Trunk biomechanics during maximum isometric axial torque exertions in upright standing

Activities involving axial trunk rotations/moments are common and are considered as risk factors for low back disorders. Previous biomechanical models have failed to accurately estimate the trunk maximal axial torque exertion. Moreover, the trunk stability under maximal torque exertions has not been investigated.

A nonlinear thoracolumbar finite element model along with the Kinematics-driven approach is used to study biomechanics of maximal axial torque generation during upright standing posture. Detailed anatomy of trunk muscles with six distinct fascicles for each abdominal oblique muscle on each side is considered. While simulating an in vivo study of maximal axial torque exertion, effects of antagonistic coactivities, coupled moments and maximum muscle stress on results are investigated.

Predictions for trunk axial torque strength and relative muscle activities compared well with reported measurements. Trunk strength in axial torque was only slightly influenced by variations in coupled moments. Presence of abdominal antagonistic coactivities and alterations in maximum strength of muscles had, however, greater effect on maximal torque exertion. Abdominal oblique muscles play crucial role in generating moments in all three planes while back muscles are mainly effective in balancing moments in sagittal/coronal planes. Trunk stability is not of a concern in maximum axial torque exertions nor is it improved by antagonistic abdominal coactivities.

In contrast to previous biomechanical model studies, the Kinematics-driven approach accurately predicts the trunk response in maximal isometric axial torque exertions by taking into account detailed anatomy of abdominal oblique muscles while satisfying equilibrium requirements in all planes/directions. In maximal torque exertions, the spine is at much higher risk of tissue injury due to large segmental loads than of instability.