Knee flexion, or bending the knee, is a fundamental movement that decreases the angle between the thigh and the lower leg. It is essential for countless daily activities, including sitting, standing, walking, squatting, and lunging.
Key Muscles for Knee Bending
The primary muscles responsible for bending the knee are the hamstrings, a group located on the back of the thigh. This group consists of three distinct muscles: the biceps femoris, the semitendinosus, and the semimembranosus.
The biceps femoris has two parts, a long head and a short head, located on the outer, rear thigh. Its long head originates from the ischial tuberosity, and the short head from the femur. Both connect to the head of the fibula. This muscle primarily bends the knee and, when slightly bent, helps rotate the lower leg outwards. The long head also extends the hip.
The semitendinosus is a long, superficial muscle on the inner, back of the thigh. It originates from the ischial tuberosity and inserts onto the medial surface of the tibia. This muscle contributes to knee bending, hip extension, and inwardly rotates the tibia when the knee is flexed.
The semimembranosus is located deeper than the semitendinosus, on the inner, rear thigh. Like the semitendinosus, it originates from the ischial tuberosity and inserts into the medial condyle of the tibia. It functions to bend the knee, extend the hip, and inwardly rotate the tibia when the knee is bent.
Several other muscles assist in knee bending, although they are not the primary movers.
The gastrocnemius, a large calf muscle, crosses the back of the knee. Its main role is to point the foot downwards, but it also contributes to knee flexion.
The popliteus is a small, deep muscle located behind the knee. It assists in knee flexion and plays a specific role in “unlocking” the knee from a fully straightened position by rotating the femur on the tibia.
The sartorius, a long, strap-like muscle across the front of the thigh, and the gracilis, located on the inner thigh, also contribute to knee flexion. The sartorius assists with hip flexion and external rotation, while the gracilis aids in hip adduction and internal rotation of the knee. These muscles, along with the semitendinosus, share a common insertion point on the tibia.
Understanding How Knee Bending Works
The process of knee bending involves a coordinated effort between muscles, tendons, and bones.
Skeletal muscles, like those in the hamstrings, move the body by contracting and shortening their length. This contraction is triggered by signals from the nervous system, which initiate chemical reactions within the muscle fibers.
Muscles do not directly attach to bones across a joint. Instead, they connect to bones via strong, fibrous tissues called tendons. Tendons act as a mechanical bridge, transmitting the force generated by muscle contractions to the bones.
When the hamstrings and other knee flexor muscles receive a signal to contract, they pull on their respective tendons. These tendons then pull on the bones of the lower leg, such as the tibia and fibula, causing them to move closer to the thigh bone (femur). This pulling action results in the bending motion at the knee joint. The knee joint itself functions like a hinge, allowing for this specific range of motion.