The purpose of the study was to investigate the difference in the force and EMG characteristics between flexor and extensor muscles of the elbow and knee joints. The subjects in this study were six healthy adults (male, 22.8±3.1 years). Each subject was asked to perform isometric flexion and extension for elbow and knee at each force level of 25, 50 and 75% of his maximum voluntary contraction (MVC), and then to sustain them at the three different force levels to exhaustion. The surface EMG was recorded from the m. biceps brachii, m. triceps brachii, m. biceps femoris and m. rectus femoris. An endurance time, IEMG and the reduction ratio of the mean power frequency (MPF) for each second were computed from the recorded EMG at each of the three different force levels. Results were summarized as follows: 1) With an increase in the normalized force, greater normalized IEMGS were observed in the flexor muscles than in the extensor muscles for both arm and leg. Differences between flexor and extensor muscles in the normalized IEMGs were all significant throughout the three different levels of the percent MVC force. 2) Flexor muscles had a longer endurance time than extensor muscles for both arm and leg. For the arm a significant difference was found at each of 25 and 50% MVC force levels. For the leg, on the other hand, a significant difference was observed at each of 25 and 75% MVC force levels. 3) The reduction ratio of the MPF for each second in the extensor muscles was significantly larger than that in the flexor muscles at each of the three different levels of the percent MVC force. Within the limitation of the study, it can be concluded that physiological characteristics of the flexor muscles in the arm and leg are different from those of the extensor muscles. Such a difference may be due to . the effects of a different muscle fiber composition in the extensor and flexor muscles.