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The Outer Unit

In the previous article on The Inner Unit, we discussed the function of the transversus abdominis, multifidus, diaphragm and pelvic floor musculature with regard to their significant functions as stabilizers of both the spine and extremities. The main message of this article was that stabilization of the core via the inner unit must always precede force generation by the core or extremities.

The scope of this article will be first to explain the anatomy of the outer unit, second to describe the function of the four sling systems of the outer unit and finally to demonstrate exercises targeting one or all the sling systems in a methodical manner.


The outer unit consists primarily of phasic muscles (Table 1), although there are many muscles such as the oblique abdominals, quadratus lumborum, hamstrings and adductors which serve a dual role, acting in a tonic role as stabilizers and a phasic role as prime movers. To be technically correct, we may say that outer unit functions are predominantly phasic functions (geared toward movement).





Movement/Gross Stability*


a-2 motoneuron

a-1 motoneuron

Susceptibility to fatigue



Reaction to faulty loading



TABLE 1. Properties of Tonic and Phasic Muscles
© Paul Chek 1999

* Phasic muscles are commonly recognized as those muscles primarily responsible for movement, although as presented in this article, many outer unit muscles serve to provide both movement and gross stability.
** Clinical experience shows that muscles prone to weakening are commonly lengthened in relation to their optimal resting length and the relative length of their antagonists.

Modified from: Stretching and Strengthening Exercises, Thieme 1991

Superficial to the musculature of the inner unit are the outer unit systems, sometimes referred to as slings. The Deep Longitudinal System (1,2) (DLS) is composed of the erector muscles of the spine and their investing fascia. The spinal erectors communicate with the biceps femoris through the sacrotuberous ligament of the pelvis and to the lower extremity via the peroneus longus muscle (Figure 1).

Superficial to the musculature of the inner unit are the outer unit systems, sometimes referred to as slings. The Deep Longitudinal System (1,2) (DLS) is composed of the erector muscles of the spine and their investing fascia. The spinal erectors communicate with the biceps femoris through the sacrotuberous ligament of the pelvis and to the lower extremity via the peroneus longus muscle (Figure 1).

the_outer_3.jpg (12331 bytes) FIGURE 1. THE DEEP LONGITUDINAL SYSTEM
© Paul Chek 1999

Here the actions of the DLS can be seen in a boy running bases in a baseball game. As the right leg goes through swing phase there is posterior rotation of the right ilium relative to the sacrum (A), assisting in what is termed form closure or passive closure of the SI joint. In preparation for heel strike in the late swing phase of gait, the biceps femoris works to control both hip flexion and knee extension. The action of the biceps femoris is transferred upward through the sacrotuberous ligament (B), assisting in force closure of the sacroiliac joint. There is a dual action in the lower leg with the contraction of the biceps femoris causing tension through the peroneus longus (C), which in concert with the anterior tibialis stabilize the foot and ankle, creating a working platform the body can move across. When the foot strikes the ground, kinetic energy will be captured in the thoracolumbar fascia (D) for use in the propulsive phase. Kinetic energy will be dissipated through the paraspinal system and should be nullified before reaching the occiput (1,2,4,6).

The Posterior System (1,2,4) (PS) or sling consists primarily of the latissimus dorsi and the contralateral gluteus maximus (Figure 2).

© Paul Chek 1999

In the propulsive phase of gait, there is a phasic contraction of the gluteus maximus, which occurs in concert with that of the contralateral latissimus dorsi as it extends the arm as a means of counter rotation. This timed contraction produces tension in the thoracolumbar fascia that will assist in stabilizing the sacroiliac joint of the stance leg. Vleeming quotes Margaria, who states that the posterior oblique system may act like a smart spring, storing and releasing energy in the thoracolumbar fascia mechanism (A), which would reduce the metabolic cost of walking (4).

The Anterior Oblique System (AS) (1, pg. 59) consists of a working relationship between the oblique abdominal muscles and the contralateral adductor musculature and the intervening anterior abdominal fascia (Figure 3).

wpe15.jpg (5341 bytes) FIGURE 3. THE ANTERIOR OBLIQUE SYSTEM
© Paul Chek 1999

The adductors work in concert with the internal oblique and opposite external oblique abdominal muscles to both stabilize the body on top of the stance leg and to rotate the pelvis forward, positioning the pelvis and hip optimally for the succeeding heel-strike.

