In order to understand why these polyurethane or nitrile foams are ideal for football helmets, it is helpful to review the properties of these materials. They can all be broadly categorized as viscoelastic foams. When a stress is applied to these types of materials, there are three main regions of deformation: linear elastic, plateau and densification. In the linear elastic region a material deforms reversibly in proportion to its young's modulus, or stiffness. In the plateau region the foam begins to collapse, and this is also a type of recoverable deformation. When the material reaches its densification region it begins to change its internal structure permanently and cannot return to its original shape. Because football helmets are used repeatedly, they needed to be designed, so the strain stays in the elastic or plateau region. The area under the curve in stress strain curve (shown in the figure to the right) represents the energy absorbed. Foams should be strained a precise amount, so they absorb energy without permanent densification.