What is a primary muscle involved in shoulder joint extension?

The primary muscle involved in shoulder joint extension is the posterior deltoid. This muscle plays a crucial role in bringing the arm backward, which is the action associated with shoulder joint extension. The posterior deltoid runs along the back of the shoulder and is specifically designed for this movement. It works in concert with other muscles, such as the teres major and latissimus dorsi, but the posterior deltoid is the most significant for extension.

In the context of the other muscles listed, while the coracobrachialis primarily assists in shoulder flexion and adduction, the medial deltoid mainly contributes to shoulder abduction. The supraspinatus, though vital for arm elevation and stabilization of the shoulder joint, does not play a major role in extension. Therefore, these muscles either support different movements or function in smaller capacities related to shoulder extension, making the posterior deltoid the key muscle for this action.