  We address the controversial hot question concerning the validity of the
loose-coupling versus the lever-arm models in the actomyosin dynamics by
re-interpreting and extending the washboard potential model proposed by some of
us in a previous paper. In the new theory, a loose-coupling mechanism co-exists
with the deterministic lever-arm model. The synergetic action of a random
component, originating from the harnessed thermal energy, and of the
power-stroke generated by the lever-arm classical mechanism is seen to yield an
excellent fit of the set of data obtained in T. Yanagida's laboratory on the
sliding of Myosin II heads on actin filaments under various load conditions.
Our theoretical arguments are complemented by accurate numerical simulations,
and the robustness of theory is tested via different combination of parameters
and potential profiles.
