The importance of quadriceps and hamstring muscle loading on knee kinematics land in-situ forces in the ACL

This study investigated the effect of hamstring co-contraction with quadric eps on the kinematics of the human knee joint and the in-situ forces in the anterior cruciate ligament (ACL) during a simulated isometric extension mo tion of the knee. Cadaveric human knee specimens (n = 10) were tested using the robotic/universal force-moment sensor (UFS) system and measurements of knee kinematics and in-situ forces in the ACL were based on reference posi tions on the path of passive flexion/extension motion of the knee. With an isolated 200 N quadriceps load, the knee underwent anterior and lateral tib ial translation as well as internal tibial rotation with respect to the fem ur. Both translation and rotation increased when the knee was flexed from f ull extension to 30 degrees of flexion; with further flexion, these motion decreased. The addition of 80 N antagonistic hamstrings load significantly reduced both anterior and lateral tibial translation as well as internal ti bial rotation at knee flexion angles tested except at full extension. At 30 degrees of flexion, the anterior tibial translation, lateral tibial transl ation, and internal tibial rotation were significantly reduced by 18, 46, a nd 30%, respectively (p < 0.05). The in-situ forces in the ACL under the qu adriceps load were found to increase from 27.8 +/- 9.3 N at full extension to a maximum of 44.9 +/- 13.8 N at 15 degrees of flexion and then decrease to 10 N beyond 60 degrees of flexion. The in-situ force at 15 degrees was s ignificantly higher than that at other flexion angles (p i 0.05). The addit ion of the hamstring load of 80 N significantly reduced the in-situ forces in the ACL at 15, 30 and 60 degrees of flexion by 30, 43, and 44%, respecti vely (p < 0.05). These data demonstrate that maximum knee motion may not ne cessarily correspond to the highest in-situ forces in the ACL. The data als o suggest that hamstring co-contraction with quadriceps is effective in red ucing excessive forces in the ACL particularly between 15 and 60 degrees of knee flexion. (C) 1999 Elsevier Science Ltd. All rights reserved.