A device prosthetic foot device (10, 110, 210, 410) and method with variable stiffness response includes a variable energy transfer medium (14, 114, 414) disposed between first and second foot members (18, 22, 118, 122, 218, 222, 322, 324, 418, 422) to transfer at least some energy from the second member to the first member during use. The energy transfer medium includes a variable viscosity fluid (50) such that the variable viscosity fluid, and thus the energy transfer medium, variably transfers energy between the first and second members to vary stiffness of the prosthetic foot device. The variable viscosity fluid can include a shear stiffening material (60) that increases in viscosity as a load or strain, or rate of loading or rate of strain, applied, or a magneto or electro rheologic fluid (90, 70) responsive to a magnetic or electric field. In addition, a socket device (510) to attach a prosthetic to a stump of an amputee can include an internal liner (558) with a variable viscosity fluid (566).
