What does the research say about Type IIa muscle fibers?

Type IIa muscle fibers

There are 2 papers on type IIa muscle fibers in aging research: 1 observational study in people, 1 in animals.

2 papers study it4 more mention it2 findings

“a higher proportion of type IIa fibers in women”

Endurance training

upin rats1

1 study

Endurance training increases type IIa muscle fibers in rats (life-long endurance training) (near to 70%).
“percentage of type IIA fibres increased from a mean value of below 40% in one-month-old rats to above 50% in the untrained and near to 70% in the trained rats at the age of 24 months.”
Animalslife-long endurance trainingrectus femoris muscle
Effects of ageing and physical training on rat skeletal muscle. An experimental study on the properties of collagen, laminin, and fibre types in muscles serving different functions
Acta physiologica Scandinavica. SupplementumJan 1, 1989

Female sex

upin humans1

1 study

Female sex is associated with type IIa muscle fibers in humans.
“a higher proportion of type IIa fibers in women”
Humansvastus lateralisfemales
Differences in Type 2 Fiber Composition in the Vastus Lateralis and Gluteus Maximus of Patients with Hip Fractures
Endocrinology and metabolism (Seoul, Korea)Jun 11, 2024