A binary mechanical model. (A) Two-state scheme in which actin (gray helix) binding to a single myosin motor (ovals) in the MDP state (green) with bound ADP, D, and phosphate, P, induces a myosin lever arm rotation upon P release that displaces elements external to that myosin a distance, d, to form the AMD state (orange). The reversible binding reaction occurs with forward, f₊, and reverse, f₋, rates. Myosin motors irreversibly detach from actin through an active (ATP-dependent) process at a rate v. (B) Muscle force is represented by a single spring with stiffness κ_sys that on one end (left) is reversibly stretched a distance d/(aN) with each binding step while the other end is defined by macroscopic mechanics (here held at a fixed force or length).