What action does the psoas major primarily assist with?

The psoas major is primarily known for its role in hip flexion, making it crucial for actions that involve bringing the thigh closer to the torso. This muscle connects the lumbar vertebrae to the femur and is a key player in activities such as walking, running, and climbing, where hip flexion is essential. By helping to flex the thigh, the psoas major supports movements that require lifting the legs, which is necessary for many athletic and daily activities.

The other options do not relate to the primary function of the psoas major, as they involve different muscle groups and actions that are not associated with this specific muscle's anatomical role or location.