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).
two adjacent vertebrae and the associated soft tissues; the functional unit of the spine
The motion segment, composed of two adjacent vertebrae and the associated soft tissues, is the functional unit of the spine.
• The spine may be viewed as a triangular stack of articulations, with symphysis joints between vertebral bodies on the anterior side and two gliding diarthrodial facet joints on the posterior side.
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.