Kinesin walks processively on microtubules in an asymmetric hand-over-hand manner with each step spanning 16 nm. We used molecular simulations to determine the fraction of a single step due to conformational changes in the neck linker, and that due to diffusion of the tethered head. Stepping is determined largely by two energy scales, one favoring neck-linker docking and the other, ε_h~(MT-TH), between the trailing head (TH) and the microtubule. Necklinker docking and an optimal value of ε_h~ (MT-TH) are needed to minimize the probability that the TH takes side steps. There are three major stages in the kinematics of a step. In the first, the neck linker docks, resulting in ～(5–6) nm movements of the trailing head. The TH moves an additional (6–8) nm in stage II by anisotropic translational diffusion. In the third stage, spanning ～(3–4) nm, the step is complete with the TH binding to the ab-tubulin binding site.
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