Is mainly caused by:

Dragging your feet.
Moving your leg too late or not moving it at all (unconsciously)
Problems with starting to walk from sitting or standing pasture (unconsciously).
You are forced to keep thinking and watching your legs in order to know which leg go.

By PLS there are problems with the central motor neurons. PLS manifests itself in stiffness and increased muscle tone with increased reflexes usually starting in both legs.