In this paper, we demonstrate that a musculoskeletal model with an anatomically-inspired abdomen model can estimate human-like abdominal muscle tension, in contrast to previous musculoskeletal models or models based on joint elasticity. We first model the abdominal structure based on anatomical knowledge. The anatomically-inspired abdomen model consists of a Tendinous Intersection model that connects the muscle bellies of Rectus Abdominis and represents its pathway, a Rectus Abdominis model that represents the muscle contraction part, and a balloon-type abdominal cavity model that represents the volumetric abdominal cavity. These models compute the intra-abdominal pressure and its effect on the lumbar vertebrae joint torques. We use these models with our musculoskeletal model to estimate the abdominal muscle tension during sitting up motion with optical motion capture, inverse kinematics, and inverse dynamics computation. Our computational results show that our abdomen model estimates the muscle tension with a waveform similar to the muscle activity measured by electromyography, while the previous musculoskeletal model and the joint elasticity model estimate obviously different muscle tension patterns. These results imply that the intra-abdominal pressure is critical in estimating the abdominal muscle tension rather than the inertial properties and the joint elasticity.