Overweight, obesity, and sex differences in the acquisition of aquatic motor competence among 4–5-year-old preschool children: a quasi-experimental study

Background Childhood overweight and obesity are associated with impaired motor competence, yet evidence on how weight status relates to structured aquatic motor skill learning in preschoolers—and whether sex moderates this relationship—remains scarce. This study compared changes in aquatic motor competence between overweight/obese and healthy-weight 4–5-year-old preschoolers following a common 8-week structured aquatic programme, and examined sex differences. Methods Sixty 4–5-year-old children (30 overweight/obese, 30 healthy-weight; 15 boys and 15 girls per group) without prior systematic aquatic exposure were recruited from two kindergartens in Taiyuan, China. Weight status was classified using the Chinese Growth Standard for Children Under 7 Years (WS/T 423–2022) and WHO criteria. All children completed a 16-session, 8-week structured aquatic motor skill programme (45 min/session, 2 sessions/week, 1:5 instructor-to-child ratio). Aquatic motor competence was assessed pre- and post-intervention using the Actual Aquatic Skills Test (AAST; 17 items, range 17–51). The primary analysis examined differential change over time using a 2 (Group) × 2 (Sex) × 2 (Time) mixed-design ANOVA; estimated mean differences are reported with 95% confidence intervals (CIs), and effect sizes as Cohen's d and partial η 2 . A 2 × 2 factorial ANOVA on post-intervention scores was retained as a secondary analysis. Results Pre-intervention AAST scores did not differ between weight-status groups (t = −0.76, p = 0.448) or between boys and girls (t = −0.94, p = 0.353). The Group×Time interaction was significant (F(1,58) = 31.88, p < 0.001, partial η 2 = 0.355): healthy-weight children showed greater mean improvement than overweight/obese children (mean Δ 27.50 vs. 23.40 points; mean difference 4.10, 95% CI 2.65–5.55; Cohen's d = 1.46, 95% CI 0.89–2.03). The Sex×Time interaction was also significant (F(1,58) = 5.15, p = 0.027, partial η 2 = 0.082): girls showed greater mean improvement than boys (mean Δ 26.43 vs. 24.47; mean difference 1.96, 95% CI 0.23–3.70; Cohen's d = 0.59, 95% CI 0.07–1.11). The Group×Sex interaction on post-intervention scores was not significant (F(1,56) = 0.22, p = 0.645, partial η 2 = 0.004; interaction contrast on change scores 0.34 points, 95% CI −1.10 to 1.78). Conclusion In this convenience sample of Chinese preschoolers, healthy-weight children and girls showed greater mean improvement in aquatic motor competence than overweight/obese children and boys, respectively, following a common structured aquatic programme. Because the design lacked a non-intervention control group, these findings are associative and do not establish that weight status caused the differential response. Longer and more individualised aquatic curricula, potentially combined with nutritional and psychological support, warrant prospective evaluation in randomised trials for overweight/obese preschoolers.