Quadriceps exercises are used sparingly in the early rehabilitation of ACL reconstructions because of concern about prematurely stretching the ACL graft. The aim of this study was to determine if a maximum isometric quadriceps contraction significantly translates the tibia anteriorly at 15 degrees, 30 degrees, 45 degrees, 60 degrees, and 75 degrees of flexion. Secondly, the role of the ACL in knee stability was analyzed by comparing the amount of tibial translation in normal, ACL deficient, and reconstructed knees. Thirdly, the location in the motion arc where a quadriceps contraction produces anterior tibial translation was determined. Anterior tibial translation was measured using an arthrometer (KT-1000) during an 89 N and manual maximum translation applied to the knee at rest. The manual maximum translation test determines the magnitude of anterior tibial translation produced by a high anterior force applied directly to the proximal calf. These translations were compared to the tibial translation intrinsically induced by a quadriceps contraction. Testing was performed in normal (N = 22), ACL deficient (N = 10), and reconstructed (N = 10) knees. Anterior tibial translation produced by a maximum quadriceps contraction was measured at 15 degrees, 30 degrees, 45 degrees, 60 degrees, and 75 degrees of flexion. The extension exercise resulted in less anterior tibial displacement than an 89 N drawer and half the translation produced by a manual maximum translation (P less than 0.001). Instrumented laxity testing produced greater anterior translation of the tibia than a maximum isometric quadriceps contraction. Anterior tibial translation was the same during maximum isometric knee extension in all tested knees.(ABSTRACT TRUNCATED AT 250 WORDS)