Background: Muscular fitness and technical ability are key determinants of swimming performance. Several mechanical, physiological, and morphological factors have been associated with 400-m swimming performance. However, most previous investigations have focused on adult swimmers, limiting our understanding of the relative contribution of these factors to performance in young swimmers. Aim: To investigate the influence of mechanical, physiological, and body composition parameters on 400-m swimming performance in young swimmers using a stepwise multiple regression approach. Methods: Thirty-two swimmers (13 female; 347 ± 105 WA points; age: 15 ± 3 years) performed a maximal 400-m effort (t400). Stroke mechanical parameters (stroke frequency [SF] and distance per cycle [DpC]) were determined using kinematic analysis. Physiological parameters included peak oxygen consumption (VO*peak) and anaerobic contributions (alactic, and lactic [AnLa]), determined using the backward extrapolation technique and blood lactate concentrations. Body composition was assessed using dual-energy X-ray absorptiometry (fat-free mass [FFM], fat mass, and bone mineral content and density). A backward stepwise regression model was used to examine the relationships between t400 (dependent variable) and the mechanical, physiological, and body composition variables. Results: The final model explained 79% of the t400 and included SF (0.8 ± 0.1 Hz; *s = *0.431), DpC (1.5 ± 0.3 m; *s = *0.603), VO*peak (3.3 ± 1.0 L·min*¹; *s = *0.260), and AnLa (1.1 ± 0.7 L; *s = *0.226). The model showed no evidence of multicollinearity (VIF < 2.224), heteroscedasticity, influential outliers, or biased residuals. Conclusions: These findings suggest that faster swimmers are characterized by the ability to maintain greater DpC at higher SF, combined with superior aerobic power and anaerobic lactic capacity. However, the relatively low performance level and high variability of the sample limit the generalizability of these findings to more competitive or homogeneous populations
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