  We study a model of cell motility where the condition of optimal trade-off
between performance and metabolic cost can be made precise. In this model a
steadily crawling fragment is represented by a layer of active gel placed on a
frictional surface and driven by contraction only. We find analytically the
distribution of contractile elements (pullers) ensuring that the efficiency of
self-propulsion is maximal. We then show that natural assumptions about
advection and diffusion of pullers produce a distribution that is remarkably
close to the optimal one and is qualitatively similar to the one observed in
experiments on fish keratocytes.
