Functional electrical stimulation (FES) in stroke rehabilitation is hindered by early patient fatigue. Therefore the study of FES-elicited fatigue is needed to optimise its therapeutic impact. In this study, the reliability of evoked electromyography (EMG) median frequency (MDF) and peak-to-peak (PTP) fatigue indices were investigated during stimulation of the forearm. During isometric constant stimulation the MDF-time and PTP-time waveforms exhibited negative gradients in the ranges (−1.3 to −3.6) Hz · s−1 and (−0.013 to −0.016) V · s−1 respectively, most with coefficients of determination (R2) greater than 0.83. During vEMG-controlled stimulation the MDF-contraction waveforms exhibited negative gradients in the range (−2.4 to −9.0) Hz · Cont−1 while the PTP gradients ranged from (0.0070 to −0.019) V · Cont−1. For these datasets the MDF waveforms’ R2 > 0.62 and most PTP waveforms’ R2 < 0.35. These findings suggest that the MDF and PTP amplitude are suitable fatigue indices during isometric constant stimulation but the PTP-based index becomes unreliable during vEMG-controlled stimulation.