Superficial muscles of the thorax and back
While all muscles in a region are affected by a workout, when anaerobic workouts are undertaken, the superficial muscles are the ones that form the majority of the bulk that you see in hardcore athletes and bodybuilders.
There are deep “flat” muscles beneath the superficial layers in these regions, and below and lateral to those, there are the “long” muscles. These are all skeletal muscles - voluntary and striated. Surrounding the muscles is connective tissue, including the linea alba (the dividing line between the two halves of the abdomen), the fascia, and the aponeuroses. All of these consist of dense, fibrous connective tissue, and protect the body from intrusion, as well as protecting the muscles from each other, as they flex and relax in different directions.
Atlas and Text-Book of Human Anatomy. Dr. Johannes Sobotta, 1914