  We compare the semiclassical description of NEMS within and beyond the
adiabatic approximation. We consider a NEMS model which contains a single
phonon (oscillator) mode linearly coupled to an electronic few-level system in
contact with external particle reservoirs (leads). Using Feynman-Vernon
influence functional theory, we derive a Langevin equation for the oscillator
trajectory that is non-perturbative in the system-leads coupling. A stationary
electronic current through the system generates nontrivial dynamical behavior
of the oscillator, even in the adiabatic regime. The 'backaction' of the
oscillator onto the current is studied as well. For the two simplest cases of
one and two coupled electronic levels, we discuss the differences between the
adiabatic and the non-adiabatic regime of the oscillator dynamics.
