There's a proven muscle mass difference between males and females: Men on average have a higher percentage of muscle than women. However, there is great variation within each gender, and women can — and should — strength-train regularly for athletic performance and health.
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Muscle Mass and Body Fat
Science has long acknowledged a difference between muscle mass and body fat for men and women. In one of the older but more comprehensive studies, published in a 1985 issue of the Journal of Applied Physiology, researchers used magnetic resonance imaging to compare skeletal muscle mass and distribution in a sample of 468 men and women. They found significantly more muscle mass in the men; also of note, the gender difference was greater in the upper body.
Interestingly, the researchers also noted that sex-based differences in weight and height explained about half of the sex-based variation in muscle mass.
Due to the age of that study, you could theorize that at least part of the difference in muscle mass was also due to gender-based social norms for physical activity. But more recent (albeit smaller) studies continue to confirm these findings.
For example, a small study published in the October 2003 issue of the British Journal of Sports Medicine evaluated 20 Japanese college students (10 women and 10 men) and confirmed that the men had more muscle mass than the women. The study also affirmed that proportionally more of the men's muscle mass is in the upper body, while women's muscle mass is proportionally greater in the lower body.
Also of note, an article published in the January 2015 issue of Physiology, from the American Physiological Society, notes that scientists have identified more than 3,000 genes that are expressed differently in muscle for women vs. men.
Gender-Based Body Fat
As you might expect, the gender-based differences in (average) muscle mass also translate to gender-based differences in body fat percentages. Consider this: As the American Council on Exercise notes, the amount of essential body fat needed for survival is 10 to 13 percent in women, while for men it's a mere 2 to 5 percent. That difference continues throughout the spectrum, with fit or athletic women typically having 14 to 24 percent body fat, while for men the comparable range is 6 to 17 percent.
The difference in body fat required for health is due to the differing demands of the reproductive cycle on men's and women's bodies. In fact, athletic amenorrhea (the absence of menstrual periods) is a potential concern for elite female athletes, whose levels of body fat may dip too low to support regular menstruation.
Neither of these sex-based differences is an excuse for women to avoid strength training, nor do they imply that strength training is a futile effort for women. Both men and women can make significant gains at the gym — and both genders can reap other health benefits from regular strength training, including stronger bones, better quality of life, more stamina and improved cognitive function.
Muscle: Women vs. Men
The U.S. Department of Health and Human Services doesn't differentiate between men and women in its physical activity guidelines for Americans. They say that in addition to regular aerobic exercise, adults of both genders should also strength-train all their major muscle groups at least twice a week.
There's plenty of scientific confirmation for this guideline. For example, as reported in a June 2016 issue of Physiological Reports, researchers evaluated a cohort of both men and women over 65 years of age. After 18 weeks of resistance exercise training, it is true that the men had experienced greater adaptations (gains) in both muscle quality and maximal torque. However, the improvements in quality and torque were significant for both genders.
And ladies, you can put down those water bottles and soup cans you were previously told to lift. If you want to see significant results you can — and should — lift using the same techniques men do. And just like the men, you should focus on finding the right challenge for your body, as opposed to whoever's next to you in the weight room.
The accepted "starting place" for general health is doing one to three sets of eight to 12 repetitions for each exercise, twice a week. No matter your gender, as you build skill, strength and confidence you can either increase the weight, add more sets, or add more exercises for that muscle group.
Read more: Your Ultimate Guide to Gaining Lean Muscle
Things Change With Age
Scientists are still working to learn more about the mechanisms by which your body gains — and loses — muscle mass. Women and men both tend to lose muscle mass with age, but even there, a gender-based difference applies. Of particular note, in a study published in the June 2009 issue of Anthropologischer Anzeiger, scientists evaluated "elderly" men and women between the ages of 59 and 92 years old.
They confirmed the prevalence of sarcopenia (age-related decline in skeletal muscle) for both genders, but found that at ages below 70 years old, the women showed more sarcopenia; in ages greater than 80 years old, the men showed more sarcopenia. Although the mechanism isn't understood, these results make it clear that just as gender influences muscle gain, it also influence muscle loss with age. Or perhaps it would be more accurate to say that aging affects muscle differently, depending on your sex.
Mass Isn't Everything
It's tempting to assume that when it comes to muscles, bigger is always better — but that isn't always the case. As a small study published in the October 2010 issue of Medicine and Science in Sports and Exercise notes, women are often observed to be more resistant to fatigue — that is, to have better endurance — than men for tasks of the same relative intensity.
However, that study of 32 subjects (16 female and 16 male) found that the difference in muscular endurance differed depending on which muscle group was being evaluated. The female subjects had significantly more fatigue resistance than the male subjects when evaluated at the elbow (biceps), but not when evaluated for ankle dorsiflexion (which uses the muscles at the front of your shin).
The bottom line? Although men do typically have more muscle mass, women are quite capable of building significant amounts of muscle too. And as scientists continue to expand their understanding of gender-based muscle differences, greater nuances in the difference between male muscle and female muscle may become clear.
- Journal of Applied Physiology: "Skeletal Muscle Mass and Distribution in 468 Men and Women Aged 18-88 yr"
- Physiological Reports: "Sex Differences in the Response to Resistance Exercise Training in Older People"
- American Council on Exercise: "Percent Body Fat Calculator"
- Physiology: "Sex-Based Differences in Skeletal Muscle Kinetics and Fiber-Type Composition"
- British Journal of Sports Medicine: "Sex Differences in Whole Body Skeletal Muscle Mass Measured by Magnetic Resonance Imaging..."
- Victoria State Government: "Menstruation - Athletic Amenorrhoea"
- Mayo Clinic: "Strength Training: Get Stronger, Leaner, Healthier"
- Health.gov: "Physical Activity Guidelines for Americans"
- Anthropologischer Anzeiger: "Gender and Age Differences in Lean Soft Tissue Mass and Sarcopenia Among Healthy Elderly"
- Medicine and Science in Sports and Exercise: "Sex Differences in Fatigue Resistance Are Muscle Group Dependent"