The training effects of bench press on the arm and shoulder muscles were electromyographically examined. The subjects used in the experiment were six helthy young males and the muscles examined were the lateral and long heads of m. triceps brachii, the sternocostal portion of m. pectoralis major and the anterior portion of m. deltoideus. The EMG recordings were made with a 4 channel electroencephalograph (Nihon-Koden Inst. Co., Ltd.) utilizing surface electrodes, 10 mm in diameter. The training was performed twice a week and continued for six months. During the training period, the electromyographic recordings and the measurements of the extension strength at the elbow joint and the flexion strength at the shoulder joint were taken once a month. The results obtained were summarized as follows: 1)All subjects showed no visible changes in the discharge patterns of the lateral head of m. triceps brachii and the sternocostal portion of m. pectralis major throughout the training period. 2)The discharge patterns of the long head of m. triceps brachii and the anterior portion of m. deltoideus varied among the six subjects and were classified into three different types. Type A: The first two subjects showed the co-active discharges in the long head of m. triceps brachii and the anterior portion of m. deltoideus. Type B: In the next subjects, the discharge of the long head of m. triceps brachii appeared at the latter half period of the bench press motion. Type C: The discharge patterns of the last two subjects shifted from type A to type B during the training. When the maximum weight was loaded, however, the subjects in type C showed the similar discharge pattern to that of type A. 3)In type A and type C, the extension strength at the elbow joint, the flexion strengh at the shoulder joint and the maximum weight of the barbell for one-bout lift increased up to 132%-165%, 150%-184% and 147%-173% of the control, respectively. 4)In spite of little or no increase in the extension strength at the elbow joint and only 2 to 16% increase in the flexion strength at the shoulder joint. the subjects in type B showed 33 to 48% increase in the maximum weight of barbell for one-bout lift. 5)The decrease in the electrical discharges of the muscles at the minimum (16kg) and the light (31 or 36kg) loads observed during the training period might be due to the increase in the contractile force of the muscles at the level of individual motor unit.