Functions of the Knee

For Young Learners

Function

Try this. Stand up, then sit down again slowly. Your knee went from almost straight to bent past a right angle, holding your whole weight the entire way. It did not wobble sideways and it did not collapse.

That is three jobs at once. Move through a big arc. Carry a heavy load. Stay steady while doing both. Most machines are built to do one of those well. The knee does all three.

There is a fourth job that is easy to miss. The knee softens landings. When you jump down from a wall, your knees bend as you land. That bending spreads the impact out over a longer moment instead of letting it hit all at once.

For Patients

Function

The knee bends to roughly 135 degrees in most people and straightens fully, sometimes a few degrees beyond. Losing either end of that range causes trouble. Full straightening is needed to stand and walk efficiently. Bending beyond 100 degrees is needed to rise from a low chair, climb stairs comfortably and get in and out of a car.

At the same time it carries load. Walking on the flat puts several times body weight through the joint. Stairs and squatting put more. That load is spread across surfaces designed to share it.

Stability is the third job. The knee resists being pushed sideways and resists the shin sliding forwards or backwards under the thigh. It allows a small amount of rotation and resists more than that.

Clinical Relevance

Losing full extension matters more than most people expect. A knee that will not straighten makes walking tiring, because the thigh muscle has to work continuously instead of resting at the top of each step.

Losing flexion shows up in specific tasks: low chairs, stairs, kneeling, getting into a car. Rehabilitation after injury or surgery usually restores extension first for that reason, then builds flexion.

A knee that gives way is reporting a stability problem rather than a movement one, and that distinction shapes what is investigated and what is treated.

For Medical Students

Function

The knee serves three mechanical demands simultaneously: a large arc of sagittal motion, transmission of substantial axial load, and maintenance of stability in the coronal and transverse planes throughout that arc.

Range is typically zero to about 135 degrees of flexion, with a few degrees of hyperextension in many individuals. Terminal extension is associated with a small obligatory external rotation of the tibia on the femur, which locks the joint into its most stable configuration and allows standing with minimal quadriceps activity.

Load transmission is shared between the articular cartilage and the menisci, which increase contact area and lower peak contact stress. Shock absorption derives from eccentric muscle work, from the deformable soft tissues and from subchondral bone.

Clinical Relevance

The three demands conflict, and the compromises explain much of knee pathology. Mobility requires incongruent surfaces, which sacrifices bony stability and places the burden on soft tissue restraints that can tear.

Function is best assessed against those three demands in turn. A deficit of range, a deficit of load tolerance and a deficit of stability have different causes and different treatments, and a history that separates them is more useful than one that records pain alone.

The detailed kinematics of rolling, gliding and the screw home mechanism are taken up in the biomechanics section.