Variability in vastus lateralis fiber type distribution, fiber size, and myonuclear content along and between the legs

Abstract

Human skeletal muscle characteristics such as fiber type composition, fiber size and myonuclear content are widely studied in clinical and sports related contexts. Being aware of the methodological and biological variability of the characteristics is a critical aspect in study design and outcome interpretation, but comprehensive data on the variability of morphological features in human skeletal muscle is currently limited. Accordingly, in the present study, m. vastus lateralis biopsies (10 per subject) from young and healthy individuals, collected in a systematic manner, were analyzed for various characteristics using immunohistochemistry (n=7) and SDS-PAGE (n=25). None of the analyzed parameters; fiber type % (FT%), type I and II CSA (fCSA), percentage fiber type area (fCSA%), myosin heavy chain composition (MyHC%), type IIX content, myonuclear content or myonuclear domain varied in a systematic manner longitudinally along the muscle or between the two legs. The average within subject coefficient of variation for FT%, fCSA, fCSA%, and MyHC% ranged between 13-18%, but was only 5% for fiber specific myonuclear content, which reduced the variability for myonuclear domain size to 11-12%. Pure type IIX fibers and type IIX MyHC were randomly distributed and present in <24% of the analyzed samples, with the average content being 0.1 and 1.1%, respectively. In conclusion, leg or longitudinal orientation does not seem to be an important aspect to consider when investigating human vastus lateralis characteristics. However, single muscle biopsies should preferably not be used when studying fiber type and fiber size related aspects given the notable sample to sample variability.

Microbiopsy Sampling for Examining Age-Related Differences in Skeletal Muscle Fiber Morphology and Composition

“…Muscle fiber CSA values were similar to several Bergström studies ( Verdijk et al, 2007 ; Nilwik et al, 2013 ; Horwath et al, 2021 ). The present study found no evidence of age-related muscle fiber atrophy, and while similar findings have been reported for type II muscle fibers in males ( Trappe et al, 2003 ; Roberts et al, 2018 ), this is in disagreement with consistent reports of preferential type II muscle fiber atrophy ( Lexell et al, 1988 ; Walker et al, 2012 ; Nilwik et al, 2013 ).…”

“…Particularly for findings derived from the microbiopsy, our sample size was limited and may not have possessed the necessary statistical power for between groups analyses. Despite our muscle fiber CSA values being similar to previous data obtained from the Bergström biopsy (Verdijk et al, 2007;Nilwik et al, 2013;Horwath et al, 2021), it is possible the 30-min delay in freezing samples affected muscle fiber size. Given the post-biopsy procedures were identical for all subjects and sample wet weight was similar between young and older males, it is unlikely any influence of the 30-min delay was age dependent.…”

“…For example, we extracted ∼45 mg of tissue from three microbiopsy punches, whereas Hughes et al (2015) retrieved ∼80 mg (and up to 120 mg) after 5 punches. Further, due to the well documented heterogeneity of muscle fiber types ( Lexell and Taylor, 1989a , b ; Horwath et al, 2021 ), it is recommended that more than one biopsy be conducted per visit ( Lexell and Taylor, 1989b ; Horwath et al, 2021 ) which is more feasible with the microbiopsy given it is minimally invasive.…”

Resistance training variable manipulations are less relevant than intrinsic biology in affecting muscle fiber hypertrophy

“…To minimize the number of muscle biopsies and reduce the burden to our subjects, we performed a unilateral biopsy only at baseline. The baseline muscle samples harvested from one leg may be a limitation in our design since a variability may exist between legs, although previous studies have not demonstrated significant/important differences for fiber cross‐sectional area, myonuclei content, and gene expression between legs in a within‐subject design 47,48 …”

Daily protein-polyphenol ingestion increases daily myofibrillar protein synthesis rates and promotes early muscle functional gains during resistance training

“…Nonetheless, despite samples being normally distributed, taking a stereological approach with regard to fCSA, and correcting all data to the untrained leg, we observe high variability in both measures. Indeed, recent work reported that fCSA coefficient of variation was ∼12% in repeated biopsy samples obtained over a 24-h period with resistance exercise ( 82 ). Future work testing these possible associations is therefore required.…”