Trunk rotation coordination in front crawl: which segment leads? Three coordination strategies and their dynamic changes across 400-m split progression in elite swimmers.

Objective: To identify upper-trunk-pelvic roll coordination strategies during 400-m front-crawl swimming and examine their kinematic characteristics, segment-wise distribution, and associations with swimming speed. Methods: Twenty-two elite male swimmers completed a maximal 400-m front-crawl trial while two inertial measurement units recorded upper-trunk and pelvic roll. Cycle-level cross-correlation lag was used to classify coordination into upper-trunk-leading (UL), pelvic-leading (PL), and near-synchronous (NS) strategies. Kinematic differences among strategies and changes in strategy probability across 50-m segments and swimming speed were examined using Bayesian multilevel models. Results: Across 2,796 valid roll cycles, NS was the most frequent strategy (53.04%), followed by PL (25.86%) and UL (21.10%). Strategy use varied substantially between swimmers. UL showed greater torso twist range than NS (NS-UL = -0.71, 95% CrI: -0.96 to -0.42) and greater torso twist velocity than both NS (NS-UL = -0.61, 95% CrI: -0.95 to -0.26) and PL (UL-PL = 0.62, 95% CrI: 0.19-1.04). In contrast, upper-trunk and pelvic roll ranges were larger in NS and PL than in UL, with no clear difference between NS and PL. Faster within-athlete speed increased the odds of UL relative to both NS (OR = 2.38, 95% CrI: 1.76-3.26) and PL (OR = 2.07, 95% CrI: 1.31-3.33). No clear segment effects or between-athlete mean speed effects were observed. Conclusion: Elite 400-m front-crawl swimmers used three distinct upper-trunk-pelvic lead-lag strategies, with marked inter-individual variability. NS was most common, whereas UL was more likely at faster within-athlete speeds. These findings suggest that trunk coordination in elite swimmers is individualized rather than organized around a single optimal pattern.