One of the 4 muscles of core stabilization and strengthening is the transverse abdominis. The abdominal muscles are the rectus abominis, internal obliques, external obliques and the transverse abdominis. The deepest of the 4 abdominal muscles, the transverse abdominis muscle runs horizontally or transversely across the abdominal region. It connects the back, sides and front of the abdominal and lower back region.
Nicknamed "the vanity muscle", this muscle pulls the navel in toward the spine. It can whittle the waist. When done appropriately, there is no movement of the rib cage or the pelvis. There is no breath holding or shoulder shrugging involved in its activation. If the ribs move, you have activated the external oblique muscle. If the abdominal wall pouches out, then you have activated the rectus abdominis, or six pack muscle.
When utilized correctly, this muscle gives the definition on the sides of the abdominal wall (the concave line on the side of the six pack muscle). Beyond the aesthetics of this, the transverse abdominis is crucial in lumbar stability in that it activates with the small spinal segmental muscles, known as the multifidi. Activation of all of these muscles together means that the spinal column “sets" before movement of the trunk or extremities. It provides stability, which allows for full and pain-free range of motion and movement during your daily activities as well as any recreational activities. And it makes your waist look fit and slim.