Rectus abdominus is a broad, flat muscle on the anterior abdomen with fibers that connect the pubic bone to the arch of the ribs. It is wider, laterally near the ribs and thicker anterior-toposterior at near the pubic bone. The muscle is divided into sections by 3, sometimes 4, rarely only 1, tendon-like bands across the fibers of the muscle. The top band is usually just under the xiphoid, the bottom one is near the umbilicus and the third is usually between those. This can be quite variable.
These paired muscles are wrapped in an aponeurotic sheath. The linea alba is the strip of connective tissue that joins the xiphoid process to the pubic symphysis along the mid-line of the body. It also binds the medial edges of those sheaths. The lateral edges of these sheaths provide attachments for the obliques and transversus
The origin and insertion are argued based on whether one is standing or swinging from their arms. Usually, the origin is considered the pelvis.
The rectus abdominus originates along the crest of
the pubic bone.
The rectus abdominus inserts along the fascia over
the costal cartilage of ribs 5, 6, and 7. The muscle
fibers in the abdominal section of the pectoralis major
may overlap this section of rectus abdominus and
weave into its fascial sheath
Transverse view of the abdominal sheath
The rectus abdominus is encased in the rectus sheath, which connects it to the other rectus abdominus medially via the linea abla, and to the obliques and transverse abdominus laterally.
This muscle draws the hips and ribs together which flexes the trunk. During the sit-up, it is active in all phases but most active when the lower thoracic and lumbar spine are flexing between the time that the scapulae come off the ground and the iliac crest comes off the ground.
When the trunk is stabilized by the spinal directors, it compresses the distended abdomen. This assists in activities like defecation or labored breathing. This muscle stabilizes the trunk when moving the head and/or legs. When supine, rectus abdominus engages to lift the head. It engages when the legs are lifted. It is active when carrying a backpack or pulling down and forward with the arms.
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