  The fluctuation theorem is a representative theorem in non-equilibrium
statistical physics actively studied in the 1990's. Relating to entropy
production in non-equilibrium states, the theorem has been used to estimate the
driving power of motor proteins from fluctuation in their motion. In this
review, usage of the fluctuation theorem in experiments on motor proteins is
illustrated for biologists, especially those who study mechanobiology, in which
force measurement is a central issue. We first introduce the application of the
fluctuation theorem in measuring the rotary torque of the rotary motor protein
F1-ATPase. Next, as an extension of this application, a recent trial estimating
the force generated during cargo transport in vivo by the microtubule motors
kinesin and dynein is introduced. Elucidation of the physical mechanism of such
transport is important, especially for neurons, in which deficits in cargo
transport are deeply related to neuronal diseases. Finally, perspectives on the
fluctuation theorem as a new technique in the field of neuroscience are
discussed.
