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The Inner Unit

How many ways can you do a crunch? Well, if you have been reading the muscle tabloids for the past 20 years you could probably come up with well over 100. Today we have classes devoted to nothing but TRASHING people’s abdominals, complete with every variation of crunch, jack knife, side bend and leg raise known to man. Are these classes, or these exercises, really improving the way you look or function, or reducing your chances of back pain?

To find the answers to these questions, in 1992 I began investigating correlation’s between abdominal exercises performed, exercise volume and the postural alignment, pain complaints and overall appearance of my clients. To ensure objective observations of postural alignment and responses to specific exercises, I designed and patented calibrated instruments to measure structural misalignment.

In the first year of recording such information as forward head posture, rib cage posture, pelvic tilt and overall postural alignment, it became evident that those performing high volume sit-up/crunch exercise programs were not showing promising results (see Figure 1)! Not only were those attending "Ab Blast" classes and/or performing high repetition/high volume abdominal routines having a harder time recovering from back pain, they were also having slower or nonexistent improvement in their postural alignment.

FIGURE 1. Poor Posture and Abdominal Training

© Paul Chek Seminars 1999

Those regularly performing crunch and sit-up type exercises frequently demonstrate forward head posture (A); note that when head carriage is normal the dotted line through the cheekbone should fall in the same vertical plane as the sternum and pubic symphysis. (B) As the rectus abdominus becomes chronically shortened, it pulls the chest downward, increasing first rib angle; this is commonly associated with shoulder dysfunction and impingement of the nerves feeding the arm as they exit the cervical spine. (C) As the hip flexors strengthen and shorten from chronic exposure to the sit-ups, leg extension and leg lowering exercises commonly used in abdominal workouts the lower abdominal and hamstring muscles are lengthened, frequently demonstrating positional weakness. The postural changes demonstrated here are common among today’s athletes and can be corrected through improved control and strengthening of the inner unit musculature.

While studying patients and clients who performed high volume abdominal routines, it became very evident that there was a common link. About 98% of those with back pain had weak lower abdominal and transversus abdominis muscles, while those with no current or history of back pain were frequently able to activate the transversus abdominis and scored better on lower abdominal strength and coordination tests. Frequently, to alleviate back pain, I had to suggest that clients stay completely away from any form of sit-up or crunch type exercises. When this advice was adhered to, and exercises for the lower abdominal and transversus abdominis practiced regularly, back pain decreased or was completely alleviated and posture routinely improved.

All the while, some "experts" in the health and fitness industries could be found stating that, "There is no such thing as lower abdominal muscles," while others suggested that the best treatment for back pain was to exercise on machines that isolated the low back muscles. My clinical observations led me to believe both theories were wrong.

In 1987, "Clinical Anatomy of the Lumbar Spine" by Nikolai Bogduk and Lance Twomey was published (1). This book is important in this story because it was Bogduk who made the first real clinical observations concerning how the abdominal and back muscles work together as a functional unit. This occurs via the connection of the transversus abdominis and internal oblique to the envelope of connective tissue (thoraco-lumbar fascia) surrounding the back muscles (Figure 2). Only a few years later, Australian researchers Richardson, Jull, Hodges and Hides began making significant headway into the understanding of how the deep abdominal wall works in concert with other muscles, creating what they would later call THE INNER UNIT (2).

FIGURE 2. Thracolumbar Fascia and Inner Unit

The thoracolumbar fascia system envelops the inner unit musculature to create the body’s own natural weight belt. Activation of the transversus abdominis aids in stabilization of the lumbar spine. When wearing a weight belt, the natural tendency is to push outward against the belt, which inhibits the transversus abdominis and may lead to faulty motor programming and destabilization of the spine! ©Paul Chek Seminars 1999


The Inner Unit became a term describing the functional synergy between the transversus abdominis and posterior fibers of the obliquus internus abdominis, pelvic floor muscles, multifidus and lumbar portions of the longisssimus and iliocostalis, as well as the diaphragm (Figure 3). Research showed that the inner unit was under separate neurological control from the other muscles of the core (2, pg. 49). This explained why exercises targeting muscles such as the rectus abdominis, obliquus externus abdominis and psoas, the same muscles exercised with traditional abdominal conditioning programs used in gyms and athletic programs worldwide, were very ineffective at stabilizing the spine and reducing chronic back pain.


© Paul Chek Seminars 1999

The inner unit is under separate neurological control from the larger outer rectus abdominis, external obliquue and anterior fibers of the obliquus internus. Traditional gym exercises do not condition these key muscles such that their ability to improve spinal stability is enhanced until their recruitment is under automatic reflex control. To accomplish automatic reflex control of the inner unit requires specific isolation training to enhance sensory-motor control. Once control is established, activation of the inner unit must be programmed into all movement patterns commonly used by the host. Failure to condition the inner unit to a high level of specificity often results in spinal injury due to instability.

Exercising the big muscles (prime movers), was not providing the correct strengthening for essential small muscles, such as the multifidus, transversus abdominis and pelvic floor. When working properly, these muscles provide the necessary increases in joint stiffness and stability to the spine, pelvis and rib cage to provide a stable platform for the big muscles. In a sense, as the big muscles (outer unit) became stronger and tighter the delicate balance between the inner and outer units becomes disrupted. This concept is easier to understand using the pirate ship model (Figure 4).


Although the large guy wires (outer unit) support the mast of the pirate ship, its functionality is completely dependant upon the support provided by the small guy wires which are representing the multifidus and inner unit muscles in this analogy.

