Orthopaedic surgeons routinely use passive tests, in which the displacing force is applied externally, to evaluate the integrity of the ligaments of the knee. Using a quadriceps active test, in which the muscle contractures of the subject served as the displacing force, tibial displacement was measured with an arthrometer in ninety-two subjects: sixty-seven who had an acute or chronic rupture of the posterior or anterior cruciate ligament and twenty-five who had normal knees. With the knee joint in 90 degrees of flexion, contraction of the quadriceps resulted in anterior translation of the tibia in forty-one of forty-two knees that had a documented disruption of the posterior cruciate ligament. This anterior translation did not occur in the contralateral, normal knee of the same subjects; in the knees of the twenty-five normal subjects; or in twenty-five knees that had a known unilateral anterior cruciate-ligament disruption.