The 'road to gold' in open-water swimming: insights on performance and stroke mechanics regulation from longitudinal race analysis of the 5-km 2024 world champion.

Open-water swimming (OWS) performance relies on reshaping stroke mechanics as race demands escalate towards a decisive end-spurt. How world-class swimmers implement this high-grading strategy in competition remains unknown. This study establishes the first biomechanical framework of a World Championship (WCH) victory by integrating inter-swimmer benchmarking, longitudinal intra-swimmer profiling, and 3D kinematic variabilities analysis of the 2024 5-km WCH winner (FONLO). FONLO and seven elite swimmers were monitored over 2022-2024 during 15 international races using a sacrum-mounted inertial sensor. Cycle-to-cycle stroke rate (SR) and jerk cost (JC) were modelled with locally estimated scatterplot smoothing to derive an international top-4 stroke mechanics template and to profile FONLO's races, including end-spurt dynamics. During the WCH win, intra- and inter-cyclic 3D kinematic variabilities were quantified via double functional decomposition. The WCH victory accentuated the top-4 J-shaped dynamics, with delayed time above mean SR and JC and greater modulation ranges across race phases. Maximal stroke mechanics variability characterised the win, including a 340% JC surge triggering a 27-s sprint. Kinematic modelling revealed stable intra-cyclic patterns with breathing-related asymmetries and increased inter-cyclic variability in the final lap. World-class OWS success emerges from precisely timed stroke regulation and controlled variability, preserving smoothness until an explosive final sprint.