What is the primary action of the rectus abdominis?

The primary action of the rectus abdominis is flexion of the spine. This muscle, often referred to as the "six-pack," is located in the front of the abdomen and plays a crucial role in bending the torso forward, such as when performing a sit-up or curling the trunk. During the flexion movement, the rectus abdominis contracts, pulling the ribcage and pelvis closer together, which effectively reduces the angle between the upper and lower body. This action is vital for various activities that involve bending forward, lifting, or twisting motions.

In contrast, the other options describe movements that are not primarily associated with the rectus abdominis. Extension of the spine involves actions performed by muscles located at the back of the body, rotation of the torso involves oblique muscles, and stabilization of the pelvis is a function that includes several muscles working in conjunction, not solely the rectus abdominis. Understanding these roles helps clarify the unique function of the rectus abdominis in core stability and movement.