Characterising skeletal muscle under large strain using eccentric and Fourier Transform-rheology

Characterising the passive anisotropic properties of soft tissues has been largely limited to the linear viscoelastic regime and shear loading is rarely done in the large deformation regime, despite the physiological significance of such properties. This paper demonstrates the use of eccentric rheology, which allows the anisotropy of skeletal muscle to be investigated. The large amplitude oscillatory strain properties of skeletal muscle were also investigated using Fourier Transform-rheology. Histology was used to qualitatively assess the microstructure changes induced by large strain. Results showed that skeletal muscle was strongly anisotropic in the linear regime. The storage and loss moduli were found to be significantly different (p<0.05) between the three fibre alignment groups; for the group tested with fibres perpendicular to plane of shear was 12.3±1.3 kPa and 3.0±0.35 kPa, parallel to shear direction was 10.6±1.2 kPa and 2.4±0.23 kPa, and perpendicular to shear direction was 5.5±0.90 kPa and 1.3±0.21 kPa. The appearance and growth of higher order harmonics at large strain was different in the three testing directions indicating that the anisotropy of muscle affects skeletal muscle behaviour in the nonlinear regime. Histological analysis showed an increasing destruction of extracellular matrix and the rearrangement of fibres with increasing strain indicating mechanical damage at strains of larger than 10%. These microstructural changes could contribute to the complex nonlinear behaviour in skeletal muscle. This paper demonstrates a method of characterising the anisotropic properties in skeletal muscle under large strain whilst giving meaningful information on the physical response of tissue at various strains.