The knee primarily bends and straightens, but it also allows a surprising amount of rotation when flexed. Understanding these movements helps explain everything from how you walk to why certain injuries happen.
Flexion and Extension: The Primary Movements
Bending (flexion) and straightening (extension) account for the vast majority of what the knee does. In healthy adults aged 20 to 44, the knee bends to roughly 138 to 142 degrees, with women typically having a few degrees more range than men. Full extension brings the knee to a nearly straight position, with most adults having about 1 to 2 degrees of slight hyperextension beyond perfectly straight.
These numbers shift with age. Children aged 2 to 8 can bend the knee to about 148 to 153 degrees and often have noticeably more extension. Range gradually decreases through the teenage years and into adulthood, though the changes are modest enough that most people never notice them.
When you bend your knee, the movement isn’t a simple hinge like a door. The thighbone (femur) both rolls and glides on the shinbone (tibia) simultaneously. As the knee bends, the tibia glides backward on the femur. This combination of rolling and gliding keeps the joint surfaces in proper contact throughout the full range and prevents the femur from simply rolling off the back of the tibia.
Rotation: The Hidden Movement
Most people think of the knee as a pure hinge, but it allows a significant amount of twisting when bent. Between 30 and 90 degrees of flexion, the knee permits roughly 45 degrees of outward rotation and 25 degrees of inward rotation. That’s a total rotational arc of about 70 degrees, which is far more than most people expect from a “hinge” joint.
Rotation drops dramatically as the knee straightens. At just 5 degrees of flexion (nearly straight), outward rotation falls to about 23 degrees and inward rotation to only 10 degrees. This is why a planted, nearly straight leg is more vulnerable to rotational injuries: there’s very little rotational slack available.
The Screw-Home Mechanism
During the final 20 degrees of straightening, the tibia automatically rotates outward about 15 degrees on the femur. This involuntary twist, called the screw-home mechanism, happens because the inner ridge (medial condyle) of the femur is longer than the outer ridge. The tibia essentially runs out of surface on the outer side first, forcing it to rotate externally to complete full extension. This rotation locks the knee into a stable, weight-bearing position when you stand. To initiate bending again, the knee must first “unlock” by rotating slightly inward.
Movements the Knee Prevents
Just as important as what the knee allows is what it blocks. The knee does not permit significant side-to-side bending, forward-backward sliding of the shinbone, or excessive rotation. Four major ligaments enforce these limits.
- Anterior cruciate ligament (ACL): Prevents the tibia from sliding forward relative to the femur and resists inward rotation.
- Posterior cruciate ligament (PCL): Prevents the tibia from sliding backward.
- Medial collateral ligament (MCL): Resists forces that push the knee inward (valgus stress).
- Lateral collateral ligament (LCL): Resists forces that push the knee outward (varus stress) and helps restrain outward tibial rotation and backward displacement, especially in the first 30 degrees of flexion.
When any of these ligaments tear, the knee gains abnormal movement in the direction that ligament was restraining. An ACL tear, for instance, allows the tibia to shift forward in ways it normally cannot.
How the Menisci Move With the Knee
The two crescent-shaped cartilage pads inside the knee, the menisci, aren’t stationary. They slide backward as the knee bends to stay positioned between the rolling joint surfaces. The lateral (outer) meniscus moves about 8.2 mm during full flexion, while the medial (inner) meniscus moves about 6.2 mm. The medial meniscus is more firmly anchored, which partly explains why it tears more often: it has less freedom to get out of the way during sudden twisting forces.
Muscles That Produce These Movements
The quadriceps, the four muscles on the front of your thigh, are the primary extensors. They straighten the knee against resistance and control the speed of bending when you lower yourself into a squat or walk downhill. The rectus femoris, one of the four quadriceps muscles, also crosses the hip joint, which is why your knee position affects hip flexibility and vice versa.
The hamstrings, three muscles on the back of the thigh (the biceps femoris, semitendinosus, and semimembranosus), bend the knee and also contribute to rotation. The biceps femoris rotates the tibia outward, while the semitendinosus and semimembranosus rotate it inward. These muscles work constantly against each other during movement. When the hamstrings contract to bend the knee, the quadriceps provide a braking force, and the balance between the two muscle groups is critical for joint stability.
What Normal End-Range Feels Like
Each knee movement has a characteristic feeling at its limit. Full flexion typically stops with a soft, cushioned sensation as the calf muscles compress against the back of the thigh. Full extension ends with a firmer stop as ligaments and the joint capsule pull taut. If straightening your knee produces a hard, abrupt block before you reach full extension, that can indicate something is mechanically stuck inside the joint, such as a torn meniscus fragment or loose cartilage. A springy resistance that prevents full straightening is also abnormal and worth investigating.