Muscle fatigue is a complex phenomenon that limits the application of functional Electrical Stimulation (ES), used to activate skeletal muscle in order to perform functional movements. The purpose of the present study was to track the development of neuromuscular fatigue under intermittent FES applied to the triceps surae muscle of 5 subjects paralyzed by Spinal Cord Injury (SCI). Experimental results gave evidence of neuromuscular fatigue development attributed to muscle contractile properties impairment. Classical parameters representing muscle contractile properties (peak twitch, Pt and twitch contraction and relaxation parameters) significantly decreased at the end of the protocol. These experimental data were used to identify the parameters of a previously developed physiological mathematical model describing all possible contractive states occurring in a stimulated muscle. The sigma-point Kalman filter was used for the identification of the model's parameters and simulation results prove that the model was capable to track fatigue and under the present stimulation conditions even predict muscle contractile behavior. This work reinforces clinical research with a tool allowing clinicians to monitor the current state of the stimulated muscle for its optimal solicitation.