The contribution by passive muscle tension to the structural load-bearing capacity of the anterior (cruciate) ligament in the anesthetized rat was investigated. Using a stereomicroscope, the joint capsule and ligaments of the right knee, except the anterior ligament, were cut and the menisci removed leaving the anterior ligament and the tendons of the denervated muscle to constrain the knee. The ligament was tested in tension until failure, using a loading rate of 2.5 mm s-1 (approximately 0.6 s-1). As a control, the femur-anterior ligament-tibia complex of the left knee was tested. The mean ultimate tensile load on the anterior ligament augmented by muscle tendons was 48 percent higher. The energy absorption at failure was 84 percent higher; and the stiffness 26 percent higher. The deformation remained unchanged. This investigation suggests that, when the strain rate is high, muscle may be passively stretched and thus absorb energy and increase the force needed to rupture the anterior ligament.