The Lateral System (LS) (Figure 4) consists of a working relationship between the gluteus medius, gluteus minimus and ipsilateral adductors (1,3). Porterfield and DeRosa (3) indicate a working relationship between the gluteus medius and adductors of one leg with the opposite quadratus lumborum. The author s clinical experience strongly suggests that the oblique musculature is synergistic with the quadratus lumborum during lateral sling functions such as those seen in Figure 4.

wpe17.jpg (6579 bytes) FIGURE 4. THE LATERAL SYSTEM
© Paul Chek 1999

As she raises her leg in step class, the body must be stabilized atop the left leg. Contraction of the left gluteus medius and adductors stabilize the pelvis in concert with activation of the contralateral quadratus lumborum, which works to elevate the pelvis enough to ensure adequate freeway space for the swinging leg. Should the lateral system fatigue and the exerciser continue to follow the instructor, she will be forced to progressively rely on passive supports such as ligaments and discs in the pelvis and spine. Such lateral system dysfunction is a common source of injury in the back and legs.



To better understand how the DLS and PS function, we will explore their actions in what is certainly one of our most primal movement patterns, gait (walking). While walking, there is a consistent low level activation of the inner unit muscles to provide the necessary joint stiffness to protect the joints and support the actions of the larger outer unit muscles (5). Recruitment of the inner unit muscles will fluctuate in intensity as needed to maintain adequate joint stiffness and support, as the inertial forces of limb movement, kinetic forces and intradiscal pressures increase.

As we walk, we swing one leg and the opposite arm forward in what is termed counter rotation. Just prior to foot strike, the hamstrings become active (6). The DLS, uses the thoracolumbar fascia and paraspinal muscle system to transmit kinetic energy above the pelvis, while using the biceps femoris as a communicating link between the pelvis and lower extremity. For example, Vleeming shows that the biceps femoris communicates with the peroneus longus at the fibular head, transmitting approximately 18% of the contraction force of the biceps femoris through the fascial system into the peroneus longus (4).

Interestingly, the anterior tibialis, like the peroneus longus, attaches to the plantar side of the proximal head of the first metatarsal. The significance of this relationship is appreciated when considering that there is recruitment of the biceps femoris and the anterior tibialis just prior to heel strike in concert with the peroneal muscles, which act as dynamic stabilizers of the lower leg and foot (7). Dorsiflexion of the foot and activation of the biceps femoris just prior to heel strike, therefore, serves to wind up the thoracolumbar fascia mechanism as a means of stabilizing the lower extremity and storing kinetic energy that will be released during the propulsive phase of gait (4).

As you can see by observing Figure 2, just prior to heel strike the gluteus maximus reaches maximum stretch as the latissimus dorsi is being stretched by the forward swing of the opposite arm. Heel strike signifies transition into the propulsive phase of gait, at which time the gluteus maximus contraction is superimposed upon that of the hamstrings (6). Activation of the gluteus maximus occurs in concert with activation of the contralateral latissimus dorsi, which is now extending the arm in concert with the propelling leg (1,2,4,5). The synergistic contraction of the gluteus maximus and latissimus dorsi creates tension in the thoracolumbar fascia, which will be released in a pulse of energy that will assist the muscles of locomotion, reducing the metabolic cost of gait.


The concept of the Anterior Oblique System (AS - Figure 3) appears to have become popular very recently (1,4). A review of the literature shows that spiral concept of muscle-joint action was understood as integral to human movement and corrective exercise by Robert W. Lovett, M.D. (8) and by anatomist Raymond A. Dart in the early 1900 s (9,10).

To clarify the point that movement originates in the spine (core), Gracovetsky describes torque generation by an S-shaped spinal column (11). He exemplifies the point that the legs are not responsible for gait, but merely instruments of expression, by showing that a man with no legs whatsoever can walk (2). In both these examples of what Gracovetsky calls the spinal engine (11), it is evident that the kinetic and potential energies of the oblique abdominal musculature, in concert with other core muscles, are primarily responsible for creating the torque that drives the spinal engine; the oblique abdominal being best situated to create rotary torque.

The oblique abdominals, like the adductors, serve to provide stability and mobility in gait. When looking at the EMG recordings of the oblique abdominals during gait (Basmajan, 12) and superimposing them upon the cycle of adductor activity in gait demonstrated by Inman (6), it is clear that both sets of muscles contribute to stability at the initiation of the stance phase of gait, as well as to rotating the pelvis and pulling the leg through during the swing phase of gait. As the speed of walking progresses to running, activation of the anterior oblique system becomes more prominent.

