Which abdominal muscle lies deepest in the abdominal wall and helps stabilize the trunk?

• A. External oblique
• B. Internal oblique
• C. Rectus abdominis
• D. Transverse abdominis

Answer: (D)

Stability of the trunk during movement hinges on a deep abdominal layer that acts like a natural corset. The deepest abdominal muscle lies just beneath the others and wraps horizontally around the abdomen. When it contracts, it compresses the abdominal contents and increases intra-abdominal pressure, which helps shield and stabilize the spine during lifting, twisting, and other activities. This deep layer is essential for maintaining posture and resisting excessive spinal motion as the limbs move.

The other abdominal muscles sit more superficially or run in different directions. The outermost layer has diagonal fibers that assist with trunk rotation and lateral flexion. The next layer sits closer to the inside with its own diagonal fibers and also contributes to movement. The front vertical muscle runs along the midline and primarily flexes the spine and assists in increasing abdominal pressure, but it isn’t the deepest stabilizer.