Muscles in Pink

As mentioned in our previous article, there are mainly two reasons why excessive muscle mass and volume increase do not occur in women who train with weights: -differences in muscle fibers; -differences in hormonal concentrations.

In this article, we’ll discuss the topic related to muscle fibers, which requires a brief introduction.

There are three types of fibers in humans: -Type I fiber (red, slow-twitch, slow oxidative); -Type II a fiber (intermediate); -Type IIb fibers (white, fast-twitch, fast glycolytic).  

What are the differences in muscle fibers between men and women?

Women have fewer muscle fibers than men, and their fibers are also smaller than those of men. However, regarding composition, women have the same composition of type I (slow-twitch) and type II (fast-twitch) fibers and all their subtypes.

But there’s a peculiarity: -In 75% of untrained women, type I fibers are larger than type II fibers; -The remaining 25% of untrained women, on the other hand, have a fiber configuration typical of males, with type II fibers larger than type I fibers. It could be that women who fall into this 25% are more suited to strength and power sports.

The cause of this initial difference, found in 75% of untrained women, has not yet been clarified. It has been hypothesized to be due to a lower demand for strength and power in daily activity, but it could also be a genuine gender difference.

What does this difference entail?

In any case, such a difference influences a training program, so after an initial period of stagnation (there is an initial period of slow performance increase that persists until type II fibers reach and exceed in size the type I fibers), women can also experience strength development. To achieve this effect, it is essential to initially use heavier load percentages to stimulate a more rapid increase in the size of type II fibers.

Furthermore, it has been shown that when there is a predominance of type I fibers, the de-training phenomenon can be more rapid, and therefore more frequent maintenance sessions are necessary.

Gender differences in the number of muscle fibers and the dimensions of their cross-sectional area are reflected in the differences in absolute strength. In women, the average maximal strength of the entire body corresponds to about 60% of the average maximal strength of a man’s body. In the upper body, women’s strength varies from 25% to 55% of that of a man, while for the lower body, a higher percentage, from about 70% to 75%, has been observed (Fleck, Kramer, 2004).

If you consider the records in powerlifting and weightlifting, such gender differences are observed both in individual lifts and in the total weight lifted.

These are obviously data that refer to comparisons between groups of athletes and untrained subjects. In a comparison made between individual subjects without similar variables, a woman could have greater strength than a man.

The differences are significant and highlight the diversity in terms of strength and hypertrophy between men and women, but what makes these differences much more significant are undoubtedly the differences in hormonal levels.

To learn more, don’t miss the next article.

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Sources and bibliography

Science and Practice of Strength Training Valdimi M.Zatsiorsky, William J. Kramer Citation: Lundsgaard A-M and Kiens B (2014) Gender differences in skeletal muscle substrate metabolism – molecular mechanisms and insulin sensitivity. Front. Endocrinol. 5:195. doi: 10.3389/fendo.2014.00195