The AS is very important, particularly in sprinting, where the limbs and torso must be accelerated. The demands on the AS are great in multi-directional sports such as tennis, soccer, football, basketball and hockey. In such sporting environments the AS must not only contribute to accelerating the body, but also to changing direction and decelerating it. One need not see an EMG study to appreciate the strong contribution of the AS; just ask anyone that has experienced an abdominal strain! Accelerating, decelerating and changing directions are all activities that result in immediate pain in the presence of both abdominal and groin strains or tears.

AS functions can be appreciated when running in sand. Because sand gives away during the initiation of the stance and propulsive phases of gait, the impulse timing of ground reaction forces is disrupted, resulting in poor use of the thoracolumbar fascia, or what Margaria (4) calls the smart spring system. The result is that you must muscle your way through the sand. Many athletes having performed sand sprints, will note abdominal fatigue in the following day or two after the sand sprints. This is due to the increased activation of the AS to compensate for the lost kinetic, potential and muscular energy, which is usually stored and released in part by the thoracolumbar fascia system. Gracovetsky states that wearing soft soled sporting shoes,as athletes often do today, can easily disrupt the body s timing mechanism, which could very well result in increased work and may result in injury (2).

During explosive activities, such as swinging a sledge hammer (Figure 5), the AS serves critical function, stabilizing as in gait, yet assisting in propelling the hammer. Trunk flexion and rotation, as a closed chain movement atop of the lead leg, is generated by the adductors as they assist in trunk flexion and internal rotation of the pelvis and assisted by gravity. Activation of the adductors occurs in concert with activation of the ipsilateal (stance leg side) internal oblique and contralateral (throwing arm side) external oblique, pulling the trunk in the necessary direction to propel the shoulder/arm complex. The forces of the shoulder/arm unit summate with those of the legs and trunk below to produce a powerful hammer swing. Here one can clearly see the phasic functions of the AS at work.

© Paul Chek 1999

The construction worker’s weight shifts forward from the back to front leg as he swings the hammer. With acceptance of weight on the front foot, the kinetic chain is closed, allowing the adductors to work in concert with the ipsilateral internal oblique and contralateral external oblique to explosively flex and rotate the trunk, supporting and driving arm action.


Porterfield and De Rosa (3) suggest that functional anatomy dictates that the lateral system provide essential frontal plane stability. While walking, the LS will be active at heel strike (initiation of stance phase), providing frontal plane stability. This is accomplished by a force-couple action between the gluteus medius and minimus pulling the iliac crest toward the stable femur while the opposite quadratus lumborum and oblique abdominal musculature assist by elevating the ilium. This action is necessary to help create the freeway space needed to swing the leg in gait, particularly when you consider the terrain we ambulated across during developmental years.

During functional activities such as participating in Step class (Figure 4) or simply walking up stairs (Figure 6), the LS plays a critical role, stabilizing the spine in the frontal plane. Stability in the frontal plane is very important to the longevity of the lumbar spine because frontal plane motions of the lumbar and thoracic spine are mechanically coupled with transverse plane motions; excessive amounts of either will quickly aggravate spinal joints

© Paul Chek 1999

Activation of the lateral system provides necessary support during such activities as walking up stairs. Many people are injured carrying heavy suitcases, which overload the lateral system, resulting in muscle, ligament and/or joint injury.

The LS provides stability that not only protects the working spinal and hip joints, but is a necessary contributor to overall stability of the pelvis and trunk. Should the trunk become unstable, the diminished stability will compromise ones ability to generate the forces necessary to move the swing leg quickly, as required by many work and sports environments. Attempts to move the swing leg, or generate force with the stance leg during gait and other functional activities, can easily disrupt the sacroiliac joints and pubic symphysis and cause kinetic dysfunction in joints throughout the entire kinetic chain.

A classic example of distal expression of LS dysfunction was illustrated by Sahrmann (13). She described a lateral shift of an athlete s center of gravity over the subtalar joint while going through the stance phase of gait (Trendelenburg s Sign) resulting in an inversion ankle sprain. Since attending her course in 1992, the author has found gluteus medius weakness and contralateral low back pain due to quadratus lumborum overload common among athletes exhibiting recurring ankle sprain.


