Sex differences in skeletal muscle fiber types: A meta‐analysis

Abstract

Biopsies have been acquired from living men and women to determine proportions of Type I (slow‐twitch) and II (fast‐twitch) skeletal muscle fibers since the 1970s. Sex differences have been assumed but the literature has not been submitted to meta‐analysis. Here, the aim was to generate effect sizes of sex differences in muscle fiber cross‐sectional areas, distribution percentages, and area percentages. Data from 2875 men and 2452 women, who participated in 110 studies, were analyzed. Myofibrillar adenosine triphosphatase histochemistry was used in 71.8% of studies to classify fibers as Type I, II, IIA, and/or IIX; immunohistochemistry, immunofluorescence, or sodium dodecyl sulfate‐polyacrylamide gel electrophoresis were used in 35.4% of studies to similarly classify myosin heavy chain (MHC) isoform content. Most studies involved biopsies from vastus lateralis (79.1%) in healthy individuals (92.7%) between 18 and 59 years old (80.9%). Men exhibited greater cross‐sectional areas for all fiber types (g = 0.40–1.68); greater distribution percentages for Type II, MHC II, IIA, IIX fibers (g = 0.26–0.34); greater area percentages for Type II, IIA, MHC IIA, IIX fibers (g = 0.39–0.93); greater Type II/I and Type IIA/I fiber area ratios (g = 0.63, 0.94). Women exhibited greater Type I and MHC I distribution percentages (g = −0.13, −0.44); greater Type I and MHC I area percentages (g = −0.53, −0.69); greater Type I/II fiber area ratios (g = −1.24). These data, which represent the largest repository of comparative muscle fiber type data from living men and women, can inform discussions about biological sex and its impact on pathologies and sports performance (e.g., explaining sex differences in muscle strength and muscle endurance).

Circulating extracellular vesicle characteristics differ between men and women following 12 weeks of concurrent exercise training

“…Functions with multiple arrows had varied responses. of oxidative type I fibers than men (Nuzzo, 2024) and highly oxidative muscle tissue has been found to release more EVs and release EVs with differing miRNA contents than glycolytic tissue (Estrada et al, 2021;Kargl et al, 2023;Nie et al, 2019). This is slightly counter to our findings as only men had an increase in EV abundance following training, but it may have contributed to the miRNA differences.…”

Females Present Different Single-Leg Squat Kinematics and Muscle Activation Strategies than Males Even after Hip Abductor Fatigue

“…This result is in partial agreement with our findings, as we also observed that females had a lower frequency in the VL but similar in the VM compared to males. One possible explanation for this finding is the greater distribution of fast-twitch muscle fibers (which have a higher muscle fiber conduction velocity) in males compared to females, mainly in the VL [42].…”

Evidence That Skeletal Muscles Modulate HDL-Cholesterol in Metabolic Healthy Young Adults

“…The different representation of Type I/IIa and type IIb muscle fibers in males and in females could help explaining the sex dimorphism that we observed in the correlation between HDL and SMI. In fact, type I muscle fibers are more represented in female than in male skeletal muscle making the role of muscle fibers in lipid metabolism potentially more relevant in females than in males [ 45 , 46 ] and the consequence of obesity-induced muscle metabolic dysfunction more severe in females than in males. It should also be considered that the higher representation of type I fibers in female skeletal muscles increases their ability to accumulate intracellular lipids and, potentially, to undergo lipid-mediated cell damage [ 47 ].…”