Global spinal mobility shows no consistent association with trunk strength in children: a school-based cross-sectional study

Background Given the high prevalence of low back pain from adolescence, this study examined whether global spinal mobility is associated with trunk strength in healthy schoolchildren. Objective To quantify global spinal mobility and movement speed, trunk extensor strength, and associated pain, and to examine their relationships by educational stage and sex. Methods In this cross-sectional study, 226 schoolchildren aged 9–16 years completed, in a single Physical Education session, anthropometric measurements, spinal mobility and speed assessments with an inertial sensor, the Biering-Sørensen test, an isometric deadlift with dynamometry, and the Wong-Baker pain scale. Z -scores for mobility, speed, and strength were analyzed using two-factor ANOVAs, simple and partial correlations, and linear regression, reporting effect sizes, 95% confidence intervals, and variance inflation factors. Results Secondary school students showed greater strength than primary school students (strength index: p < 0.001), with no effect of sex; mobility did not differ between stages but was greater in girls ( p = 0.002), and speed showed a Stage × Sex interaction ( p = 0.002). Mobility was moderately associated with speed, particularly in primary school, whereas correlations with the composite strength index were non-significant in three of the four subgroups. Secondary school boys showed a small negative association ( r = −0.25; 95% CI: −0.45 to −0.03; p = 0.02), driven mainly by Biering-Sørensen time ( r = −0.38; p < 0.01); after anthropometric adjustment, it was no longer significant for the composite index (partial r = −0.22; p = 0.06) but persisted for Biering-Sørensen time (partial r = −0.31; p = 0.01). The mobility–pain relationship was weak, and perceived discomfort was generally low. Conclusions Global spinal mobility showed no consistent cross-sectional association with trunk strength in healthy schoolchildren; the only association, observed in secondary school boys, was small and, after adjustment, persisted only for Biering-Sørensen time. Factors not assessed here, such as neural or coordinative aspects, might contribute to this pattern, but this hypothesis requires further investigation. The protocol proved feasible in Physical Education settings, although its screening accuracy has not yet been established.