Apportionment of lumbar L2-S1 rotation across individual motion segments during a dynamic lifting task

Segmental apportionment of lumbar (L2-S1) rotation is a critical input parameter for musculoskeletal models and a candidate metric for clinical assessment of spinal health, but such data are sparse. This paper aims to quantify the time-variant and load-dependent characteristics of intervertebral contributions to L2-S1 extension during a dynamic lifting task. Eleven healthy participants lifted multiple weights (4.5, 9.1, and 13.6 kg) from a trunk-flexed to an upright position while being imaged by a dynamic stereo X-ray system at 30 frames/s. Vertebral (L2-S1) motion was tracked using a previously validated volumetric model-based tracking method that employs 3D bone models reconstructed from subject-specific CT images to obtain high-accuracy (≤0.26°, 0.2 mm) 3D vertebral kinematics. Individual intervertebral motions as percentages of the total L2-S1 extension were computed at each % increment of the motion to show the segmental apportionment. Results showed L3-L4 (25.8±2.2%) and L4-L5 (31±3.1%) together contributed a larger share (∼60% combined) compared to L2-L3 (21.7±3.7%) and L5-S1 (22.6±4.7%); L4-L5 consistently provided the largest contribution of the measured segments. Relative changes over time in L3-L4 (6±12.5%) and L4-L5 (0.5±10.2%) contribution were minimal; in contrast, L2-L3 (18±20.1%) contribution increased while L5-S1 (-33±22.9%) contribution decreased in a somewhat complementary fashion as motion progressed. No significant effect of the magnitude of load lifted on individual segmental contribution patterns was detected. The current study updated the knowledge regarding apportionment of lumbar (L2-S1) motion among individual segments, serving both as input into musculoskeletal models and as potential biomechanical markers of low back disorders.