The abdomen muscles form the abdominal cavity in a multi-layer cover (cavum
abdominis). This cavity is formed by the straight abdomen muscle,
supported by two oblique and one transverse abdomen muscle. These three
thin muscle plates of the abdominal wall continue on from the
intercostal muscles (intercostalmuskulatur).
Towards the middle, the outer oblique abdominal muscle (musculus obliquus abdominis externus), the inner oblique abdominal muscle (musculus abdominis internus) and the transverse abdominal muscle (musculus transversus abdominis) are transformed into aponeuroses.
Here at the front of the abdominal wall they form a sheath, which encloses both straight abdominal muscles (musculus rectus abdominis). These extend to both sides of the medial line between the pubic symphysis (symphyse) and the front chest wall.
This makes the abdomen muscles into a unit, which withstands the pressure of the intestines. Simultaneous contraction of the abdominal muscles connected by the aponeuroses (aponeurosen) can reduce the abdominal cavity. This is called abdominal press. It is important, for example, during defecation.
The abdomen muscles are also involved in bending, turning and lateral movements of the trunk, thanks to the bracing of the muscles of the abdominal wall on the one hand, and the aponeuroses on the other. Together they make up a functioning unit also referred to as "oblique girdle".
The deep abdominal muscles include the major psoas muscle (musculus psoas major) and the quadratic psoas muscle (musculus quadratus lumborum). Together with the lumbar vertebrae (vertebrae lumbales), they form the rear abdominal wall.
The bone-free lower section of the pelvis consists of two muscle packages, which carry the weight of the intestines. The two pelvic muscles are the levator ani muscle (musculus levator ani) and the transverse perenei muscle (musculus transversus perinei profundus).
The rectum (rectum) and the urethra (urethra), and in women also the vagina (vagina), pass through the levator ani muscle. The urethra and in women the vagina also pass through the transverse perenei muscle. During birth, these muscles must expand to such an extent that the child's head can pass through.