the accuracy of non-intrusive prediction equations based on anthropometric measurements. Parr and colleagues compared the maturity offset method (Mirwald et al. 2002) to predict age at APHV and the three protocols to predict the percentage of mature adult height (Khamis and Roche 1994) with observed APHV (criterion) in 23, 13-year-old male football (soccer) academy players. Results showed that the prediction methods accurately assigned 65%–94% of players, with the predicted percentage of adult stature window method being the most accurate. Teunissen et al. compared three methods to predict APHV, the aforementioned maturity offset method (Mirwald et al. 2002), in addition to the methods by Fransen et al. (2018) and Moore et al. (2015) among 32 elite male youth football (soccer) players from the Netherlands. Results showed that the prediction methods did differ from attained APHV, and thus the authors stressed caution when using these methods. While these analyses need to be replicated in larger samples which include females and a wider age range, both contributions highlight the utility but also the challenges in accurately predicting maturity within a practical and applied (non-clinical) setting.

As an entity, this special issue highlights that variation in biological maturity likely plays a role in adolescent disengagement from physical activity and certainly impacts selection and sporting performance among adolescent athletes. However, the measurement of biological maturity in the field settings remains a challenge and the lack of standard measures and/or harmonisation of physical activity and biological maturity data hinders comparison across studies/populations. We hope this special issue will stimulate a dialogue in the academic field but also in practice (e.g. elite youth sport setting) about the need to truly understand, alongside psycho-social and behavioural factors, the human biological influences on physical activity and sports performance/opportunities during the important and developmentally unique period of adolescence.

Disclosure statement

No potential conflict of interest was reported by the author(s).