Myosin filaments reversibly generate large forces in cells

We present high resolution experiments performed on elementary contractile units in cells that challenge our current understanding of molecular motor force generation. The key features are the development of a force per motor considerably larger than forces measured in single molecule experiments, a force increase followed by relaxation controlled by a characteristic displacement rather than by a characteristic force, the observation of steps at half the actin filament period even though a large number of motors are at work in an elementary contractile unit. We propose a generic two-state model of molecular motor collections with hand-over-hand contractions and we find that these unexpected observations are spontaneously emerging features of a collective motor behavior.