The knee joint is a complex structure connecting the thigh bone (femur) to the shin bone (tibia, enabling a wide range of movements like walking, running, jumping, and squatting. Muscles, tendons, and ligaments work together to ensure its function and stability. Understanding the muscles behind the knee provides insight into how this joint operates.
Identifying the Major Muscles Behind the Knee
The posterior aspect of the knee, known as the popliteal region, houses several muscles. These are broadly categorized into the hamstrings in the thigh, calf muscles in the lower leg, and a smaller, deeper muscle.
The hamstring group consists of three muscles: the biceps femoris, semitendinosus, and semimembranosus. The biceps femoris is positioned on the outer side of the back of the thigh and has two heads. The semitendinosus lies in the middle of the back of the thigh, between the biceps femoris and the semimembranosus. The semimembranosus is located on the innermost side of the back of the thigh, deep to the semitendinosus. All three hamstring muscles originate from the pelvis and extend down to the knee, attaching to either the tibia or fibula.
In the calf region are the gastrocnemius and plantaris muscles. The gastrocnemius is a large, superficial muscle with two heads, medial and lateral, forming the bulk of the calf. These heads originate from the lower thigh bone, just above the knee, and connect to the heel via the Achilles tendon. The plantaris is a smaller, slender muscle with a short belly and a long, thin tendon. It originates from the lateral side of the thigh bone, near the gastrocnemius, and its tendon runs down the leg, often alongside the Achilles tendon.
Deep within the popliteal region, beneath the gastrocnemius and hamstrings, lies the popliteus muscle. This small, flat, and triangular muscle forms part of the floor of the popliteal fossa, the hollow space behind the knee. Its origin is on the outer side of the thigh bone, and it extends diagonally to insert onto the upper, posterior part of the shin bone.
How These Muscles Function
The muscles behind the knee work together to facilitate various movements and provide joint stability. Their coordinated actions contribute to everyday activities and athletic performance. Each muscle or group has specific functions in the overall mechanics of the knee.
The hamstring muscles primarily contribute to knee flexion, or bending the knee. They also extend the hip joint, moving the leg backward. The semitendinosus and semimembranosus help rotate the lower leg inward when the knee is bent, while the biceps femoris assists in outward rotation. These combined actions are important for activities like walking, running, and jumping.
The gastrocnemius muscle, while forming part of the calf, also plays a role at the knee. It assists in knee flexion, working with the hamstrings to bend the leg. Its primary action is plantarflexion of the foot, pointing the toes downward at the ankle joint. The plantaris muscle, though small, weakly aids both knee flexion and ankle plantarflexion, often working alongside the gastrocnemius.
The popliteus muscle has a specialized role in knee mechanics. It is often referred to as the “key” to unlocking the knee. When the knee is fully extended and “locked,” the popliteus muscle contracts to initiate flexion by slightly rotating the thigh bone outward on the shin bone. This muscle also contributes to the posterolateral stability of the knee joint, helping to prevent excessive rotation and movement.
Understanding Common Conditions Affecting Posterior Knee Muscles
Muscles behind the knee can be susceptible to various non-traumatic conditions, often resulting from overuse or sudden strain. These conditions can cause discomfort and limit movement, impacting daily activities.
One common condition is a hamstring strain, often called a “pulled hamstring.” This occurs when hamstring muscle fibers are overstretched or torn. Symptoms can range from mild pain and swelling to sharp pain and difficulty bearing weight, depending on the tear’s severity. Hamstring strains are observed in activities involving sudden acceleration or changes in direction.
The gastrocnemius muscle can also experience tightness or strains. Tightness can restrict ankle movement and contribute to discomfort behind the knee. Strains in this muscle, sometimes called “tennis leg,” result from forceful movements involving simultaneous knee extension and ankle flexion. General muscle soreness behind the knee can arise from overuse or unaccustomed physical activity. This type of soreness resolves with rest and gentle movement.