Physical Activity, Physical Fitness, and Motor Competence in Children Aged 5–10 Years—A Systematic Review

Background: Motor competence (MC), physical activity (PA), and physical fitness (PF) are interrelated components of child development, yet evidence on the MC–PF–PA triad in middle childhood remains inconsistent due to methodological heterogeneity. Objective: To synthesise associations between MC, PF, and PA in children aged 5–10 years (2020–2025) and to appraise methodological quality and certainty. Methods: A PRISMA 2020-compliant systematic review (PROSPERO: CRD42024617560) searched six databases (PubMed, Scopus, Web of Science, SportDiscus, SciELO, PsycINFO) from January 2020 to April 2025, updated November 2025. Eligible observational studies in English reported quantitative associations between ≥2 constructs in 5–10-year-olds using validated instruments. Risk of bias was appraised in duplicate with JBI checklists. Quantitative pooling was unfeasible under a pre-specified pooling matrix; associations were synthesised narratively and certainty rated using a GRADE-style framework. Results: Thirteen studies (n = 43–1064; 11 cross-sectional, 2 longitudinal) were included; six were low, five moderate, and two high risk of bias. MC–PA was predominantly positive, especially with objective instruments; MC–PF was consistently positive; PF–PA was less consistent but trended positive longitudinally. Mediation/moderation analyses (k = 3) supported roles for perceived competence, self-efficacy, and PF. Null/negative findings clustered among subjective PA measures and higher risk of bias. GRADE certainty was low-to-moderate. Conclusions: MC, PF, and PA are interconnected in middle childhood, but cross-sectional predominance and instrument heterogeneity preclude causal inference; findings are provisional and should drive longitudinal, methodologically standardised research.