EXcellence and PERformance in Track and Field (EXPERT)—A Mixed-Longitudinal Study on Growth, Biological Maturation, Performance, and Health in Young Athletes: Baseline Results (Part 2)

<b>Background</b>: The athletic potential of young athletes is shaped by individual and environmental factors. <b>Objectives</b>: This study examines the physical growth, body composition, biological maturation, motivation, perseverance, physical performance and contextual factors of young male and female track and field athletes. <b>Methods</b>: A total of 425 (224 girls) track and field athletes were recruited and divided into five age cohorts (10, 11, 12, 13, and 14 years respectively). Measurements were assessed across (i) individual (anthropometry, body composition, biological maturation, motivation, and perseverance), (ii) performance (motor performance), and (iii) club context domains. Data analysis used descriptive statistics for clubs' characteristics, a two-factor ANOVA for anthropometry, body composition, biological maturation, and performance and an ANCOVA for motivation and perseverance. All analyses used STATA 18.0. <b>Results</b>: Sex-related differences were identified in physical growth, maturation, psychological, and performance variables during adolescence. Girls reached their peak height velocity (PHV) around 12 years of age, compared to 14 years in boys. At all ages (except at age 11), girls had higher body fat, and at age 12 were taller and outperformed boys in right-handgrip strength and in sprint (30 m and 40 m). From age 13 years onwards, boys became taller, with greater leg length, greater fat-free mass, and superior results (<i>p</i> < 0.05) in most performance tests. Psychologically, girls reported higher levels of interest-enjoyment, effort-importance, relatedness, and perceived choice; no sex differences were found in perseverance. The clubs involved were of small size, with developing, yet qualified, coaches, with limited support staff and infrastructure. <b>Conclusions</b>: Clear sex differences in physical growth, psychological, and performance variables emerged during adolescence, and were related in part to earlier maturation in girls. Further, there was variation in clubs' infrastructure and staff that may potentially influence track and field athletes' growth and development.