Rectus abdominis is a long muscular strap extending from the ventral lower sternum to the pubis. The muscle is interrupted (but not all the way through) by three (or more) fibrous bands called the tendinous intersections, the most superior intersection is generally at the level of the xiphoid, the lower intersection is at the level of umbilicus and the middle one is at the midway between them.
It is originated from two tendons inferiorly medial and lateral;
- Medial: ligaments covering the front of symphysis pubis
- Lateral: the pubis
Sternum (xiphoid process, costoxiphoid ligament)
Ribs 5 to 7 (costal cartilages by three fascicles of differing sizes)
Spinal nerves T7 to T12 (ventral rami)
T7 innervates fibers above the superior tendinous intersection, T8 innervates fibers between the superior and middle intersections and T9 innervates fibers between the middle and lower intersection.
Superior and inferior epigastric arteries.
(Also, small terminal branches from the lower three posterior intercostal arteries, subcostal artery, posterior lumbar arteries and the deep circumflex artery may provide some contributions)
The main action for rectus abdominis is flexion of the trunk (flexion of thoracic and lumber spine), while it works by drawing symphysis and Sternum toward each other.
Also, it works on posterior pelvic tilt with other abdominal muscles.
Diastasis Recti Abdominis is a midline separation at the linea alba. A palpable midline gap of more than 2.5 cm or any visible bulging on exertion is considered as a diastasis and commonly occurs around the umbilicus, but can occur anywhere between the xiphoid process and pubic bone.
It is very common among pregnant women. The muscle opening often shrinks after giving birth, but in some studies of women with diastasis recti, the muscle wasn't back to normal even a year later, so they should bring those affected muscles back together before having another pregnancy. 
Have the patient in a supine lying position with a pillow under knees to ensure the relaxation of the muscle, palpation will be along the muscle starting from the xiphoid process and the adjacent ribs with going down to the pubis bone on both sides. Ask the patient to raise (or try to raise) the trunk and then relax.
We mainly need to assess the strength of the abdominal muscles rather than its flexibility because having weak abdominal muscles will come out later with serious of problems.
MMT for rectus abdominis is usually done by asking the patient to bring his/her trunk against gravity from supine position until the inferior angles of the scapula are off the table, and the therapist will start grading according to the patient’s performance and ability to do the movement as instructed.
Grading from 5 to 1 will be based on the level of ability to bear the resistance (the resistance here is generated by the patient's arms behind the head, arms crossed over chest or arms overstretched above the plane of the body) to only minimal contraction felt by the examiner's hand.
- Sit up exercise which required raising the trunk against gravity to the midline and slowly lower it down repeatedly in multiple sets, and in order to isolate the rectus abdominis, do the exercise while bending the knees to minimize the engagement of hip flexors. (For rectus abdominis, raising the trunk should be performed straightly)
Recent Related Research (from Pubmed)
- Helen J.Hislop Jacqueline Montgomery,Muscle Testing,2007,8th edition.
- Rectus Abdominis,https://www.mananatomy.com/body-systems/muscular-system/rectus-abdominis, (accessed 23 April 2017)
- Diastasis recti abdominis, https://www.coreconcepts.com.sg/condition/diastasis-recti-abdominis/ (accessed 14 April 2017)
- Abdominal separation (diastasis recti), http://www.webmd.com/baby/guide/abdominal-separation-diastasis-recti (accessed 14 April 2017)
- Diastasis recti, https://medlineplus.gov/ency/article/001602.htm (accessed 14 April 2017)