SHARE SHARE TWEET EMAIL. The deltoid is the large muscle that covers your shoulder joint. The muscle consists of three fiber groups: the anterior, lateral and posterior deltoids. The anterior deltoid, also known as the front delt, connects the collarbone to the humerus, the large bone of the upper arm.nternal Arm Rotation. The shoulders are the most flexible joints of your body. The ability to rotate your arms is one of the keys to the versatility of motion possible at the shoulder. Along with other muscles of the shoulder and chest, the anterior deltoids rotate the upper arm inward toward the body.