Rectus Sheath: Your Fascia's got your Front

 When you first studied the musculo-skeletal 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 our 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 shapeshifting freedom that allows you to flex, 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’s got 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). 

Each of these broad, flat muscles wrap around the midsection, with the muscle fibers transitioning into their flat tendon as they approach the front of the body. The flat tendons fuse together on either side of the RA, creating pockets for the 6-pack muscles as they pass to the midline. The result of all this converging is the rectus sheath: a multi-layered 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 all this possible. 

FORCE TRANSMISSION: Fascia At Work

Whether you’re carrying your massage table or twisting to reach your massage oil, your abdominal muscles contract to generate force. How can this muscle-generated force create stability and movement in the bone-free expanse of the belly? As it turns out, our muscles don’t need bones per se for movement. What they do need is a tissue that shares a particular property of bones: stiffness. 

Here’s where the rectus sheath comes in. Every time your obliques and transversus abdominis muscles contract, they pull on their flat tendons which tension the rectus sheath. As the rectus sheath is tensioned, it stiffens, and bam! you've got what you need for movement: a connective tissue framework that is stiff and tense for force transmission. No bones about it.

Force transmission? Check. But in order to have complex movement, we need motor coordination. 

MOTOR COORDINATION: Crossing the Midline

Stretch break! Let’s take a minute to feel how the abdominal muscles talk to each other to create motor coordination. Whether you’re seated or standing, take a twist to the right, then to the left. When we twist, the external oblique (EO) of one side contracts at the same time as the internal oblique (IO) of the opposite side, creating coordinated movement together. How? The rectus sheath holds some clues. 

There’s one part of the rectus sheath that’s almost never left out of our anatomy books: the linea alba. At the midline, the flat tendons of the EO, IO, and TrA from both the left and right sides merge, creating a longitudinal reinforcement. 

Our anatomy books describe the linea alba as the insertion point for EO, IO, and TrA. When we zoom in closer, the collagen fibers of the linea alba tell an additional story. But zooming in to the level of collagen fibers, it’s clear that the linea alba is not the end point for these muscles. Analyzing fiber-by-fiber, researchers found that fibers from every layer of the rectus sheath pass across the midline and even interweave strategically (Axer 2001). This three-dimensional meshwork of fibers allows both force transmission and communication with the opposite side. 

If we return to our twist, we now see that the EO and IO are connected by the fibers of their flat tendons intermingling and crossing through the midline. The EO can actually feel the movement of the IO on the opposite side through its sensory nerves and vice versa, enabling motor coordination. 

Motor coordination? Check. On to the rectus sheath’s next feat –  trunk stability.

TRUNK STABILITY: Front, Meet Your Back

If you asked somebody to touch their abs, chances are they’d touch the front of their abdomen, or perhaps laterally along the waistline. But if they touched their low back, technically, they’d also be touching their abs. How’s that possible? Looking at the torso in cross-section, we see that the combined muscle and fascia of EO, IO, and TrA circle around to the back, anchoring to thoracolumbar fascia and the spine.  

We also see interesting organizational similarities between the front and back of the trunk. The rectus sheath housing the longitudinal RA muscles in the front is paralleled in the back by the thoracolumbar fascia housing the longitudinal erector spinae muscles. With loose connective tissue between the EO, IO, and TrA, the abdominal muscles keep the front and back connected in an organization of “gliding rings” of flat muscles and tendons. 

This organization allows for dynamic movement of the trunk, but also takes the stability game next level. As our EO, IO, and TrA tension the rectus sheath, they also tension the thoracolumbar fascia amplifying their contractile efficiency and creating an integrated tensional ring of stability. So basically, your fascia’s got your front AND your back. 

Stability? Check. 

TRADE OFF OR UPGRADE?

The bone free abdomen is not left unsupported after all. The fascial system allows for the tradeoff of less bony protection for structural freedom, and offers some important upgrades. The specific organization of the rectus sheath allows force transmission, motor coordination, and stability. Understanding the fascial anatomy of the abdomen helps us better understand how the muscles of our trunk work together.

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Further Reading:

Axer, H. et al. “Collagen Fibers in Linea Alba and Rectus Sheaths.” Journal of Surgical Research 96, no. 1 ( 2001): 127–34. 

Rizk, N. “A New Description of the Anterior Abdominal Wall in Man and Mammals.” Journal of Anatomy 131, no. 3 (1980).

Stecco, Carla. Functional Atlas of the Human Fascial System. Edinburgh: Churchill Livingstone Elsevier, 2015.

Vleeming, A. et al. “The Functional Coupling of the Deep Abdominal and Paraspinal Muscles: The Effects of Simulated Paraspinal Muscle Contraction on Force Transfer to the Middle and Posterior Layer of the Thoracolumbar Fascia” Journal of Anatomy 225, no. 4 (October 1, 2014): 447–62. 

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