The region directly behind the knee, known as the popliteal fossa, is a densely packed area containing nerves, blood vessels, and strong soft tissues. The tendons in this posterior space are extensions of large muscles in the thigh and calf, connecting them securely to the bones of the lower leg. These fibrous tissues are fundamental to the power and stability of the lower extremity, enabling complex movements like walking, running, and climbing.
Identifying the Major Tendons
The tendons at the back of the knee primarily originate from the hamstring muscles in the thigh and the gastrocnemius muscle in the calf. The hamstring group includes three main muscles whose tendons insert around the knee joint at different locations. On the medial (inner) side of the posterior knee, the tendons of the semimembranosus and semitendinosus muscles attach to the tibia. The semitendinosus tendon, along with two others, forms the pes anserinus complex, which inserts on the upper, inner surface of the tibia, offering medial stability.
The lateral (outer) side of the posterior knee is dominated by the biceps femoris tendon. This tendon, formed by the union of the muscle’s two heads, attaches to the head of the fibula and the lateral tibia. The placement of the hamstring tendons on both the medial and lateral sides creates a robust attachment system at the back of the knee.
The upper tendons of the gastrocnemius muscle originate just above the knee joint. The gastrocnemius has two heads (medial and lateral) which arise from the posterior surfaces of the medial and lateral condyles of the femur. These proximal tendons transition into muscle belly and then merge distally with the soleus muscle to form the large Achilles tendon, which connects to the heel bone.
Role of These Tendons in Leg Movement
The primary function of the hamstring tendons is to initiate knee flexion (bending the knee). Since the hamstring muscles also cross the hip joint, their tendons are involved in hip extension, the motion of drawing the leg backward. This dual action makes the hamstrings powerful movers for activities requiring hip and knee coordination, such as running and jumping.
The specific insertions of the hamstring tendons also contribute to rotational stability of the knee. The semimembranosus and semitendinosus tendons on the medial side assist with internal rotation of the tibia, while the biceps femoris tendon on the lateral side aids in external rotation. This rotational control stabilizes the joint during quick changes in direction and deceleration.
The proximal gastrocnemius tendons contribute to both knee and ankle movement because the muscle crosses both joints. While the gastrocnemius is best known for its role in plantar flexion (pointing the foot downward), it also assists the hamstrings in knee flexion. The muscle is particularly active during fast, powerful movements like sprinting and jumping, where it provides propulsive force.
Understanding Common Posterior Knee Tendon Injuries
Injuries to the posterior knee tendons typically fall into two categories: acute strains and chronic tendinopathy. An acute strain involves a sudden tear of the muscle fibers or the tendon itself, most often affecting the hamstring group. These injuries commonly occur during explosive movements, such as sprinting, which places high eccentric loads on the muscle-tendon unit. A strain can range from a minor pull to a complete rupture, with pain usually localized to the back of the thigh or the area just behind the knee joint.
Chronic tendinopathy refers to the degeneration or breakdown of the tendon structure, often due to repetitive stress or overuse. Distal hamstring tendinopathy produces pain near the tendon insertion points at the back of the knee, worsening with activities like running or prolonged sitting. Similarly, the proximal tendons of the medial gastrocnemius head can develop tendinosis, causing localized pain.
Management for these tendon issues involves rest and consultation with a physical therapist to implement a progressive loading program. This approach focuses on strengthening the muscle and tendon to tolerate greater stress over time. With appropriate rehabilitation and modification of activity, most individuals can return to full function.