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April 20, 2022

What is (and is not) the “Core”

After recently hearing a yoga teacher insist that foot strengthening exercises were “core exercises”, it got us thinking: what exactly is the core? And how can we help the yoga community apply this term more precisely?

“Core” is not a medical term, and does not have a universal, clear definition. It can (and will) be used loosely — especially by fitness and movement instructors, including yoga teachers. 

Over the years, we have heard yoga teachers call all of the following poses “core poses”:

  • Tadasana
  • Navasana*
  • Hasta Padangustasana

Since the muscles of your trunk are often working in any position outside of savasana, it quickly gets silly how many different poses are referred to as “core poses”.

Clinical perspective

Clinicians usually define the core by the muscles that control the pressure in the abdomen, including:

  • the respiratory diaphragm,
  • pelvic floor muscles,
  • abdominal wall muscles,
  • and multifidus (a lumbar extensor / stabilizer muscle)

Responsive abdominal pressure control is fundamental to the stability of the spine. That, in turn, enables the whole of the body to transmit forces that would be unsustainable without this pressurized system (imagine an aluminum can containing a carbonated drink — the walls of the can maintain the pressure).

In particular, the abdominal wall and the posterior muscles (the multifidi) are repeatedly used in research definitions of “core.” 

Here’s a link to one research example, “​​Trunk muscles on the L4-L5 scan were classified into 2 groups: 1) psoas and 2) core muscles”. Here’s another link to published research distinguishing psoas from core stabilizing muscles. 

To be fair, one can also find published research that includes psoas, or other muscles unrelated to the pressure system of the trunk, in the definition of “core”. 

Again, “core” is not clearly defined, even in the scientific literature, but the pressure system is an excellent, medically accepted clarification.  

What about the glutes?

Some insist that the gluteal muscles, or other muscles around the hip, are also part of the core. While the gluteal muscles work closely with the trunk, and often co-contract (gluteals with lumbar extensors or obliques, adductors with transversus abdominus, etc.), they do not directly move the spine or contribute to pressure in the trunk.

Is Psoas a Core Muscle?

What about Psoas? Isn’t psoas in the abdomen?

Psoas major and psoas minor connect directly to the spine. They can move the spine or stabilize the spine, depending on the relationship. 

Your definition of core is, of course, allowed to include psoas. However, if it does, one should understand the role of iliacus (which shares a tendon with psoas), open vs. closed chain movement, and position vs. effort. (These subjects are more in depth than can be covered in this post). 

Like a photon, psoas plays more than one role in the body. Its role depends on the position, the angle and the direction of any resistance (like gravity). It is not exclusively, nor predominantly, a “core” muscle as it does not change the pressure within the torso.

However you define core, everything is not everything

At Yoga Anatomy Academy, we use “core” to indicate the muscles that pressurize the torso. However, there is wiggle room in how any individual yoga instructor could, or should, use this term.

Core itself is not a medical term, which makes it ok that it morphs.

But everything is not everything. When a yoga instructor speaks of foot arch strengthening as “core” strengthening, that is outside of any anatomic understanding of “core”.  (Or else my fingers would be giving me a core workout as I type this.)

Knowing fully well that these exercises will not strengthen your core, we still recommend foot strengthening! Try out our cheekily named online course, SOLEstice, for 2 hours of guided foot exercises that will make you *wish* you were doing core work.

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