Recent mathematical nonlinear models allow to predict the muscular response to functional electrostimulation (FES) in the isometric case or non-isometric case, based on the seminal work of V. Hill. Physical controls are Dirac pulses and this leads to a sampled-data control system, where the output is the force response. In this article, our aim is to provide a finite dimensional approximation of the force response for the non-fatigue dynamics to provide fast optimizing schemes for ongoing device, in particular the design of a smart electrostimulator for muscular reinforcement or rehabilitation or to track a reference trajectory in the non-isometric case.