Analysis of experimental data on two muscles demonstrates that, in contracting striated muscle, the total rate of ATP splitting, nu(t) (number of ATP molecules split per active myosin head per second), comprises of three separate components: nu(p), which is required for the generation of the contractile force P which is equal to the external load; nu(v) which is devoted to the development of the velocity of shortening V; and nu(w), which is responsible for the production of the mechanical power (PV). Nu(p) is proportional to P and nu(v) to V, which means that the sliding distance is independent of P. The mechanical power was found to be equal to the free energy change associated with the hydrolysis of nu(w), which means that the thermodynamic efficiency of the power-producing component is practically 100%. It is concluded that ATP hydrolysis is actually three different reactions. The analysis leads to Hill's force-velocity relationship. Its empirical constants a and b are expressed by thermodynamic and molecular parameters. The constant a was found to be inversely proportional to the sliding distance. The same considerations and conclusions should apply also to other muscles and to the movement under load of microtubules interacting with, e.g. kinesin.