Sex differences in semitendinosus muscle fiber-type composition

Area of Science:

• Muscle physiology
• Human anatomy
• Sex differences in sports science

Background:

• Muscle fiber-type composition varies across different muscle groups.
• The hamstring muscle fiber-type composition, specifically the semitendinosus, is not well-characterized.
• Understanding sex-based differences in muscle composition is crucial for sports performance and injury prevention.

Purpose of the Study:

• To compare the semitendinosus muscle fiber-type composition between men and women.
• To investigate potential sex differences in muscle fiber size and proportion.
• To explore the functional implications of observed sexual dimorphism in hamstring muscles.

Main Methods:

• Biopsy samples were collected from the semitendinosus muscle of 12 men and 12 women during anterior cruciate ligament reconstruction surgery.
• Succinate dehydrogenase (SDH) and ATPase activities were analyzed to determine muscle fiber types.
• Muscle fiber size and the proportion of fibers expressing specific myosin heavy chain (MyHC) isoforms (MyHC-I, MyHC-IIA, MyHC-IIX) were quantified.

Main Results:

• Men exhibited a significantly lower proportion of SDH-positive fibers and a higher percentage of fast muscle fibers compared to women.
• Men's semitendinosus muscles showed a lower proportion of MyHC-I and higher proportions of MyHC-IIA and MyHC-IIX fibers relative to women.
• Muscle fibers expressing MyHC-I, MyHC-IIA, and MyHC-IIX were significantly larger in cross-sectional area in men than in women.

Conclusions:

• Significant sex differences exist in the semitendinosus muscle composition.
• Men display a faster muscle fiber phenotype and larger muscle fiber size compared to women.
• These identified sexual dimorphisms in hamstring muscle characteristics may have functional consequences for athletic performance and biomechanics.