The sartorius muscle is the longest muscle in the human body. It is a long, thin, superficial muscle that runs down the length of the thigh in the anterior compartment.

The sartorius muscle can move the hip joint and the knee joint, but all of its actions are weak, making it a synergist muscle. At the hip, it can flex, weakly abduct, and laterally rotate the thigh. At the knee, it can flex the leg; when the knee is flexed, sartorius medially rotates the leg. Turning the foot to look at the sole or sitting cross-legged demonstrates all four actions of the sartorius.

More Info: en.wikipedia.org