As you learn how to throw a sinker, keep in mind that it is like any other type of fastball. Hence, it is not harmful to your arm when you master the trick of throwing it. According to American Sports Medicine Institute (ASMI), most fastballs such as sinkers put more stress on the arm than other pitches because of the high velocity.   However, always keep in mind that pitching is basically an injurious and dangerous game. Always practice good arm care and avoid pitching too often. Besides, baseball organizations like Major Baseball Leagues provide helpful information on how pitchers can keep their arms healthy.