What is the primary function of the rectus abdominis muscle?

The rectus abdominis muscle's primary function is to flex the lumbar spine. This muscle runs vertically along the front of the abdomen and is crucial for bending forward at the waist, which involves the flexion of the spinal column. When the rectus abdominis contracts, it pulls the rib cage and pelvis toward each other, effectively creating a forward bending motion, a key movement in various exercises and everyday activities.

While stabilization of the vertebral column and assistance in breathing are important roles of abdominal muscles, these functions are more associated with other muscles within the core and diaphragm rather than being the primary action of the rectus abdominis. Additionally, although the torso can rotate due to other muscles within the abdominal region, that movement is not a primary function of the rectus abdominis itself. Thus, flexing the lumbar spine accurately describes the main action of this muscle.