  The mechanics of crawling cells on a substrate is investigated by using a
minimal model that satisfies the force-free condition. A cell is described by
two subcellular elements connected by a linear actuator that changes the length
of the cell cyclically in time, together with periodic alternation of adhesive
characters at the interface between the cell and the substrate. Here the key
model parameters are the phase shifts between the elongation of the actuator
and the alternation of the adhesion of the two elements. We emphasize that the
phase shifts determine not only the efficiency of the crawling motion but also
its direction.
