OBJECTIVE: To determine whether muscle fatigue affects knee joint proprioception. METHOD: Thirty healthy subjects (18 male and 12 female) with an average age of 22.1 years were selected for this study. Angular errors were measured to test propriocetive function in the knee joint. In a sitting position, the subject's dominant leg was passively positioned to the 45degrees knee flexion stimulus point for 2~4 second, while the subject concentrated on this position, the leg was passively returned to the 90o flexion position, afterward the subject was asked to duplicate the stimulus point actively. The difference, in degrees between the stimulus point and the reproduced point is an angular error. All subjects were blindfolded and underwent proprioception measurement before and after the muscle fatigue. Isokinetic exercises performed at 180 degree/sec on the Cybex Orthotron II(Cybex, a division of Lumex, Inc. USA) were used to induce the muscle fatigue. All subjects performed an isokinetic exercise with their dominant leg. Exercise was continued until 50% of the initial peak torque was reached. When the subjects were fatigued to less than 50% of the initial peak torque, the isokinetic exercises were discontinued. Then we immediately retested the proprioception using the same method. RESULTS: The mean and standard deviation of the angular error increased from 2.79+/-1.17degrees to 6.40+/-3.42degrees after muscle fatigue. CONCLUSION: The proprioception significantly decreased after the muscle fatigue(p<0.001). This result suggests that the muscle fatigue must be considered when an injured professional athlete a patient with fatigable disease is set in a rehabilitation program. Further studies are required to determine the physiological mechanisms of the role of muscle fatigue for the decreasing proprioception.