Every joint in the body wiggles a little bit. This wiggle is called joint play. Some joints have a lot more wiggle room(joint play) than other joints. Like for instance the kneecap joint. You can move the kneecap(patella) all around. The joint between the thigh bone(femur) and the kneecap has a lot of joint play. The joint between the shin bone(tibia) and femur – not nearly as much joint play. Joint play is a way to test if a ligament, the thing that connects bone to bone, is torn or messed up somehow. You know how football players love to hurt their knees and then an athletic trainer runs onto the field and does some stuff? Well here’s some of what they do. I was never an athletic trainer so I certainly don’t know everything that they do but I know that this is part of what they do. So when a football player twists their knee and falls down and can’t walk the athletic trainer rushes out and performs some special tests on the knee. He tests the joint play of a joint and compares it to the other side to see if it’s a lot more. If the painful knee has a lot more joint play then the ligament inside that joint is probably torn or stretched and well….that’s not good. Also, putting the legs in certain positions will stress certain ligaments and if that produces pain well….that’s not good. There are tests like this for every joint in the body, they’re called special tests. Special tests are something that I will most likely never, ever do again. Oh well.