The back of the knee is known anatomically as the popliteal fossa. This area is anchored by several powerful tendons that manage the intense forces generated by the upper and lower leg muscles. When people search for “the tendon in the back of the knee,” they are typically referring to the structures that define the boundaries of this dynamic space. This article will identify these specific tendons, explain their function, and discuss the common problems that can affect them.
Identifying the Key Tendons
The back of the knee is framed by a distinct group of three major hamstring tendons. These tendons form the superior boundaries of the popliteal fossa, creating the distinct ridges that can be felt when the knee is bent.
On the outer, or lateral, side of the knee is the tendon of the Biceps Femoris muscle, which is often the most palpable structure in this area. This tendon extends to insert onto the head of the fibula.
The inner, or medial, side of the back of the knee is defined by two separate tendons: the Semitendinosus and the Semimembranosus. The Semitendinosus tendon is the more superficial of the two, while the Semimembranosus tendon lies deeper, closer to the bone. Both of these medial tendons travel down to insert on the top portion of the tibia on its inner surface.
A secondary set of tendons forms the lower boundaries of the popliteal fossa, specifically the two heads of the Gastrocnemius muscle. These tendons originate just above the knee joint on the lower end of the thigh bone.
How These Tendons Facilitate Movement
The primary biomechanical function of the hamstring tendons is to act as powerful movers across two major joints: the hip and the knee. Their most recognized action at the knee is flexion, which is the motion of bending the knee to bring the heel toward the buttocks. This action is constantly used during activities like walking, running, and climbing stairs.
In addition to bending the knee, these hamstring tendons also contribute significantly to hip extension. These tendons transmit the force generated by the contracting hamstring muscles.
Beyond simple bending and extending motions, the medial and lateral hamstring tendons play a role in rotational stability of the knee. By inserting on opposite sides of the joint, they function like opposing restraints. The medial tendons, for example, assist in internally rotating the lower leg, while the lateral Biceps Femoris tendon assists in external rotation. This coordinated action is important during dynamic movements, such as quickly changing direction while running.
Common Conditions Related to Posterior Knee Tendons
Pain in the back of the knee is often traced back to issues affecting these hamstring tendons, with tendinopathy and muscle strain being the most frequent diagnoses. A hamstring strain occurs when the muscle fibers or the tendinous fibers tear, often resulting from a sudden, forceful contraction or eccentric loading, such as sprinting or lunging. This type of injury commonly occurs at the musculotendinous junction, the point where the muscle transitions into the tendon.
Tendinopathy, or inflammation of the tendon, is typically an overuse injury caused by repetitive stress or a rapid increase in physical activity. This condition results in pain and tenderness directly over the tendon insertion points, such as where the Biceps Femoris attaches to the fibula. Poor flexibility or insufficient warm-up can increase the friction and stress placed on the tendon fibers, leading to chronic irritation and pain.
Another specific condition related to the medial hamstring tendons is Pes Anserine Bursitis, which affects a fluid-filled sac located on the inner, upper part of the tibia. This bursa reduces friction where the Semitendinosus tendon inserts onto the bone. Inflammation of this bursa is often caused by repetitive motions, tight hamstrings, or underlying conditions like knee osteoarthritis, leading to localized tenderness and swelling.