The purpose of this study was to document the difference between eccentric and concentric fatigue of the quadriceps femoris muscle during a maximal effort 40 repetition bout at 180 degrees /sec. Sixteen female volunteers aged 22 to 32 years were tested twice, at least 7 days apart. The left lower extremity of each subject was used during testing on the Kin-Com(R) dynamometer. Subjects were tested through a range of motion of 0 to 90 degrees of knee flexion while seated. Percent decline in peak torque production was then calculated. The results demonstrated that peak torque production during the 40 repetition bout decreased an average of 0.3% for the eccentric trial and an average of 47.7% for the concentric trial. A significant difference in percent decline in peak torque production between concentric and eccentric trials was found to exist (p < 0.0001). The findings of this study indicate that eccentric training may require the performance of many more repetitions than concentric training in order to obtain muscle fatigue. J Orthop Sports Phys Ther 1989;10(8):309-314.