To break it down, the core is composed of the muscles that run the length of your trunk. This includes the back, abs, glutes, and even the diaphragm. To get more technical, your multifidus, erector spinae, rectus abdominis, transverse abdominis, obliques, and pelvic floor play a large role—and to a lesser degree, your latissimus dorsi, trapezius, and gluteus muscles.