  Peristalsis by actively generated waves of muscle contraction is one of the
most fundamental ways of producing motion in living systems. We show that
peristalsis can be modeled by a train of rectangular-shaped solitary waves of
localized activity propagating through otherwise passive matter. Our analysis
is based on the FPU-type discrete model accounting for active stresses and we
reveal the existence in this problem of a critical regime which we argue to be
physiologically advantageous.
