The longest muscle in the human body is the sartorius muscle. Running down the length of the thigh, it originates from the anterior superior iliac spine of the pelvis and extends to the medial side of the tibia in the lower leg. The sartorius is involved in several movements, including hip flexion, abduction, and external rotation, as well as knee flexion. Often referred to as the "tailor's muscle," it plays a role in actions like crossing the legs, reinforcing its importance in everyday movements.