Before revision:  This functional switch arises because dissociation after the powerstroke is suppressed by force (catch bonding) and because the ensemble shifts to the pre-powerstroke state in a soft environment. We also find that in rigid environments, symmetric myosin II minifilaments perform a random walk with an effective diffusion constant which decreases with increasing ensemble size, in marked contrast to the behavior of ensembles of processive motors that function in cargo transport .
After revision:  This functional switch arises because dissociation after the powerstroke is suppressed by force (catch bonding) and because ensembles can no longer perform the power stroke at large forces. Symmetric myosin II minifilaments perform a random walk with an effective diffusion constant which decreases with increasing ensemble size, in marked contrast to the behavior of ensembles of processive motors that function in cargo transport .