[1] Bone is a composite of the bio-polymer collagen and the bio-ceramic hydroxyapatite.
Bone has two structural forms; cortical and cancellous.
[2] The viscoelasticity of bone can therefore arise from the void collapse and deossification of cancellous bone and the natural viscoelastic response of collagen as a polymer.
[1][2] Viscoelasticity[3] is the phenomena of time-dependent strain exhibited by amorphous materials such as polymers or glasses.
In turn, constitutive equations can relate the mechanical interpretation of viscoelasticity to the materials properties and strain rate.