The mechanism of normal level walking is presented in this paper in terms of the displacements of and the force systems at the leg joints. The data on four normal subjects were obtained from simultaneous recording of the positions of the leg in space and the floor reactions during level walking. The mass moments of inertia of the lower extremity were determined experimentally and the effects of gravity and inertia were included in the analysis. The forces and moments are presented in terms of the space components referred to a system of horizontal and vertical orthogonal axes.