Muscle fatigue due to functional electrical stimulation still prevents its widespread use as a gait rehabilitation tool for Spinal Cord Injured subjects. Although there is an active research towards optimization of pulse parameters to delay muscle fatigue, changes in stimulated muscle’s performance during repeated contractions due to fatigue have not been yet determined. A study of muscle fatigue of knee extensors and flexors of incomplete SCI during isometric contractions is presented. The outcome of this study is the proposal of objective criteria for detecting muscle fatigue from initial changes has been extracted. Also a comparison between flexors and extensors knee muscle fatigue models is given.