Which of the following muscles is primarily involved in trunk flexion?

The rectus abdominis is the muscle primarily involved in trunk flexion. This muscle is located in the front of the abdomen and plays a key role in movements that bring the rib cage and pelvis closer together, such as bending forward. When the rectus abdominis contracts, it flexes the spine, allowing for actions like sitting up from a lying position or performing a crunch.

The external abdominal oblique, while also involved in flexing the trunk, primarily supports rotational movements and lateral flexion of the trunk rather than trunk flexion in isolation. The splenius cervicis and splenius capitis, located in the back of the neck and upper back, are more involved in neck extension, rotation, and lateral flexion. Their primary functions do not focus on trunk flexion, highlighting why the rectus abdominis stands out as the primary muscle for that specific movement.