The longest muscle in the human body is the sartorius muscle. This muscle runs from the anterior superior iliac spine of the pelvis down to the medial aspect of the knee. It is involved in actions such as flexing the hip and knee, and it assists in the outward rotation of the thigh. Because of its length and unique position, the sartorius is sometimes referred to as the "tailor's muscle," as it helps in the seated position traditionally used by tailors.