Mti

1200 N

Figure 12-12 The lumbar biomechanics during standing (A) and sitting (B). The dashed line crosses the center of gravity (CG).

falling forward. The total force acting on the disc is 400 N + 400 N, or 800 N. Sitting positions produce greater pressure on the disc than standing. When a person sits, the center of gravity is about 15 cm in front of L3, whereas the muscle's lever arm is still 5 cm behind, the same as when standing. Therefore, to maintain equilibrium and keep the body from falling, the back muscles need to exert a force of 1200 N:

muscles

The force acting on the disc is 1200 N + 400 N, or 1600 N. Sitting in a chair with back support may reduce the distance of 15 cm, thereby diminishing the pressure on the disc. The back muscles must pull harder (static tension) when the body sits. Pressure on the disc is caused by the body weight (mg) acting on the disc from above and by the contraction force (F) of the surrounding muscles.

The total compressing force in Figure 12-13 is mg + F. The disc of a young person can withstand

Figure 12-13 The disc pressure is the joint effect of the body weight (mg) and the muscular contraction forces (F).

a stress of 800 kg, or 8000 N. The total area of the L3 vertebra in an adult is about 10 cm2. That means the disc of a young adult can withstand a pressure of 8000 N/10 cm2, or 800 N/cm2. An elderly person can withstand half of this pressure.

Low back pain can have many different causes besides those related to tumor or infection. If the annulus fibrosus tears and the nucleus pulposus is pressed backward, the posterior longitudinal ligament, which runs posteriorly along the vertebral bodies, is stretched. When the ligament stretches, pain is produced through its sensory nerve endings. The disc itself harbors very few sensory nerve endings and so may not be the source of pain. This type of pain can be relieved if the patient prevents the back from stretching by avoiding certain activities, such as lifting heavy objects, leaning forward while working, or sitting still. If the nucleus pulposus bulges too far out, it can press against the nerve root, which passes through the intervertebral foramen. Pain is then felt in the muscles that are supplied by this nerve. For the same reason, pain can be felt in the shoulder when a cervical disc is injured. Tense muscles, small vertebra displacements, and worn-down intervertebral cartilage can put pressure on the nerves and cause pain. As another example, if the sciatic nerve is affected, the pain is felt in the leg muscles supplied by this nerve.

Some nerves (e.g., the lumbar plexus) that pass out from the intervertebral foramen and into a muscle can be drawn out a little farther by stretching of the back muscles. This can cause the nerves to be pressed against a protrusion on the disc, producing severe pain in the leg. The Lasegue test is used to confirm sciatica: The patient is placed on the back and lifts the painful leg so that the clinician can determine whether the sciatic nerve is irritated. This type of pain must not be confused with that felt by people with tight hamstrings.

Lifting something heavy and simultaneously twisting the trunk, which happens in shoveling snow, is very dangerous for people who suffer from back complaints because it creates the greatest pressure in the posterior part of the disc, which is not protected by extra ligaments (Fig. 12-14).

In the following example, a person who weighs 80 kg—of which 40 kg lies above the level of L3—lifts a 10-kg object. It is possible to calculate the stresses to which the spinal column is subjected in different postures. Distance is measured in centimeters and force in newtons.

A posture with flexed knees (Fig. 12-15, A) is the least stressful position. To do the specified work, the muscles of the back require the force (Fm) as follows:

A posture with straight knees (see Fig. 12-15, B) requires

A posture in sitting position (see Fig. 12-15, C) requires

This calculation is purely mechanical. If a person has well-trained abdominal muscles and diaphragm, the mechanical stress of the back muscles can be reduced by about 40%. When lifting a weight, a person can build up pressure in the abdomen by tensing the diaphragm and abdominal muscles (Fig. 12-16), creating an up-and-down motion, which counteracts the tendency for abdominal collapse. The disc, which is a part of the abdominal cavity's back wall, is thus protected. This shows how important it is to have well-trained abdominal muscles, strong leg muscles, and correct posture in order to lift a weight correctly with support for the back. When practitioners consider any exercise that engages the back muscles, they must always account for the stabilizing effect of the abdominal muscles.

Intervertebral disc, anulus fibrosus

Posterior longitudinal ligament

Joint capsule

Intervertebral disc, anulus fibrosus

Posterior longitudinal ligament

Joint capsule

Anterior longitudinal ligament

Meningeal branch

(Medial branch) (Lateral branch)

Posterior branch

Figure 12-14 The nerves of the vertebral column and the posterior longitudinal ligament.

Anterior longitudinal ligament

Meningeal branch

(Medial branch) (Lateral branch)

Posterior branch

Ganglion of sypathetic trunk White ramus communicans Grey ramus communicans

Anterior branch

" Trunk of spinal nerve

Spinal sensory ganglion

Figure 12-14 The nerves of the vertebral column and the posterior longitudinal ligament.

5 cm Fm

500 N

30 cm

30 cm

500 N

500 N

40 cm

5 cm

40 cm

5 cm

C 500 N

Figure 12-15 Physical tensions on the lumbar spine with different ways of lifting weights. Fm, Muscle force.

C 500 N

Figure 12-15 Physical tensions on the lumbar spine with different ways of lifting weights. Fm, Muscle force.

Figure 12-16. Well-trained abdominal muscles reduce tension of lumbar muscles.

1501

Figure 12-17 Weight-lifting techniques. A, Symmetric lift. B, Asymmetric lift. CG, Center of gravity.

1501

Figure 12-17 Weight-lifting techniques. A, Symmetric lift. B, Asymmetric lift. CG, Center of gravity.

Figure 12-17 shows the difference between symmetric and asymmetric lift. Suppose the body weighs 40 kg above the level of L3 and the load is 30 kg. The lever arm of the back muscles (for lateral bending) is 5 cm. In a symmetric lift, the total load is 700 N (150 + 150 + 400 N). For asymmetric lift, suppose the common center of the body plus load is 10 cm to the side of L3 on the right, even if the body tilts to the left. The back muscles must then contract

10 cm), and so the total load on the disc is 700 N + 1400 N, or 2100 N. It is possible to calculate the forces that are necessary to swing a leg forward during running, jumping, hurdling, and other activities involving the legs and back. The weight and acceleration of the leg may cause the force of muscle contraction to reach about 4000 N. The iliopsoas is the major muscle responsible for this flexion.

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