Sartorius and the cross-legged sitting position

The sartorius is the longest muscle in the body. It is long and thin, and it runs obliquely (at a slant) across the front of the thigh. It is the most superficial muscle of the anterior compartment, the group of muscles at the front of the thigh, supplied by the femoral nerve.

It starts at the anterior superior iliac spine (the bony point at the front of the pelvis) and runs downwards and towards the midline to the upper medial tibia (the shin bone). Its tendon passes behind the inner side of the knee and joins the tendons of gracilis (a strap-like muscle on the inner thigh) and semitendinosus (a hamstring) at the pes anserinus, their shared attachment on the inner side of the upper shin bone.

Because it crosses both the hip and the knee, it acts at both joints. At the hip it flexes (bends the thigh forwards), abducts (moves the thigh away from the midline) and laterally rotates (turns the thigh outwards). At the knee it flexes. Those movements together produce the cross-legged sitting position. Each of these actions is individually weak, which makes the sartorius a synergist, meaning a helper that acts alongside stronger muscles. The name comes from the Latin sartor, meaning tailor, probably because tailors once sat cross-legged.

Other muscles each match only part of the pattern. Rectus femoris (the quadriceps muscle that crosses both the hip and the knee) bends the hip but straightens the knee. Gracilis flexes the knee but adducts the thigh. The hamstrings extend the hip.