What is the function of the deltoids?

The deltoids move the upper arm up and away from a neutral position at the side. They each have three different major regions, each with different attachment points around the shoulders. Each of these regions moves the arm in a different direction when flexed.

All three major regions of the deltoids attach to the same point on the humerus. The region most toward the front of the body moves the arm up and toward the collarbone. The middle region moves the arm straight up and forward, as when throwing a ball underhand. The rear region lifts the arm directly out to the side.