ATLAS OF HUMAN ANATOMY - G.L. Bilich - 2014

Muscular System

MUSCLES AND FASCIAE OF THE ABDOMEN

Functions of the Abdominal Muscles

The Abdominal Muscles perform multiple Functions, most often working in conjunction with other Muscle groups (such as the back muscles, gluteal muscles, and Diaphragm). The primary Functions of the abdominal muscles include:

✵ maintaining muscle tone: strengthening the abdominal wall and compressing the Abdominal cavity Organs (abdominal press);

✵ stabilizing the spine and reducing load upon it;

✵ executing body and trunk movements;

✵ participating in the act of Respiration;

✵ increasing intra-abdominal pressure through the contraction of the abdominal wall, pelvic floor muscles, and diaphragm (Fig. 287).

A. Contraction of the abdominal muscles results in a decreased volume of the abdominal cavity, increased intra-abdominal pressure, compression of the abdominal organs, and Maintenance of the abdominal viscera in a fixed position. These muscles also participate in defecation, vomiting, and urination. The abdominal press plays a vital role in childbirth.

The abdominal press contributes to spinal stabilization by increasing intra-abdominal pressure.

B. Simultaneous contraction of the diaphragm, abdominal muscles, and pelvic floor increases intra-abdominal pressure. Due to the hydrostatic effect, this stabilizes the trunk, reduces stress on the spine (especially its lumbar region), and tensions the trunk wall. This occurs automatically during heavy lifting, easing the load on the intervertebral discs by 50% in the upper lumbar spine and by 30% in the lower lumbar spine. Furthermore, the load on the deep back muscles is reduced by more than 50%. This underscores Structure/19.html">The Importance of well-developed, strong abdominal muscles in the Prevention and Treatment of spinal disorders.

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Fig. 286. Origins and insertions (A — iliopsoas muscle, B — rectus abdominis muscle and pyramidalis muscle, B — quadratus lumborum muscle) (diagrams):

1 — Iliacus; 2 —- Psoas major; 3 — Rectus abdominis; 4 — Quadratus lumborum; 5 — Pyramidalis

Fig. 286. Functions of the abdominal muscles (A — frontal plane, B — sagittal plane) (diagram)

Trunk movements are executed by the oblique and rectus abdominis muscles (Figs. 288, 289).

A. Fiber orientation and Organization OF THE rectus and oblique abdominal muscles.

B. The trunk is bent to the right with a simultaneous rotation to the left due to the contraction of the right external oblique abdominal muscle and the left internal oblique abdominal muscle.

Fig. 288. Involvement of the abdominal wall muscles in pelvic movements: active and passive postures (A — normal active posture, B — active stable posture, C — passive "slumped" posture) (diagrams):

1 — Quadriceps femoris; 2 — Anterior superior iliac spine; 3 — Psoas major; 4 — Posterior superior iliac spine; 5 — Gluteus maximus; 6 — Iliopsoas

Fig. 289. Involvement of the anterolateral abdominal muscles in trunk movements (diagrams):

1 — Sternum; 2 — Rectus abdominis; 3 — External oblique; 4 — Transversus abdominis; Transverse abdominal; 5 — Internal oblique; 6 — Linea alba; 7— Pubic Symphysis

C. Rotation of the trunk to the right is performed by the Muscles of the right lateral abdominal wall and the left deep back muscles.

D. Lateral flexion of the trunk to the right occurs through the contraction of the right lateral abdominal wall (facilitated by the right deep back muscles as well).

E. Trunk flexion is accomplished through the bilateral contraction of the rectus abdominis, the lateral abdominal wall muscles, and the iliopsoas muscles.

Imbalance between the deep back muscles and the abdominal muscles is most evident in the reduction of lumbar lordosis and the alteration of the pelvic tilt angle. In a normal active posture, the pelvis is tilted forward by approximately 12° (see Fig. 288-A). Upon adopting a stable posture (Stomach in, chest out), the pelvis is maintained in a more vertical position, aligning the anterior and posterior superior iliac spines in the same horizontal plane (see Fig. 288-B). The most active muscles include the abdominal wall muscles, gluteals, calf muscles, and quadriceps femoris. When the abdominal muscles are weak, the result is a passive, slumped posture (see Fig. 288-C) with an excessive anterior pelvic tilt. In this case, the curve of lumbar lordosis becomes more pronounced due to strong contraction of the deep back muscles. This posture is further reinforced by the contraction of the iliopsoas muscle.