We studied the effect of quadriceps or hamstring contractions on the anterior tibial shear load to anterior cruciate ligament failure in an in vivo rat model. In both knees of 20 Wistar rats the joint capsule and ligaments, except the anterior cruciate ligaments, were cut and the menisci resected. In 10 rats the right ligament was loaded to failure by anterior translation of the tibia relative to the femur in a testing device during hamstring contraction induced by electrical stimulation of the ischiatic nerve. In the other 10 rats, the right ligament was loaded to failure correspondingly during quadriceps contraction induced by electrical stimulation of the femoral nerve. The loading rate was 2.5 mm s-1 (approximately 0.6(-1)). The knee flexion during testing was 30 degrees. As control, the anterior cruciate ligament of the left knee was loaded correspondingly with relaxed muscles. The ultimate load for the ACL tested during hamstring contraction was 1.5 times higher than when tested with the hamstrings relaxed, more than double the energy was absorbed at failure and the linear stiffness was 1.2 times higher. During testing with quadriceps contraction, no differences in the structural properties were found, compared to testing with the quadriceps relaxed. Our findings show that hamstring contraction helps to resist anterior tibial shear force at 30 degrees knee flexion in rats thus protecting the anterior cruciate ligaments. Quadriceps contraction in this situation does not affect anterior shear force to ligament failure.