The Deep Muscle System ("Inner Core")

The terms, "inner core muscles", "inner core", and "the deep muscle system", are often used interchangeably in rehabilitation, fitness, and wellness settings. However, what does one really mean when he or she refers to this group of muscles? A Google search shows many different descriptions, lists, and explanations of muscles that make up our deep muscle system.

Many physical therapists refer to the deepest muscles located in the region between our pelvis and diaphragm as "the inner core" muscles. When we help patients train and strengthen these muscles, we focus on the following: the breathing diaphragm in the upper most part of the core; the deep and superficial muscles of the pelvic floor located in our pelvis; the transversus abdominis in the front abdominal muscle wall; and the deep lumbosacral multifidi in the low back.

The breathing diaphragm is a dome shaped muscle in our chest. It connects to the mid and lower ribs as well as several other structures. Here it is in its relaxed state, following an exhalation of air:

https://commons.wikimedia.org/wiki/File:Lung_and_diaphragm.jpg

When you inhale, your chest cage expands and your diaphragm contracts and flattens out. When you exhale, your chest cage contracts and your diaphragm relaxes:

https://frcemsuccess.com/respiration/

The pelvic floor muscles span the bottom of the pelvis. They consist of a superficial and a deep layer and help support our pelvic organs. Beside their important role in bowel and bladder control, they also work with the other deep muscles of the inner core to provide stability in the low back, pelvis, and hip when we move. It is fairly common to find dysfunction in these muscles with clients who have bowel and bladder problems as well as pain in their low back, pelvis, or hip region.

Here is a picture looking down at the pelvic floor from above:

PelvicGuru.com

The transversus abdominis is the deepest muscle in our abdominal wall and wraps around the abdomen horizontally. Physical therapists and wellness professionals often refer to it as “the TA” or the “deep abdominals.” Most of our clients are not as familiar with this muscle as they are with the external obliques, internal obliques, and rectus abdominis ("6 pack muscle"), which lie more superficial to the transversus abdominis.

PelvicGuru.com

The deepest muscles in our back, the deep lumbosacral multifidi, connect one vertebra to another in the spine and span one to two intervertebral segments down to the sacrum and pelvis. These muscles play a large roll in controlling shear and torsional stresses in the spine and sacroiliac joints (“SI joints”), which is important for spinal and SI joint stability for those clients who have too much mobility in these regions.

https://en.wikipedia.org/wiki/Multifidus_muscle

The role of the deep muscle system has been heavily researched. Recent studies show that these muscles function differently when compared to the more commonly known superficial front and back muscles of the trunk, such as the “obliques” and rectus abdominis muscles. In clients with low back, pelvic girdle, and / or hip pain, the deep muscles may either be difficult to activate and use, or they may be too tight, or hypertonic. Both are problematic and should be addressed in treatment by your physical therapist. For these types of muscle impairments, a training program is initiated to ensure that they can be appropriately activated; once this occurs, a strengthening program is commenced.


To learn more about an appropriate training program for the deep muscle system, please refer to my blog post, A Training Program for the Deep Muscle System ("Inner Core"). If you have questions regarding your inner core muscles, please consult your physical therapist.