So long as the line of gravity from the center of mass falls within the base of support, a structure is stable. Stability is improved by lowering the center of gravity or increasing the base of support. In the upright position, the body is relatively unstable because it is a tall structure with a small base of support. When the center of gravity falls outside the base of support, the structure either falls or some force must act to keep the structure upright. Both inert and dynamic structures support the body against gravitational and other external forces. The inert osseous and ligamentous structures provide passive tension when a joint reaches the end of its range of motion (ROM). Muscles act as dynamic guy wires, responding to perturbations by providing counterforces to the torque of gravity as well as stability within the ROM so stresses are not placed on the inert tissues.

Postural Stability in the Spine

Spinal stability is described in terms of three subsystems: passive (inert structures/bones and ligaments), active (muscles), and neural control. The three subsystems are interrelated and can be thought of as a three-legged stool; if any one of the legs is not providing support, it affects the stability of the whole. Instability of a spinal segment is often a combination of tissue damage, insufficient muscular strength or endurance, and poor neuromuscular control.

Inert Structures: Influence on Stability

Penjabi described the ROM of any one segment as being divided into an elastic zone and a neutral zone. When spinal segments are in the neutral zone (midrange/neutral range) the inert joint capsules and ligaments provide minimal passive resistance to motion and therefore minimal stability. As a segment moves into the elastic zone, the inert structures provide restraint as passive resistance to the motion occurs. When a structure limits movement in a specific direction, it provides stability in that direction. In addition to the inert tissues providing passive stability when limiting motion, the sensory receptors in the joint capsules and ligaments sense position and changes in position. Stimulation of these receptors provides feedback to the central nervous system, thus influencing the neural control system summarizes the stabilizing features of the osteoligamentous tissues in the spine.

Muscles: Influence on Stability /Role of Global and Core Muscle Activity

The muscles of the neck and trunk not only act as prime movers or as antagonists to movement caused by gravity during dynamic activity, they are important stabilizers of the spine. Without the dynamic stabilizing activity from the trunk muscles, the spine would collapse in the upright position. Both superficial (global) and deep (core) muscles function to maintain the upright posture. The global muscles, being multisegmental, are the large guy wires that respond to external loads imposed on the trunk that shift the center of mass. Their reaction is direction-specific to control spinal orientation. The global muscles are unable to stabilize individual spinal segments except through compressive loading because they have little or no direct attachment to the vertebrae. If an individual segment is unstable, compressive loading from the global guy wires may lead to or perpetuate a painful situation as stress is placed on the inert tissues at the end of the range of that segment.

The deeper, core muscles, which have segmental attachments, respond regardless of direction of motion. They provide dynamic support to individual segments in the spine and help maintain each segment in a stable position so the inert tissues are not stressed at the limits of motion. Both the global and core musculature play critical roles in providing stability to the multisegmental spine.

Role of Muscle Endurance

Strength is critical for controlling large loads or responding to large and unpredictable loads (such as during heavy labor, sports, or falls); but only about 10% of maximum contraction is needed to provide stability in usual situations. Slightly more might be needed in a segment damaged by disk disease or ligamentous laxity when muscles are called on to compensate for the deficit in the passive support. Greater percentages of type I fibers than type II fibers are found in all back muscles, which is reflective of their postural and stabilization function. Inactivity has been shown to change muscle fiber composition and may be one reason for decreased function in patients with low back pain.

Focus on Evidence

In a study that looked at 17 mechanical factors and the occurrence of low back pain in 600 subjects (ages 20 through 65), poor muscular endurance in the back extensors muscles had the greatest association with low back pain.

Muscle Control in the Lumbar Spine

The focus of recent research has been on the role of the transversus abdominis (TrA) and multifidus muscles and their function as core stabilizers. These deep muscles have segmental attachments in the lumbar spine and are therefore able to provide segmental control and stiffness. Studies have shown that the deep fibers of the multifidi and TrA are the first muscles to become active when there is postural disturbance from rapid extremity movements. Other deep muscles that theoretically play a role in segmental stability but to this point in time have been difficult to assess because of their depth include the intersegmental muscles (rotators and intertransversarii muscles) and deep fibers of the quadratus lumborum.

Abdominal muscles. The rectus abdominis (RA), external oblique (EO), and internal oblique (IO) muscles are large, multisegmental global muscles and are important guy wires for stabilizing the spine against postural perturbations. The transversus abdominis (TrA) is the deepest of the abdominal muscles and responds uniquely to postural perturbations. It attaches posteriorly to the lumbar vertebrae via the posterior and middle layers of the thoracolumbar fascia and through its action develops tension that acts like a girdle of support around the abdomen and lumbar vertebrae. Only the TrA is active with both isometric trunk flexion and extension, whereas the other abdominal muscles have decreased activity with resisted extension. This is attributed to the stabilization function of the TrA.

Early electromyographic research studies of the activity of the deeper abdominal muscles in their stabilization function was done with surface electrodes and did not discriminate activity between the TrA and IO. By using ultrasound imaging techniques, insertion of fine-needle electrodes into the various muscles has produced evidence of differing functions between these two muscles with perturbations to balance in healthy individuals as well as those who have low back pathology. The TrA responds with anticipitatory activity, with rapid arm and leg movements, no matter in which direction the limb movement occurs, and coordinates with respiration during these stabilizing activities. The TrA also has a coordinated link with the perineum and pelvic floor muscle function.

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