Our observations point to a direct connection between the action of a mechanic force and the state of the solvent, and suggest that in muscle passive and active shortening of the sarcomere promote the change of the water-water and of the water-protein interactions. Muscle contraction has to be described as a chemo-osmo-elastic transduction, where the analysis of the energy partition during the power stroke requires consideration of the osmotic factor in addition to the chemo-elastic ones. The rule is that the increase of protein osmotic pressure increases the stiffness of the structure and this latter increases the force that the structure may support or, in the course of the contraction, the force that the structure may deliver. The reason why we consider viscosity an indissoluble partner of muscle contraction stems from our definition of muscle as a highly non-ideal solution. This means that, in muscle, water activity is lower than expected from the water molar fraction, that bulk water decreases in favour of hydration water, that thickness of the water protein shell increases, that the boundaries of the hydrated proteins are getting closer, hence that the mutual friction increases. Thus muscle contraction is ruled by the interplay between water activity, viscosity and stiffness. These parameters change cyclically with the cross-bridge attaching-detaching process. A model is presented that describes muscles contraction on purely physical grounds. The rate of the contraction is modulated by a parameter, k, related to the visco-elastic hindrances of the contractile apparatus.