Regional variation in the mechanical properties of the skeletal muscle

This study quantified how passive skeletal muscle mechanics vary by excision location and test temperature, focusing on transverse-plane stiffness that is largely dictated by extracellular matrix (ECM) collagen rather than myofiber load-bearing. Biceps brachii muscles from 9-week-old Wistar rats were sectioned into distal, middle, and proximal regions along the proximo–distal axis, with samples cut so myofibers were orthogonal to the loading plane. Equibiaxial tensile tests were performed in saline at either room temperature (22°C) or physiological temperature (37°C), and the resulting stress–stretch curves were used to fit a first-order Ogden hyperelastic model (μ, α). The authors found strong regional heterogeneity: distal samples exhibited substantially higher stiffness than medial and proximal samples, consistent with greater collagenous structures (e.g., epimysium/aponeurosis continuity) in the distal region; on average, stiffness-related metrics could approximately double depending on sampling location (p < 0.001). Temperature also increased stiffness (p < 0.05), which the authors attribute to accelerated onset of rigor mortis at 37°C. Although samples were cut transversely, anisotropy increased at 37°C for some regions, while distal samples remained near-isotropic. Overall, the work highlights that excision zone and temperature are critical experimental variables for reproducible skeletal muscle ECM mechanics and for building accurate computational models.