Assessing the microstructural response to applied deformation in porcine passive skeletal muscle

The passive micro-structural mechanical response of muscle tissue is important for numerous medical applications. However, the recently observed tension/compression asymmetry in porcine muscle remains poorly explained. In particular there remains a lack of understanding of how external tension or compression applied in the fibre or cross-fibre direction translates internally to deformation of muscle fibres and the extra-cellular matrix. Accordingly, fresh porcine skeletal muscle tissue was harvested, deformed by 30% in uniaxial tension or compression in both the fibre and cross-fibre directions and prepared for optical microscope, polarised light microscope and SEM analysis. The average deformed specimen results were compared to the average control results in each case. For compressive or tensile stretch applied in the muscle fibre direction the average measured muscle fibre cross-sectional area changes are in close correspondence with predictions based on global Poisson's ratio measurements and these deformation modes did not cause shape changes in the muscle fibre cross-sections. However, muscle tissue reacted to the applied cross-fibre direction deformations as follows: compression flattened muscle fibre cross-sections, aligning them perpendicular to the direction of the applied deformation while tensile deformations stretched the cross-sections of muscle fibres, aligning them parallel to the direction of applied deformation. No evidence of structural reorganisation of endomysium collagen fibres in response to applied stretch was observed. The observed micro-structural responses do not appear to be influenced by the surrounding endomysium, but appear to be significantly influenced by proximity to the perimysium network. It is hypothesised that the perimysium and its interaction with the surrounding muscle fibres is therefore likely to be predominantly responsible for the tension/compression asymmetry observed in macroscopic tests of passive skeletal muscle stress strain behaviour.