  Kinesins move processively toward the plus end of microtubules by hydrolyzing
ATP for each step. From an enzymatic perspective, the mechanism of mechanical
motion coupled to the nucleotide chemistry is often well explained using a
single-loop cyclic reaction. However, several difficulties arise in
interpreting kinesin's backstepping within this framework, especially when
external forces oppose the motion of kinesin. We review evidence, such as an
ATP-independent stall force and a slower cycle time for backsteps, that has
emerged to challenge the idea that kinesin backstepping is due to ATP
synthesis, i.e., the reverse cycle of kinesin's forward-stepping
chemomechanics. Supplementing the conventional single-loop chemomechanics with
routes for ATP-hydrolyzing backward steps and nucleotide-free steps, especially
under load, gives a better physical interpretation of the experimental data on
backsteps.
