Is There a Difference Between Female and Male Muscles?

With the increasing popularity of women's sports — Nielsen reports that 84 percent of sports fans are interested in women's sports, a figure that is 51 percent male, as of 2018 — it'd be lovely to believe that widespread acceptance of equal gender strength is the norm. But despite promising progress, issues like the controversy over transgender athletes at the 2016 Olympic Games still rear their heads.

Cutting through the cultural and sociopolitical talk, you'll find persistent questions about potential male and female muscle differences that have been asked for an eternity. Fortunately, a ton of scientific studies continue to do the heavy lifting.

When it comes to male and female muscle differences, it turns out that size really does matter. That is to say, it's not really so much that there's a difference in male and female muscles, but more of a difference in how much muscle different genders pack. Trainer and record-holding powerlifter Greg Nuckols puts it on his site, Stronger By Science, "Most of the major differences in performance and metabolism between genders can be explained by size and body composition, not gender itself."

Nuckols expands on that thought, noting that, "A woman and a man with similar training and similar amounts of muscle and fat will perform similarly." Naturally, then, the commonly recorded differences in strength and performance among men and women can most often be explained by the fact that members of the genders usually do not share similar amounts of muscle or fat.

The way that muscle and fat interact during exercise greatly influences both strength and performance, however, and that process does vary between men and women in a variety of ways.

"A woman and a man with similar training and similar amounts of muscle and fat will perform similarly." — Greg Nuckols, Head of Content at Stronger by Science

Read more: How Women Can Build Muscle Fast

To get some of the concrete, physiological differences in male muscle vs. female muscle out of the way right off the bat: According to Fair Play for Women, a woman's body is typically about 30 to 35 percent muscle by weight, while a man's body is about 40 to 50 percent muscle by weight. Of course, these numbers vary by age, fitness level and genetics.

Likewise, healthy women typically have a higher body fat percentage than men. However, that mitochondria-rich fatty tissue tends to be more metabolically active than male fatty tissue — which often resides near the stomach, as opposed to the common distribution across hips and thighs on a women's body. This is why you might sometimes hear that women have "healthier" fat than men. Those metabolic tendencies factor into athletic performance too.

General differences in fat and muscle proportion affect weight loss patterns as well. Because men generally have more muscle and muscle burns more calories than fat, they often burn more calories when at rest and get leaner faster.

More muscle mass to support also means that men can typically take in more calories while still losing weight, compared to women. Overall, though, potentially quicker weight loss doesn't mean that men lose more weight than women given similar amounts of effort.

Read more: 5 Simple Steps to Make Fat Loss Faster and Easier

Over the years, you may have heard of men outperforming women in terms of raw strength stated as simple fact. While there is some factual evidence here, it's important to keep in mind that the sorts of tests of strength measured tend to be pretty specific and simple.

Among the more numerous comparisons, you'll find that men greatly outperform women in tests of grip strength, which the Journal of Lifestyle Medicine once again reiterated in July 2017 with a study of maximal grip strength among aging adults.

A popular and often-cited 1993 study of 16 men and women from the European Journal of Applied Physiology and Occupational Physiology asserts that "the women were approximately 52 percent and 66 percent as strong as men in the upper and lower body respectively."

This study chalks the gender strength difference up to a greater proportion of lean muscle in men, but even it notes that women have between 25 and 45 percent smaller muscles in areas ranging from the biceps brachii to the knee extensors, giving further credence to the more modern assertion that muscle mass really is key.

Of course, muscles are just one part of your body's mechanics — how they factor in to overall performance in the comparison of male to female training is much more complex, not to mention more revealing.

While it's true that differences in muscle mass account for the most significant differences between the muscles of a man and woman, male and female muscles do feature some innate compositional differences.

Perhaps most notably, women tend to have about 27 to 35 percent more type I muscle fibers than men. More commonly known as slow-twitch muscle fibers, as per the American Council on Exercise, type I fibers are aerobic muscles rich in blood-carrying myoglobin. By the same token, women's muscles have a greater capillary density.

The combo of more slow-twitch fibers and more capillaries makes for an increased ability to deliver more blood to the muscle, while it also increases fatty acid oxidation.

According to ACE, "Because they can provide their own source of energy, slow-twitch fibers can sustain force for an extended period of time, but they are not able to generate a significant amount of force." Overall, a higher proportion of fat and more type I fibers means that women's muscles process glucose more efficiently.

Speaking of fat content, a woman's body commonly has significantly more than a man's, proportionally speaking. That greater fat concentration also lends women more intramuscular triglycerides, which has a positive correlation with increased insulin sensitivity, explains a February 2018 study published in the journal Endocrinology and Metabolism. Women also have higher levels of stearoyl CoA desaturase-1, a protein that converts saturated fatty acid into unsaturated fatty acid.

Read more: The Three Types of Muscles in Human Body

It's not uncommon for studies to conclude that men exhibit greater athletic performance than women. Like the differences seen in maximal strength, this variation is not necessarily due to a difference in muscle itself — it once again comes down to a difference in muscle mass.

A seminal March 1986 study from the journal Ergonomics comparing the sex difference in muscular strength among about 100 equally trained men and women lays it out clearly: "The findings suggest that the sex difference in muscular strength in equally trained men and women is almost entirely accounted for by the difference in muscle size." The importance of muscle mass gets a little more layered than that, though.

The same study determines that for sports and other strength-reliant activities, fat-free mass — that is, the mass of all the body's components except for fat, including muscle — is a much more valid qualifier for strength than gender. In fact, adjusting strength measures for fat-free mass eliminates the sex difference in all the strength tests performed by the Ergonomics study, with the exception of curls and bench presses.

In January of 2015, Physiology, the official journal of the American Physiological Society, published a study on the differences among skeletal muscle kinetics between the sexes. It identified more than 3,000 genes expressed differently in male and female skeletal muscles, those movement-propelling muscles connected to the skeleton.

Researchers concluded that the prevalence of slower-moving muscle fibers in female muscles (type II A and type I) compared to those of males enhanced endurance and recovery thanks to lower contractile velocity, or the speed of muscle contraction.

"Sex difference in muscular strength in equally trained men and women is almost entirely accounted for by the difference in muscle size." — Ergonomics, Vol. 30, Issue 4

Read more: 5 Ways Women Are Smashing Stereotypes in the Weight Room

Across all training experiences, intensity levels and workout types, women use more fat as an energy source during exercise than men do. As an energy pathway, fat is predominantly used during low-to-moderate intensity exercise of a longer duration. In the big performance picture, this generally makes women less susceptible to fatigue when compared to men (backing up Physiology's findings about skeletal muscle endurance characteristics in women).

On an athletic level, the fat and muscle tissue in a woman's body is generally better equipped for performance, with the exception of short, high-intensity bursts. Men are often more suited to the latter type of activity because of greater glycolytic capacity.

Basically, men burn through more glucose in the absence of oxygen as they power through intense bursts of physical activity, but also experience more lactate accumulation during exercise. At the end of the day, that lactate accumulation can lead to longer recovery times.

Read more: Muscle Recovery After Weight Lifting

Men may tend to have the edge in explosive strength, but women take that edge for insulin sensitivity and fat burning, both of which are powerful performance factors. Nuckols puts a bow on the big picture: "Of the differences that do exist, the largest contributing factors are fiber type differences and sex hormone differences. And, in essence, they set women up to be more metabolically suited to just about everything."