Mathematical models have been developed to predict fatigue during functional electrical stimulation, but the predictive accuracy at different muscle lengths is unknown. The objectives of our study were to: (1) experimentally determine the relationship between knee extension angle (20 degrees, 40 degrees, 65 degrees, and 90 degrees) and fatigue of the quadriceps muscles, and (2) predict that relationship using a mathematical model. A computer-controlled stimulator sent trains of pulses to surface electrodes on the thighs of five subjects while forces were measured at the ankle. A two-component mathematical model was developed. One component accounted for force, and the other accounted for fatigue. The model was fit to the data, and parameters were identified at 90 degrees . The fitted subject-averaged r(2) value was 0.89. The model was used to predict fatigue at the remaining angles, and the subject-averaged r(2) values were >0.75. Therefore, at least 75% of the variability in the measurements was explained by the model. The force model is explicitly dependent on angle, and the fatigue model is explicitly dependent on force; therefore, the dependence of fatigue on knee angle was implicit.