Supporting player development during the youth-to-senior transition process: A mixed-methods case study of a complex adaptive system in professional soccer

Through a Complex Adaptive Systems (CAS) framework, this study aimed to [1] quantify the seasonal physical loading of a Major League Soccer (MLS) homegrown player (age = 18 y; stature = 1.73 m; body mass = 68.7 kg; position = fullback) undergoing the youth-to-senior transition and [2] explore club sport scientists’ perspectives on the working dynamics of the player's training program during the transition process. A mixed-methods single case study design was adopted, integrating external load data (total duration of activity, total distance covered, and distance running at 14.4–19.7 km/h, high-speed running at 19.8–25.1 km/h, and sprinting at >25.1 km/h) from both the player's club and national federation using global positioning systems (GPS) technology, alongside semi-structured interviews with sport science practitioners from the player's MLS organization. Descriptive statistics were used to summarize the GPS data, while hierarchical content analysis was used to formulate categories. Findings addressed relevant features of a CAS within this youth-to-senior transition process, detailing a complex developmental pathway to first team (FT) soccer. The player's physical loading exhibited large variability during the preseason (Coefficient of Variation; CV = 19–58%) and in-season (CV = 25–50%) phases, reflecting the influence of contextual factors such as squad-shifting, international call-ups, and extensive travel when oscillating between disparate training environments. Practitioners also highlighted the dual role of the reserve team (RT), providing the player consistent competitive match exposure to support his individual development. Overall, this study demonstrates the value of a holistic, systems-level approach to better understand optimizing long-term player development initiatives.