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Biomechanics of the Lumbar Spine

    Human spine complex structure


Major function is to protect the spinal cord and transfer load from head and trunk to pelvis

Providing the mechanical linkage between the upper and lower extremities

Each of the vertebra articulate with the adjacent ones to permit motion in three planes.

The lumbar region of the spine is of particular interest because low back pain is a major medical and socioeconomic problem in modern times.

Spinal stability

spine gains stability from the intervertebral discs

and from the surrounding ligaments and muscles;

Intrinsic stability

Discs and ligaments provide intrinsic stability

Extrinsic stability

Muscles provide extrinsic support


Vertebral Column

The spine consists of a curved stack of 33 vertebrae divided structurally into five regions.

Proceeding from superior to inferior, there are 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 fused sacral vertebrae, and 4 small, fused coccygeal vertebrae.

Cervical vertebras

The first two vertebrae are specialized in shape and function.

The first, known as the atlas, provides a receptacle for the condyles of the occiput of the skull.

The atlantooccipital joint is extremely stable

Allow flexion/extension of about 14–15°

but no motion in any other plane.

A large range of axial rotation is provided at atlantoaxial joint.

Motion at the atlantoaxial joint  includes 75° of rotation, 14° of extension, and 24° of lateral flexion

Lumber vertebra

vertebral size increases from the cervical region down through the lumbar region.

The lumbar vertebrae are larger and thicker than the vertebrae in the superior regions of the spine.

This is for a functional purpose when the body is in upright position each vertebra t support the weight of not only the arms and head but all the trunk positioned above it.

An increased surface area of the lumbar vertebrae reduces the amount of stress to which these vertebrae would otherwise be subjected.

The weight-bearing surface area of the intervertebral disc also increases with the weight supported.

Intervertebral Discs

fibrocartilaginous discs that act as cushions.

Healthy intervertebral discs in an adult account for approximately one-fourth of the height of the spine.

When the trunk is erect, the differences in the anterior and posterior thicknesses of the discs produce the lumbar, thoracic, and cervical curves of the spine.

Structure of discs

Two functional structures:

A thick outer ring composed of fibrous cartilage called the annulus fibrosus, or annulus, surrounds a central gelatinous material known as the nucleus pulposus, or nucleus

Mechanical properties

Annulus acts like a coiled spring whose tension holds the vertebral bodies together against the resistance of the nucleus pulposus

Nucleus pulposus acts like a ball bearing composed of an incompressible gel.

What happened during the movement.


Flexion and extension

vertebral bodies roll over the nucleus while the facet joints guide the movements. spinal flexion, extension, and lateral flexion produce compressive stress on one side of the discs and tensile stress on the other.

spinal rotation creates shear stress in the discs.

Stress higher with flexion than rotation approximately 450 times greater than the twisting.

During daily activities, compression is the most common form of loading on the spine.


In Bending

◦Increased tensile force posteriorly

◦Increased compressive force anteriorly

In Rotation

◦Reorientation of collagenous fibers

◦Tightening of fibers traveling in one direction

◦Loosening of fibers traveling in the opposite direction

Effect of loading on disc

When loaded in compression, it simultaneously loses water and absorbs sodium and potassium til it’s internal electrolyte concentration is sufficient to prevent further water loss.

At this point, internal disc pressure is equal to external pressure.

Continued loading over several hours results in slight

a decrease in disc hydration

For this reason, the spine undergoes a height decrease of 2 cm over the course of a day, with 54% of this occurring during the first 30 minutes after an individual gets up in the morning

Effect of unloading

Once pressure relieved, the discs quickly reabsorb water, disc volumes and heights are increased.

On earth, disc height and volume are typically greatest when a person first arises in the morning. Increased disc volume also translates to increased spinal stiffness, so greater risk of disc injury early in the morning.

Effect of injury and aging

Irreversibly reduce the water-absorbing capacity as well as shock-absorbing capability.

Results of MRI degenerative changes to be the most common at L5-S1

Fluid content of all discs begins to diminish around the second decade of life

A geriatric disc has a fluid container that is reduced by approximately 35%

As a result of a degenerative change, abnormal movements occur between adjacent vertebral bodies

Effect of injury and aging

More compressive, tensile, and shear loads on the spine.

Reduced height of the spinal column.

The normal lordotic curve of the lumbar region may be reduced as an individual attempts to relieve compression on the facet joints by maintaining a posture of spinal flexion

Factors affecting disc health negatively and positively.
Negative factor
  • Habitual smoking
  • Exposure to vibration can negatively affect disc nutrition.
Positive factor
  • Regular exercise can improve it.