Final answer:
The posterior deltoid and triceps brachii are primary muscles responsible for extension at the glenohumeral joint (GHJ), with additional assistance from the latissimus dorsi and teres major.
Step-by-step explanation:
The muscles responsible for extension at the glenohumeral joint (GHJ), commonly known as the shoulder joint, include several key players that work together to perform this action. At the forefront are the posterior deltoid and triceps brachii. The posterior deltoid muscle, which covers the shoulder, is involved in moving the arm backward, hence assisting in extension. Likewise, the triceps brachii, situated on the back of the upper arm, plays a crucial part in extending the forearm at the elbow, and also assists in extending the shoulder when the arm is being moved in a posterior direction.
Other muscles that contribute to this movement include the latissimus dorsi, which adducts, extends, and internally rotates the arm at the shoulder, and the teres major, also known for assisting in these actions. It is important to note that these muscles coordinate to smoothly extend the arm at the shoulder joint.