Which of the following muscles is NOT involved in shoulder extension?

The infraspinatus muscle is primarily known for its role in shoulder external rotation and stabilization of the shoulder joint rather than for shoulder extension. This muscle is part of the rotator cuff group, which is predominantly involved in the dynamic stabilization of the glenohumeral joint and assists with movements that require rotation.

In contrast, the latissimus dorsi is a major contributor to shoulder extension, along with the posterior fibers of the deltoid, which also assist in extending the arm. The pectoralis major, particularly its sternal head, plays a role in extending the shoulder when the arm is flexed. Therefore, while the other muscles mentioned in the options are actively engaged in extending the shoulder, the infraspinatus does not participate in this motion, making it the correct answer.