Mercury strain gauges were sutured onto the tibial collateral anterior and posterior cruciate ligaments to quantitatively determine the relative strain or deformation of each of these ligaments as a function of joint position. The results were obtained on 5 amputation specimens by subjecting them to flexion, extension, rotation, valgus--varus and anteroposterior forces. The tibial collateral ligament is most lax in full flexion and stretches with extension, valgus and external rotation. The cruciate ligaments are most lax at 35 degrees flexion and stretch with both flexion and extension. Internal rotation and varus stretch and anterior cruciate ligament. These principles allow us a better understanding of injury patterns. The most advantageous position for immobilization following acute injuries or reconstructions is better understood knowing that minimal tension on ligamentous fibers occurs as follows: Anterior cruciate, 35 degrees; Posterior cruciate, 35 degrees; Tibial collateral ligament, 45--90 degrees (or as much flexion as the patient will tolerate).