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

Background: 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. Purpose: This study aimed to provide evidence of construct validity and score reliability for the MOBAK 3–4 in a Portuguese sample. Study Sample: 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. Results: 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. Conclusions: 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.