Breathing, core, and postural control - Part 3: Adaptive guy wires

In the previous article, I mentioned that a direct connection between some of our deep core muscles and thoracolumbar fascia provides the pathway that the proactive modulation of intra-abdominal pressure is coordinated with whole-body motion.

Now let's shift our attention to other abdominal muscles that form superficial layers of the abdominal wall. They are rectus abdominis and external obliques. There are also internal obliques, but it is not clear wheather or not they are strictly classified as superficial muscle. I will talk about this near the end of this posting. Rectus abdominals, internal and external obliques are attached to the lower ribcage and to the pelvis. When they contract, due to their downwardly directed fiber orientation, they create the force that can bend the spine forward as well as pulling the ribcage down. Obliques can also side bend and rotate the trunk when one side is activated. For this reason, superficial abdominal muscles are often classified as prime movers of the spine. However, they actually play a huge role in stabilizing the spine.

Dr. Stuart McGill compared the superficial abdominal muscles to a guy wire system to explain how they provide stability to our spine. As you can see in the first picture from the left, guy wires are used to keep a rod upright while successfully allowing the structure to absorb compressive force. In this case, the tension between the two wires has to be the same to keep the rod perfectly upright. According to Dr. McGill, we have multiple guy wires in our body including the muscles that connect directly to the spine which is shown in the second picture from the left and the superficial abdominal muscles which are shown in the third picture from the left. And they are working together to maintain the same amount of tension coming from front and back as well as between the left and right sides. You must be wondering which muscles oppose the tension from the abdomen that creates forward flexion of the spine? As you can see in the fourth picture from the left, activation of the front abdominal muscles should be accompanied by co-contraction of the extensors of the spine if our goal is to stabilize the spine. Unlike the rigid rod, our spine is a movable structure. So we must be able to bend and rotate it while keeping the whole structure stable. Unlike the mechanical guy wires, our muscles can eccentrically contract to stabilize our spine even when the length of the "guy wire" muscles are imbalanced. For example, when we bend our spine to the right side, left superficial abdominal muscles lengthen to allow the movement. And they also eccentrically contract so that we can bend the spine just how much we want to bend without letting it buckle.

In relation to this, it was highlighted that direction-specific stabilization is one of the unique functions of the superficial abdominal muscles that differentiate their role from deeper muscles such as transversus abdominis. Drs. Paul Hodges and Carolyn Richardson found that transversus abdominis was always activating on both sides with consistent timing prior to a limb motion no matter which direction the motion occurred or how variable the reaction time of that movement was. On the other hand, the superficial abdominal muscles showed various activation timing and magnitude between the left and right side depending on the direction and reaction time of the movement. This indicates that superficial abdominal muscles provide highly adaptable stability which is the key for controlling whole-body movement. Importantly, direction-specific stabilization of superficial abdominal muscles is known to occur prior to the onset of a movement if it is a high threshold movement such as lifting a heavy weight or throwing a baseball. This means that predictive stabilization is not reserved only for deep core muscles. Our central nervous system assesses the upcoming challenge and proactively recruits whatever muscles available in a context-dependent manner. Clearly, coordination between deep core and superficial abdominal muscles is necessary to prepare for a high threshold movement.

Asides from the neuromuscular evidence, anatomical features of the superficial abdominal muscles also tell us that they are suitable for providing stability throughout the global movement of the body. According to Dr. Andry Vleeming, our abdominal fascia connects superficial abdominal muscles to other important lower body flexors and rotators so that the force between lower and upper body can be smoothly transferred while successfully stabilizing the pelvic junction. In addition, Drs. Logan and McKinney described that obliques are part of the myofascial network that wraps around the back and crosses in the front of the body like a ribbon. This myofascial network is essential for generating powerful movement of the body while tightly stabilizing the central part of our body.

If we look into the abdominal fascia more closely, we can understand superficial and deep abdominal muscles are truly designed to work together. This picture shows cross-sections of abdominal fascia that was cut in a transverse plane. As you can see, all of the fascial sheaths that separate the oblique muscles and transversus abdominis eventually combine below the arcuate line. The arcuate line is located around the level of the belly button. This anatomical feature would allow all of the abdominal muscles to work together especially around the pelvic junction to optimally stabilize the central area of our body while effectively transferring the force between the upper and lower body. Just like thoracolumbar fascia, abdominal fascia is connected to many more muscles and deserves a more in-depth discussion dedicated to it. But I will talk about it another time so that we don't go too much tangent here.

If we look into the internal oblique more in detail, we can appreciate that a smooth transition between various functions of the abdominal network is possible. As you can see in the first picture from the top, internal oblique connects directly to the thoracolumbar fascia just like the transversus abdominis. In addition, internal oblique is also known to provide proactive stabilization just like the transversus abdominis as shown in the second picture. So in a sense, internal oblique can facilitate proactive modulation of intra-abdominal pressure. Interestingly, this muscle is also one of the vital guy wires in our body just like the external obliques and rectus obliques. The third picture shows that the fascial sheath of the internal oblique splits into half to blend in with the fascial sheaths of the external oblique and the transversus abdominis. This may indicate that our body is designed to use both the intra-abdominal pressure and guy wires together rather than relying on any one of them separately.

Why does our body need both the guy wire and intra-abdominal pressure? According to Dr. Stuart McGill, the intra-abdominal pressure is actually what keeps the longevity of the guy wire function of the superficial muscles. The way that superficial muscles stabilize the spine is by exerting a downward force on both sides. This inevitably increases the compressive force to the spine, which could be detrimental to the body if sustained a long time. Intra-abdominal pressure provides decompression against this force. In picture B, Dr. McGill brilliantly explained the relationship between the superficial muscles and deep core muscles by comparing the intra-abdominal pressure to a pneumatic piston located between two guy wires.

In this section, we looked into how superficial abdominal muscles work like adaptable guy wires that not only coordinate with intra-abdominal pressure but also support movement and stability throughout the whole-body motion. I'd like to emphasize that our entire core is designed to support intra-abdominal pressure, guy wire function, and movement control through the distributed activation rather than through the individual effort of specific muscles. Therefore, our training should aim to optimize such coordinative functions of the core.

References

Cholewicki, J., Juluru, K., & McGill, S. M. (1999). Intra-abdominal pressure mechanism for stabilizing the lumbar spine.Journal of biomechanics,32(1), 13-17.