Trunk Muscle Performance, Spinal Alignment, and Postural Profiles in 9–11-Year-Old Children Participating in Intensively Organized Sport Assessed with Non-Radiographic Tools

Comprehensive data on sagittal spinal alignment, regional mobility, trunk muscle strength and imbalances in 9–11-year-old children participating in intensively organized sport (IOS) are limited. This cross-sectional study evaluated the association between IOS participation and the aforementioned postural parameters, compared with a non-intensive comparison cohort. A total of 143 children were included (IOS: n = 66; controls: n = 77). Sagittal spinal curvatures (thoracic kyphosis and lumbar lordosis) and regional mobility were assessed using Spinal Mouse®, software-generated sagittal spinal curvature and trunk muscle imbalance profiles were assessed using Moti Physio 3D system, and absolute trunk muscle strength was measured using a K-Push dynamometer. Statistical analyses included multivariable ANCOVA, multivariable ordinal logistic regression, Fisher’s exact tests, and Benjamini–Hochberg (BH) adjustments, with significance set at p ≤ 0.05. In adjusted ANCOVA models, the IOS group demonstrated significantly higher absolute trunk flexor (20.1 ± 0.5 vs. 16.8 ± 0.5 kgf) and extensor (25.5 ± 0.5 vs. 22.7 ± 0.5 kgf) strength than controls (both p < 0.001), independent of demographic and anthropometric covariates. At the univariate level, the comparison group showed a significantly higher prevalence of software-classified weakness of the m. rectus abdominis (88.3% vs. 72.7%; BH-adjusted p = 0.05) and tightness of the m. psoas major (96.1% vs. 83.3%; BH-adjusted p = 0.05). In contrast, sagittal spinal curvatures and mobility did not differ between groups. After BH adjustment, m. quadratus lumborum differences showed only a non-significant trend (BH-adjusted p = 0.06). Structural parameters were mainly associated with biological factors: age showed an independent association with lumbar lordosis severity (BH-adjusted p = 0.03), whereas sex was independently associated with thoracic (BH-adjusted p = 0.01) and lumbar spine mobility (BH-adjusted p = 0.03). In summary, IOS participation in children aged 9–11 years is associated with greater absolute trunk strength, whereas structural sagittal spinal profiles and software-based muscle-imbalance classifications appear to be driven primarily by age and sex. These findings support integrating non-invasive posture screening with trunk strength assessment when monitoring young athletes and planning developmentally appropriate exercise interventions.