The knee joint is the point at which the tibia and femur come together. The fibula, the thinner and weaker bone of the lower leg, runs parallel to the tibia. Although it has no direct effect on knee mobility, the fibula is attached to the two ends of the tibia via ligaments. These connections provide some stability to the ankle. The femur and tibia are two long bones that form the upper part of the leg. The hip and knee joints allow for extensive movement in the sagittal plane (back and forth), while the ankle provides rotational freedom in the transverse plane (side to side). In anatomy classes, you often hear the terms "weight-bearing bone" or "supporting bone." This means that these bones play a role in providing support for the body's weight as well as absorbing the shock of walking and other weight-bearing activities. The fibula is a weight-bearing bone, whereas the talus (which sits below the fibula) is a non-weight-bearing bone. The fibula receives most of its strength and stability from the ligaments and muscles that attach to it. However, if a severe injury disables these structures, then the bone itself may be forced to take up some of the load instead. The bone marrow inside the fibula is responsible for producing red blood cells; therefore, any disease or trauma that affects this tissue can lead to anemia.