In this study, a detailed uncertainty analysis was performed to place probability limits on model predictions for both uninjured and ACL-reconstructed joints. We investigated the effect of uncertainty in connective tissue intrinsic properties on the predicted kinematics and kinetics of the human joint under combined quadriceps and external loading observed during the load acceptance phase of the gait cycle. Our findings indicated that, on average, the ACL-reconstruction procedure using a patellar tendon graft increased the tibiofemoral joint adduction and internal rotation angles over the load acceptance phase. Our findings highlight one of the main features of the current construct; that is, model predictions are described in terms of their probabilistic properties. In the context of surgical evaluations, such properties define the level of likelihood that a given intervention can recover normal joint function.