What is the primary action of the transversus abdominis muscle?

• A. Abducts the trunk by lateral flexion of the vertebral column
• B. Compresses the abdominal contents (increases intra-abdominal pressure)
• C. Flexes the lumbar vertebral column
• D. Extends the vertebral column

Answer: (B)

Verified step by step guidance

1. Step 1: Identify the anatomical location and fiber orientation of the transversus abdominis muscle. It is the deepest of the abdominal muscles, with fibers running horizontally (transversely) across the abdomen.
2. Step 2: Understand the function of muscle fiber orientation. Since the fibers run horizontally, the muscle is well-suited to compress the abdominal contents rather than produce movement like flexion or extension.
3. Step 3: Recognize that the transversus abdominis acts primarily to increase intra-abdominal pressure by compressing the abdominal cavity, which supports the trunk and stabilizes the spine.
4. Step 4: Differentiate this action from other abdominal muscles: for example, the rectus abdominis flexes the lumbar vertebral column, and the obliques assist in lateral flexion and rotation.
5. Step 5: Conclude that the primary action of the transversus abdominis is to compress the abdominal contents, thereby increasing intra-abdominal pressure, rather than moving the vertebral column.