Which muscle flexes the knee, extends the hip, and medially rotates the hip?

Which muscle flexes the knee, extends the hip, and medially rotates the hip?

• A. Semimembranosus
• B. Semitendinosus
• C. Biceps Femoris
• D. Sartorius

Answer: (A)

The muscle you’re looking for is a member of the hamstring group that crosses both the hip and knee joints. It extends the hip, flexes the knee, and, because it pulls the thigh inward, medially rotates the hip. Semimembranosus fits all three actions: it originates on the ischial tuberosity, crosses the hip and knee, and, in function, extends the hip, flexes the knee, and internally rotates the thigh when the hip is extended. The other options don’t match all three actions together. The long head of the biceps femoris extends the hip but tends to laterally rotate the thigh, not medially. The semitendinosus similarly extends the hip and flexes the knee and medially rotates the knee, but its hip internal rotation is less pronounced. The sartorius flexes the hip and knee and helps with lateral rotation of the hip rather than extending it, so it isn’t the best fit for all three actions.

The muscle you’re looking for is a member of the hamstring group that crosses both the hip and knee joints. It extends the hip, flexes the knee, and, because it pulls the thigh inward, medially rotates the hip. Semimembranosus fits all three actions: it originates on the ischial tuberosity, crosses the hip and knee, and, in function, extends the hip, flexes the knee, and internally rotates the thigh when the hip is extended.

The other options don’t match all three actions together. The long head of the biceps femoris extends the hip but tends to laterally rotate the thigh, not medially. The semitendinosus similarly extends the hip and flexes the knee and medially rotates the knee, but its hip internal rotation is less pronounced. The sartorius flexes the hip and knee and helps with lateral rotation of the hip rather than extending it, so it isn’t the best fit for all three actions.