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Sex Differences in the Weightroom
Men and women are different, so exercise “gender equity” initiatives are misguided.
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There are a number of reasons to be interested in the extent to which men and women differ in their physical strength, their preferences and motivations for exercise, and their physiological and anatomical adaptations to strength training exercise. For instance, data on sex differences in muscle strength and endurance might have implications for physical fitness standards set by the military and by police and firefighting services for male and female candidates. Such information might also have implications for understanding to what extent biological men have an advantage over biological women in athletic competition.
Moreover, understanding sex differences in preferences and motivations for exercise might lead to the development of strategies aimed at making physical exercise more appealing to men and women who are sedentary or who are infrequent exercisers. Also, information on preferences and motivations for exercise could inform current debates on whether “gender equity” in participation in physical activity and in exercise science experiments are reasonable goals given that such goals assume little or no sex differences in interests and preferences.
Finally, knowledge of whether men and women can be expected to respond similarly to exercise programs could be utilized by personal trainers, strength coaches, and physical therapists to deliver exercise prescriptions that will maximize health and performance benefits for their male and female clients.
In my recent review paper, I aimed to bring together all of these topics into one story. I examined the extent to which men and women differ in their muscle size and strength, their preferences and motivations for exercise, and their physiological and anatomical adaptations to strength training exercise. My review included data from hundreds of studies conducted over several decades.
Below, I highlight some of the key findings.
How much stronger are men than women?
Men are stronger than women. During childhood, there is little difference in grip strength between boys and girls. Boys become notably stronger than girls at 15 years of age, with girls’ grip strength being approximately 75 percent of boys’ grip strength at this age. This sex difference in grip strength becomes larger with further maturation and by the time grip strength peaks—between the ages of 30 and 39—women have approximately 60 percent of men’s grip strength.
Sex differences in muscle strength are more pronounced in upper- than lower-body muscles. In upper-body exercises, such as the bench press and biceps curl, the maximal amount of weight that a woman can lift is approximately 50-55 percent of what a man can lift, whereas for the squat and deadlift exercises, the maximal amount of weight that a woman can lift is around 60-65 percent of what a man can lift. Moreover, for the calf muscles, female strength rises to approximately 70 percent of male strength, indicating that the muscles men and women use similarly for activities of daily living (e.g., walking, stairclimbing, squatting up and down) exhibit the smallest sex differences.
What causes the sex difference in muscle strength?
The sex difference in muscle strength is due, primarily, to greater muscle mass in men than women. Men have more muscle mass than women in absolute terms and also as a proportion of total body mass. Men also carry a greater proportion of their muscle mass in their upper bodies, which helps explain the larger disparity in muscle strength in upper-body compared to lower-body muscles between men and women.
Another anatomical contributor to the sex difference in muscle strength is that men’s total muscle area is occupied by a greater proportion of Type II muscle fibers. Type II muscle fibers, also called “fast twitch” fibers, are able to generate more force than Type I (“slow twitch”) muscle fibers, which comprise a greater area of women’s than men’s muscle area. Thus, both the size and composition of muscles contribute to sex differences in muscle strength.
There is however no evidence that men and women differ in their ability to voluntarily activate their muscles with their nervous system—a measurement called voluntary activation.
Sex differences in exercise interests and motivations
Men participate in strength training more frequently than women. This was found in nearly all 27 population-level surveys in my review. Participation in strength-based sports, such as competitive powerlifting and Olympic weightlifting, is also more common among men than women. Men also rate strength training higher on their list of preferred exercise activities than do women, while women are more likely than men to prefer activities such as walking, aerobics, yoga, stretching, and dance.
Why do men participate in strength training more than women? The drive for muscularity is a key factor. Questionnaires on the drive for muscularity have consistently shown that this desire is rated higher among men than women. Mate attraction and intrasexual competition likely underlie the high drive for muscularity among men. In fact, many exercise preferences and behaviors that men and women exhibit are likely linked to mate attraction.
Some motivators for exercise participation are rated equally by men and women. They include the desire to improve health and fitness, to socialize with others, to have fun, and to relax or manage one’s mood. However, clear sex differences also exist. Men are motivated more by challenge, competition, social recognition, and a desire to increase muscle size and strength, whereas women are motivated more by improved attractiveness, muscle “toning,” and body weight management. Men also express greater preference for exercise that is competitive, high-intensity, and for upper-body muscles. Women, on the other hand, express greater preference for exercise aimed at lower-body muscles and that is supervised and involves instruction.
Do men and women respond similarly to strength training programs?
The first study to examine whether men and women experience similar changes in muscle size and strength from strength training was published in 1974. Subsequent decades of research have generated several dozen studies exploring this same question. When results from these studies have been submitted to meta-analysis, the findings have been that both men and women increase muscle size and strength after weeks of strength training. However, percent increases in upper-body, but not lower-body, strength are greater in young adult women than young adult men, which might be attributed to the more pronounced sex difference in upper-body than lower-body strength at baseline between adult women and men. No sex difference in the percent increase in muscle size after weeks of strength training is evident in younger or older adults.
An important caveat to keep in mind with these findings is that they come from studies in which men and women completed strength training programs that were identical in terms of training frequency and exercise volume and intensity. However, as my review has also shown, when men and women are left to their own choices, they usually do not perform the same types of strength training programs.
Conclusion: what was learned?
Many of us know through everyday observations that men and women differ in many ways. Yet, implicit in some of the political activism that seeks to promote the position and interests of women, including in the areas of sport and exercise, is the idea that men and women have the same interests, preferences, and abilities. My review shows that while men and women do exhibit some similarities, there are many pronounced differences when it comes to the activity of strength training. These differences and similarities have various implications.
Men, due to their greater muscle mass, are physically stronger than women. Thus, as occupational tasks involve lifting and moving absolute loads, women, on average, will not be able to perform such tasks to the degree that men can. Moreover, greater muscle mass and strength in men places biological women at a disadvantage (and perhaps in danger) when competing against biological men in certain athletic competitions.
Men and women also exhibit many differences in their preferences and motivations for exercise as well as their exercise behaviors. Men, due to their greater drive for muscularity, are more likely than women to prefer strength training as an exercise modality, and men are also more likely to prefer exercise that involves upper-body muscles, is high-intensity, and involves challenge and competition (e.g., rankings or points systems that are exhibited for social recognition). Women have a greater preference for activities such as walking, dance, and yoga, and are more likely to prefer supervised and lower-body exercise.
Understanding these sex differences might help to optimize enjoyment and adherence to exercise programs. Moreover, sex differences in preferences and motivations for exercise also support my previous conclusions that initiatives that strive to achieve “gender equity” in participation in physical activity and in exercise science experiments are misguided, as such initiatives assume that social/environment factors, rather than biological and psychological factors, are the reasons that “gender equity” does not exist for these outcomes.
Finally, from the review, we also learned that both men and women can increase the size and strength of their muscles after weeks of strength training. Thus, as lower levels of muscle strength and less frequent participation in muscle-strengthening activities are associated with earlier mortality in both men and women, all men and women should be encouraged to participate regularly in strength training exercise.
All up, my review, which spans 43 journal pages, includes 13 tables, 19 figures, and 502 supporting references. It was the amount of information I thought was necessary to bring clarity to a topic (i.e., sex differences) that seems to be associated with a great deal of angst and confusion in contemporary society.
I hope my review helps in giving reality one last stand!