Your knee — it’s the biggest joint in your body, made up of a lot of parts that can get injured in all kinds of sports. The knee connects the lower end of the thighbone (femur) to the upper end of the shinbone (tibia). Several large ligaments — strong, elastic bands of tissue that join bone to bone — make this connection, bracing and controlling the motion of the joint. The kneecap (patella) slides in a groove at the end of the thighbone; it protects the knee and gives leverage to various leg muscles. Cartilage at the ends of the leg bones helps to cushion the joint, like a shock absorber.

Sprains — stretched or torn ligaments — are an especially common knee injury; the "anterior cruciate ligament" (ACL) is the one most often damaged. This can happen, for example, when you change direction quickly, twist a leg or fall, slow down when running, or land from a jump. Typically, a torn ACL will be immediately disabling: you’ll feel or hear a "pop," and the knee seems to "give way." Even though it might not be painful, see a doctor right away. Surgery may be necessary.

Knee pain can also come on gradually from overuse of the joint. For example, "patello-femoral syndrome," or kneecap pain caused by the kneecap being pushed against the sides of its groove, can occur if you do a lot of running. It usually causes pain, either sharp or dull, in the front of the knee. Squatting or walking down stairs often makes it worse, and you might feel a grinding or "clicking" in the joint. A sports medicine specialist can help you to rebalance the kneecap and prevent further pain through proper training, stretching and strengthening exercises.