What is the primary action shared by all groups of fibers in the deltoid muscle?

The primary action shared by all groups of fibers in the deltoid muscle is abduction of the glenohumeral joint. The deltoid muscle has three distinct sets of fibers: anterior, lateral, and posterior, and while each group can contribute to various movements, they collectively work to abduct the arm.

Abduction refers to the movement of raising the arm away from the body, and the deltoid is the main muscle responsible for this action after the first 15 degrees, which is primarily facilitated by the supraspinatus muscle. The lateral fibers of the deltoid are especially adept at this movement, but the anterior and posterior fibers also assist in abduction, albeit with some additional functions such as flexion and extension of the shoulder.

Understanding the primary role of each group's fibers helps clarify that abduction is indeed the common action they all share, distinguishing it from other movements like flexion, adduction, or external rotation, which do not involve all fiber groups uniformly.