The effect of quadriceps contraction on the structural capacity of the knee joint loaded to anterior cruciate ligament (ACL) failure was studied in an in vivo rat model. In both knees of 20 rats the joint capsule and ligaments, except the ACL, were cut and the menisci resected by microsurgery. The rats were randomized to destructive testing of their ACLs either by femorotibial distraction or by anterior tibial translation at a displacement rate of 2.5 mm s-1 (60% s-1). The knee flexion during testing was 60 degrees. During loading of the right ACL, quadriceps contraction was induced by electrical stimulation of the femoral nerve. As control, the ACL of the left knee was loaded with relaxed quadriceps. The ultimate load for the knee to ACL failure when tested in tension by femorotibial distraction during quadriceps contraction was 140% higher than tested with the muscles relaxed (p = 0.0001). Energy absorption at failure during muscle contraction was 274% higher (p = 0.0001), and the linear stiffness increased by 59% (p = 0.0004). During testing by anterior tibial translation, neither linear stiffness nor ultimate load changed significantly, but the energy absorbed at failure was 46% (p = 0.02) higher during quadriceps contraction compared to testing with the quadriceps relaxed. These results showed that quadriceps contraction substantially increased the load carrying capacity of the rat knee subject to ACL failure when loaded by femorotibial distraction, but less when it was loaded by anterior tibial translation.