Women find pull-ups more difficult than men do because they have less muscle mass in their upper extremities, said Tim Hewett, director of research in the department of sports medicine at Ohio State University Wexner Medical Center, Fox News reports. Magnetic resonance imaging (MRI) studies show that women have about 40 percent less upper-body mass than men do, Hewett said.

On the other hand, women's lower-body strength is closer to men's. Women also have more endurance than men, he said.

"Women do have their physical advantages; it's just the one glaring difference in performance is muscle-strength measures, especially in the upper extremities," Hewett said.

Of course, each individual is different, and there are certainly women who can do many pull-ups.

One female Marine said that last year, she could hardly complete a single pull-up, but now, she can do eight, and is working toward her goal of 12.

And last year, three female Marines became the first to complete the Corps' infantry training, which is considered one of the toughest training courses in the U.S. military.