The influence of task constraints on football players' performance: non-linear trends during goalkeeper training

In the realm of sports, task constraints shape players' adaptation processes and serve as crucial regulators of performance. While previous studies have examined the influence of task constraints on performance using simple linear models, potential non-linear relationships remain poorly understood, especially in highly complex training contexts such as football goalkeeping. Our study involved monitoring the performance coefficients of six female youth goalkeepers across 22 training tasks, which were characterized by eight task constraints encompassing interactions among players and properties of the training environment. We used Generalized Additive Mixed Models (GAMMs) to examine potential non-linear relationships between task constraints and goalkeeper performance. GAMMs revealed significant non-linear effects for several task constraints, i.e., the number of players, field dimensions, spatial degrees of freedom and playing time (edf = 2.1-3.3, P < 0.001), explaining 20.8-30% of deviance. Landscape representativity showed a significant overall effect on performance (P = 0.003). The results indicated that task constraints may influence goalkeepers' performance with potential threshold-like effects and, thus, the manipulation of task constraints during training might be partially context-dependent. Overall, the results advocated for a constraint-led approach that facilitates representative training designs high in player interactions to devise impactful training environments for players' skill development.