The Arnold Press (also called the Arnold Dumbbell Press) is a shoulder exercise made popular by Arnold Schwarzenegger that primarily works all three deltoid muscles.
It is sometimes confused with the Scott Press (named after the first Mr. Olympia Larry Scott). Although similar, the two exercises are slightly different, mainly in the range of motion used.
The similarities are probably because Arnold may have seen Larry Scott doing the exercise and then Arnold modified it to what felt best to him.
There’s also a good chance that Larry Scott originally learned his version of the exercise from Vince Gironda, a trainer to many successful bodybuilders, including Arnold.
Regular shoulder presses tend to over-emphasize the front deltoid muscle since it is the primary muscle needed to extend the arm overhead.
The side and rear deltoid muscles help draw the arm up to the side and behind the body but that action is limited in a normal overhead press without purposely trying to do it.
The key to getting all three deltoid muscles involved is to focus on the elbow. The direction the elbow moves determines which of the three muscles is doing the work to move the weight rather than acting more as a stabilizer muscle.
The choice is yours if you want to do a standing or seated Arnold press. Generally the seated version is harder since it minimizes cheating.
Either way you will start with a pair of dumbbells in front of the body with your palms facing you. Unless you’re a freak of nature, you’re not going to be able to do that holding a barbell.
Now instead of just pressing overhead as you normally would putting the most emphasis on the front delts, you will start the press by first rotating your elbow outward to the side by twisting the weight so the palm turns towards the front.
This outward motion will draw the elbow from the front to the back of the body so that the side and rear delts are more involved in the movement.
As you rotate you will then simultaneously start pressing overhead which requires the front delts The idea is to continue to rotate the arm (and elbow) as far behind the body as you can while extending the arm overhead all in one motion.
Since the elbow is now moving through space rather than in more of a fixed direction, the benefit is all three deltoid muscles are more equally helping to move the weight overhead.
Simply return the weight to the starting position following the same path for the next rep.
Another trick Arnold (and Larry Scott) used to further involve the rear delts specifically was to tilt the weight downward by keeping the little fingers higher than the thumb.
With the weight tilted, the rear delts need to work harder to hold the arm in place to keep from dropping the weight on your head. You can see in the pictures how Arnold has the weight tilted.