Training Induced Changes to Skeletal Muscle Passive Properties Are Evident in Both Single Fibers and Fiber Bundles in the Rat Hindlimb

Abstract

Introduction: The passive mechanical behavior of skeletal muscle represents both important and generally underappreciated biomechanical properties with little attention paid to their trainability. These experiments were designed to gain insight into the trainability of muscle passive mechanical properties in both single fibers and fiber bundles. Methods: Rats were trained in two groups: 4 weeks of either uphill (UH) or downhill (DH) treadmill running; with a third group as sedentary control. After sacrifice, the soleus (SOL), extensor digitorum longus (EDL), and vastus intermedius (VI) were harvested. One hundred seventy-nine bundles and 185 fibers were tested and analyzed using a cumulative stretchrelaxation protocol to determine the passive stress and elastic modulus. Titin isoform expression was analyzed using sodium dodecyl sulfate vertical agarose gel electrophoresis (SDS-VAGE). Results: Single fibers: passive modulus and stress were greater for the EDL at sarcomere lengths (SLs) ≥ 3.7 μm (modulus) and 4.0 μm (stress) with DH training compared to UH training and lesser for the SOL (SLs ≥ 3.3 μm) with DH training compared with control; there was no effect of UH training. Vastus intermedius was not affected by either training protocol. Fiber bundles: passive modulus and stress were greater for the EDL at SLs ≥ 2.5 μm (modulus) and 3.3 μm (stress) in the DH training group as compared with control, while no affects were observed in either the SOL or VI for either training group. No effects on titin isoform size were detected with training. Conclusion: This study demonstrated that a trainability of passive muscle properties at both the single fiber and fiber bundle levels was not accompanied by any detectable changes to titin isoform size.

Influence of weighted downhill running training on serial sarcomere number and work loop performance in the rat soleus

“… Noonan et al (2020) demonstrated this with lower passive stress and passive elastic modulus in soleus single fibres of rats that ran downhill compared to uphill. Noonan et al (2020) used the same rats as Chen et al (2020) , therefore these lower passive properties appeared to be due to a 6% greater SSN.…”

“…With a greater SSN, individual sarcomeres may stretch less during muscle excursion, leading to less passive force generated by sarcomeric proteins such as titin (Herbert and Gandevia, 2019). Noonan et al (2020) demonstrated this with lower passive stress and passive elastic modulus in soleus single fibres of rats that ran downhill compared to uphill. Noonan et al (2020) used the same rats as Chen et al (2020), therefore these lower passive properties appeared to be due to a 6% greater SSN.…”

An increase in serial sarcomere number induced via weighted downhill running improves work loop performance in the rat soleus

“…Noonan et al (2020) demonstrated this with lower passive stress and passive elastic modulus at longer SLs in soleus single fibers of rats that ran downhill compared to uphill. Noonan et al (2020) used the same rats as Chen et al (2020), therefore the lower passive stress and elastic modulus appeared to be due to a 6% greater SSN.…”

“…With a greater SSN, individual sarcomeres may stretch less during muscle displacement, leading to less passive force generated by sarcomeric proteins such as titin (Herbert and Gandevia, 2019). Noonan et al (2020) demonstrated this with lower passive stress and passive elastic modulus at longer SLs in soleus single fibers of rats that ran downhill compared to uphill. Noonan et al (2020) used the same rats as Chen et al (2020), therefore the lower passive stress and elastic modulus appeared to be due to a 6% greater SSN.…”

Anisotropic Muscle-like Conductive Composite Hydrogel Reinforced by Lignin and Cellulose Nanofibrils

“…Owing to the abundant water content and highly oriented muscle fibers, skeletal muscles exhibit outstanding physical properties, such as good fatigue resistance, high mechanical strength, excellent flexibility, and so forth. , Although some features of skeletal muscles in conductive hydrogels have been successfully imitated through various methods, it is still impossible to reproduce all the desired properties in one material system. , Liang et al designed a strategy to fabricate hydrogels with preferentially aligned micro/nanostructures, which endowed the hydrogel with a more than 100-fold increase in fatigue thresholds. The prepared fatigue-resistant hydrogels were alternatives to soft materials in robots and artificial muscles .…”