The psoas major and the rectus abdominis muscles have an important relationship inside the body even though they don’t literally work together.

Muscles relate to each other in many different ways. Sometimes they work in opposition—for the hamstring to lengthen the quadriceps must shorten. This happen through the process of reciprocal inhibition and most often occurs in muscles that live on opposite sides of a bone. Muscles are not elastic and therefore cannot naturally retract after stretching.

Muscles also work synergistically—they help the opposite pairs fulfill their function when needed. The tensor fascia latae synergistically helps the psoas major with hip flexion and internal rotation.

Other muscles work together though they aren’t literally connected- the levator ani (pelvic floor) and the diaphragm should do the same thing at the same time to achieve optimal function. Some of these relationships are clear-cut like the hamstring and quadriceps, but other like the psoas major and rectus abdominis are more complicated.