Bending the trunk forward (spinal flexion), such as when performing an abdominal crunch, primarily uses which muscle?

• A. Erector spinae
• B. External oblique
• C. Rectus abdominis
• D. Gluteus maximus

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the movement described — bending the trunk forward is known as spinal flexion, which decreases the angle between the thorax and the pelvis.
2. Step 2: Identify the primary muscles involved in spinal flexion. These muscles are located on the anterior (front) side of the abdomen and work to pull the rib cage towards the pelvis.
3. Step 3: Review the function of each muscle option: Erector spinae extends the spine (opposite of flexion), External oblique assists in trunk rotation and lateral flexion, Gluteus maximus primarily extends the hip, and Rectus abdominis is the main muscle responsible for spinal flexion.
4. Step 4: Recognize that during an abdominal crunch, the Rectus abdominis contracts to flex the spine, making it the primary muscle used in this movement.
5. Step 5: Conclude that the Rectus abdominis is the correct answer because it is the main muscle that produces spinal flexion during trunk bending movements like abdominal crunches.