Hamstrings cocontraction reduces internal rotation, anterior translation, and anterior cruciate ligament load in weight-bearing flexion

Strengthening of the hamstrings is often recommended following injury and r econstruction of the anterior cruciate ligament. It has been suggested that hamstrings activity stabilizes the knee and reduces anterior cruciate liga ment load during weight-bearing flexion; however, the effects of hamstrings cocontraction on the kinematics and mechanics of the normal knee have not been assessed at physiological load levels. The aim of this study was to de termine whether the addition of hamstrings force affects knee rotations, tr anslations, and joint and quadriceps force during flexion with loads at phy siological levels applied to the muscles and joints, Eight cadaveric knee s pecimens were tested with a servohydraulic mechanism capable of applying co ntrolled dynamic loads to simulate quadriceps and hamstrings muscle forces throughout a physiological range of motion. A constant vertical load of phy siologic magnitude was applied to the hip, and quadriceps force was varied to maintain equilibrium throughout flexion, Two conditions were tested: no hamstrings force and a constant hamstrings force equivalent to the vertical load. Hamstrings force significantly reduced internal rotation (p < 0.0001 ) and anterior translation (p , 0.0001), increased quadriceps force (p < 0. 0001) and normal resultant force on the tibia (p < 0.0001), and reversed th e direction of the shear force on the tibia (p < 0.0001). These results sug gest that hamstrings strengthening following anterior cruciate ligament inj ury may benefit anterior cruciate ligament-deficient and reconstructed knee s by reducing the load in the ligament; however, they also imply that this comes at the expense of efficiency and higher patellofemoral and joint forc es.