Anatomic considerations and electromyographic investigations show that the Mm. vasti medialis and lateralis both have a synergistic function with regard to the extension of the knee and an antagonistic function in stabilizing the rotation of the femur against the tibia. The rotational stabilisation for the femur by the Mm. vasti is very important in preventing knee injuries, especially ligamentous ruptures. On the other hand, extreme reflectoric muscle contractions, caused by a rotational trauma can cause chondromalacic lesions in the femoro-patellar joint, broadening the syndrome of the "unhappy triad" to an "unhappy tetrad".