Anti-directional segmental motion during flexion and extension in the adolescent thoracolumbar spine: a normative study of 166 participants.

PURPOSE: Anti-directional segmental motion has been documented in the cervical spine but never quantified in the thoracolumbar region. We aimed to determine its prevalence and determinants in adolescents and test its independence from coronal deviation.

METHODS

Idiag M360 data from 166 adolescents (103 males, 63 females; 14.4 ± 0.9 years) were retrospectively analysed. Segmental angles (Th1/2–L5/S1) in upright, flexion and extension were recorded. Anti-directional motion was defined as segmental movement opposite to the intended trunk direction. Bonferroni-corrected correlations, kyphosis tertile analysis, regression and segment-level frontal-plane comparisons were performed (STROBE).

RESULTS

All 166 participants exhibited anti-directional motion (mean 10.4 ± 2.9 segments). During flexion, 4.0 ± 1.9 segments were affected, almost exclusively thoracic. Thoracic flexion ROM was the dominant determinant (r = − 0.701; regression β = − 1.31, R² = 0.52, adjusted R² = 0.51). A sharp prevalence decline across the lower thoracic spine mapped the rib cage constraint gradient: Th8/9 (39%) → Th11/12 (1.2%), corresponding to the transition from false to floating ribs. Higher kyphosis was associated with more flexion paradox (H = 14.28, p < 0.001), with a threshold near 50°. No association with coronal deviation was found at any thoracic segment (all p > 0.14).

CONCLUSIONS

Anti-directional segmental motion is a universal feature of adolescent thoracolumbar kinematics. Its distribution maps the rib cage constraint gradient, and its magnitude is governed primarily by thoracic mobility and secondarily by sagittal curvature through an end-range positioning mechanism. These normative data provide a reference framework for future case–control studies investigating segmental motion sharing in adolescent spinal pathology.