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When you first studied the musculoskeletal system, you probably learned that most muscles create movement through contraction that pulls on their tendons, which then transmits force to the bones. But what happens in regions like the anterior abdomen, where there are no bony insertions? The potentially vulnerable boneless configuration in this region is a trade-off for an impressive amount of shape-shifting freedom that allows you to f lex, twist, and expand your belly with ease. But how do the abdominal muscles leverage themselves across the midline? Does the structural freedom gained leave the abdomen completely unsupported? Spanning the center of the abdomen, from the rib cage above to the pubic symphysis below, we find a different kind of connection where muscles insert into and pull upon a three-dimensional network of deep fascia. Meet the rectus sheath, the fibrous fascia that fills in the gap where no bony skeleton exists. Your fascia has your back! Or in this case, quite literally, your front. ANATOMY: THE BELLY'S SOFT SKELETON The rectus sheath is a major structural component of the abdominal wall, yet it's often missing from anatomical drawings. While named for its sheath-like envelopment of the rectus abdominis (RA) muscles, the rectus sheath is actually created by the converging fascia of our other three major abdominal muscles: the external obliques (EO), internal obliques (IO), and transversus abdominis (TrA). 68 m a s s a g e & b o d y wo r k s e p te m b e r/o c to b e r 2 0 2 3 critical thinking | ANATOMY FOR TOUCH The Rectus Sheath Making Complex Abdominal Movement Happen By Nicole Trombley and Rachelle Clauson Rectus sheath. As the internal obliques, external obliques, and transversus abdominis transition into their respective flat tendons, or aponeuroses, they form what is called the rectus sheath of the abdomen. In this image, we see the anterior surface of the rectus sheath at the level of the external obliques. Image courtesy of AnatomySCAPES.com. Each of these broad, f lat muscles wraps around the midsection, with the muscle fibers transitioning into their f lat tendon as they approach the front of the body. The f lat tendons fuse together on either side of the RA, creating pockets for the "six-pack" muscles as they pass to the midline. The result of this converging is the rectus sheath, a multilayered fascial hub that allows: (1) force transmission, (2) motor coordination, and (3) stability in our trunk. With a skill set like that, we're tempted to ask, "Who needs bones?" Let's explore how the rectus sheath's anatomy makes this possible. 1