This paper introduces a simplified nonlinear model to study sitting control for persons with Spinal Cord Injury. The system obtained via Lagrangian techniques is defined with a nonlinear descriptor form and written under an exact Takagi-Sugeno model. In order to stabilize this class of system that includes a time-varying input delay, a state feedback control extended from robust control theory is derived. Then, the main goal consists in being able with an Unknown Input Observer to estimate the state as well as the internal delayed control force applied.