Anatomy Made Simple: Inner Muscles vs Outer Muscles

Written by:
Tyler Joyce
December 26, 2022

In my last blog post I talked about how to simplify explaining anatomy to clients by way of the myofascial system. Before we dig into each myofascial chain, I would like to discuss another simple concept that has also dramatically improved the way I treat and educate my clients.

The concept of inner muscles versus outer muscles was introduced by Anders Bergmark in 1989. A professor of solid mechanics at the University of Lund in Sweden, Bergmark was interested in how the spine stabilizes itself. In his study, he explains that the spine must be mechanically stable in the same way as a purely passive engineering structure, and that its stability comes from tension in the muscles, not just the bones themselves.

Bergmark was the first to describe and break down the body into two distinct systems: the local system and the global system. Each system plays an important role in maintaining joint health. Basically the local system stabilizes the joints; and the global system moves the joints.

The Local System

So let’s first look at the characteristics of the local system. It primarily consists of smaller muscles located near the joints, such as the pelvic floor, deep hip rotators, transverse abdominis, multifidi, deep neck flexors, and rotator cuff. Their primary job is to stabilize the spine and other joints to allow for upright posture and functional mobility.

These inner muscles should always be on through continuous muscle activation. They are mostly made up of slow twitch or tonic muscle fibers which are highly resistant to fatigue. So they are ideal for providing joint stability at all times.

These inner muscles stabilize the joints independent to the direction of movement. Whether you move your arms or legs forward or backward, or twist your spine right or left, they will stabilize and protect the joints during any movement pattern.

Additionally, these inner muscles are smart. They have a high density of muscle spindles for more communication with the brain. In fact, they possess a feedforward mechanism which stabilizes the joints even before the movement is initiated. It is this mechanism that protects the joints from being overloaded and injured. 

The Global System

The global system primarily consists of larger muscles which are closer to the surface of the body. Some examples include the lats, pecs, biceps, triceps, quads and hams. They are the muscles that make you look good in the mirror. Their primary job is to move the joints.

These outer muscles have fast twitch or phasic muscle fibers which give them power. But they fatigue quickly. They turn on to perform a specific job (walk, run, squat, lift, push, pull, etc.). Then they turn off, relax and recover until the next job begins.

These outer muscles have a lower density of muscle spindles, compared to the inner stabilizers. So there’s generally less communication with the brain.

Unlike inner muscles, these outer muscles move dependent to the direction of movement. If you want to bend forward and touch your toes, or reach up to the sky with your arms, the global system helps you do that. Meanwhile the local system stabilizes your spine and other joints during these movements to protect them from overload and injury.

Neuromuscular Deactivation

So what happens to these two systems under stress?

Many studies have shown that trauma (injury, concussion, pregnancy, surgery, cancer, etc.) tends to shut down the local system. It’s a process known as neuromuscular deactivation. Basically your inner muscles go dormant and thus your joints are no longer stabilized and protected by the feedforward mechanism that I described earlier.

To make up for this loss of joint stability, the global system will naturally take over. This means the outer muscles must take on two jobs: moving and stabilizing. But they are not built to stay on constantly like the inner muscles. So it’s only a matter of time for them to tighten up like a knot.

A classic example of this is tight hamstrings (outer muscles) compensating for a weak core (inner muscles). It is the body's innate way to protect the spine, by stiffening up muscles further down the myofascial chain. In other words, your hamstrings might be tight due to joint instability at the spine. If not properly addressed, this muscle imbalance will set in motion a cycle of pain that could lead to more serious injuries, like pulling or tearing your hamstrings while running.

Neuromuscular Activation

So how do we treat this? At Activcore, we apply suspension exercises to wake up the dormant stabilizer muscles and reverse the pain cycle. Our approach is called NEURAC (NEURomuscular ACtivation). It's a method of rebooting the body's inner stabilizing system through proprioceptively enriched exercises. Essentially, by having you perform repeated joint motions in a painless "zero-gravity" environment, NEURAC tells your brain that it is safe to turn the joint stabilizers back on. It's like turning up a light switch to muscles that went dim.

Final Thoughts

The moral of my story is to pay homage to Anders Bergmark for making this discovery more than 30 years ago. It has changed the way I look at the body. It has also helped me explain to my clients why they are in pain. Most importantly, regardless of how degenerative someone's spine may look on an X-ray or MRI, it has shown me that joint health relies more on how well the inner muscles are working.

Check out the Redcord suspension system to learn more about how to test and treat the body's inner stabilizing system.


Bergmark, A. Stability of the Lumbar Spine. ACTA Orthopaedica Scandinavica Supplementum No. 30, Vol. 60. 1989

Disclaimer:  The views expressed in this article are based on the opinion of the author, unless otherwise noted, and should not be taken as personal medical advice. The information provided is intended to help readers make their own informed health and wellness decisions.

Tyler Joyce

Co-Founder | Physical Therapist
Tyler Joyce is a 20+ year experienced physical therapist who specializes in helping competitive baseball players and weekend athletes overcome chronic pain and injury through application of the Redcord suspension system. He works at Activcore in Princeton, New Jersey, located just 2 miles from Princeton University.


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