By Fleur van Griensven
When a girl tells people she just started lifting, they often think that she will wake up as a Hulk the next day. They think that lifting makes women look bulky, so they shouldn’t train like men. Another thing you often hear, is that women can never get as big as men, but is that true? Do women naturally have less muscular potential than men? Or can we finally acknowledge the fact that women should train heavy too and that a lot of girls are not living up to their potential by lifting 2 kg pink dumbbells?
The natural muscular potential of women.
Popular opinion is that men have more muscular potential than women and thus can gain more muscle. However, some research indicates that women naturally have roughly the same muscle building potential. A prospective intervention study examined the influences of gender on muscle size responses to strength training. The results were that women can gain the same percentage of muscle mass compared to men during strength training .
Studies on protein synthesis came also to the same conclusion. Women have similar muscle protein synthesis rates and thus seem to build the same amount of muscle protein after training compared to men . This is however the conclusion of only a few studies, so more research is definitely needed to be 100% clear that women do naturally have the same muscle building potential. However, this is difficult because these studies take a long time and need to be well-controlled.
When women start training they do have a different body composition. Most of the times, women have less muscle mass and more fat mass compared to men. Women have +/- 12% essential body fat compared to just +/- 3% fat in men . Essential body fat is all the fat which we can not lose without it negatively affecting our physiological functions. It surrounds our organs and nerve tissues. Men and women do have a different starting point when it comes to muscle mass, but they seem to be gaining muscle mass at the same rate. Keep in mind that for the rest of this article I always compare men with a starting point to women with the same starting point, so a same body composition.
How about testosterone?
Testosterone is the most important male sex hormone. Women however do also produce testosterone, even though they have 15 times less active testosterone than men, under normal circumstances . Besides the known functions of testosterone in our body, like development of primary/secondary sexual characteristics and production of sperm, it also plays a positive role in muscle building. Testosterone is an anabolic hormone, which means that it stimulates muscle protein synthesis and thus muscle growth.
Is it then true, because women have less testosterone they also have less potential to build muscle mass? No, this is probably not the case. The testosterone functions are different in men and women. What comes next might be a bit of a complicated story with terms you never heard about, but do not worry about that and try to see the big picture in why having less testosterone as a woman is not that bad at all.
It seems that testosterone is not needed for muscle development in women because growth factors like IGF-1 and growth hormone take over the anabolic role that testosterone plays in men . This has been found in an animal study done with mice, so more studies are necessary to test if this is the case in humans too. However since women can gain muscle with lower testosterone levels, it can be safely assumed that other hormones besides testosterone are involved in this muscle building process.
Women have just as much IGF-1 and produce +/- 3 times as much growth hormone as men . This study shows that despite the fact that women have lower testosterone levels than men, they do have higher growth hormone levels. It could be possible that in women growth hormone partly takes over the role of testosterone. This explains why having less testosterone does probably not limit how much muscle women can build. People think that testosterone is the most important hormone, but there are more hormones that play a role in muscle growth.
The other (sex)hormones combined with the advantages/disadvantages women have.
Where testosterone is the most important male sex hormone, estrogen is the most important female sex hormone. Estrogen is commonly seen as the hormone that makes you fat and frail. However, its positive effects should not be disregarded. Some of these positive physiological effects are:
It is anti-catabolic, which means that it prevents muscle loss .
It aids in muscle repair .
It is good for connective tissue (bones, ligaments etc.).
The bad reputation of estrogen is based on nothing more than the assumption that if testosterone is anabolic, estrogen must be catabolic. There is a lot of ongoing research and there are indications that it plays a role in muscle growth and the well-being of skeletal muscle. However, more research is needed in the future.
One big disadvantage for a lot of women these days is the use of the anti-contraceptive pill. This holds especially true for the ones containing a lot of progesterone. They do have a negative effect on muscle growth compared to not using an anti-contraceptive pill, because progesterone competes with testosterone for the androgen receptor. Basically, this comes down to less active testosterone when you take in an anti-contraceptive pill with a lot of progesterone . How much of a negative effect occurs is hard to tell, because it is very difficult to study.
