Lifting weights makes slow-twitch (endurance) muscle fibres grow more rapidly among men than women. However, the training benefit for fast-twitch muscle fibres did not differ between the sexes. The new research results can help to optimise training outcomes for both sexes.
A new study published in the Journal of Applied Physiology concludes that resistance training affects the muscle fibres of men and women differently.
The researchers investigated how this type of training affects the muscle fibres of men and women and found that the outcomes for men and women clearly differed.
This finding contributes to improving our understanding of sex differences in adaptations to exercise and may have implications for how men and women should optimally train.
“Our study is one of several showing that men and women may adapt differently to exercise. Specifically, we found that men’s slow-twitch muscle fibres grow more, but other studies have shown that women may recover faster from the same bout of exercise. This knowledge may be relevant in structuring their training in order to maximise adaptations,” explains a researcher involved in the study, Lukas Moesgaard, a PhD Fellow from the Department of Nutrition, Exercise and Sports, University of Copenhagen.
Differences in muscle fibres
The researchers wanted to determine whether there are sex differences in the growth of both the fast-twitch (type 2) muscle fibres and the slow-twitch (type 1) fibres.
Type 1 muscle fibres contract slowly and are highly resistant to fatigue, which is particularly well suited for long-distance running.
“There has been an idea that men build more muscle, but we wanted to investigate this more closely for the individual muscle fibres. In addition, we wanted to determine whether we could identify some of the underlying mechanisms,” says Lukas Moesgaard.
12 men and 12 women participated in resistance training
Twelve men and 12 women who had not previously participated in a resistance training programme performed resistance training three times a week for 8 weeks supervised by the researchers to ensure that all participants exercised the same amount.
The researchers collected muscle biopsies from the participants before and after the training intervention and analysed them for differences.
The researchers stained the biopsies to identify the type 1 and type 2 muscle fibres and analysed them for fibre type and size, satellite cells and myonuclei.
Previous research has shown that muscle growth is associated with more myonuclei appearing in the muscle fibres, but the researchers wanted to investigate whether men and women differ in their ability to add new myonuclei.
Men’s muscle fibres grew more than women’s
The results showed that men and women differed in how the muscle fibres react to resistance training.
Although the participants performed resistance training, which primarily stimulates the growth of the type 2 muscle fibres, the researchers surprisingly found a difference in the type 1 muscle fibres.
- The type 1 muscle fibres grew by an average of 6% among women versus 23% among the men.
- The type 2 muscle fibres grew by an average of 26% among women versus 29% among men, but this difference was not statistically significant.
“These are exciting results because they suggest that exercise affects men and women differently by type of muscle fibre. This indicates that the type 1 muscle fibres fundamentally differ between men and women, and investigating this difference further in the future will be interesting,” explains another researcher behind the study, Morten Hostrup, Associate Professor, Department of Nutrition, Exercise and Sports, University of Copenhagen.
Men and women might consider exercising differently
In the second part of the study, the researchers investigated whether men and women differ in the addition of new cell myonuclei in the type 1 muscle fibres but found no differences.
Morten Hostrup says that some studies have found differences in the quantity of androgen receptors between the different muscle fibre types, and since the androgen receptors bind to testosterone, this might explain the difference in muscle growth between the sexes.
“The next step will be to find out the reason for the difference. We are also performing other exploratory studies to determine whether certain proteins influence the observed difference,” says Morten Hostrup.
Lukas Moesgaard elaborates that the research can become relevant in many areas related to exercise and training differences between the sexes.
“Within physiology, there is great interest in understanding how to plan exercise to target the type 1 muscle fibres. Some hypotheses indicate the need for more repetitions. Investigating whether this hypothesis is more relevant for women than for men may be interesting, since women seem to have more difficulty in increasing the growth of type 1 muscle fibres. Further, some literature also shows that women recover better than men, so women may simply need higher training volumes to grow type 1 muscle fibres,” he concludes.