Arm angle is the angle at which a pitcher throws the ball. The lower the arm angle the more lateral movement a pitcher’s breaking pitches and fastball have.
There are four common arm angles used by pitchers:
Overhand: the ball is released with the pitcher’s arm near perpendicular to the ground.
3/4: the ball is released at about a 45 degree angle with the ground; more diagonal to the ground than the overhand slot. There is also a low 3/4 arm slot where the ball is released with the pitcher’s arm slightly above shoulder level.
Submarine: the ball is released with the pitcher’s arm below shoulder-level and close to the ground.
The overhand arm angle is considered to be the safest arm action, while side-arm is the most violent angle and mostly used by relief pitchers.