Introduction: The purpose of this study was to propose a method that allows extraction of the current muscle state under electrically induced fatigue. Methods: The triceps surae muscle of 5 subjects paralyzed by spinal cord injury was fatigued by intermittent electrical stimulation (5x5 trains at 30Hz). Classical fatigue indices representing muscle contractile properties [peak twitch (Pt) and half relaxation time (HRT)] were assessed before and after each 5-train series and were used to identify 2 relevant parameters (Fm, Ur) of a previously developed mathematical model using the Sigma-Point Kalman Filter. Results: Pt significantly declined during the protocol, while HRT remained unchanged. Identification of the model parameters with experimental data yielded a model-based fatigue assessment that gave a more stable evaluation of fatigue than classical parameters. Discussion: This work reinforces clinical research by providing a tool that clinicians can use to monitor fatigue development during stimulation.