Smith machine squats pose high risk to ACL graft integrity after the ACL reconstruction and conventional squats are a safer alternative.

Purpose: The purpose of this study was to evaluate the impact of squats after the anterior cruciate ligament (ACL) reconstruction on the ACL graft, considering new data on biomechanics, posterior tibial slope (PTS) and anterolateral ligament (ALL).

Methods: Utilising finite element analysis on the new 14-component knee joint model, we have evaluated stresses on the knee elements separately for the knee with a native double-bundle ACL and with a single-bundle ACL graft for the 5° and 14° PTS variants during both conventional and Smith machine horizontal squats.

Results: Replacing a native ACL with a single-bundle graft causes an overstrain on the graft compared to the intact ACL under all conditions.