The pes anserinus is an anatomical structure located on the inner side of the knee that plays a supporting role in lower limb movement and stability. Its name is derived from the Latin phrase for “goose’s foot,” which aptly describes the three-pronged, fan-like shape of its tendinous insertion. This structure represents a convergence point for muscles originating from different parts of the hip and thigh, allowing for coordinated action across the knee joint. The integrity of the pes anserinus is important for normal biomechanics, and problems in this area can lead to pain and movement dysfunction.
Defining the Pes Anserinus Structure and Location
The pes anserinus is a common tendinous insertion found on the medial, or inner, side of the proximal tibia. This attachment point is located approximately 5 centimeters below the medial tibial joint line, just beneath the knee joint. The three distinct muscle tendons combine here to form a broad, flat structure that resembles the webbing of a goose’s foot. This insertion point lies superficial to the tibial attachment of the medial collateral ligament (MCL). The combined tendons anchor the muscles to the anteromedial surface of the tibia, creating a mechanical advantage for their combined actions.
The Three Key Attaching Muscles
The pes anserinus is formed by the conjoined tendons of three separate muscles: the Sartorius, the Gracilis, and the Semitendinosus. These muscles originate from different compartments of the thigh—anterior, medial, and posterior—but share this final common insertion point. The arrangement is often described as a layered structure, with the Sartorius tendon being the most superficial.
Sartorius
The Sartorius muscle is the longest muscle in the human body, originating high on the pelvis at the anterior superior iliac spine. Its tendon is the most anterior of the three at the insertion site. Its primary action is to flex the hip and the knee.
Gracilis
The Gracilis muscle is a long, slender muscle from the medial compartment of the thigh, originating from the ischiopubic ramus of the pelvis. This muscle is primarily an adductor, drawing the thigh toward the midline of the body. Its tendon lies in the middle of the three conjoined tendons.
Semitendinosus
The Semitendinosus muscle is one of the three hamstring muscles, originating from the ischial tuberosity of the pelvis. Its main functions are to extend the hip and flex the knee. Its tendon is the most posterior of the three contributing to the pes anserinus insertion.
Functional Significance of the Insertion
The convergence of the Sartorius, Gracilis, and Semitendinosus at the pes anserinus provides a unique and powerful mechanical function for the lower limb. The combined contraction of these muscles results in simultaneous knee flexion. Furthermore, the angle of the conjoined tendon allows the muscles to contribute significantly to the internal, or medial, rotation of the tibia, especially when the knee is bent.
Because the Gracilis also acts as a hip adductor, the combined structure facilitates the movement of the leg toward the body’s center line. This collective action is important for dynamic knee stability, acting as a secondary restraint against valgus stress. The muscles provide tension on the inner side of the knee, helping to prevent the lower leg from rotating outward and maintaining proper alignment.
Common Conditions Affecting the Pes Anserinus
The most common condition involving this structure is Pes Anserine Bursitis, which is the inflammation of the bursa located beneath the conjoined tendons. This fluid-filled sac normally acts as a cushion to reduce friction between the tendons and the bone. Inflammation causes pain and tenderness specifically on the inner side of the knee, typically about two inches below the joint line.
This irritation can be caused by overuse, repetitive movements, or mechanical issues that increase friction on the area. Common contributing factors include tight hamstring muscles, obesity, and underlying knee conditions like osteoarthritis. The condition is frequently seen in athletes, particularly runners, but also affects individuals with abnormal knee alignment, such as knock-knees. The tendons themselves can also be affected, leading to a condition known as tendinopathy or tendinitis.