A new model of the mechanochemical actin-activated myosin ATPase cycle is proposed. In active muscle, the collective behavior of a large number of myosins in muscle can be described with a set of chemical kinetic equations. The non-equilibrium steady state solution of equations shows that the fraction of myosin heads in any given biochemical state is independent of both the concentrations of ADP and Pi. Combining muscle mechanics data of Pate and Cooke, the muscle state equation is deduced. The theoretical results are consistent with Baker's experimental data but some what different from conventional muscle theory. Based on the knowledge of special structure of muscle, the muscle state equation is discussed thoroughly.