A prosthetic foot device with variable stiffness response includes a variable energy transfer mechanism disposed between first and second foot members to transfer a variable amount of energy between the members during use. A chamber is associated with one of the first and second foot members, and a piston is associated with another of the first and second foot members and is movable in the chamber. At least one aperture is formed between the piston and the chamber. A variable viscosity fluid is disposed in the chamber and displaceable through the at least one aperture between the piston and the chamber to allow fluid to flow within the chamber between opposite sides of the piston. The variable viscosity fluid has a viscosity that is variable to vary an ability of the variable viscosity fluid to flow through the at least one aperture.