Although the outer unit is thought of as a phasic system, (a system for moving the body) by most, it does provide crucial stabilizer functions. We must remember that the muscles of the inner unit are relatively small, with less potential to generate force than the large outer unit muscles.

The inner unit muscles are concerned with providing joint stiffness and segmental stability. They work for extended periods of time at low levels of maximal contraction. The outer unit muscles, while very well oriented for moving the body, are also very important to stability, often serving to protect the inner unit muscles, spinal ligaments and joints from damaging overload. For example, consider this common scenario:

The coach instructs two football players to engage in an oblique medicine ball toss drill. One player is much bigger and stronger than the other, as the other player finds out as he attempts to catch the 8 kg. (17.5 lbs.) medicine ball traveling at him at over 60 kph (40mph)! The smaller player does not have the strength in his outer unit to decelerate the ball and is forced into end-range trunk flexion and rotation, traumatizing his lower lumbar discs, ligaments and intrinsic spinal muscles (multifidus, rotatores, intertransversarii and interspinales).

Regardless of how well conditioned the inner unit of the smaller player may have been, lack of strength in his outer unit relative to his partner, or the demands of the task at hand resulted in inner unit overload and injury! With careful scrutiny of most activities in the work or sports environment, you will find that good eccentric strength in the outer unit systems is critical to protecting the inner unit from damage. Protection of the inner unit through proper conditioning of the outer unit is a worthy goal when one considers that optimal proprioception is dependant upon the health of the inner unit muscles and the joints they protect!


Now that we have taken a detailed look at the anatomy and function of the outer unit, it should be clear that modern exercise technology has taken us a long way from conditioning the outer unit systems the way they were designed to work! For example, can you see any way the following exercises condition the outer unit systems in such a way that they could provide carryover to most functional work or sport activities?

I could go on, filling the page with exercises that do very little to enhance function. Many of you will no doubt recognize the above exercises as traditional bodybuilding exercises. What has happened? Only a few years back in the days of Bill Pearl, bodybuilders were building beautiful physiques with functional exercises like squats, lunges, barbell rows, cable rows, deadlifts and the like. Today, we are overrun by the machine era, the era of the aesthetic emotional hook so carefully used by the machine manufacturers to convince you that you will look better using their machines.

Our bodies were not designed to exercise on machines, they were designed to function in the wild. We are designed for three-dimensional freedom, not two dimensional guided, unrealistic exercise that encourages muscle imbalance between those muscles used to stabilize and those used in a phasic manner for any given movement. The motor programs developed on machines are of little use to the body for anything other than pushing or pulling the levers of that very machine during that very exercise. This limits functional carryover to those that operate cranes, excavators, bulldozers, and buses for a living; they are about the only people that must apply force to levers in a seated, supported, two-dimensional environment.


Using your new understanding of the outer unit systems, carefully analyze such functional pushing and pulling exercises as the single arm standing cable row (Figure 7) (14) and standing single arm cable push (Figure 8) (15). You will see all outer unit systems being conditioned simultaneously, just as they are used in most of our work and sport environments.

wpe1B.jpg (5255 bytes) FIGURE 7. STANDING CABLE ROW
© Paul Chek 1999

Although the Standing Cable Row exercise conditions the anterior and lateral systems, it is an excellent form of conditioning for the posterior oblique system. As you can see, puling on the cable with a split stance provides an excellent opportunity for the latissimus dorsi and contralateral gluteus maximus to work together in accordance with functional anatomy.

wpe1C.jpg (5163 bytes) FIGURE 8. STANDING CABLE PUSH
© Paul Chek 1999

As the exerciser pushes the handle of the cable machine forward there is a concerted effort by the anterior oblique system to produce the necessary leg and trunk stabilization and motion to support the smaller shoulder and arm musculature.

Medicine ball exercise, like free weight training, was much more popular in the 40s, 50s, 60s, and 70s than it is today. Great athletes of those decades used exercises such as the oblique medicine ball toss and push-pass (16), not to mention almost 100 other variations of medicine ball exercises (17).

