Which muscle is primarily responsible for scapular depression?

The correct answer is the inferior trapezius. This muscle plays a critical role in scapular depression, which involves moving the scapula downwards towards the ribcage. The inferior fibers of the trapezius originate from the lower thoracic vertebrae and insert onto the spine of the scapula, specifically the medial portion. When these fibers contract, they pull the scapula down, working in conjunction with other muscles for proper scapular movement.

In contrast, the pectoralis major primarily functions in shoulder flexion, adduction, and internal rotation rather than scapular depression. The quadriceps are a group of muscles located in the thigh and are involved mainly in knee extension; they do not play a role in movements of the scapula. Lastly, the manubrium is a part of the sternum and does not have any muscular function related to the movement of the scapula. Understanding the anatomy and function of these muscles highlights why the inferior trapezius is essential for scapular depression.