Knee specimens were placed in an apparatus which imposed cyclic anterior-posterior or rotatory forces, with various compressive loads applied to the joint. Force-displacement graphs or torque-rotation graphs were automatically plotted, giving the laxity under various conditions. The ligaments, capsule, and menisci provided joint stability under no-load conditions. However, under compressive loads, the conformity of the condylar surfaces was an important factor in stabilizing the knee. The mechanism proposed was the uphill movement of the femur as the femur and the tibia were displaced or twisted relative to one another.