Anterior-posterior knee laxity in the sagittal plane indicates the functional state of cruciate ligaments. Particularly, the anterior cruciate ligament or ACL is damaged frequently during strenuous activities like in sports. Following injury and treatment, not only a significant percentage of patients are not able to return to their pre-injury level activity, but also they continue to have knee related complications in medium to long term. Therefore, there is a need for better understanding of the knee. In the present study, anterior laxity of the knee resulting from anterior tibial translation is related to strain developed in the fibres of the ACL while the joint is flexed. The knee was modelled in the sagittal plane using anatomical data and material properties to represent the joint structures mathematically and simulate the joint motion during 0-120o flexion. The ACL was represented as bundles of non-linear elastic fibers. A laxity test with 130N anteriorly directed external force on the tibia was simulated at several flexion angles. For selected values of anterior tibial translation, strain in different fibres of the ACL was calculated. The results from model calculations showed agreement with experimental observations from literature. 130N anterior force translated the tibia anterior to the femur non-linearly throughout the flexion range. Fibres of the ACL slackened or stretched differently depending on their relative insertions that altered due to changes in flexion angle and tibial translation. The anterior fibre showed increased levels of strain with flexion for each value of tibial translation. The intermediate and posterior fibre stretched at low and high flexion positions depending on the applied tibial translation and flexion angle. Anterior bundle of the ACL stretched increasingly with flexion and with anterior tibial translation. The posterior bundle stretched mainly near extension or in high flexion. The analysis has relevance to ACL-reconstruction and ACL rehabilitation.