How does tissue preparation affect skeletal muscle transverse isotropy?

Abstract

The passive tensile properties of skeletal muscle play a key role in its physiological function. Previous research has identified conflicting reports of muscle transverse isotropy, with some data suggesting the longitudinal direction is stiffest, while others show the transverse direction is stiffest. Accurate constitutive models of skeletal muscle must be employed to provide correct recommendations for and observations of clinical methods. The goal of this work was to identify transversely isotropic tensile muscle properties as a function of post mortem handling. Six pairs of tibialis anterior muscles were harvested from Giant Flemish rabbits and split into two groups: fresh testing (within four hours post mortem), and non-fresh testing (subject to delayed testing and a freeze/thaw cycle). Longitudinal and transverse samples were removed from each muscle and tested to identify tensile modulus and relaxation behavior. Longitudinal non-fresh samples exhibited a higher initial modulus value and faster relaxation than longitudinal fresh, transverse fresh, and transverse rigor samples (p<0.05), while longitudinal fresh samples were less stiff at lower strain levels than longitudinal non-fresh, transverse fresh, and transverse non-fresh samples (p<0.05), but exhibited more nonlinear behavior. While fresh skeletal muscle exhibits a higher transverse modulus than longitudinal modulus, discrepancies in previously published data may be the result of a number of differences in experimental protocol. Constitutive modeling of fresh muscle should reflect these data by identifying the material as truly transversely isotropic and not as an isotropic matrix reinforced with fibers.

An experimental and computational investigation of the effects of volumetric boundary conditions on the compressive mechanics of passive skeletal muscle

“…The model also accounted for the experimental overshoot in strain applied by the Instron during fast compression. Parameter determination was performed in two steps: a Monte Carlo simulation followed by a nonlinear least-squares deterministic optimization (lsqnonlin in MATLAB) [27], [28], [30], [42]- [45]. In the Monte Carlo simulation, the six parameters ( 1−3 and 1−3 ) were randomly varied for 100,000 simulations, ensuring 0 < 1 + 2 + 3 < 1 [30].…”

“…Stress relaxation tests have been thus used extensively to characterize the stress-strain and stress-time behavior of the tissue, and are typically accompanied by various viscoelastic modelling approaches [3], [4], [17], [27]- [29]. Inverse finite element methods are effective in determining material properties through parameter optimization to experimental data [27], [30]- [32]. In previous continuum mechanics based modeling of skeletal muscle, the assumption of near incompressibility leads to a decoupling of the volumetric (volume changing) and isochoric (shape changing) responses of the hyperelastic model [23], [33].…”

A new model of passive muscle tissue integrating Collagen Fibers: Consequences for muscle behavior analysis

“…Most of the passive muscle models in the literature do not consider any distinction between CFs and MFs, or at least do not consider the mechanical effects of CFs on the 3-D behavior of tissue. These models cannot provide a realistic account of the observed stress-stretch responses in muscle tissue, especially in tissue that amazingly shows stronger resistance against elongation in a direction different from the MF alignment , Hernández et al 2011, Mohammadkhah et al 2016and Wheatley et al 2016. Recently, Grasa et al (2016) have introduced the CFs in their passive muscle model that can describe such an amazing stress-stretch behavior, but this model does not include the 3-D distribution of the CFs.…”

“…(3-D) passive behavior, especially for those that show less resistance in loading along the MF direction (Gindre et al 2013, Nie et al 2011and Wheatley et al 2016. Takaza and colleagues (2013) measured passive stress-stretch responses of the pig longissimus dorsi muscle under uniaxial tensile tests.…”

“…Depending on the sarcomere length, the mean CF angle with respect to the MF axis varies in the range of 20° to 80°. This explains why mechanical passive properties of some muscles are stronger in the direction transverse to the MF direction , Mohammadkhah et al 2016and Wheatley et al 2016. For these muscles, the available CLs, in which the CFs and the MFs are identified with the same direction, cannot describe the observed stress-stretch behavior.…”