The major controversy about the anterior cruciate ligament (ACL) is the appropriate time for use of terminal knee extension and exercises that place a subluxing force. Recent studies have delineated the area in which the quadriceps mechanism provides a subluxing force on the tibia and across the knee joint. An active rehabilitation program encourages progressive recovery of function while protecting the knee against subluxing forces. The program includes rehabilitation for nonsurgically treated but torn cruciate injuries, and injuries of the ACL repaired by various surgical operations.