Updated: May 15, 2020
This blog is made into both video and text formats for your convenience. Please pick and choose what works better for you. The contents are exactly the same.
In the previous section, I mentioned that interaction between activation of stabilizing muscles in our trunk and the pressure within the abdominal cavity determines intra-abdominal pressure. By nature, this intra-abdominal pressure is a changing force that is utilized to stabilize our spine and posture in a context-dependent manner. So, let's look into more details with respect to how our body modulates the intra-abdominal pressure. To do so, let's first talk about the anatomy of the abdominal muscles in detail. I am going to start with a deep muscle called transversus abdominis. Transversus abdominis is like a built-in corset in our body. As you can see in this picture, its fiber runs in the transverse plane. In addition, the left and right sides of this muscle are well connected through its connective tissues so that they can work together to put pressure into the abdominal cavity. Transversus abdominis attaches to the cartilage of the entire lower ribcage, the iliac crest of the pelvis, and inguinal ligaments. It also wraps around the trunk and connects with thoracolumbar fascia in the back. So, transversus abdominis truly encircles the entire abdominal walls.
Well, transversus abdominis has received lots of spotlight during the past decade. Particular attention has been focused on its role of predictively providing stability for voluntary movement. As you can see in this graph from Dr. Paul Hodges' review article, transeversus abdominis always activated before the shoulder muscles turned on when the healthy participants were asked to raise their arms. There has been many follow up research that showed that people with recurrent low back pain often showed delayed activation and decreased tone of the transversus abdominis. Such research evidence led many clinicians to believe that altered control of transversus abdominis contributes to chronic low back pain. For this reason, rehabilitation exercises that focus on selectively activating transversus abdominis have become very popular. However, recently more evidence came out to show that diminished control of transversus abdominis is actually the result of the pain rather than the cause of the pain. So, many clinicians are rethinking the current method for training transversus abdominis.
Moreover, the way that the transversus abdominis training is usually instructed may disrupt the natural coordination of the whole abdominal complex. For example, clients are often asked to draw in their belly-button to contract the transversus abdominis while keeping outer layer muscles such as rectus abdominis and obliques relaxed. While this method can be useful to help people become more conscious of proactive activation of transversus abdominis, it is unrealistic to apply this method to many of our daily movements. The problem is that it isolates the transversus abdominis too much when in fact its coordinative function is the key for providing stability.
So let's look further into the coordinative function of the transversus abdominis. Although transversus abdominis had received lots of attention for its predictive activation, let's not forget that diaphragm also plays a role in predictive activation. Another very impactful research by Dr. Paul Hodges and colleagues showed that diaphragm activated before the shoulder muscle was activated when healthy participants raised their arms. And they also said that the onset of diaphragm activation matched with the timing of transversus abdominis onset.
This makes sense because as we have looked at it before, our abdominal cavity is like a pressure chamber. And the diaphragm is the lid of the chamber with the transversus abdominis forming the abdominal wall. If our goal is to increase the pressure for a healthy amount and maintain that pressure level, the movement of the lid and the wall should be coordinated preferably through simultaneous action.
Well, then what about the floor of the chamber? If you have paid attention to what I've been saying, you must be thinking that 'I bet this guy will say that pelvic floor also predictively activates during voluntary movement.' And you are absolutely right! Again, Dr. Paul Hodges and his research team has found that pelvic floor muscles increased its activity level in advance of shoulder muscle activity during arm elevation. Importantly, they suggested that this is pre-programmed anticipatory postural activity.
This means that our body knows how much pressure we need to put into our abdominal cavity to prepare for destabilizing forces that would come from the upcoming movement of our own limbs. And our nervous system chooses to use the diaphragm, transversus abdominis, and pelvic floor to create and modulate this preparatory pressure. Well, that makes sense as they are one of the deepest muscles in our body and they are the ones that enclose the pressure chamber in our body. So far, now we know how diaphragm, transversus abdominis, and pelvic floor coordinate to initiate the preparatory stabilization through intra-abdominal pressure. Now let's look into how this intra-abdominal pressure can be modulated.
As I showed you before, transversus abdominis connects to the thick connective tissue called thoracolumbar fascia in the back to complete the abdominal wall in 360 degrees. Such a connection between transversus abdominis and thoracolumbar fascia forms a brilliant feedback system that keeps our spine stable.
As we talked about in the previous section, outwardly directed pressure within the abdominal cavity pushes the spine backward. This pressure also stretches the transversus abdominis. Lengthening of transversus abdominis, in turn, tightens the thoracolumbar fascia by pulling on it from both sides. Tensioning the thoracolumbar fascia provides the force that pushes the spine forward. As a result, the pressure within the abdominal cavity that pushes the spine backward is countered by the tension of the thoracolumbar fascia that pushes the spine forward. In other words, these two opposing forces form a strong buttress to stabilize the spine. I think this is a brilliant structural design; because the increase of the internal pressure leads to a proportional increase of the thoracolumbar fascia tension by the simple law of physics so that our central nervous system doesn't have to use complex calculations.
Tensioning the thoracolumbar fascia has another advantage which is related to whole-body stability. As you can see in this picture, thoracolumbar fascia is located at the junction between the upper and lower bodies. Due to this anatomical advantage, it connects important extensors of our upper and lower body. This allows the force between the lower and upper body to be transferred smoothly while firmly stabilizing the junction between them. Tensioning of the thoracolumbar fascia by transversus abdominis occurs at a slightly different layer of the thoracolumbar fascia compared to the tensioning through the upper body and lower body muscles shown in the picture on the left. The fact that the thoracolumbar fascia provides stability through multiple layers tells us that we are anatomically designed to coordinate the core stability with whole-body postural stability during movement. There are so many more muscles that are connected to the thoracolumbar fascia and many more benefits of having this fascia in our body. I won't go into too many details here to keep within the topic of interest. However, thoracolumbar fascia deserves another extensive discussion dedicated to it.