Walking is the motion realized by humans and other footed animals when the legs alternate to establish steps. A step is a single movement of the leg in order to move the foot from its current landing space to another landing space.
The equilibrium needed to establish steps; the containership of the steps; the work done by the force exerted by the leg muscles to realize walking and the motion expressed by walking; are all expressions of Pj Problems. Consequently, walking is an expression of Pj problems.