Biomedical Engineering Reference
In-Depth Information
6.6 Forces on the Spinal Column
After the age of 40, rare are the persons who have no lower back pain or lumbar
region pain caused by incorrect posture during their life. Many persons may come
to have lumbar region pain, due to incorrect lifting of weight, for example, lifting a
child from the ground. To understand a little about the origin of this pain, let us
analyze the forces involved in such cases.
A human spinal column is divided in four parts, from top to bottom: the cervical,
made up by seven vertebrae; the thoracic, by twelve vertebrae; the lumbar, by five
fused vertebrae in the sacrum; and four fused vertebrae in the coccyx. As already
discussed in Chap.
1
, the vertebrae of the human spinal column increase in size
continuously from top to bottom to bear increasingly heavy weight. Between the
vertebrae there are intervertebral discs made of fibrous material with the purpose of
damping the forces and the impacts suffered by the spinal column, for example, in
footraces, when, briefly, the person is in the air and lands on one foot, differently when
the person walks and never leaves the ground. The average spinal column length of a
normal adult is 70 cm, which at the end of the day can be shortened by 1.5 cm, but is
recovered after one night in a horizontal position. This fact is easily observed by the
driver when he or she notes that the adjustment of the rear view mirror inside the car
differs between the morning and at the end of a day. As the years pass, unhappily, the
spinal column is shortened little by little, due mainly to osteoporosis.
The principal muscles used to bend the back or to lift objects from the ground are
spinal erector muscles. They connect the ileum and the lower part of the sacrum to
all of the lumbar vertebrae and to four thoracic vertebrae. Research performed by
L.A. Strait, V.T. Inman and H.J. Ralston and published in the Amer. J. Phys.
15, 377-378 (1947) has shown that during a back flexion, the forces exerted by
the spinal erector muscles on the spinal column can be represented by a single
muscle force. The spinal column can be considered a rigid body, and the force is
applied at a point at 2/3 of its length from the sacrum, and forming an angle of 12
with the column. The axis of rotation is located at the fifth lumbar vertebra. The
contact (reaction) compressive force of largest intensity is applied exactly there
during the bending of the back. If this force surpasses a certain limit, the interver-
tebral disc flattens and its diameter increases, pressing on the nerve and, as a
consequence, causing lumbar pain.
6.6.1 Forces Involved in the Spinal Column
When the Posture Is Incorrect
One of the incorrect postures is that in which the spinal column is curved (without
bending of the knees) whether to brush the teeth over a washbasin or to lift a heavy
object from the floor. For these situations, it is truly impossible not to curve the
spine, if the knees are not bent. Even in the case of brushing the teeth over a low
