Myocellular differences between the soleus and vastus lateralis
Authors: Skiles Chad , Gouw Aaron , Boyd Gerard , Kuszmaul Dillon , Minchev Kiril , Chambers Toby , Trappe Todd , Trappe Scott 
Published: May 2023

Objective: The purpose of this study was to compare myocellular characteristics between the soleus (SOL) and the vastus lateralis (VL) muscles including fiber distribution and fiber size along with myonuclear (MN) and satellite cell (SC) content. Hypotheses: SOL would have a greater distribution of myosin heavy chain (MHC) I fibers than the VL while the VL would contain a greater distribution of MHC IIA fibers. Fibers of the SOL would be larger than the fibers of the VL. Furthermore, there would be no difference between the VL and SOL MN content. In contrast, the VL would have a greater SC density than the SOL. Methods: Nine healthy males and one female subjects (n=10; 34±8 yrs) were recruited for this study. Muscle biopsies from the SOL and VL were sectioned, immunofluorescently stained, and analyzed for fiber distribution, size, MN and SC content. Results: The total number fibers studied was 2,977 for the SOL (298±43 per subject) and 2,729 for the VL (273±34 per subject). The percentage of MHC I fibers was higher in the SOL compared to the VL (71.0±4.1 vs 27.9±2.6%; P<0.05). Whereas, the SOL had a lower percentage of I/IIA (0.2±0.1 vs 1.8±0.7%; P<0.05), IIA (26.0±3.2 vs 54.8±3.3%; P<0.05) and IIA/X (2.8±1.8 vs 15.3±2.0%; P<0.05) fibers. There was no difference in MHC IIX content between the SOL and VL (0.0±0.0 vs 0.2±0.1%; P>0.05). Fiber size was not significantly different between the two muscles within MHC I (6,551±546 vs 5,825±468 μm2; P>0.05), MHC IIA (8,068±827 vs 6,762±474 μm2; P>0.05), and all fibers (6,979±607 vs 6,374±493 μm2; P>0.05). There were no differences in the amount of MN within MHC I (3.44±0.20 vs 3.41±0.36 MN/fiber; P>0.05), MHC IIA (3.82±0.23 vs 3.76±0.29 MN/fiber; P>0.05), and all fibers (3.55±0.20 vs 3.60±0.31 MN/fiber; P>0.05). There were no differences in SC content in MHC I (0.12±0.01 vs 0.10±0.01 SC/fiber; P>0.05) and IIA fibers (0.07±0.01 vs 0.06±0.01 SC/fiber; P>0.05). However, within all fibers the SOL trended to exhibit a greater SC density than the VL (0.10±0.01 vs 0.07±0.01 SC/fiber; P=0.063). Conclusion: In summary, there were myocellular similarities and differences observed between the SOL and VL muscles. SOL is a postural muscle and contains more MHC I fibers, whereas the VL is a phasic muscle and can rely more heavily on MHC IIA fibers. However, there was no difference in fiber size which was contrary to our hypothesis. A novelty from this study was that this project, to the best of our knowledge, was the first to investigate MN and SC in human SOL, but no differences were observed between the two muscles. The myocellular similarities across the different muscles may be related to the shared tasks that skeletal muscles perform, however, the observed difference in fiber distribution may be associated with the different functions and roles related to their respective muscles. This research was supported by the National Aeronautics and Space Administration, NNJ06HF59G; the Institute for Translational Sciences at the University of Texas Medical Branch, supported in part by a Clinical and Translational Science Award (UR1TR000071) from the National Center for Advancing Translational Sciences, National Institutes of Health; and the NASA Flight Analogs Project. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.