What is the action of the piriformis?

• A. Abduct the hip when hip is flexed
• B. Adduct the hip when hip is flexed
• C. Flex the hip
• D. Extend the knee

Answer: (A)

The action being tested is how the piriformis works differently depending on the hip angle. This muscle is one of the deep external rotators of the hip, and its fibers pull on the femur in a way that changes with hip position. When the hip is flexed, the piriformis acts to abduct the thigh, pulling it away from the midline to help stabilize the pelvis during movements with the leg bent. In contrast, when the hip is extended, its pull mainly causes external rotation rather than abduction. That’s why abducting the hip when the hip is flexed is the correct description of its action. The other options don’t fit: the piriformis doesn’t flex the hip, isn’t a knee extensor, and isn’t primarily an adductor in this position.