A matter of time or timing: longitudinal analysis of sensitive periods for linear running speed performance and development in elite youth soccer.

The objective of the study was to characterize variability in the development of maximal running speed among elite youth soccer players by evaluating its non-linear trajectory relative to both chronological age and biological maturation, and to determine how these trajectories vary relative to estimated maturity timing. A mixed-longitudinal sample of 117 male youth soccer players (10.7-15.9 years, 461 observations) was considered. Maximal running speed was measured via a 20-meter flying sprint. Biological maturity status was estimated as the percentage of predicted adult height (%PAH), and players were classified as early, average, or late maturing. Non-linear developmental trajectories were analyzed using Generalized Additive Mixed Models. Relative to estimated biological maturity status, a consistent peak rate of speed improvement at ~94.4% PAH was noted and it was preceded by a phase of accelerated improvement between ~87.6-90.7% PAH. However, relative to chronological age, the timing of the accelerated developmental phase was related to estimated maturity timing; early and average maturing players experienced this interval of accelerated gains at a significantly younger age than late-maturing peers, who showed no sustained period of acceleration. Maximal running speed performance in youth soccer players is related to biological maturity status rather than chronological age. The identified phases of accelerated improvement represent developmentally meaningful periods that may help contextualize individual differences in the development of sprinting performance and support maturity-based approaches to long-term athlete development, talent identification, and performance monitoring.