The knee joint is a complex structure central to human movement, enabling activities from walking to jumping. Many muscles surround and act upon the knee, working together to facilitate various movements and provide essential support. These muscles contribute to both mobility and stability, allowing for a wide range of motion while protecting the joint from excessive stress.
The Major Extensors
The primary muscles for straightening, or extending, the knee are the quadriceps femoris group, located on the front of the thigh. This group consists of four muscles: the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. All four quadriceps heads converge into a common tendon attaching to the kneecap (patella) and then to the tibia, the larger lower leg bone.
The rectus femoris is unique as it crosses both the hip and knee joints, allowing it to flex the hip in addition to extending the knee. The vastus muscles originate directly from the thigh bone (femur) and primarily focus on knee extension. Collectively, these muscles are important for actions like standing up, walking, running, and jumping. The quadriceps are considered the sole extensor muscle of the knee.
The Primary Flexors
Located on the back of the thigh, the hamstring group serves as the main muscles for bending, or flexing, the knee joint. This group comprises three muscles: the biceps femoris, semitendinosus, and semimembranosus. These muscles originate from the pelvis and extend down to attach near the knee joint on the tibia and fibula.
In addition to flexing the knee, the hamstrings also contribute to hip extension, important for activities such as walking, climbing stairs, and squatting. The interplay between the hamstrings and the quadriceps is important for controlled movement and stability around the knee, as they act as opposing muscle groups.
Muscles for Stability and Rotation
Beyond the primary flexors and extensors, several other muscles contribute to the knee’s stability and rotational movements. The gastrocnemius, a prominent calf muscle, assists with knee flexion, especially when the knee is already bent. While its main role is ankle plantarflexion, it also helps stabilize the knee and can influence forces on the anterior cruciate ligament (ACL).
The popliteus is a small, triangular muscle located at the back of the knee, often called the “key” to unlocking the knee joint. When the knee is fully extended and “locked,” the popliteus initiates flexion by subtly rotating the thigh bone on the shin bone. It also helps stabilize the knee during rotational movements and supports the posterior cruciate ligament (PCL).
The iliotibial band (IT band), a thick band of connective tissue running along the outside of the thigh, plays a role in lateral knee stability. The tensor fasciae latae (TFL), a muscle located at the hip, connects into the IT band and helps stabilize the knee, particularly when fully extended. The TFL and IT band work together to maintain pelvic and knee stability during movement.
Finally, the pes anserinus group contributes to the medial (inner) stability of the knee. This group consists of the sartorius, gracilis, and semitendinosus muscles, whose tendons converge and insert together on the inner side of the shin bone, below the knee. This conjoined insertion helps protect the knee against inward-directed forces and excessive rotation.