Chronic and Acute Mediators of Passive Viscoelasticity in Human Skeletal Muscle Fibers

Cellular viscoelastic modulus in skeletal muscle tissue responds dynamically to chronic stressors, such as age and exercise. Passive tissue mechanics may also be sensitive to acute stimuli such as mechanical loading and/or activation-induced muscle fatigue. These insights are largely derived from preclinical studies of age and acute muscle activation. Therefore, we sought to understand the relative responsiveness of muscle cellular passive mechanics to chronic (resistance training) and acute (muscle fatigue) stressors in healthy young males and females categorized as "resistance trained" or "untrained". We measured passive mechanics to test the hypothesis that Youngs Modulus and stress would be greater in fibers from trained versus untrained participants and both would be reduced following fatigue. We further assessed the translation of these findings to composite tissue in a sub-set of volunteers where muscle tissue bundles, containing both fibers and extracellular matrix, were analyzed in addition to single fibers. We report a main effect of training such that cellular passive mechanical measures were increased in single fibers from trained versus untrained participants. We likewise report reductions in passive mechanical measures following fatiguing exercise. Surprisingly, both training and acute fatigue only impacted muscle fiber passive measures in males, whereas females showed a more variable response across conditions. Last, we provide preliminary evidence supporting the translation of per-individual cellular differences to the tissue level. Together, these data suggest males respond more dynamically to acute and chronic stressors of muscle tissue mechanics, potentially linking cellular response and sex-dependent differences in musculotendinous injury risk. New and noteworthyWe report that passive stress and modulus in single muscle fibers was higher in resistance trained healthy adults and fatiguing exercise reduced passive stress and modulus. In each case, dynamic responsiveness of muscle fibers to chronic and acute stressors was observed consistently in males, whereas responses in females varied considerably. We provide further evidence that cellular mechanisms may contribute to multicellular muscle tissue samples, suggesting these findings have relevance to in vivo tissue mechanics.