Here is a practical tip which you can use to take advantage of the benefit we have being a woman. Hopefully this and more research in the future about some other topics relating the effects of hormones on muscle growth will help you get the most out of your training sessions!
We can use a higher rep range.
There are two types of muscle fibres: Type I&II. Type I muscle fibers are known as slow-twitch muscle fibers, which makes them able to contract for a long period of time and more resistant to fatigue. Being able to contract for a long period of time could mean that women can benefit from a higher rep range. This could mean that for women to grow to their full potential, more reps per set need to be done to benefit from the type I fibres which are more resistant to fatigue and can contract longer. One study came to the conclusion that during exercise in women, a potential conversion to type I muscle fibers or no conversion at all takes place . This study looked at the muscle fibre adaptations during execution of a knee-extension exercise in both young men and women. They found a significant increase in percentage of type I fibres in young women. The study has its limitations: if the muscle fibres had been splitted into type Ia, IIa, IIb etcetera, no significant result would have been found. This also is anecdotal evidence and more research needs to be done.
Conclusion, do women naturally have less muscle building potential than men?
There is some research which indicates that women can gain the same percentage of muscle mass compared to men during strength training [1,2]. However more research is needed.
Why you see more men with a significant amount of muscle mass compared to women and why women aren’t 100% living up to their potential can be contributed to a lot of factors:
Mostly social-cultural. There are not as many women compared to men training with weights. If they do train, some of them just do not want to build as much mass as possible. Everyone has their own goal and idea of how they want to look. That is okay, as long as you do what makes you happy and never have someone telling you what you shouldn’t or can’t achieve.
If women go to the gym, they spend countless hours on the treadmill or playing around with pink dumbbells, that is for sure not 100% optimal if you want to build as much muscle mass as possible.
Oral-contraceptives, which have a negative effect on muscle growth. Part of the anabolic role of testosterone will be taken over by other hormones in women, so testosterone production probably does not limit how much muscle women can build.
More information about this topic for sure will come available in the future as more and more studies are being done on this interesting topic. When that time comes we will update this article and bring you the latest conclusions.
One take home message for all the women out there busting their ass off (or on) in the gym: you probably are not less capable of putting on muscle mass than men, and the only limitations you have are the ones you put on yourself!
 Roth, SM (2001). Muscle size responses to strength training in young and older men and women. Journal of the American Geriatrics Society, 49(11), 1428-33.
 O’Hagan, FT (1995). Response to resistance training in young women and men. International journal of sports medicine, 16(5), 314-21.
 Vehrs, P (2013). Assessment and interpretation of body composition in physical education. Journal of Physical Education, Recreation & Dance, 46-51
 Wisse, B (2016, 2 March). Testosterone. Retrieved from https://medlineplus.gov/ency/article/003707.htm
 MacLean, HE (2008). Impaired skeletal muscle development and function in male, but not female, genomic androgen receptor knockout mice. FASEB journal, 22(8), 2676-89.
 Van den Berg, G (1996). An amplitude-specific divergence in the pulsatile mode of growth hormone (GH) secretion underlies the gender difference in mean GH concentrations in men and premenopausal women. Journal Clinical endocrinal Metab, 81(7), 2460-2467
 Hansen, Mette (2014). Influence of Sex and Estrogen on Musculotendinous Protein Turnover at Rest and after exercise. Exercise & Sport Sciences Reviews, 42(4), 183-192.
 Velders, M (2013). How Sex hormones promote skeletal muscle regeneration. Sports Medicine, 43(11), 1089-1100.
 Woock, C (2009). Oral Contraceptive use impairs muscle gains in young women. The FASEB Journal, 23(1).
 Martel, G (2006). Age and sex affect human muscle fibre adaptations to heavy resistance strength training. Experimental Physiology, 91(2), 457-464