The transversus abdominis is like a corset around your abdomen. It's the muscle you work if you pull in your stomach.

The multifidus is a group of muscles that lie along your spine from your neck to your pelvis, with the short fibers connecting one bone (vertebra) of the spine to other vertebrae near it.

The muscles of the pelvic floor are most noticeable when you squeeze to keep yourself from urinating.

The diaphragm is a big, flat muscle inside your rib cage, in between your chest and your belly. The diaphragm can be thought of as a deep core muscle. It helps with breathing but also works with the other core muscles to make the trunk more stable.