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Panjabi introduced a spinal stability system that includes passive, active, and control subsystems. The passive subsystems involve the osteoarticular and ligamentous components, which provide restraint primarily in the end range of motion. The active subsystems are of great interest in the research and clinical fields for looking at the role of muscle and fascia and their ability to control the mechanical components of the spine segments.

The control subsystems introduce neurological function as well as its effect and timing on motor control. The role of the neural subsystem is to coordinate feedback from the active and passive subsystems, with appropriate levels of muscular contraction to balance against destabilizing forces between the subsystems.

The emotional or psychological subsystem has been added to account for psychological and social factors influencing the patients’ pain and “their capacity to increase the central nervous system mediated drive of pain via the forebrain”.

The passive, active, and control subsystems are interdependent, with each one being capable of compensating for a deficit in another. Back pain may be related to a deficit in control of one subsystem and to an inability of the remaining subsystems to compensate for this deficit. A deficit in one subsystem may produce back pain as a result of decreased control of a spinal segment, with compression of neural and articular structures.

Panjabi described articular motion in the lumbar spine with the terms neutral and elastic zones: “The neutral zone is a region of intervertebral motion around the neutral posture where little resistance is offered by the passive spinal column”. The elastic zone is the part of the motion “from the end of the neutral zone up to the joint’s physiological limit”.

He used the concept of a ball in a bowl to describe the neutral and elastic zones. The ball resting in the center of the bowl represents the neutral zone within a joint that has a higher degree of laxity. As the ball rolls toward the ends of the bowl, increasing resistance from the soft tissue is seen, as at the end of physiological range of motion in a joint with a stiffening effect. “The neutral zone of the spinal segment is dependent on the muscles for control and proprioceptive feedback”.

Research indicates that the “lumbar spine’s vulnerability to instability is greatest in the neutral zone of motion, under low load conditions and where motor control of the spine is compromised” . The risk of injury in the elastic zone increases as the motor system relaxes to allow passive systems to provide restraint in the spinal segment .

Hypomobility is described as restricted movement within the articular surfaces of a joint. Hypermobility is described as excessive movement within a joint that is beyond a normal degree of movement before the stabilizing structures provide restraint.

According to Panjabi, “Clinical instability is a significant decrease in the capacity of the stabilizing system to maintain the intervertebral neutral zone within physiological limits so that there is no neurological dysfunction, no major deformity and no incapacitating pain.”

He describes clinical instability as a lack of motor control within the neutral zone . Motor control also has been described as “the way in which a task is performed. Altered motor control describes the manner by which the movement or posture has changed” . Clinical instability at a segmental level is seen with increased movement in the neutral zone and excessive motion at the end of range .

“Instability of the lumbar motion segments is closely related to deep muscles which support and control intersegmental movement during functional tasks” (15). The size of the neutral zone may increase with disc degeneration, spinal injury, or poor muscle control, which will all affect stability.Bergmark  introduced the concept of two muscle classifications. These are the local and global stabilizing systems.

The local muscle groups provide specific segmental spinal stability through their attachment directly to the lumbar vertebrae. These muscles provide control of intersegmental motion and position of the lumbar spine. This group would include the transversus abdominis (TrA) , multifidus, and internal oblique (fiber insertion into thoracolumbar fascia) .

Additional studies have found that the pelvic floor muscles, the diaphragm , and the intertransversarii, interspinales, longissimus thoracis pars lumborum, iliocostalis lumborum pars lumborum, and medial fibers of the quadratus lumborum also are included as local stabilizing muscles. The TrA and internal oblique muscles provide stability for the lumbar spine through the thoracolumbar fascia along with the control of the intra-abdominal pressure.

The global muscle groups provide large trunk movements and general trunk stability through their attachments between the thorax and pelvis and/or pelvis and legs, with no specific attachment to the lumbar spine .

This group would include the rectus abdominis, external oblique, and thoracic erector spinae muscles, longissimus thoracis pars thoracis, iliocostalis lumborum pars thoracis, quadratus lumborum lateral fibers, and internal oblique . The global muscle group also has been described as being divided into four slings between the thorax, pelvis, and lower extremity.

The posterior oblique sling would include the latissimus dorsi and the gluteus maximus via the thoracolumbar fascia. The anterior oblique sling would include the external oblique, the anterior abdominal fascia, the contralateral internal oblique abdominal muscle, and adductors of the lower extremity. The longitudinal sling involves connections between the peroneii, the biceps femoris, the sacrotuberous ligament, the deep lamina of the thoracodorsal fascia, and the erector spinae. The lateral sling includes the gluteus medius and minimus, the tensor fascia latae, and the lateral stabilizers of the thoracopelvic region .

The integration between and within these slings provides stability and effective transfer of loads between the spine and the extremities. The local stabilizing muscle system sometimes is referred to in the literature as the inner unit, with the global stabilizing system being referred to as the outer unit.

A significant neurophysiological difference exists in the timing of the contraction of these two local and global muscular systems. Motor control refers to the “timing of specific muscle action and inaction” .

When loads are predictable, the local system contracts in anticipation before the movement, regardless of the direction of movement. The global system contracts later and is direction dependent. Local muscles are controlled independently of the global system.

With low-load situations, the local muscle system is associated with low levels of intra-abdominal pressure and relaxed respiration. Activity is seen in the muscles of the pelvic floor, the transverse abdominal wall, the psoas, and the multifidus. The psoas acts synergistically with the lumbar multifidus to control the position of the pelvis on the hips and lordosis.

The global muscle system is involved in posture and initiation of movement and is associated with low levels of intra-abdominal pressure and relaxed respiration. With high-load situations, the local and global systems co-contract, in association with higher levels of intra-abdominal pressure, to act as a splint and to restrict the movement of the pelvis and thorax .

Lee has focused on the local system of the lumbopelvic region, including the pelvic floor, diaphragm, multifidus, and TrA, in looking closely at the evaluation and treatment of pelvic girdle dysfunction. Sapsford et al.It reported that “in healthy subjects, voluntary activity in the abdominal muscles results in increased pelvic floor muscle activity.” Difficulties in establishing a stable base within the pelvic girdle will affect stabilization strategies of the lumbar spine and, therefore, need to be addressed concurrently.

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