According to BodyBuilding, the main muscles used when swinging a baseball bat are the abdominal muscles, back muscles, leg muscles and forearms muscles. However, in order to increase overall bat swing power, the entire body should be trained so as to help the body work together more efficiently.
The abdomen and back are used to create the twisting motion during the swing. The three main areas to focus on in the abdomen are the upper abdominals, lower abdominals and rotational abdominals. BodyBuilding also states that leg strength is important because the force used to hit the baseball comes mainly from the legs. Finally, grip and forearm strength are vital for finishing off a powerful swing.