Multiscale Passive Mechanical Characterization of Rodent Skeletal Muscle

Purpose: To experimentally measure selected passive properties of skeletal muscle at three different scales (macroscopic scale: whole muscle, microscopic scale: single skinned fiber, and submicron scale: single myofibril) within the same animal model (mice), and to compare a primarily slow-twitch fiber muscle (soleus) and a primarily fast-twitch fiber muscle (extensor digitorum longus, EDL) for each scale. Methods: Healthy 3 months old wild-type C57BL6 mice were used. To characterize each scale, soleus (N = 11), EDL (N = 9), slow fibers (N = 17), fast fibers (N = 16), and myofibrils from soleus (N = 11) and EDL (N = 11) were harvested. Passive mechanical (ramp, relaxation) tests were applied at each scale to compare the passive properties (Young's modulus, static and dynamic stresses) within a given scale, across scales and between muscle types. Results: The soleus and EDL showed significant passive mechanical differences at the macroscopic scale while no variation was observed between both tissues at the microscopic and submicron scales. The results highlight the importance of the scale that is used to mechanically characterize a multiscale tissue. Conclusion: The present work will allow for a better understanding of the multiscale passive mechanical properties for two muscles with vastly differing physiological and metabolic properties. This study provides referent data to the body of literature that can be built upon in future work.