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Core Anatomy

Anatomy of the  Core

As you can see in the diagram above the core is a cylinder like group of muscles which provide stability and strength to the trunk and spine.  We can take a very general non-specific view of some of the musculature and general muscular actions which provide these functions.  This is not meant as an in depth anatomy lesson but as a simple laymans primer to give a basic understanding of core anatomy.

Abdominal Wall: The abdominal wall encircles the trunk, it is made of three layers which include the Transverse Abdominis, the External Oblique and the Internal Oblique muscles.


Photo: Public Domain

These three muscular units encircle the trunk.  They are involved in trunk flexion, although the main trunk flexor is the Rectus Abdominis.  More importantly these muscles are involved in torso twisting, lateral bending, lumbar stabilization and spinal stability.  Although the Transverse Abdominis has received most of the attention in recent core research it is in reality a combination of these three muscles which provides much of our trunk stability.  It’s important to note that although they do assist with flexion this is not their primary function.  Trunk flexion in rehabilitating the core and treating low back pain is actually counter intuititive for many reasons, one of which is the increased disc pressure caused by trunk flexion movements such as sit ups and even crunches.  We will discuss the “how” of rehabilitation later on and specifically discuss which flexion exercises should not be performed.  These muscles also provide function to breathing, which we will discuss a bit more.  A very simple way to visualize these muscles is to compare them to a low back brace often given when you have low back pain.

Diaphragm: The top of the “core” is your Diaphragm muscle which primarily functions in breathing.  The Diaphragm is a thin sheetlike muscle that divides your thoracic cavity from your abdominal region.  The Diaphragm helps provide intra abdominal pressure which is one of the main stabilizing benefits of the “core”.

Photo: Diaphragm CC-BY-SA-2.5

Pelvic Floor:  The pelvic floor muscles are a group of muscles which forms the bottom of our core cylinder, they are essential for controlling intra abdominal pressure within the core, they also provide important muscular and stabilization functions in relation to the sacroiliac joints.

Lumbar Extensors and Stabilizers

Rehabilitation of the core muscles should always go hand in hand with rehabilitating the back extensors.  A common mistake to see is adherence to an exercise program which only addresses the core without addressing the extensors, and the posterior chain which we will discuss later on as well.  The main goal of lumbar extension rehabilitation is not to increase strength, but rather to increase extensor endurance, balance the extensor muscles with the trunk flexor muscles, and provide trunk stability during high risk activities such as trunk flexion.  Keep in mind that the lumbar musculature actually contributes the minority of trunk extension, something which is often overlooked but is actually a huge mistake.  Commonly you will see lumbar extension machines in most gyms, but isolating lumbar extension without incorporating it with thoracic flexion, posterior chain muscle activation, hip hinging and even support from the Latissimus Dorsi musculature can be at best counter productive and at worst cause more injury.

Isolating the lumbar extensors can be detrimental to rehabilitation of low back pain, this commonly seen exercise in a typical gym can be detrimental.

Photo: Extension Isolation CC-BY-SA-3.0

Extensors:  Thoracolumbar extension incorporates primarily three muscle groups, the Longissimus, Iliocostalis and Multifidus muscle groups.

Longissimus/Iliocostalis:  These are the primary extensors of the thoracolumbar region, they both extend from the sacrum at the bottom of the spine and insert into the lumbar and thoracic vertebrae.  Instead of thinking of them as two separate muscle groups it is easier to classify them by function, in this case the thoracic and lumbar portions which each contain both muscles.  The thoracic portion has a much stronger extension potential than the lumbar portion, this portion also is made up of more endurance type muscle fibers than the lumbar portion.  While the lumbar portion does provide extension this extension comes with more disc compression than the thoracic portion’s extension, making the thoracic portion very important in a rehabilitation program especially when we consider the large amount of disc compression which accompanies trunk flexion.  Remember that during trunk flexion your extensors are still working to offset the flexion, this is why a balance between the two opposing muscle groups is crucial and this imbalance is one of the primary dysfunctions noted in low back pain patients.

Photo:  Public Domain

Multifidus:  The Multifidus are small muscles which span from two to three vertebrae which to a small extent are involved in producing extension.  Because of their size they do not produce much extension torque, rather their importance lies in providing minor but important corrections and segmental stabilization within smaller vertebral to vertebral motion versus the more global trunk extension of the longissimus and iliocostalis muscles.  Some of the Multifidus muscles, specifically the deeper layers, are connected to the vertebral joints, they provide direct support and stabilization of the vertebral joints.

The Multifdus muscles are a major part of the rehabilitation process and incredibly important to trunk stability.  Later on we will devote a lot of discussion to these small but important muscles.

Photo: Public Domain

Posterior Chain

The posterior chain is a group of muscles in the back of your trunk, hips and legs.  The posterior chain is the force generating area of the body.  While the core is what stabilizes and protects the spine the posterior chain can be seen as what propels us, from sitting down to sprinting muscular endurance in these muscles is very important.  Additionally it is the posterior chain which allow hip hinging.  Hip hinging is an important concept and it means basically the majority of your trunk flexion comes from the hips and not the low back itself.  Remember when we talked about the thoracic extensor muscles were a more efficient extender of the trunk because of it’s line of action and how it also has more endurance fibers to support prolonged work.  In addition to concentrating more on thoracic extension muscular endurance now we can add rehabilitation to the posterior chain to enhance the stability.  By hip hinging both in exercise and in work we can keep the low back in a “neutral” position and minimize the compressive forces on the spine and discs.

