Core training part I: Inner and outer unit

Research defined two major stabiliser systems of the body, the inner unit and outer unit.

The inner unit tonic muscles

The inner unit refers to the functional synergy between the deep muscles of the core.
The inner unit is composed of the Transversus abdominis, the posterior fibres of the Obliquus internus abdominis, the diaphragm, the pelvic floor muscles, the Multifidus and lumbar portions of the Longissimus and Iliocostalis. These muscles have their origin or insertion at the vertebrae and generate little or no movement during activation. Contraction of these deep core muscles provides segmental stabilisation of the spine.

The outer unit phasic muscles

The outer unit consists of many muscles such as the Obliquus externus, Obliquus internus, Erector spinae, Latissimus dorsi, Gluteus muscles, the Quadratus lumborum, adductors and hamstrings. The outer unit muscles have an important stability function when the body is under load (lifting weights) or during high-speed movements.
The outer unit controls the range of motion, generates movement and provides gross stability.

The inner unit

The inner unit muscles are tonic muscles that function as stabilisers. They effectively stabilise the spine and sacroiliac joint at low levels of contraction with low susceptibility to fatigue.

Coordination is critical for proper stabilisation. The ability of the inner unit muscles to contract prior to force production of phasic muscles (geared toward movement) is more important than their strength.

Research shows that in people with no history of low back pain, the Transversus abdominis fires 30 milliseconds before arm movements and 110 milliseconds before leg movements.

Prior to movement of the body the inner unit is activated, contracting the Transversus abdominis and the Multifidi. Because the fibres of the Transversus abdominis are horizontally orientated, the umbilicus is drawn in toward the spine on contraction. This drawing in of the abdominal wall compresses the internal organs. The up- and downward pressure generated by the compressed internal organs activates the diaphragm and the pelvic floor muscles. This simultaneous activation of the inner unit muscles stiffens the spine and provides segmental stabilisation.

Contraction of the Transversus abdominis plays a very important role in inner unit stabilisation for the following reasons:

  1. The drawing in of the abdominal wall on Transversus abdominis contraction increases the intra-abdominal pressure.
  2. The Transversus abdominis and Obliquus internus abdominis are connected with the thoracolumbar fascia. The contraction of these muscles tightens the thoracolumbar fascia in a weight-belt like fashion. The thoracolumbar fascia attaches to the spinous and transverse processes of each lumbar vertebra. The generated lateral tension on the thoracolumbar fascia by the contraction of the Transversus abdominis and Obliquus internus abdominis, stabilises each vertebra.
  3. The Multifidus and extensors of the back are enveloped in the thoracolumbar fascia. When these muscles contract, they will expand within the confined area of the thoracolumbar fascia. The increased intra-compartmental pressure produces an extension force. This is referred to as the hydraulic amplifier mechanism. It is mathematically proven that the hydraulic amplifier mechanism increases the strength of the back extensors by 30%.
    To better understand this mechanism, imagine the inner tube of a bicycle being glued on the back of the vertebra. On inflating the inner tube, it will stiffen and cause the spine to extend.

The outer unit

Although the outer unit is a phasic system, with large muscles that are very well oriented to produce force and move the body, it also plays an important role in stabilisation. The outer unit consists of four myofascial systems that generate movement and stabilise the body:

The deep longitudinal system

In the deep longitudinal system the Biceps femoris (muscle of the hamstring group) is coupled with the spinal erectors through the sacrotuberous ligament. At the end of the swing phase the hamstrings eccentrically contract to control hip flexion and knee extension. The contraction of the Biceps femoris strains the sacrotuberous ligament, assisting in stabilisation of the sacroiliac joint (=force closure of the sacroiliac joint). Kinetic energy is dissipitated by the Erector spinae through rotary action on the spinal column.

The posterior oblique system

In the posterior oblique system the gluteus maximus is connected to the Latissimus dorsi of the opposite side through the thoracolumbar fascia. During walking or running the Gluteus maximus contracts on foot strike in concert with a contraction of the opposite Latissimus dorsi (the Latissimus dorsi extends the arm as a means of counter rotation). This countered contraction creates tension on the thoracolumbar fascia, stabilising the sacroiliac joint (force-closure). Some authors described that the posterior oblique system may act like a smart spring. The stored energy in the thoracolumbar fascia can be released with a subsequent contraction, minimizing muscle action and the metabolic cost of locomotion

The anterior oblique system

In the anterior oblique system the thigh adductors are coupled with oblique abdominal muscles through the adductor-abdominal fascia. This system rotates the pelvis forward during the swing phase, playing an important role in locomotion.

The lateral system

The lateral system stabilises the body in the frontal plane. During a single-leg stance the hip abductors and adductors of the supporting leg work in concert with the opposite Quadratus lumborum to stabilise the pelvis. The oblique (both internal and external) musculature is also synergystic to secure a stable spine and pelvis. Deficiency of the lateral system is a common source of injury in the back, sacroiliac joint and supporting leg. Most sports are single-leg dominant in nature. During running and sprinting the body is propelled forward through powerful single-leg actions. A strong and functional lateral system will help improve athletic performance, preserve energy and prevent injuries.


The inner and outer unit function synergistically to stabilise the body and create powerful and economic movement.

Without efficient functioning of the inner unit there is no stability of the spine and sacroiliac joints. The core will not provide a firm base of contraction for the phasic muscles, which results in lost limb power, low economy of movement and susceptibility to injury.

A well-conditioned inner unit depends on strong outer unit systems to protect the inner unit muscles, the spinal ligaments and joints. For example, consider a basketball player who, after a lay-up or a dunk, lands on one leg. The forces on landing, multiplying several times bodyweight need to be countered by the lateral system, to prevent the pelvis from tilting. If the lateral system does not have the adequate eccentric strength to prevent the pelvis from tilting more than seven or eight degrees from parallel, an overload is created on the sacroiliac joint and the spine, but also on the hip and knee of the stance leg. Tilting of the pelvis on landing will force the spine to flex laterally. Lateral flexion at the lumbar and thoracic level is coupled with rotation. Lateral flexion coupled with rotation outside the neutral zone (range of motion in which the strain on ligaments is not excessive) will cause injuries to the spinal ligaments and joints, no matter how well conditioned the inner unit. It is obvious that machine exercises, where stability is provided by the machine, do not condition the outer unit systems. This also explains why traditional abdominal workouts, targeting the superficial abdominal muscles, are ineffective to enhance stabilisation of the spine and reduce lower back pain.


Functional training requires the inner and outer units to function synergistically, resulting in joint and body stability, economy of movement, enhanced power and decreased susceptibility to injury.

Part II of this article addresses how to functionally train the core, focusing on movement patterns rather than muscles. A functional approach to core training means that the core is trained to do what it does