  Active solids such as cell collectives, colloidal clusters, and active
metamaterials exhibit diverse collective phenomena, ranging from rigid body
motion to shape-changing mechanisms. The nonlinear dynamics of such active
materials remains however poorly understood when they host zero-energy
deformation modes and when noise is present. Here, we show that stress
propagation in a model of active solids induces the spontaneous actuation of
multiple soft floppy modes, even without exciting vibrational modes. By
introducing an adiabatic approximation, we map the dynamics onto an effective
Landau free energy, predicting mode selection and the onset of collective
dynamics. These results open new ways to study and design living and robotic
materials with multiple modes of locomotion and shape-change.
