The influence of physical dimension on apparent stress–strain behaviour of in vitro passive skeletal muscle samples

Abstract

The stress–strain behaviour of skeletal muscle is affected by many factors, leading to varied results reported in the literature. This article examines how the physical dimension of samples in in vitro compression tests affects the muscle stress for a given stretch ratio, in both quasi-static and dynamic loading. It is proposed that physically larger samples display a higher stress response due to the greater inclusion of complete muscle fascicles and also a reduction in percentage fluid exudation during compression. In the case of quasi-static loading, this was evaluated by testing nominally cubic samples of fresh and aged porcine tissue of characteristic lengths between 10 and 40 mm in compression at 0.05%/s strain in the fibre and cross-fibre directions using a Zwick Z005 universal testing rig. For the dynamic tests, a custom instrumented drop tower test rig was used to achieve average strain rates of 12,500%/s, and the stress responses at stretch ratios of $\lambda =$ 0.8 and $\lambda =$ 0.5 of nominally cubic samples of aged porcine tissue of characteristic lengths between 10 and 30 mm compressively loaded in the cross-fibre direction were evaluated. Both static and dynamic results clearly indicate that the muscle stress for a given stretch ratio of aged skeletal muscle tissue increases as the characteristic sample dimension increases from 10 to 30 mm. The effect was somewhat stronger for the dynamic tests. In the case of quasi-static testing, the strain rate of 0.05%/s limited the influence of the viscoelastic properties of the muscle, and sample dimension effects in the static tests are mostly attributable to the greater proportion of complete fascicles in the physically larger samples. In dynamic testing, in addition to the proportion of complete fascicle inclusion, the smaller percentage of fluid exudation for the larger samples compared to the smaller samples may also influence the size effect.