Intramuscular deltoid injection. Artwork showing the correct site for a deltoid injection. The deltoid muscle is in the upper arm. To find the correct site, the person giving the injection should first find the acromion process, the part of the scapula (shoulder blade) that forms the shoulder joint with the humerus (upper arm bone). The injection site is two to three finger widths below this bone. When giving the injection the needle should be held at an angle of 90 degrees.