Arm elevation involves changes in the whole spine: an exploratory study using EOS imaging

Abstract

Background Few studies have assessed the participation of the spine in arm elevation. The primary aim of this exploratory study was to specify spinal movements during unilateral arm elevation. Methods We used an EOS imaging system to assess 2D global posture (Sagittal Vertical Axis [SVA], T1 and T9 tilt and Central Sacral Line [CSL]) and segmental spine curves (C3-C7 in the sagittal plane only, and T1-T6, T7-T12 and L1-L5 in the sagittal and frontal planes) for four different left arm elevation levels: in the sagittal (Sa) plane (30°Sa: reference position, 140°Sa and 180°Sa), and in the scapular (Sc) plane (180°Sc), in ten right-handed asymptomatic participants (5 women; mean age 24.6 SD 3.0 years]. In addition, we estimated C1, head and pelvic orientation and head and pelvic linear displacement. We used Bayesian statistics (BF 10 > 3 indicates a significant variation: moderate, strong, very strong or extreme evidence). Results From 140°Sa to 180°Sa or Sc, the significant decrease in SVA and the T1-T9 tilt angles indicated a global backward spine bending (moderate to very strong evidence). The significant reversal of the C3-C7 lordosis at 30°Sa (-1.34 [2.53]°) to kyphosis at 180°Sa (13.88 [3.53]°, strong evidence) and 180°Sc (11.85 [2.75]°, extreme evidence) and the significant decrease in the T7-T12 kyphosis (26.58 [2.84]°at 30°Sa to 16.40 [2.65]° at 180°Sa and 17.60 [2.78]° at 180°Sc [all extreme evidence]) showed a global spine straightening. We found significant pelvic anteversion between 30°Sa and 140°Sa (moderate evidence) and persistent right spine bending and leftward head displacement (extreme evidence). The change in C1 orientation (extreme evidence) showed an atlanto-occipital extension. Conclusion Simple unconstrained movements of unilateral arm elevation involve the whole spine, pelvis and head, including significant backward spinal bending, a reduction in the low cervical spine lordosis and the thoracic kyphosis, and atlanto-occipital extension. Supplementary Information The online version contains supplementary material available at 10.1186/s12891-024-08093-7.