This paper is concerned with the design of an application-oriented control law for a biomechanical model of people living with spinal cord injury sitting stability. A major difficulty is to stabilize the resulting open-loop unstable nonlinear system with a delayed control input. A solution is proposed based on Padé approximation of delays to obtain a set of design conditions in the form of linear matrix inequalities which can be effectively solved by convex optimization techniques. Numerical simulations are conducted to show the effectiveness of the proposed control law compared to previous works.