The spine and abdominal wall
The spinal column has to fulfil many functions. It has to be weight bearing, provide stability and support, act as a shock absorber, protect the spinal cord and allow movement. The spinal column possesses three types of joint:
- A cartilaginous joint between the individual vertebrae
- A gliding joint between the vertebral arches
- A pivot joint formed by the first two cervical vertebrae (the atlas and axis). The atlas articulates with the occipital bone of the skull and allows flexion and extension, such as when nodding. The atlas and axis articulate and allow rotation, such as when shaking the head to say ‘no’.
Several ligaments hold the vertebrae together to make the column more stable. The movements possible at the spine include flexion, extension, lateral flexion and rotation. The combination of flexion, lateral flexion and hyperextension results in circumduction. Movement is not uniform throughout the spine:
- Most flexion occurs in the cervical and lumbar regions, but very limited in the thoracic. The sacrum and coccyx are fixed.
- Extension is quite free in the cervical and lumbar regions, but very limited in the thoracic.
- Lateral flexion takes place in all regions of the spine, but more so in the cervical and lumbar regions.
- Rotation is good in the cervical and upper thoracic regions, but minimal in the lumbar region.
Extension (or hyperextension) of the trunk is caused by the back muscles around the vertebral column. These deep muscles of the back form a broad, thick column which extend from the sacrum up to the skull. The largest of these muscles is the erector spinae. The origins and insertions of the various deep back muscles overlap extensively, and when they contract, entire regions of the vertebral column can be moved simultaneously (causing extension or hyperextension). When these muscles contract on only one side of the vertebral column, lateral flexion occurs. When lateral flexion occurs, there is some degree of vertebral column rotation.
There are also a large number of short muscles that extend from one vertebra to the next. These include the rotatores, multifidus, interspinalis and intertransversarius muscles. These act primarily as synergists for extension and rotation of the spine, as well as also acting as spine stabilizers. Trunk muscles also maintain the normal curvatures of the spine, and act as postural muscles.
The anterior and lateral abdominal wall has no bony reinforcements, but is made up of four paired muscles. Three broad flat muscle pairs, layered one on the other, make up the lateral abdominal wall – the external oblique muscle fibres run at right angles to those of the internal oblique muscle, which lies beneath it. The fibres of the transversus abdominis muscle run horizontally across the abdomen at an angle to both, and this is the deepest muscle. These three muscles join (insert) at the front onto an aponeurosis. An aponeurosis is a fibrous or membranous sheet which connects a muscle and the part it moves. The aponeuroses enclose the fourth muscle, the rectus abdominis, which is joined (fused) in the middle to form the linea alba (a tendinous seam) that runs from the sternum to the pubic symphysis.
The abdominal muscles protect and support the internal organs when they are well toned. Other functions include lateral flexion and rotation of the trunk, and anterior flexion against resistance (as in sit-ups or crunches).