The knee is the joint located between the bones of the upper and lower leg. It is the reason the leg is able to bend, and it also provides stability so that the legs can support the weight of the body. The knees help support common, everyday motions like walking, running, crouching, jumping, and turning. Knee pain is a common complaint and it affects people of all ages. It can manifest as a result of an injury, like a ruptured ligament or torn cartilage. Knee pain also has causes that aren't a result of underlying diseases. It is commonly caused by heavy physical activity, lack of use, injuries like sprains or strains, or sitting in a confined area.