Characterization of time-dependent and history-dependent mechanical behaviour of human masseter muscle.

The human masseter muscle is one of the primary muscles responsible for mastication and mandibular movement; however, its intrinsic mechanical properties remain insufficiently characterized. In this experimental study, the nonlinear, viscoelastic, and history-dependent behaviour of the human masseter muscle was investigated using ex vivo uniaxial cyclic tensile tests. Masseter muscle samples prepared from fresh and formalin-preserved cadavers were tested under two loading protocols: a stepwise stress-relaxation protocol with progressively increasing stretch levels and a constant stretch protocol with repeated loading to a fixed maximum stretch, at two strain rates. The influence of strain rate and formalin preservation on the mechanical behaviour of the tissue was investigated. The results showed that formalin-preserved samples showed higher stiffness than fresh samples. Under cyclic loading, energy dissipation, stress-softening, residual deformation, and cyclic conditioning progressively changed during the initial loading cycles and reached stabilization during the final cycle. These findings provide experimental evidence that the human masseter muscle exhibits nonlinear, viscoelastic, and history-dependent mechanical behaviour under cyclic tensile loading. The proposed model provides baseline experimental data that may support future biomechanical modelling and related clinical investigations.