The “core muscles,” which are the muscles responsible for giving us strength and stability when bending or twisting, can essentially be broken down into seven different sets: the rectus abdominis, the external obliques, the internal obliques, the transversus abdominis, the multifidus, the quadratus lumborum and the lumbar erector spinae. Each of these sets of muscles performs a different function, as follows:
Rectus abdominis – Responsible for flexing and rotating the trunk, the rectus abdominis is the set of muscles made famous by the term “six pack” abs. The muscle extends out from the xiphoid process and adjacent costal cartilages, and attaches to the pubic bone at the crest and symphysis.
External obliques – These contribute to spinal stability and allow the trunk to flex and bend toward the side. They also enable the trunk to rotate toward the opposite side of the body. It begins at the front lateral part of the lower seven ribs, and attaches at the pubic tubercle, the linea alba and the front part of the iliac crest.
Internal obliques – Provide stability to the spine and allow for the flexion and rotation of the trunk toward the same side of the body. It originates from the inguinal ligament, the thoracolumbar fascia and the anterior iliac crest, and attaches at the linea alba and lower four ribs.
Transversus abdominis – Plays a key role in trunk stabilization and compresses the abdominal wall. It starts at the thoracolumbar fascia, the lower six costal cartilages and the iliac crest, and attaches at the middle of the linea alba.
Multifidus – Stabilizes and allows for the rotation and flexion of the spine by supporting the vertebral joints. They begin at the posterior sacrum, the superior iliac spine, the mamillary processes of the lumbar vertebrae, the transverse processes of the thoracic vertebrae and the articular processes of the cerebral vertebrae. Each connects at the spinous processes of the vertebrae, two to four bones above where they originate.
Quadratus lumborum – Connects the spine to the pelvis and is responsible for movements related to bending and twisting the spine in relation to the pelvis, such as bending while rotating. It begins at the top of the iliac crest, and attaches to the top of the lumbar spine and the lowest ribs.
Lumbar erector spinae – These are long muscles that allow the spine to extend, bend and twist. They extend from the lumbar spine to the neck. They begin at the iliac crest, the sacrum, the lower seven ribs, and the spinous and transverse processes of the thoracic vertebrae, and attach at the angles of the ribs, the transverse processes of the vertebrae and the base of the skull.