MOBAK 3–4: Construct Validity and Score Reliability in an 8–10-Year-Old Portuguese Sample Within the Cascais Municipality

<b>Background:</b> Developing children's motor competence (MC) is central to fostering physical literacy and constitutes a core aim of high-quality physical education. Accurate and valid assessment tools are therefore essential. The MOBAK 3-4, following the MOBAK 1-2, was designed to assess basic motor competencies (BMC) in 8-10-year-olds. <b>Purpose:</b> This study aimed to provide evidence of construct validity and score reliability for the MOBAK 3-4 in a Portuguese sample. <b>Study Sample:</b> A total of 436 pupils (M = 9.4 ± 0.6 years; 53% boys) were assessed by trained test administrators with excellent inter- and intra-rater agreement. <b>Results:</b> Confirmatory factor analysis supported a two-factor correlated model-Object Movement (OM) and Self-Movement (SM)-including residual covariances between Dribbling-Running and Balancing-Jumping. Stepwise measurement invariance testing across sex supported partial thresholds and loadings invariance (Throwing and Running freed). Latent mean comparison indicated boys scored significantly higher in OM (d = 0.87 [0.86, 1.63]), but similarly in SM (d = -0.29 [-0.57, 0.06]) A Multiple Indicators Multiple Causes model with age evidenced the moderating effect of sex: age predicted higher OM and SM in girls, but negligible gains in boys. Score reliability was acceptable for OM (Ω = .69) but inadequate for SM (Ω = .39), limiting its interpretability as a stand-alone scale, particularly in girls. Regression-based OM and SM subscores are recommended over a single global index. <b>Conclusions:</b> MOBAK 3-4 is a feasible and psychometrically supported tool for assessing children's BMC. Results highlight age- and sex-specific patterns in MC, with implications for research, policy, and practice in physical education.