Gluteal muscles:  The gluteal muscles comprise of the Gluteus Maximus, the Gluteus Medius and the Gluteus Minimus.  The Maximus is the largest muscle in the body, it functions to extend and laterally rotate the hip, also providing a balance against the hip flexor the Iliapsoas.  The Gluteus Medius and the Gluteus Minimus support the pelvis providing stabilization during activity, they are especially important during any activity in which you stand or move with one leg, consider that when simply walking there is a portion of the walking gait where you are only supporting your weight with one leg for example.

Hamstrings:  The hamstrings group comprised of the Semitendinosus, the Semimembranosus and the Biceps Femoris. This group flexes the knee and extends the hip.  When the feet are fixed they act to assist with the extension of the trunk.  As a posterior chain muscular group they also act to balance the force generated by the anterior chain muscles.  For example when squatting down they serve to contract against the anterior chain.  They are another habitually shortened and over active muscle group in today’s sedentary lifestyles.

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Iliopsoas:  The Iliopsoas muscle is really a combination of the Iliacus muscle and the Psoas muscle.  Discussion of the Iliopsoas is important because it balances the spinal extensors.  The Iliopsoas runs from the pelvis to the lumbar and vertebrae and is involved in hip flexion, or trunk flexion if the legs are held down.  The Iliopsoas is common to see as a tight muscle which can put pressure in the lumbar vertebrae and discs causing low back pain.  It’s usually contracted and overly activated due to sedentary lifestyles which involve prolonged sitting, driving and prolonged trunk flexion.  What is often overlooked is that the Iliopsoas while tight may only be half of the problem, the other half is the spine extensors which lacks the endurance or coordination to counteract the pressure of the Iliopsoas.  Scrutinizing both of these opposing muscle groups is important.

Photo: Public Domain

Quadratus Lumborum:  The Quadratus Lumborum is very important to lumbopelvic stability.  Running from the lumbar vertebrae to both the rib cage and the pelvis it provides at first glance mainly lateral stabilization, although testing has shown it to be isometrically active with other ranges of motion as well.  What’s interesting about this muscle is that typically this muscle contracts isometrically, that is it does not go through much of a range of motion but rather it stiffens to provide a large global stabilizer.

Photo: Public Domain

Latissiumus Dorsi:  The Latissimus Dorsi runs from the lumbar vertebrae and the thoracolumbar fascia to the humerus.  It is involved significantly in lumbar stabilization.

Photo: Public Domain

Thoracolumbar Fascia:  This is a ligament system which is superficial to the spine, connecting to the thoracic and lumbar vertebrae and the pelvis and connecting laterally to the Transverse Abdominis and Internal Oblique, it is what finishes the “back brace” which encircles our trunk.  As a ligament it is rich in proprioceptive receptors, which we will discuss later on.  The Thoracolumbar Fascia serves as an anchor point for some of the core and extensor musculature, but is also part of a feedback system discussed more in the proprioceptive system.  Note Lumbodorsal Fascia and Thoracolumbar Fascia are used interchangeably.

Photo: Public Domain

Proprioceptive System

Proprioception very simply is our ability to recognize where our body is in time and space and to be able to modulate things like velocity and force based on neural input.  In a very simple sense we can consider the brain sending a message to a muscle to contract, the muscle contracting activates certain receptors in the muscle itself and surrounding ligaments which are concerned with force, velocity, stretch, etc., these receptors then send this signal back to the brain so it can modulate the specific action to avoid injury while accomplishing the task.  Think of this two way communication as very quick and very often, it’s a constant feedback and modulation system.

Proprioception is incredibly important in the rehabilitation of injuries, or simply in the training of a patient/client even in the absence of injury.  Training or rehabilitation which does not have proprioceptive evaluation and rehabilitation as part of its goal has already lost most of its advantage.

Ligaments:  A discussion of proprioception and core stability would be grossly inadequate without a brief discussion of ligaments.  Ligaments are fibrous tissues which connect bone to bone, they are designed for support and stabilization, but are usually also rich in proprioceptive receptors which gives them a large role in the proprioceptive feedback system.  Ligaments typically cross joints where they hold together bones, although ligaments can also surround the joints themselves such as the joint capsules which surround the articular joints in the spine, these joint capsules are extremely important as we will soon discuss.

A simple focus on strengthening stabilizing muscle structures is not enough if there has been tissue damage to a ligament.  The ligament is an elastic band which allows a certain amount of movement, but if the ligament is overstretched it becomes damaged and loses both it’s ability to limit and guide movement, but to also modulate and feedback with the brain about that movement.  Once the ligament has been damaged we can begin to see altered movement patterns not only globally but also in specific joints which can lead to increased stress on surrounding tissues which  must take up functions they were not meant for, and can lead to abnormal wear of the joint resulting in things like osteoarthritis which is the joint eventually breaking down.

Photo: Public Domain

 

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