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

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 (BF₁₀ > 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.