Chronic and acute mediators of passive viscoelasticity in human skeletal muscle fibres.

The cellular viscoelastic modulus in skeletal muscle tissue responds dynamically to chronic stressors, such as age and exercise. Passive tissue mechanics can 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 (exercise-induced muscle fatigue) stressors in healthy young males and females categorized as 'resistance trained' or 'untrained'. We measured passive mechanics to test the hypothesis that Young's modulus and stress would be greater in fibres from trained versus untrained participants and that both would be reduced following fatigue. We also assessed the translation of these findings to composite tissue in a subset of volunteers where muscle tissue bundles, containing both fibres and extracellular matrix, were analysed in addition to single fibres. We found that resistance-trained individuals demonstrated enhanced passive elastic and viscous modulus compared with non-trained individuals. We also report reductions in passive mechanical measures following fatiguing exercise. Surprisingly, both chronic and acute effectors of passive mechanics were observed in muscle fibres only from 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 that males respond more dynamically to acute and chronic stressors of muscle tissue mechanics, potentially linking cellular response and sex-dependent differences in functional outcomes across the lifespan.