Which muscle is responsible for adduction and downward rotation of the scapula?

The rhomboid muscles, located between the spine and the scapula, are primarily responsible for the adduction (or retraction) and downward rotation of the scapula. This means that when the rhomboids contract, they pull the scapulae together towards the spine, and they also play a crucial role in rotating the scapula downward, which is important for movements like pulling or lifting.

Understanding the specific actions of the rhomboids helps clarify their role in upper body movements. They are essential for maintaining good posture and stabilizing the scapula during various activities, such as rowing or performing pull-ups. The downward rotation of the scapula facilitated by the rhomboids is vital for effective arm movement especially in exercises that involve lowering the arms or bringing them across the body.

Other muscles mentioned in the options serve different functions: the upper trapezius is largely involved in elevation and upward rotation of the scapula; the lower trapezius assists with depression and upward rotation; and the sternocleidomastoid primarily acts on head movement rather than scapular mobilization. Thus, the rhomboids are the specific muscles that match the required actions of adduction and downward rotation of the scapula.