Three muscles attach to the pes anserinus: the sartorius, gracilis, and semitendinosus. Their tendons converge and insert together on the inner (medial) surface of the shinbone, about 5 cm below the knee joint line. The name “pes anserinus” is Latin for “goose foot,” describing the webbed, fan-like shape these three tendons create where they meet the bone.
The Three Muscles and Where They Come From
Each of the three muscles originates from a different part of the hip and thigh, travels a different path, and is controlled by a different nerve. What they share is a common destination: the inner side of the upper tibia.
Sartorius is the longest muscle in the body, running diagonally from the front of the hip bone down across the thigh to the inner knee. Despite its length, it is relatively weak and acts mostly as a helper muscle during movements like crossing your legs. Its tendon sits in the most superficial (closest to the skin) layer of the pes anserinus.
Gracilis is a long, thin strap muscle on the inner thigh. It originates from the pubic bone and runs straight down the medial compartment of the thigh. It is supplied by the obturator nerve. At the knee, its tendon inserts just behind the sartorius tendon, in a slightly deeper layer.
Semitendinosus is one of the three hamstring muscles. It originates from the ischial tuberosity (the “sit bone” at the base of the pelvis), sharing a starting tendon with the long head of the biceps femoris. It courses down the back of the thigh and wraps around to the inner knee, where its tendon inserts behind the gracilis tendon in the deepest layer of the pes anserinus. It is innervated by the sciatic nerve.
How the Tendons Are Layered
The three tendons do not simply merge into one flat sheet. They stack in a consistent front-to-back order on the medial tibia. The sartorius tendon is the most superficial, with its fascia intimately attached to the overlying connective tissue layer. The gracilis and semitendinosus tendons lie deeper, beneath that fascial layer. Although the gracilis and semitendinosus are separate structures higher up in the thigh, they converge as they approach their insertion on the tibia.
What These Muscles Do Together
All three muscles flex the knee, and all three help rotate the lower leg inward (internal tibial rotation). You use this combined motion whenever you sit cross-legged. Because the tendons attach on the inner side of the tibia, they also brace the knee against forces that push it inward (valgus stress) and against excessive rotation. This makes the pes anserinus an important dynamic stabilizer of the medial knee.
Each muscle also has a secondary role at the hip. The sartorius flexes and externally rotates the hip. The gracilis adducts the thigh (pulls it toward the midline). The semitendinosus extends the hip. So while they converge at the same insertion point, they contribute to very different movements higher up in the leg.
The Bursa Underneath
A small, fluid-filled sac called the pes anserine bursa sits between these converging tendons and the tibial bone. Its job is to reduce friction as the tendons glide over bone during knee bending and rotation. When this bursa becomes irritated or inflamed, the result is pes anserine bursitis, which causes pain and tenderness on the inner knee, typically a few centimeters below the joint line. Tight hamstrings are a common contributing factor, since increased tension on the semitendinosus tendon adds friction over the bursa.
Why Surgeons Care About These Tendons
The gracilis and semitendinosus tendons are commonly harvested as graft material for ACL reconstruction. Surgeons identify the converging attachment at the tibia, then trace the tendons back up the thigh to where they separate, harvesting them individually. These hamstring grafts produce good functional outcomes with relatively little lasting impact at the donor site, which is one reason the pes anserinus is among the most clinically relevant tendon structures in the knee.
Understanding which muscles feed into the pes anserinus also matters for rehabilitation. Stretching and strengthening programs for inner knee pain typically target all three muscles, since tightness or weakness in any one of them can alter how much load the shared insertion point absorbs. Hamstring flexibility exercises are especially emphasized because the semitendinosus tendon, being the deepest layer, creates the most direct friction against the bursa when it is chronically tight.