The effects of strain amplitude and localization on viscoelastic mechanical behaviour of human abdominal fascia.

PURPOSE: The purpose of the paper is to examine and compare the viscoelastic mechanical properties of human transversalis and umbilical fasciae according to chosen strain levels. METHODS: A sequence of relaxation tests of finite deformation ranging from 4 to 6% strain with increment 0.3% was performed at strain rate 1.26 mm/s. Initial and equilibrium stresses T0, Teq, initial modulus E and equilibrium modulus Eeq, reduction of the stress during relaxation process ΔT, as well as the ratio (1 - Eeq /E) were calculated. RESULTS: The range in which parameters change their values are (0.184-1.74 MPa) for initial stress, (0.098-0.95 MPa) for equilibrium stress, (43.5-4.6 MPa) for initial modulus E. For Eeq this interval is (23.75-2.45 MPa). There are no statistically significant differences between the values of these parameters according to localization. The differences in viscoelastic properties of both fasciae are demonstrated by reduction of the stress during relaxation process and ratio (1 - Eeq /E). The values of ΔT and (1 - Eeq /E) ratio for umbilical fascia are significantly greater than that of fascia transversalis. An increase of 2% in strain leads to change of the normalized relaxation ratio of fasciae between 28%-66%. There is a weak contribution of viscous elements in fascia transversalis samples during relaxation, while in umbilical fascia the contribution of viscous component increases with strain level to 0.66 at 5.3% strain. CONCLUSIONS: This study adds new data for the material properties of human abdominal fascia. The results demonstrate that in chosen range of strain there is an influence of localization on visco-elastic tissue properties.