Origin: Inner surfaces of cartilages of the lower six ribs, interdigitating with the diaphragm; thoracolumbar fascia; anterior3/4 of internal lip of the iliac crest; and lateral '/? of the inguinal ligament.
Insertion: Linea alba by means of a broad aponeurosis, pubic crest, and pecten pubis.
Direction of Fibers: Transverse (horizontal).
Action: Acts likes a girdle to flatten the abdominal wall and compress the abdominal viscera; upper portion helps to decrease the infrastemal angle of the ribs, as in expiration. This muscle has no action in lateral trunk flexion, except that it acts to compress the viscera and to stabilize the linea alba, permitting better action by the an-terolateral trunk muscles.
Nerve: T7-12, LI iliohypogastric and ilioinguinal, ventral divisions.
Weakness: Permits a bulging of the anterior abdominal wall, which indirectly tends to increase lordosis. (See accompanying photograph.) During flexion in the supine position and hyperextension of the trunk in the prone position, a lateral bulge tends to occur if the transversus abdominis is weak.
Weakness: Moderate or marked weakness of both the external and internal obliques decreases both respiratory efficiency and support of the abdominal viscera.
Bilateral weakness of external obliques decreases the ability to flex the vertebral column and tilt the pelvis posteriorly. In standing, it results in either anterior pelvic tilt or anterior deviation of the pelvis in relation to the thorax and lower extremities. (See p. 71.)
Bilateral weakness of the internal obliques decreases the ability to flex the vertebral column.
Cross-sectional weakness of the external oblique on one side and of the internal oblique on the other allows separation of the costal margin from the opposite iliac crest, resulting in rotation and lateral deviation of the vertebral column. With weakness of the right external and left internal obliques (as seen in a right thoracic, left lumbar scoliosis), there is a separation of the right costal margin from the left iliac crest. The thorax deviates toward the right and rotates posteriorly on the right. With weakness of the left external and right internal obliques, the reverse occurs.
Unilateral weakness of lateral fibers of the external and internal obliques on the same side allows separation of the thorax and the iliac crest laterally, resulting in a C-curve that is convex toward the side of weakness. Weakness of the lateral fibers of the left external and internal obliques gives rise to a left C-curve.
Shortness: Bilateral shortness of anterior fibers of the external and internal oblique muscles causes the thorax to be depressed anteriorly, contributing to flexion of the vertebral column. In standing, this is seen as a tendency toward kyphosis and depressed chest. In a kyphotic-lor-dotic posture, the lateral portions of the internal oblique are shortened, and the lateral portions of the external oblique are elongated. These same findings occur in a sway-back posture with anterior deviation of the pelvis and posterior deviation of the thorax.
Cross-sectional shortness of the external oblique on one side and of the internal oblique on the other causes rotation and lateral deviation of the vertebral column. Shortness of the left external and right internal obliques (as seen in advanced cases of right thoracic, left lumbar scoliosis) causes rotation of the thorax forward on the left.
Unilateral shortness of lateral fibers of the external and internal obliques on same side causes approximation of the iliac crest and thorax laterally, resulting in a C-curve that is convex toward the opposite side. Shortness of the lateral fibers of the right internal and external obliques may be seen in a left C-curve.
Anterior view of abdomen showing division of right external oblique into a, b. and c portions and left internal oblique into a', b', and c' portions.
Anterior view showing left and right portions (L. R. and R. R.) of rectus abdominis, and left and right portions (L. T. and R. T.) of trans versus abdominis.
Posterior view showing posterior fibers of transversus abdominis.
The terms upper and lower differentiate two important strength tests for the abdominal muscles. More often than not, there is a difference between the grades of strength attributed to the upper abdominals compared to those attributed to the lower abdominals.
If the same muscles entered into both tests and the difference in strength resulted from a difference in the difficulty of the tests, there should be a fairly constant ratio between the two measurements.
In order of frequency, the following combinations of strength and weakness are found:
1. Upper strong and lower weak.
2. Upper and lower both weak.
3. Upper and lower both strong.
The difference in strength may be remarkable. A subject who can perform as many as 50 or more curled-trunk sit-ups may grade less than fair on the leg-lowering test. This same subject can increase the strength of the lower abdominals to normal by doing exercises specifically localized to the external oblique.
Because the oblique abdominal muscles are essentially fan-shaped, one part of a muscle may function in a somewhat different role than another part of the same muscle. Knowledge of the attachments and the line of pull of the fibers, along with clinical observations of patients with marked weakness and those with good strength, leads to conclusions regarding the action of muscles or segments of abdominal muscles.
The rectus abdominis enters into both tests. There is a distinct difference, however, between action of the internal oblique and that of the external oblique as exhibited by the two tests.
When analyzing which muscles or parts of muscles enter into the various tests, it is necessary to observe the movements that take place and the line of pull of the muscles that enter into the movement.
As trunk flexion is initiated by slowly raising the head and shoulders from a supine position, the chest is depressed, and the thorax is pulled toward the pelvis. Simultaneously, the pelvis tilts posteriorly. These movements obviously result from action of the rectus abdominis muscle. (See figure below.)
Along with depression of the chest, the ribs flare outward, and the infrasternal angle is increased. These movements are compatible with the action of the internal oblique.
No test movement can cause an approximation of parts to which the lower transverse fibers of the internal oblique are attached, because these fibers extend across the lower abdomen from ilium to ilium like the lower fibers of the transversus abdominis. In posterior pelvic tilt and in trunk-raising movements, however, this pan of the internal oblique will act with the transversus to compress the lower abdomen.
