The knee joint is a hinge-type synovial joint that allows your lower leg to move relative to your thigh while supporting your body’s weight. This means that it is able to bend, straighten and slightly rotate. This movement is what enables you to walk, run, sit and stand.
There are four main movements that can be seen in the knee joint:
- Flexion – your hamstrings (back thigh muscles), gracilis (groin muscle), sartorius (anterior thigh muscle) and popliteus (small knee joint muscle) work together to flex your knee.
- Extension – your quadriceps (front thigh muscles) and tibial tuberosity (the flat, elevated surface where your tibia and knee joint meets) work together to extend the knee.
- Lateral rotation – the biceps femoris (back thigh muscles which form part of hamstrings) enable the lateral rotation of your knee.
- Medial rotation – semimembranosus (flat, long, back thigh muscle), semitendinosus (long, superficial, back thigh muscle), gracilis, sartorius and popliteus enable the medial rotation of your knee.
These movements depend on the four bones that make up the knee:
- Femur (thighbone)
- Patella (kneecap)
- Tibia (shinbone)
- Fibula (calf bone)
How movement occurs
The muscles in your thigh (quadriceps and hamstrings) enable your knee to move. They are assisted by tendons, which connect them to muscles.
When you straighten your leg, your quadriceps (front thigh muscles) contract and pull on their quadriceps tendon. The quadriceps tendon then pulls on the patella using the patellar tendon. This extends the knee.
When your hamstrings contract and pull on their tendons, which in turn pulls the femur, flexion occurs.
Extension and flexion
Extension is one of the main movements of your knees that takes place above the menisci (which is a piece of cartilage that serves as a cushion between your thighbone and shinbone).
Extension takes place around the transverse (side to side) axis of the knee joint, which isn’t fixed. The axis moves forward and upward.
Flexion is the other main movement of your knee. Like extension, it also takes place above the menisci around the transverse axis of the knee joint. During flexion, however, the axis moves in the reverse direction than extension.
Rotation
Rotation in the knee takes place around a vertical axis. This movement happens below the menisci and is controlled by the twisted cruciate ligaments (ligaments arranged in an x shape).
Locking and unlocking
The locking mechanism allows your knee to remain fully extended without relying on your muscles too much. This is a result of medial rotation of the thighbone.
The knee joint can only be flexed after it has been unlocked. This is a result of lateral rotation of the femur.
Your knee joints are the largest and most fragile joints in your body. They are made up of the tibia, fibula, femur and patella bones that enable them to extend, flex and rotate both laterally and medially. Your knees can also lock to hold you in position and unlock when its time to move again. This movement enables you to walk, run, sit, stand and many more daily tasks.
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