The kinematics of the human knee joint and the strain of the ligaments as a function of flexion are determined analytically and experimentally. The experimental results were obtained in 13 tests of four knee joints in which the strain in each of the two collateral and two cruciate ligaments was measured with mercury strain gauges while the tibia was rotated through a flexion angle of 130 deg. The values of the relative ligament strain obtained from the analytical model are in good agreement with the experimental results.