A mixed-kinetic model describes unloaded velocities of smooth, skeletal, and cardiac muscle myosin filaments in vitro.

In vitro motility assays, where purified myosin and actin move relative to one another, are used to better understand the mechanochemistry of the actomyosin adenosine triphosphatase (ATPase) cycle. We examined the relationship between the relative velocity () of actin and myosin and the number of available myosin heads () or [ATP] for smooth (SMM), skeletal (SKM), and cardiac (CMM) muscle myosin filaments moving over actin as well as from actin filaments moving over a bed of monomeric SKM. These data do not fit well to a widely accepted model that predicts that is limited by myosin detachment from actin (/), where equals step size and equals time a myosin head remains attached to actin.