The Swiss Ball can be used to effectively condition the outer unit systems in three-dimensional movement while providing unloading opportunities for those recovering from injury. For example, analyze the Supine Lateral Ball Roll (18) (Figure 9) and see if you can determine which outer unit systems are being used and categorize them in the order of demand during this exercise. This will be a great start toward a better understanding of functional exercise.

wpe1D.jpg (7187 bytes) FIGURE 9. SUPINE LATERAL BALL ROLL
© Paul Chek 1999

Can you determine which sling systems are most active and prioritize them in order of contribution during this exercise?


The outer unit consists of four systems, the deep longitudinal, posterior oblique, anterior oblique and lateral. These systems are dependent upon the inner unit for the joint stiffness and stability necessary to create an effective force generation platform. Failure of the inner unit to work in the presence of outer unit demand often results in muscle imbalance, joint injury and poor performance. The outer unit can not be effectively conditioned in patterns of movement that carryover to function when using modern bodybuilding machines. Effective conditioning of the outer unit should include exercises that require integrated function of the inner and outer units, using movement patterns common to any given client s work or sport environment.


  1. 1. Lee, D. (1999). The Pelvic Girdle. 2nd Ed. (pg. 60) Churchill Livingstone.
  2. 2. Gracovetsky, S.A. (1997), Linking the spinal engine with the legs: a theory of human gait. (pg. 243 251). In: Movement, Stability & Low Back Pain The essential role of the pelvis. Vleeming, A., Mooney, V., Dorman, T., Snijders, C. & Stoeckart, R. (Eds.) Churchill Livingstone.
  3. 3. Porterfield, J.A. & DeRosa, C. (1991). Mechanical Low Back Pain Perspectives in Functional Anatomy. W.B. Saunders Co.
  4. 4. Lee, D. & Vleeming, A. (1999). Movement, Stability and Low Back Pain.(Course Notes). St. Charles Hospital and Rehabilitation Center, New York January 30, 1999 February 1, 1999.
  5. 5. Richardson, C., Jull, G., Hodges, P. & Hides, J. (1999). Therapeutic Exercise For Spinal Segmental Stabilization In Low Back Pain. Churchill Livingstone.
  6. 6. Inman, V.T., Ralston, H.J. & Todd, F. (1981). Human Walking. Williams and Wilkins.
  7. 7. Travell, J.G.& Simons, D.G. (1992). Myofascial Pain and Dysfunction The Trigger Point Manual Vol. 2. Williams and Wilkins.
  8. 8. Lovett, R.W. (1912). Lateral Curvature Of The Spine and Round ShouldersP. Blakiston s Son & Co. Philadelphia.
  9. 9. Dart, R.A. (1947). The Double-Spiral Arrangement Of The Voluntary Musculature In The Human Body. Surgeons Hall Journal Vol. 10 Number 2, October, 1946 March 1947.
  10. 10. Dart, R.A. (1950). Voluntary Musculature In The Human Body The Double-Spiral Arrangement. The British Journal Of Physical Medicine December, 1950 Vol. 13. No. 12 New Series
  11. 11. Gracovetsky, S. (1988) The Spinal Engine. Springer-Verlag Wien, New York.
  12. 12. Basmajian, J.V. & De Luca, C.J. (1979). Muscles Alive Their Functions Revealed by Electromyography 5 th . Ed. (pg. 386-387). Williams and Wilkins.
  13. 13. Sahrmann, S. (1992). Diagnosis and Treatment Of Muscle Imbalances Associated With Regional Pain Syndromes Level I (Course Notes) Los Angeles, CA. September 19-20, 1992
  14. 14. Chek, P. (1997). Gym Instructor Video Series Volume 3 Rows, Pulls, Chins and the Deadlift. Video. C.H.E.K Institute, Encinitas, CA.
  15. 15. Chek, P. (1997). Gym Instructor Video Series Volume 2 Pushing and Pressing Exercises. Video. C.H.E.K Institute, Encinitas, CA.
  16. 16. Chek, P. (1996). Paul Chek s Medicine Ball Workout Video C.H.E.K Institute, Encinitas, CA.
  17. 17. Chek, P. (1996). Paul Chek s Medicine Ball Training Correspondence Course. C.H.E.K Institute, Encinitas, CA.
  18. 18. Chek, P. (1996). Swiss Ball Exercise Correspondence Course C.H.E.K Institute, Encinitas, CA.

609 South Vulcan Avenue, Suite 101, Encinitas, CA 92024, U.S.A. Ph: (760) 632-6360 Fax: (760) 632-1037 1-800-552-8789 e-mail: web page:

© Paul Chek 1999