While the Latissimus Dorsi (“Lats”) is more well-known as a key muscle for widening the back... perhaps even more important for the upper back specifically is the Teres Major!
Anatomy: As you can see in the anatomy graphic (see first image below), the Teres Major sits on the lateral aspect (side) of the upper back, just under the shoulder joint, and attaches on the humerus, making up a significant portion of the “upper-outer” back (also, it can be much larger proportionally than it appears in that graphic if trained).
How to Build It: The best way to specifically target the Teres Major is via shoulder adduction (bringing the upper arm in towards the middle of the body – see second image below). Why? Because while the Teres Major also performs shoulder extension (pulling the upper arm back / down), the Latissimus Dorsi is a much more powerful shoulder extensor, so it takes most of the load in those exercises (such as lat pull-downs, pull-ups, etc.). On the flip side, while the Lats also do shoulder adduction, the Teres Major is better at it since it’s oriented laterally across the body, so it will take more of the load in shoulder adduction exercises! (And it’s actually the medial (middle) portion of the Lats that performs shoulder adduction anyway – see Research Article 2 below)
Check out the third image below for an example of one of the exercises I use to target the Teres Major via shoulder adduction!! (There are more than a dozen different shoulder adduction exercises in my Total Back Program, each with it’s own unique benefits & use-cases 💪)
Extra Tip: One way to increase the relative amount of load on the Teres Major during pull-ups or lat pull-downs is to use a WIDE GRIP, since that brings more shoulder adduction into the pull-down motion! 👊
Related Dr. Gains YouTube Videos:
1) EVERYTHING You NEED to Know BEFORE Working Shoulders! (Part 1)
Relevant Research Articles:
1) Anatomy, Shoulder and Upper Limb, Teres Major Muscle
2) Differential activation of parts of the latissimus dorsi with various isometric shoulder exercises
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Dr. Gains 
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