This dissertation describes a research effort undertaken to study the intra- and inter-individual differences in the fatigue characteristics of quadriceps muscles using EMG power spectrum analysis. Specifically, the relationships between the temporal shift of the mean power frequency (MPF) and certain normalized load levels are investigated. The MPF linear decay expressed as the ratio of MPF linear slope to the initial MPF is considered as quantitative expression of fatigue. The actual applied force, the ratio of force to MVC, the ratio of force to body weight, and the ratio of force to estimated quadriceps muscle mass are considered as the muscle load parameter. Fourteen healthy young male subjects in the age range of 21-34 participated in the experiment requiring application of knee extension type of exercise at four different muscle load levels. The model relating the MPF linear decay to the load level normalized by the estimated quadriceps muscle mass is found to best describe the fatigue process in all the four load levels considered for this study. The intra-individual differences are found to be not significant (level of significance = 0.05) for the four models considered. However the inter-individual differences are found to be significant. The results also indicate that the fatigue rate varies over time during the load application. The fatigue rate of the rectus femoris part of the muscles is found to be at a higher rate than the other parts of the quadriceps muscle. Further, it is indicated that the initial MPF is independent of the load levels and test days.