Joints, particularly hinge joints like the elbow and the knee, are complex structures made up of bone, muscles, synovium and cartilage and ligaments, designed to bear weight and move the body through space. The knee consists of the femur (thigh bone) above, and the tibia (shin bone) and fibula below. The patella, or kneecap rides on top of the lower portion of the femur and the top portion of the tibia. The muscles and ligaments connect these bones and the space between them is cushioned by fluid-filled capsules (synovia) and cartilage. When muscles are exercised, they pull on the bones, strengthening them. The range of motion of a joint represents how far it can be flexed (bent) and extended (stretched).