Electromyographic studies may confirm or modify the conclusions drawn from clinical observations.
As the trunk curl is completed and the movement enters the hip flexion phase, one will observe that the rib cage, which had flared outward, is now being pulled inward and that the infrasternal angle decreases. The anterior fibers of the external oblique now come into play.
If the internal oblique and rectus are strong (as indicated by the ability to perform numerous curled-trunk sit-ups), and if part of the external oblique is also brought into action during this movement, where is the weakness that accounts for the marked difference in the test results of the upper and lower abdominals?
The posterolateral fibers of the external oblique are elongated as the thoracic spine flexes during the trunk curl. (See figure below.) These fibers of the external
With the trunk held in flexion during the hip flexion phase of trunk raising, the rectus abdominis, anterior fibers of the external oblique, and upper anterior and lateral fibers of the internal oblique shorten. In contrast, the posterolateral fibers of the external oblique, elongate. This helps explain why an individual may be able to perform many sits-ups but fail the leg-lowering test.
oblique help to draw the posterior rib cage toward the anterior iliac crest, and in so doing, they tend to extend, not to flex, the thoracic spine. The photographs on page 71 indicate the line of pull of the posterolateral fibers of the external oblique in good alignment and the elongation of these fibers in faulty positions.
The action of the external oblique may also be observed in cases of scoliosis with muscle imbalance between the right and left external oblique muscles. It is not uncommon to observe that flexion of the spine may begin with a rather symmetrical pull; however, as the effort is made to raise the trunk in flexion toward the thighs, there will be forward rotation of the thorax with extension of the thoracic spine on the side of the stronger external oblique.
This photograph shows a subject with strong external oblique muscles doing a sit-up with the trunk held straight and the lower abdomen pulled upward and inward. This is in sharp contrast to a curled-trunk sit-up, as shown in the illustration on left, or to an arched-back sit-up, as shown at bottom of page 204.
ANALYSIS OF THE TRUNK-RAISING MOVEMENT
Before doing this test, examine the flexibility of the back so that any restriction of motion is not interpreted as muscle weakness.
The trunk-raising movement, when properly done as a test, consists of two parts: spine flexion (i.e., trunk curl) by the abdominal muscles and hip flexion (i.e., sit-up) by the hip flexors.
During the trunk-curl phase, the abdominal muscles contract and shorten, flexing the spine. The upper back rounds, the lower back flattens, and the pelvis tilts posteriorly. On completion of the curl, the spine is fully flexed, with the low back and pelvis still flat on the table. The abdominal muscles act to flex the spine only. During this phase, the heels should remain in contact with the table.
The trunk curl is followed by the hip flexion phase, during which the hip flexors contract and shorten, lifting the trunk and pelvis up from the table by flexion at the hip joints and pulling the pelvis in the direction of anterior tilt Because the abdominal muscles do not cross the hip joints, they cannot assist with the sit-up movement If the abdominal muscles are strong enough, however, they can continue to hold the trunk curled.
The hip flexion phase is included in this test because it provides resistance against the abdominal muscles. The crucial point in the test is the moment at which the hip flexion phase is initiated. At this point, the feet of some subjects may start to come up from the table. The feet may be held down if the force exerted by the extended lower extremities does not counterbalance that exerted by the flexed trunk. However, if the feet are held down, attention must be focused on whether the trunk maintains the curl because at this point, the strength of the hip flexors can overcome the ability of the abdominals to maintain the curl. If this occurs, the pelvis will quickly tilt anteriorly, the back will arch, and the subject will continue the sit-up movement with the feet stabilized.
The trunk-raising test for the upper abdominal muscles is valuable when performed correctly. However, if the ability to perform a sit-up—regardless of how it is done—is equated with good abdominal strength, this test loses its value. (See facing page and p. 104.)
During a curled-trunk sit-up with the legs extended, the pelvis first tilts posteriorly, accompanied by flattening of the low back and extension of the hip joints. After the trunk-curl phase is completed, the pelvis tilts anteriorly (i.e., forward), toward the thigh, in hip flexion, but it remains in posterior tilt in relation to the trunk, maintaining the flat-back position (see Figures C and I), p. 191).
During a sit-up with the low back arched, the pelvis tilts anteriorly, toward the thigh, as the sit-up begins, and it remains tilted anteriorly.
Patient: Supine, with legs extended. If the hip flexor muscles are short and prevent posterior pelvic tilt with flattening of the lumbar spine, place a roll under the knees to passively flex the hips enough to allow the back to flatten. (Arm positions are described below under Grading.)
Fixation: None necessary during the initial phase of the test (i.e., trunk curl), in which the spine is flexed and the thorax and pelvis are approximated. Do not hold the feet down during the trunk-curl phase. Stabilization of the feet will allow hip flexors to initiate trunk raising by flexion of the pelvis on the thighs.
Test Movement: Have the subject do a trunk curl slowly, completing spine flexion and, thereby, the range of motion that can be performed by the abdominal muscles. Without interrupting the movement, have the subject continue into the hip flexion phase (i.e., the sit-up) to obtain strong resistance against the abdominal muscles and, thereby, an adequate strength test.
Resistance: During the trunk-curl phase, resistance is offered by the weight of the head and upper trunk, and by the arms placed in various positions. However, the resistance offered by the weight of the head, shoulders and arms is not sufficient to provide an adequate test for strength of the abdominal muscles.
The hip flexion phase provides strong resistance against the abdominals. The hip flexors pull strongly downward on the pelvis as the abdominals work to hold the trunk in flexion and the pelvis in the direction of posterior tilt. (See facing page.)
Grading: See facing page.
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