muscles... what are

The muscular system of the lower abdomen, including the pelvis, is separated from that of the chest by the muscular wall of the diaphragm - so critical to breathing - and lies somewhat exposed between the protective bones of the ribs above and the pelvic girdle behind. As such, skeletal muscles across the lower abdomen play a critical role in protecting the delicate organs within the abdomen, in addition to providing propulsion and stability.

Attached to the pelvis are muscles of the abdominal wall, the buttocks, the lower back, and the insides and backs of the thighs. This includes the 

gluteus maximus and the rest of the posterior thigh muscles that connect 

the lower abdomen with the legs in support of the body's weight and in its propulsion. 

Other abdominal muscles serve as flexors of the trunk on the hip as well as of the thigh at the hip joint, or rotate the hip and thigh together.

Inner organs within the abdominal cavity are also part of the muscular system. For one, the uterus or wombs a hollow muscular organ that forces the baby through the birth canal with great force. The bladder too and intestines are hollow muscular organs of the digestive and excretory systems, located within the abdomen.

Protecting those vital organs, the specific muscles commonly referred to as the abdominal muscles also aid in locomotion but are more critical to the body's stability, working in conjunction with the muscles of the lower back. Broad and flat, they flex, bend the vertebral column sideways, and rotate it while assisting in the digestive process and in forced expiration.