Sagittal plane translations of the knee in anterior cruciate deficient subjects and controls. Med. Sci. Sports Exerc., Vol. 29, No. 12, pp. 1561-1566, 1997. Static anterior-posterior (AP) laxity is one of the commonly used criteria in selecting patients for cruciate ligament reconstructions, but in reality dynamic AP laxity plays a more important role. The aim of thisin vivo study was to compare the sagittal translation of the knee during active and passive motion, signifying dynamic AP laxity, with static AP laxity in healthy subjects (controls) and patients with anterior cruciate ligament deficiency. The sagittal plane knee translations were recorded and compared in both knees of nine healthy subjects (Controls) and seven patients with confirmed unilateral ACL deficiency during dynamic and static situations with an electrogoniometer system. In all groups during the ascents the tibia moved anteriorly in relation to the femur, whereas during the decents it moved posteriorly. The static anterior-posterior translation was significantly smaller in the control knee than in both healthy and injured knees of the ACL deficient group (P < 0.05). The injured knee showed the same laxity (92%) as the uninjured knee during dynamic activities, but it was 46% of static laxity. Also in the injured knees, the dynamic active laxity was larger during descents than ascents (P < 0.05). The results indicate that there is also a change in mechanics of the noninjured knee following injury to the contralateral knee and that this population of patients with ACL deficiency had good control over their abnormal anterior-posterior laxity.