Anatomy and Biomechanics of The Back

Biomechanics is the term used to describe movement of the body. This section is a review of basic spine biomechanics. In order to better understand the biomechanics of the spine it is important to understand the anatomy of the spine. Please read the section on basic spine anatomy before reading this section. It discusses the bones, ligaments, muscles and other structures that make up and support the spine.

The spine is one of the most complex parts of the body. The spine can be divided into five sections: the cervical section (the neck), the thoracic section (the upper back), the lumbar section (the lower back), the sacrum (part of the pelvis) and the coccyx (the tailbone). Each section of the spine has unique features that allow it to move certain ways and do different things.

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The Lumbar Spine

Vertebrae in the cervical, thoracic and lumbar sections of the spine are separated by a structure called the “intervertebral disc”. This disc forms part of the joint that connects the “bodies” of two vertebrae. This joint allows very little movement between two vertebrae. The facets are paired, flat areas of the vertebrae that form joints (facet joints) with the facets of the vertebrae above and below (see diagram). The majority of spine movement occurs at these joints.

The main movements of the spine are to bend forward (flex), bend backward (extend), side-bend (side-flex), and rotate.

In the cervical section of the spine there are 7 “cervical” vertebrae. The joints between the vertebrae in the upper part of the neck (above the second cervical vertebra) allow primarily neck flexion, extension and rotation. The joints between the vertebrae in the lower part of the neck allow flexion, extension, side-flexion and rotation to occur.

In the thoracic section of the spine there are 12 “thoracic” vertebrae. The joints between the vertebrae in the thoracic section of the spine allow flexion, extension, side-flexion and rotation to occur. In the thoracic spine the individual ribs attach to the vertebrae. The ribs provide stability to the thoracic spine and help to control motion.

In the lumbar section of the spine there are 5 “lumbar” vertebrae. The joints between the vertebrae in the lumbar section of the spine allow small amounts of flexion, extension, side-flexion and rotation to occur. The lumbar spine has the least amount of movement when compared to the thoracic and cervical sections of the spine.

The sacrum is a single bone that forms part of the pelvis. This triangular shaped bone is made up of 5 fused vertebrae. The coccyx is also a single bone that is made up of 4 small fused vertebrae. It attaches to the bottom of the sacrum.

There is no movement between the fused vertebrae in the sacrum but there is a small amount of movement in the joints that connect the sacrum to the left and right pelvic bones. These joints are called the sacroiliac joints. The sacroiliac joints play a role in transferring the weight of the spine and upper body to the pelvis and legs.

Finally, normal spine biomechanics is required to maintain a healthy spine. Abnormal biomechanics can be classified as hypomobile (decreased) movement between vertebrae, hypermobile (increased) movement between vertebrae or instability (severe loss of stability). Muscle weakness, ligament injury, broken bones or damage to the intervertebral disc can all lead to abnormal biomechanics, a major factor in the development of neck and back pain.

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