Finite element investigation of the intrinsic stiffness contribution of intra-abdominal pressure in a transient spine and trunk model.

Lifting velocity is a high-risk factor for low back pain in occupational settings and intra-abdominal pressure (IAP) is known to fluctuate transiently during lifting. The contribution of IAP toward spinal stability at different loading rates remains largely unaddressed in biomechanical models. This could provide insight into how lifting velocity can affect low back injury mechanics. A nonlinear, transient finite element model of the lumbar spine and surrounding anatomy was developed, including an anatomically accurate abdominal compartment. In combined loading, the spine was flexed to target in vivo intervertebral rotation patterns at 20° net L1-S1 rotation. Six load cases were then simulated transiently, driving extension back to standing posture at 80°/s, 40°/s, and 10°/s velocities, for IAP magnitudes of 0 mmHg and 75 mmHg. Statically, 75 mmHg IAP introduced a 19.9 Nm extension moment. Transiently, 75 mmHg IAP increased lumbar stiffness at all rates, with a maximum gain of 0.948 Nm/° (33%) at 80°/s. Without IAP, intrinsic stiffness was maximized at 40°/s, with 3.1 Nm/°. Between lift velocities, stiffness increased proportionally to velocity when calculated relative to the starting angle. The differences were small when offsetting the effect of faster baseline trajectories. An intrinsic stability potential for IAP was identified during lifting movement. Since faster lifting is induced by larger loads, the passive displacement-limiting gain of IAP holds greater weight with increasing velocity. Ineffective abdominal muscle activation to generate IAP or poor timing during a lift could increase low back injury risk due to insufficient lumbar spine stability.