Minimal model of the ATPase cycle. (A) Actomyosin cycle and corresponding structural states of myosin head consist of at least six biochemical states. The minimal three-state model consists of a detached state 1, weakly bound state 2 (or A.M.D.Pi), and strongly bound post-power stroke sate 3 (or A.M.D). The state 1 groups transition between A.M, A.M.ATP, M.ATP, and M.ADP.Pi states (gray box) to a single transition where forward rate is dominated by the rate of ADP release and backward rate by reverse attachment from M.ATP to A.M.ATP. These rates are slowest forward and backward rates. The compounded state transition includes ADP release, ATP binding, dissociation of myosin from actin, and ATP hydrolysis step. Note that the lever arm must move for displacement δ in order to open the nucleotide pocket and allow ADP release. (B) Free energy landscapes for myosin ATPase states of the three-state model. The detached state M.ATP is only shown for reference. The free energy of detached states is independent of strain, x, whereas the free energy of attached states is quadratically proportional to x if cross-bridge stiffness is constant.