A theoretical model of human walking has been developed in which the muscles of the swing leg are assumed to be silent after the double-support period, which serves to establish initial conditions on the angles and velocities of both lower extremities. The swing leg moves entirely under the action of gravity until heel strike. Realistic features of gait, including knee flexion of the stance leg, pelvic tilt, pelvic rotation, and plantar flexion of the stance leg have been added to the model one by one. Experiments test the predictions of the model -- how gait parameters change when specific determinants of gait are absent, as in mild and severe gait pathologies.