Do we really behave like a gigantic spring during running sports?

We are increasingly hearing terms like lower limb stiffness and "spring mass model" in the discussion of running, and especially injury. But how realistic, and accurate, is it to assume the human body, and in particular the lower leg, simply acts like a great big spring?

Given that by far the majority of lower leg mass consists of material that is essentailly non elastic, I thought it might be timely to explain the concept of lower limb stiffness, and the concept  of the spring mass theory.