Amputees often complain that prosthetic feet are quite stiff. The feet have to be to stabilize the prosthetic limb during stance.

As long as the ground reaction force passes through the prosthetic foot's solid keel, it prevents the prosthetic shank section from moving anteriorly; this retarding of shank advancement stabilizes the knee in extension.

This factor helps below-knee amputees, and is vital for above knee amputees.