Background: Muscle fatigue is an important issue in neuromuscular rehabilitation. Better control of this phenomenon would result in better prevention of its consequent physiological damages.Objective: To provide a mathematical representation of muscle fatigue as a function of time.Methods: We conducted this study by combining the EMG-based estimation methods of muscle activation with the available muscle fitness equations describing the electrically evoked muscular contractions. Two groups of experiments were devised to produce a set of isometric and dynamic contractions in two hand muscles—biceps and deltoid—in a healthy man. The relevant surface EMG signals recorded simultaneously from the target muscles, provided the data needed for this process. Secondly, a number of EMG-based fatigue indices including peak to peak amplitude, root mean square values, average rectified values, number of zero crossings and mean frequency, were evaluated for the assessment of fatigue in the devised experiments. The mentioned indices were then, plotted as a function of the calculated fatigue, so that a mathematical representation of their relationship could be achieved.Results: The results showed an overall increase in fatigue index for both groups of contractions as time passed, and, as was expected, the calculated fatigue in dynamic experiments stood at lower levels, having some fluctuations, in comparison to the isometric ones.Conclusion: The mathematical relationships between the time and frequencydomain fatigue indices and the proposed index were compatible with the previous experimental observations. These findings could be applied for the assessment and control of muscle fatigue in FES-research studies.