The purpose of this study was to compare the electromyogram (EMG) activity of the trunk musculature for long throwers and short throwers during the side medicine-ball throw. The study consisted of 2 sessions. Thirty athletes performed the side medicine-ball throw (S-MBT) in the first session. The top 5 subjects (long throwers) and the worst 5 subjects (short throwers) in the performance of the S-MBT proceeded to the second session. These subjects performed the fast side medicine-ball throw (FS-MBT) and the isometric maximal trunk rotation torque (IMTRT) test after applying surface electrodes bilaterally to the following muscles: pectoralis major, rectus abdominis, external oblique, and latissimus dorsi. The value of IMTRT in the left turn in long throwers was significantly greater (p < 0.05) than that recorded for short throwers, whereas no difference was observed in the right turn. The velocities of the medicine ball using 2-kg, 4-kg, and 6-kg medicine balls were significantly different (p < 0.01-0.001) between long throwers and short throwers The examination of EMG activity in each muscle group for long throwers and short throwers revealed that the major difference between the 2 groups is EMG activity of the left external oblique. These results suggest that the EMG activity of the external oblique on either side is an important factor for FS-MBT.