The mechanical properties of the human lumbar annulus fibrosus is investigated both theoretically and experimentally. The theory presented is based on the assumption of material incompressibility which is then justified by the experiments. Spencer's theoretical approach is followed, but the fibers of the material are considered to be extensible. A strain-energy function and the associated constitutive equations are also presented.

An optical method is employed to measure the extension ratios and the change in relative orientation of the fibers.