The knee is a complex synovial joint between the femur and tibia, and the femur and patella. Though the main movements are flexion and extension, there is a degree of rotation which helps increase the stability of the knee.
Movement of the knee is created largely through the muscles contained within the thigh, with the quadriceps being responsible for extension, acting through the patella tendon (see image); and flexion being created by the hamstring muscles and to a lesser extent the calf (or gastrocnemius to be precise). Along with creating movement, these muscles also act as ‘active stabilisers’ for the knee, and therefore their function is integral to the safety of the knee joint.
The main ‘passive stabilisers’ of the knee are its ligaments and meniscus. There are 4 main ligaments: Medial and Lateral Collateral Ligaments & Posterior and Anterior Cruciate Ligaments. Each is responsible for resisting certain lines of force acting upon the knee, and therefore when the force exceeds its capacity they are subject to injury.
The menisci act as stabilisers by increasing the contact area between the tibia and femur by 40-60%. They also have a pivotal role in acting as shock absorbers, ensuring smooth movement between the surfaces, and providing information about the knee position (proprioception).