Evaluation of trunk muscle forces and internal loads using kinematics-based modeling

Trunk muscle forces and internal loads are computed under simulated standing postures while carrying a load using a nonlinear finite element model of the T1-S1 spine with realistic nonlinear load-displacement properties. A novel kinematics-based algorithm is applied that exploits a set of a priori known spinal sagittal rotations to solve the redundant active-passive system. The loads consist of upper body gravity distributed along the spine plus 200 N held in hands either in front or on sides. Predictions are in good agreement with reported measurements of posture, muscle EMG and intradiscal pressure. Minimal changes in posture (posterior pelvic tilt and lumbar flattening) substantially influence muscle forces and internal loads. Placement of 200 N load in front of the body markedly increases the global muscle forces and internal loads which reach anterior shear and compression forces of ∼500 N and ∼1200 N, respectively, at lower lumbar levels. Coactivation in abdominal muscles (up to 3% maximum force) substantially increases extensor muscle forces and internal loads. The strength of the proposed model is in accounting for the full synergy by simultaneous consideration of passive structure and muscle forces under different postures and loads.