After completing this chapter, you
will be able to:
- Explain how anatomical structure affects movement capabilities
of the spine.
- Identify factors influencing the relative mobility and stability
of different regions of the spine.
- Explain the ways in which the spine is adapted to carry out
its biomechanical functions.
- Explain the relationship between muscle location and the nature
and effectiveness of muscle action in the trunk.
- Describe the biomechanical contributions to common injuries
of the spine.
The spine is a complex and functionally significant segment of
the human body. Providing the mechanical linkage between the upper
and lower extremities, the spine enables motion in all three planes,
yet still functions as a bony protector of the delicate spinal cord.
To many researchers and clinicians, the lumbar region of the spine
is of particular interest because low back pain is a major medical
and socioeconomic problem in modern times.
The spine consists of a curved stack of 33 vertebrae divided
structurally into five regions (Figure 9-1). 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. There may be one extra vertebra or one
less, particularly in the lumbar region.
A. Left lateral and B. posterior views of the major regions
of the spine. From Shier, Butler, and Lewis, Hole’s Human Anatomy and Physiology, © 1996.
Reprinted by permission of The McGraw-Hill Companies, Inc.
Because of structural differences and the ribs, varying amounts
of movement are permitted between adjacent vertebrae in the cervical,
thoracic, and lumbar portions of the spine. Within these regions,
two adjacent vertebrae and the soft tissues between them are known
as a motion segment. The motion segment
is considered the functional unit of the spine (Figure 9-2).
The motion segment, composed of two adjacent vertebrae
and the associated soft tissues, is the functional unit of the spine.
Each motion segment contains three joints. The vertebral bodies
separated by the intervertebral discs form a symphysis type of amphiarthrosis.
The right and left facet joints between the superior and inferior
articular processes are diarthroses of the gliding type that are
lined with articular cartilage.
A typical vertebra consists of a body, a hollow ring known as
the neural arch, and several bony processes
(Figure 9-3). The vertebral bodies serve as the primary
weight-bearing components of the spine. The neural arches and posterior
sides of the bodies and intervertebral discs form a protective passageway
for the spinal cord and associated blood vessels known as the vertebral
canal. From the exterior surface of each neural ...