superior mechanical expansion qualities a suitable commercially available composite material of this kind is cylon u00ae made by cytec fiberite inc of havre de grace md the resilient material 39 s physical properties as they relate to stiffness flexibility and strength are all determined by the thickness of the material a thinner material will deflect easier than a thicker material of the same density the material utilized as well as it 39 s physical properties are both associated with the stiffness to flexibility characteristics in the prosthetic foot keel and calf shank the thickness of the foot keel and calf shank are uniform or symmetrical in the example embodiment of fig3 5 but the thickness along the length of these components can be varied as discussed below such as by making the hindfoot and forefoot areas thinner and more responsive to deflection than the midfoot region the foot keel and shank in each of the several embodiments of the invention disclosed herein have a relatively low moment of inertia in the sagittal plane as compared with that in the frontal plane this is a result of the mechanical form of these members which are wider in the frontal plane than thick in the sagittal plane to aid in providing the prosthetic foot 1 with a high low dynamic response capability the midfoot portion 5 is.