On the scale of their individual monomers, active polymers are those that are out of equilibrium. If the chain is subject to nonequilibrium fluctuations, for example by the presence of some elements that exert forces locally on the monomers, this can be accomplished by being composed of particles that perform mechanical work on their environment at the expense of some source of free energy. An excellent illustration is a DNA fibre in living cells that is being pulled on by multiple molecular motors.