The mast of the pirate ship is made of vertebra which are held together (stiffened) by the small guy wires running from vertebra to vertebra, just like the multifidus (a member of the inner unit) do in the human spinal column. Although the big guy wires (representing the outer unit) are essential to holding up the mast of the pirate ship (our spine), they could never perform this function effectively should the small segmental stabilizers (inner unit) fail. By viewing the pirate ship’s large guy wires, it becomes easy to see how developing too much tension from the over-use of exercises such as the crunch, could disrupt the posture of the mast, or spinal column in the case of a human.

To better apply the concept of the pirate ship, let’s examine how the inner and outer units work in a common situation such as picking dumbbells up from the floor in the gym (Figure 5). Almost in synchrony with the thought, "Pick up the weights from the floor," the brain activates the inner unit, contracting the multifidus and drawing in the transversus abdominis. This tightens the thoraco-lumbar fascia in a weight belt-like fashion (Figure 2). As this is happening there is simultaneous activation of the diaphragm above and the pelvic floor below. This works to encapsulate the internal organs as they are compressed by the transversus abdominis. This process creates both stiffness of the trunk and stabilizes the joints of the pelvis, spine and rib cage, allowing effective force transfer from the leg musculature, trunk and large prime movers of the back and arms to the dumbbells.


© Paul Chek Seminars 1999

Such functional tasks as picking up dumbbells off the gym floor require synergistic function of the inner and outer units. Failure of the inner unit for any reason, predisposes the spine to forces that frequently can not be effectively stabilized and dissipated, resulting in spinal injury and/or sacroiliac joint injury.

Exercise Rest Reps Tempo Sets

4-Point Transversus Abdominis Trainer





Horse Stance Vertical / Horizontal


10 each arm/leg



Horse Stance Alphabet





When the inner unit is functioning correctly, joint injury is infrequent, even under extreme loads such as pushing a car, tackling an opponent in football or lifting large weights in the gym. When it is not functioning correctly, activation of large prime movers will be no different than a large wind hitting the sail of the pirate ship in the presence of loose guy wires running from vertebra to vertebra in the mast. Any system is only as strong as its weakest link!


The first, and most important step, toward reducing back pain and/or improving posture, which in turn generally improves aesthetics, is to stop all crunch and/or sit-up type exercises until you become proficient at activating your inner unit! Although the assessment procedures for the inner unit are beyond the scope of this article, the interested reader may find detailed information in the video series Scientific Core Conditioning (3). Because inner unit dysfunction is extremely common in today’s working and exercising population, it is safe to assume that everyone needs to start with beginning exercises, even the most elite of athletes.

To begin conditioning the transversus abdominis, use the 4 Point Transversus Abdominis Trainer (4) (Figure 6). For conditioning of the multifidus and related stabilizer and postural muscles the Horse Stance exercises may be used (4,5,) (Figures 7-9). Although the exercises may seem simple from looking at the diagrams here, they are actually very technical and must be executed with exquisite precision (see Scientific Back Training (5) or The Golf Biomechanic’s Manual (4) for more details). These exercises are only a small sample of the number of inner unit exercises available (4,5), but, when done correctly, they are sufficient to make a noticeable difference in the way your body functions.

To get the most from the inner unit exercises shown here it is suggested that the exercises be done 3-4 times per week as an individual workout. To get best results from these exercises while continuing with a traditional gym program, I suggest you stop all crunch and sit-up exercises and replace them with the exercises demonstrated here. Always perform an inner unit exercise as the last exercise of your training session, i.e. perform one exercise after each workout. Alternate through the exercises, selecting either the 4 Point Transversus Abdominis Trainer or a variation of the Horse Stance exercises after each training session. It is very important not to fatigue the stabilizer system before attempting traditional free weight exercises or injury is likely!

If you are implementing the stabilizer exercises into a machine-based program then you may intersperse the exercise among the machine exercises. Because of the inherent stability provided by machines, it is unlikely that you will become injured. As your stabilizer system improves, I suggest progressively replacing machine exercises with free weight exercises, as machine-based programs do nothing to enhance functional strength and stability. Should you begin adding free weight exercises to a machine-based program, you must always perform your stabilizer training after completion of all free weight exercises.


© Paul Chek Seminars 1999


© Paul Chek Seminars 1999


© Paul Chek Seminars 1999 Figure 8A © Paul Chek Seminars 1999 Figure 8B


© Paul Chek Seminars 1999

In a future article, I will discuss such key concepts of Outer Unit training as:


Inner unit training provides essential joint stiffness and the stability needed to provide the large prime movers of the body with a working foundation. When outer unit or prime mover exercises are executed in absence of a functional inner unit, poor posture, unwanted aesthetic changes and musculoskeletal injury are inevitable. For optimal health and performance, the inner unit must not only be functional, but must be maintained with technically correct exercise protocol.


  1. Bogduk, N. & Towmey, L. (1987). Clinical Anatomy of the Lumbar Spine. Churchill Livingstone.
  2. Richardson, C., Jull, G., Hodges, P. & Hides, J. (1999). Therapeutic Exercise For Spinal Stabilization In Low Back Pain. Churchill Livingstone.
  3. Chek, P. (1999). Scientific Core Conditioning Video Correspondence Course. Encinitas: C.H.E.K Institute.
  4. Chek, P. (1999). The Golf Biomechanic’s Manual – Whole In One Golf Conditioning. Encinitas: C.H.E.K Institute.
  5. Chek, P. (1994). Scientific Back Training Video Correspondence Course. Encinitas: C.H.E.K Institute.