The muscles that make up your body are composed of individual muscle fibres. Think of the biceps muscle of your arm as a handful of spaghetti strands. Each strand of spaghetti represents a long muscle fibre that runs from the front of your shoulder to the inner crease of your elbow. However, not all spaghetti strands are necessarily the same, nor are all of the muscle fibres that make up your biceps muscles or other muscles of your body.
Skeletal muscle fibres differ from each other based on their biochemical and physiological properties. Based on such properties, skeletal muscles have historically been categorized as two general types: Type I slow-twitch fibres and Type II fast-twitch fibres. Type II muscle fibres are called “fast-twitch” because they shorten at high velocities and create large power and force outputs. Type I fibres, on the other hand, are called “slow-twitch” because they shorten at slow velocities and create low forces and low power outputs.
However, there is a trade-off. Type II fibres, though strong and powerful, fatigue more quickly than do Type I fibres, which are known for their ability to create low levels of force for long periods. Marathon runners, for example, are known to have high proportions of Type I muscle fibres, whereas 100-meter sprinters and other power athletes are known to have high proportions of Type II muscle fibres. The way that muscle fibre type is determined is by having a biopsy or small chunk of tissue taken from one’s muscle and then treated chemically.
For many years, researchers have speculated that proportions of Type I and Type II muscle fibres might be different in the average man versus the average woman. Remarkably, though biopsies of skeletal muscles have been acquired from men and women in research studies since the 1970s, the available data have never been aggregated to determine if the average man and woman differ in their muscle fibre types. Such information could help to inform discussions about sex differences in sports performance and susceptibility to disease.
This is why earlier this year I performed a meta-analysis of the existing data on muscle fibre types in men and women. I discovered that between 1976 and 2022 a total of 110 studies reported muscle fibre type data from both male and female participants. The number of studies included in the analysis was 3-4 times greater than two previous narrative reviews on the topic – one of which was my own.
Data from 2,875 men and 2,452 women were included in the analysis. Most of the data were collected in the United States, Canada, and Scandinavian countries. Most of the participants in the included studies were healthy non-athletes who were between the ages of 18 and 59. The vastus lateralis muscle, which makes up the outer part of the thigh, was the muscle most frequently biopsied.
The results revealed many sex differences. Compared to women, men had a greater a percentage of Type II or fast-twitch muscle fibres. Women, on the other hand, had a greater percentage of Type I or slow-twitch fibres. One of the measures of Type I fibres showed that, on average, 53.2% of female muscle is comprised of Type I fibres compared to 50.6% of male muscle. A second measure of Type I fibres showed an even larger sex difference.
When considering the area or space of muscle occupied by different fibre types, sex differences were even more evident. That is, a greater area of male than female muscle was composed of Type II fibres, and a greater area of female than male muscle was composed of Type I fibres. For one of the measures of Type I fibres, the percentage area of a woman’s muscle composed of Type I fibres was 54.8% compared to 49.3% for men. Consequently, the ratio of Type I to Type II muscle fibre area was greater in women than men, and the ratio of Type II to Type I muscle fibre area was greater in men than women.
Finally, the meta-analysis also revealed large sex differences in the raw cross-sectional areas or sizes of the muscle fibres. These large differences were observed for all muscle fibre types that were assessed.
So, what do the results mean? What are their implications?
The results could have implications for understanding sex differences in physical abilities, including in sports and in physically demanding occupations. Compared to women, men have a greater proportion of Type II muscle fibres, and men have larger cross-sectional areas for all muscle fibre types. This helps to explain how men are stronger and more powerful than women and how the average man is typically more successful at completing certain physical tasks in sports and at work.
The results might also have implications for understanding sex differences in aging and development. For example, aging is known to disproportionally impact Type II fibres. Men have greater proportions of Type II fibres. Thus, men and women might be differently impacted by the aging process. In fact, a preliminary analysis in the study found that the sex differences in proportions of Type I and Type II muscle fibre types were greatest in young and middle-aged adults but then mostly disappeared in adults over the age of 60.
Also, certain diseases differently impact certain muscle fibre types. For example, Duchenne muscular dystrophy predominantly impacts Type II fibres and is a condition found primarily in boys. Other conditions, such as spinal cord injury, diabetes, heart disease, and chronic obstructive pulmonary disease cause Type I muscle fibres to transition to be more like Type II muscle fibres. How sex differences in muscle fibre types might associate with disease susceptibility and severity is something to be teased out in future research.
In closing, the recent meta-analysis on sex differences in muscle fibre types represents the largest repository of comparative muscle fibre type data from living men and women that has ever been assembled. The data show that, compared to women, men between the ages of 18 and 59 have greater cross-sectional areas of individual muscle fibres than do women, and men also have a greater percentage of Type II fast-twitch muscles fibres than do women. The implications of these findings for sex differences in aging and disease susceptibility remain to be understood. The implications for sports performance, however, are perhaps clearer. Larger whole muscle size, larger muscle fibre size, and greater proportions of Type II muscle fibres are just a couple of the many ways in which biological males, including transgender women, are naturally advantaged over biological females in the female category of sport. How muscle fibre types are or are not impacted by medical treatments administered to transgender individuals is a research topic to pay attention to in the coming months and years.
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