One of the most common places you find spiral humeral fractures are amongst baseball pitchers. Pitching in baseball is an extremely repetitive, high force action placing stress on the entire body. Huge amounts of force transfered from the upper leg, through the hips into upper torso rotation during a pitch apply massive amounts of torque force onto the shoulder, elbow and wrist joints resulting in soft tissue damage. Hard tissue damage of the humerus occurs when large torque forces between the shoulder and elbow joints create a twisting action that results in a spiral fracture.
Torque: is using a force to produce rotation around an axis. Torque forces are necessary in allowing for load bearing during stressful movement and allow for some overload without too much damage to the body. However when torque forces are too large for the joints, muscles, tendons, ligaments and bones to handle, this is when injuries occur.
Another cause of the spiral fracture is in the extreme contraction of the muscles around the bone that apply forces so strong that in fact break the bone.