  The stochastic driving force exerted by a single molecular motor (e.g., a
kinesin, or myosin) moving on a periodic molecular track (microtubule, actin
filament, etc.) is discussed from a general viewpoint open to experimental
test. An elementary "barometric" relation for the driving force is introduced
that (i) applies to a range of kinetic and stochastic models, (ii) is
consistent with more elaborate expressions entailing explicit representations
of externally applied loads and, (iii) sufficiently close to thermal
equilibrium, satisfies an Einstein-type relation in terms of the velocity and
diffusion coefficient of the (load-free) motor. Even in the simplest two-state
models, the velocity-vs.-load plots exhibit a variety of contrasting shapes
(including nonmonotonic behavior). Previously suggested bounds on the driving
force are shown to be inapplicable in general by analyzing discrete jump models
with waiting time distributions.
