At the Core of You – The Deep Front Line

We hear the word everywhere – core. Strengthen your core. Work your core. Use your core. Feel your core. But what does it really mean – core?

Many years back I joined a local gym and hired a coach to train me for a while. A few sessions in I asked him what he thought the core was. He was a bit surprised over my question. Obviously, no client ever asked him that before me. People, in general, tend to not question a fitness coach about their general knowledge of anatomy. I must admit, though, this is NOT a basic anatomical question. Most of us believe the commonly connoted idea that the core equals our abdominal wall. Some better informed people throw in the spinal muscles (although not clear on which ones those may be) and may go in deeper detail regarding the four layers of the abdominal muscles. Great! I have no issue with this idea and I am not condemning those who aren’t sure because the core I am talking about is not often, if at all, mentioned in anatomy books and certainly not taught in most reputable personal training courses.

Due to the nature of physical yoga practice the Deep Front Line (DFL)* is in center focus in asana (posture). This is the main reason why, unless taught by a skilled teacher, the majority of practitioners at the novice level (and sometimes beyond) do not grasp the true purpose of the steadily held postures. This aspect bothers some people to the point of either boredom or annoyance. These are the individuals I often hear saying, “yoga is not for me, I can’t stand the pace, I need more movement”, or something along those lines.

This is unfortunate. Yoga is the art of coming back to ourselves, both in the mental-emotional and physical sense. It is a centering process which seeks to find the “inner hub”. It may come as a surprise to most that our breathing mechanism is inseparable from the core activation process. The breath gives life to our postures through the balance of its two aspects, Brahmana and Langhana, the opposing actions of expansion (inhalation) and reduction or contraction (exhalation). Behind it all there is a muscle called the diaphragm that separates the “airy” part of our torso from the “watery” part below it. Both of those areas can be involved in the breath as the muscles activate and relax around the tissues. The initiator – and the epicenter of – the breath is our diaphragm.

Without going too deep into the mechanism of breathing, let’s just establish that the movements of the breath creates a corresponding movement in the spine. Usually, the inhale causes the spine to extend and the exhale causes it to slightly flex forward. This gets the small muscles around the spine, called the multifidi, on the fifth and deepest layers of the back that runs from the sacrum to the skull and help erect and rotate the spine, involved. However, even these deep spinal muscles don’t cut close enough to the deep core line, merely because they are too far out posterior to it. To find this fascial line you must look even deeper.

The DFL starts deep in the soles of the feet and tracks up through the inner ankle as it continues into the tibialis posterior and the two main toe flexors, the digitorum and the hallucis longus.  This deep posterior compartment flows into the ligamentous capsule of the knee that contains the popliteus muscle situated behind the knee.  The adductor group represents two tracks in this line, one posterior, one anterior. Posterior it is the adductor magnus and the connective tissue separating it from the hamstrings. This line attaches to the ischial tuberosity (sit bones). Although the deep lateral rotators are connecting to this posterior chain via the gluteus maximus, the fibers in these muscles run at almost a right angle to the ones we have listed before, so our source dismisses this group of muscles from the DFL. The obturator internus, however, is implicated due to its fascial connection to both the adductor magnus and the pelvic floor muscles. Specifically, the levator ani (anal muscles, part of the pelvic floor) are implicated as the continuation of the DFL up towards the axial skeleton. 

Anteriorly, the DFL progresses in a different direction, starting from above the knee where it takes a wide curved line up through the fascial lines of the sartorius and the adductor longus. This is not a flat track, however, but rather a “snail-like” three-dimensional one. The most important aspect of this line is its continuity into the psoas muscle which is the deep stabilizer and flexor of the pelvis. This muscle is classified as multiarticular or polyarticular due to the fact that it acts on many joints. The psoas major serves as a “guy-wire” between the spine and the leg, acting in synergy with surrounding muscles to complete its main mission of stabilization. It also serves as a deep hip flexor and our source lists it as a lumbar flexor in terms of its upper portion acting on the lumbar spine, yet a lumbar extensor as the function of its lower fibers. All this may seem like a hiatus from our main topic but it is actually a crucial point to make when looking at the argument of lower back pain and “core strength” associations. 

At the top of the psoas major (and minor – which is underdeveloped in about 50% of the population), at the 1st lumbar vertebra or often the 12th thoracic vertebra, there is an important junction point of these muscles: the crura and similar posterior attachments of the diaphragm, called the thoracolumbar junction. Of this meeting point many functions originate: breathing, walking, assimilation and elimination, gut reaction (via the celiac or solar plexus), etc. It is also a very important point of stability and balance; the true core of the core, if you will. 

There are some local muscles that support the iliolumbar structure and provide further stabilisation here alongside the psoas. The iliacus is a functional part of the psoas complex, covering the inner crest of the ilium (hip bone) It functions as a hip flexor and external rotator, and it plays a key role in maintaining correct posture. The quadratus lumborum, better known as the QL, is also a main stabilizer that aids the body in standing erect, and it plays a key role in backward bending and side flexion as well. Perhaps a lesser known fact is that it participates in the breathing process as it acts on the 12th ribs during forced exhalation. 

Before we head higher, there is a “tail” of the DFL that we must mention. Instead of following the two separate lines down the path of the psoas, we track straight down through the lumbar spine onto the sacral fascia and the anterior surface of the coccyx (tailbone). From here, this line that forms the pelvic floor proceeds forward to the inner surface of the pubis (pubic bone) and connects with the lower fibers of the transversus abdominis, the deepest layer of the four layered abdominal wall. This also includes the umbilicus and the myofascia and viscera leading up to the front ribs. 

From the line of the diaphragm, the DFL splits into 3 tracks, posterior, middle, and anterior. The posterior track travels up along the anterior longitudinal ligament (the front line of the spine) all the way to the occiput, the back of the skull. The muscles involved here are the longus capitis, longus colli, rectus capitis anterior (collectively abbreviated as “ALL”), as well as the scalene muscles which play a role in the stabilization of the neck and head. 

The middle track of the upper DFL follows the central tendon that flows into the pericardium (the fascial sack of the heart) and also includes the parietal pleura of the lungs, the esophagus and the pulmonary vasculature of the heart. This track of tissues joins the ALL muscles of the neck and part of it splits and continues into the deep front arm lines through its neurovascular bundles. However, most of this bundle of fascia travels up to the posterior side of the pharynx, including the pharyngeal constrictor muscles and goes as high as the occiput via the stylohyoid muscles (short muscles arching between the neck to the skull).

The upper anterior track reaches from the diaphragm to the sternum (breast bone) and continues up, through the sternohyoid muscle, to the hyoid bone and to the bottom of the jaw and to the root of the tounge. It is interesting to note that our source likens this area to the structure of the pelvic floor musculature. The DFL ends in the fascia of the scalp via the muscles that connect our jawline to the skull, the masseter and temporalis muscles. 

As you can see, the deepest musculature of the body that connects us to our center and keeps us erect and vital, amongst other very important roles it has, cannot be simplified down to the abdominals. It is naive to believe that any muscle in the body can act alone without its continuous connections to a fascial line and thus other muscles. If you think about it a bit, working with the human body necessitates this knowledge. Leaving out any aspect of the DFL will shortchange the person with an aim to better his or her condition of posture, stability, strength, agility, and speed. 

Sources:

Anatomy Trains, Myofascial Meridians for Manual & Movement Therapists (3rd edition, 2014, Elsevier Ltd) – Thomas W. Myers;

Yoga Anatomy Principles – Lectures by Leslie Kaminoff