Injury of the anterior cruciate ligament, which works for anterior restraint, is especially important among the knee joint ligaments because it can result in the knee to giving way and requires surgical reconstruction before the patient can return to sports activities. On the other hand, injury of the posterior cruciate ligament, which works for posterior restraint, rarely results in the knee joint giving way and so the patient's comeback to sports is easier. Our motion analysis of the muscular skeletal model indicated that general motions mainly need posterior restraint and hardly require anterior restraint at all. Anatomy also shows that the anterior knee does not have a muscular tissue which works for restraint while the posterior knee has the stout gastrocnemius, which takes charge of posterior restraint. From the above facts, the assumption is that properly balanced motions of the knee joint only need posterior restraint with the posterior cruciate ligament and the gastrocnemius while an unexpectedly ill-balanced motion in the middle of sports activity can cause an anterior restraint which results in the knee joint giving way if the anterior cruciate ligament is insufficient. Accordingly, it is suggested that rehabilitation after anterior cruciate ligament reconstruction can be safely performed if only losing balance in an unnatural motion is avoided. In addition, newly developed methods are hoped to serve as powerful additions with benefits such as an increase in muscle size and strength by rehabilitation with vascular occlusion and neuro-function improvement by applying whole body vibration. Lastly, an issue of equal importance remains, which is to develop a method of preventing anterior cruciate ligament injuries.
