The knee is often described as the largest and most complex joint in the human body. Unlike the hip, which sits deep within a bony socket, the knee joint relies heavily on surrounding muscle tissue for both stability and movement. These muscles are organized into distinct groups, each fulfilling a specific function of straightening, bending, stabilizing, or rotating the leg.
The Primary Extensors of the Knee
The primary action of straightening the leg at the knee joint, known as extension, is performed by the powerful Quadriceps Femoris group, located on the front of the thigh. This group is a complex of four distinct muscles that merge into a single tendon inserting at the kneecap (patella) and shinbone (tibia). The four components are the Vastus Lateralis, Vastus Medialis, Vastus Intermedius, and the Rectus Femoris.
The three vasti muscles—Lateralis, Medialis, and Intermedius—originate solely from the femur and are dedicated to extending the knee. Their combined force is responsible for everyday movements such as rising from a chair, standing upright, and pushing off the ground while walking. The Vastus Medialis also plays a specific role in tracking and stabilizing the patella during these movements.
The fourth muscle, the Rectus Femoris, is unique because it is the only one of the four to cross both the hip and knee joints. While it contributes to knee extension, its origin on the pelvis also allows it to assist with hip flexion, which is the action of lifting the thigh forward. This dual-joint function is essential for dynamic activities like sprinting and kicking, where the leg must be simultaneously extended at the knee and flexed at the hip.
The Primary Flexors of the Knee
The primary muscles responsible for bending the leg at the knee, or knee flexion, are the Hamstring group, situated along the back of the thigh. This group consists of three main muscles: the Biceps Femoris, the Semitendinosus, and the Semimembranosus. The Biceps Femoris is typically divided into a long head and a short head, bringing the total number of distinct muscle bodies to four.
The Semitendinosus and the Semimembranosus are found on the medial (inner) side of the back of the thigh, while the Biceps Femoris is on the lateral (outer) side. Most hamstrings are biarticular, meaning they cross both the hip and the knee joints. This anatomical arrangement gives the hamstrings a dual responsibility in lower body movement.
Their main action at the knee is flexion, such as pulling the heel toward the buttock, but they are also powerful extensors of the hip, working to drive the leg backward. This hip extension function is particularly important during the propulsive phase of walking and running. The short head of the Biceps Femoris is the exception, acting only across the knee joint to produce flexion.
Stabilizers and Rotators of the Knee Joint
Beyond the major muscle groups that extend and flex the knee, several smaller, accessory muscles provide rotational control and stability. One of the most important of these is the Popliteus, a small, flat muscle located deep in the back of the knee. The Popliteus is often referred to as the “key to the knee” because its primary function is to “unlock” the joint from its fully extended, weight-bearing position.
When the knee is fully straight, the femur and tibia are slightly twisted, or “locked,” for stability. The Popliteus initiates flexion by producing internal rotation of the tibia, effectively “unlocking” the joint.
The Gastrocnemius, the large, two-headed muscle that forms the bulk of the calf, is primarily an ankle plantar flexor. However, its upper attachments cross the knee joint, allowing it to contribute a supplementary force to knee flexion, especially when the knee is almost fully extended.
On the medial side of the knee, the Sartorius and Gracilis muscles contribute to both flexion and internal rotation. The Sartorius, the longest muscle in the body, works with the Gracilis and the Semitendinosus to form the pes anserinus group. This group’s combined tendon insertion on the tibia helps stabilize the inner knee against excessive side-to-side forces and assists with internal rotation of the lower leg.