The lower limb of a paraplegic patient can be analyzed as a dynamically determinate system, since the muscles there are isolated from voluntary control. Hence, when activated by functional electrical stimulation (FES), the only nonzero active muscle forces are those of the actually stimulated muscles. This unique situation allows the calculation of the muscle force from the externally measured torques and the correlation of this direct muscle output to parameters of another nature, such as metabolic or myoelectric. In the present work we have studied the relation between force and myoelectric activity of the FES-activated quadriceps muscles of paraplegics during the course of fatigue. Stimulation was provided by means of surface electrodes. The electromyographic (surface EMG) activity was represented by the peak-to-peak (PTP) amplitude of the M-wave signal obtained. It was important to suppress the stimulus artifact preceding the EMG signal and we, therefore, designed an amplifier capable of monitoring the artifact-free signal. The surface EMG and the force were sampled on line for analysis. The PTP amplitudes were correlated with the corresponding force values, simultaneously measured and a parallel decay in these two parameters was noticed. An exponential curve was found adequate to express the relationship between force and EMG PTP. The results obtained indicate the conditions under which surface EMG can be used to noninvasively monitor the quadriceps muscle fatigue during stimulation.