Evaluating the influence of trunk intra-muscular and intra-abdominal pressure on spinal geometric compensation.

Biomechanical modelling studies have revealed the impact of passive mechanical properties of spinal soft tissues on spinal configuration. This study extends prior work by evaluating the involvement of trunk abdominal and intramuscular pressure (IMP), on spinal geometric compensation, using a validated finite element spine model. The model included the vertebrae, rib cage, IVD, pelvis, ligaments, abdominal cavity and abdominal and spinal muscles. Over a fixed pelvis, the model underwent a 60° forward flexion. Muscles and the abdominal cavity were modelled as fluid-filled solid entities containing hydrostatic pressure elements, enabling IMP and intra-abdominal pressure (IAP) quantification. Changes in lumbar segmental rotations, spine range of motion (RoM) and curvature (thoracic kyphotic (TKA) and lumbar lordotic angle (LLA)) were analyzed following a 1) 10-fold increase and decrease in paraspinal IMP and a 2) 20-fold increase in IAP, relative to the validated model. A 10-fold increase in paraspinal IMP or 20-fold increase in IAP decreased lumbar ROM by a maximum of 8.4° and increased the TKA and LLA by a maximum of 5.8° and 4.7°, respectively during forward flexion. Heightened IAP correlated with decreased paraspinal IMP. Conversely increased paraspinal IMP correlated with IAP reductions. This investigation showed a synergistic interplay between paraspinal IMP and IAP on segmental mobility and spine geometry. The shared influence may suggest a clinical impact of targeting both muscle groups in scenarios involving lower back dysfunction.