The knee joint connects the femur to the tibia, allowing mobility for daily activities. Knee flexion is the primary bending movement that decreases the angle between these bones, bringing the heel closer to the buttocks.
Understanding the Biomechanics of Knee Flexion
Flexion is an angular movement in the sagittal plane, opposite of extension (straightening the leg). The knee is classified as a modified hinge joint, meaning its movement is not a simple rotation around a single, fixed axis.
The joint requires a combination of motions between the femur and tibia during flexion. As the knee bends, the femur rolls and slides backward on the tibia. This coupled motion prevents the femur from rolling off the back of the tibia. During the initial degrees of flexion, a small rotation known as the “unlocking” of the knee must occur to permit smooth bending.
The Muscles and Structures Responsible
The primary power source for knee flexion comes from the hamstring muscle group, located along the back of the thigh. This group consists of three distinct muscles: the biceps femoris, the semitendinosus, and the semimembranosus. When these muscles contract, they pull the lower leg backward toward the thigh, initiating and controlling the bend of the knee.
The semitendinosus muscle is noted for its strong activation during flexion exercises. Secondary muscles also contribute to this movement, including the popliteus, a small muscle located behind the knee that helps to “unlock” the joint from its fully extended position. The gastrocnemius, a calf muscle, also crosses the knee joint and assists in flexion, especially when the ankle is free to move.
Ligaments provide stabilization as the muscles execute the movement. The cruciate ligaments, specifically the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL), cross each other inside the joint and manage the forward and backward sliding of the tibia on the femur. This ligamentous control ensures the bones remain aligned during the contractions of the hamstring muscles.
Measuring Normal Movement and Daily Necessity
The standard range of motion (ROM) for knee flexion in a healthy adult is between 135 and 140 degrees. This measurement is often taken by a healthcare professional using a goniometer. The center of the device is placed over the knee axis while the arms align with the femur and tibia to quantify the maximum bend.
Achieving and maintaining this full range is directly linked to the ability to perform basic life functions. Activities like sitting down in a chair or rising from a seated position require approximately 90 to 100 degrees of flexion. More demanding movements, such as climbing stairs, require about 110 degrees of flexion.
The ability to squat deeply, tie shoes, or get out of a bathtub requires the full range of motion approaching the maximum 135 degrees. A loss of even a few degrees can significantly impact the quality of life, making everyday tasks difficult.
Specific Conditions That Limit Knee Flexion
A variety of medical conditions can physically impede the ability of the knee to flex fully. Osteoarthritis, a degenerative joint disease, is a common culprit where the breakdown of cartilage leads to joint space narrowing and the formation of bony growths known as osteophytes or bone spurs. These structural changes can create a mechanical block, preventing the joint from achieving its full bend.
Meniscal tears can also restrict movement if a torn piece of the C-shaped cartilage becomes displaced within the joint, physically blocking the smooth motion of the femur on the tibia. This is often described as the knee “catching” or “locking.” Following a surgery or significant injury, the body may overproduce fibrous tissue, leading to a condition called arthrofibrosis.
This build-up of scar tissue can essentially glue structures together, causing stiffness and a reduction in the joint’s flexibility. The resulting inability to achieve full flexion is sometimes referred to as a contracture, where the soft tissues have shortened and become resistant to stretching.