The core refers to the space between your ribcage and your pelvis. It is the inner container between the bottom of your rib cage, where your diaphragm is, and the muscles at the bottom of your pelvis, the pelvic floor muscles. There are four muscles that comprise the core, they are the:
We are familiar with our diaphragm, because it is also the breathing muscle and the diaphragm separates the chest and lungs, from the abdomen. The pelvic floor muscles are the muscles at the bottom of the pelvis. They span from the tailbone at the back of the pelvis to the pubic bone at the front.
The Transverse abdominus is the deepest abdominal muscle; it is the abdominal muscle deep to the rectus abdominus (sometimes known as the 6-pack ab muscle) and the oblique abdominal muscles. The multifidus is a deep back muscle that connects into the spinal vertebrae. These four muscles constitute the deep muscular core.