Now I have something else to blame my uninspired biceps on. Turns out, my haphazard gym schedule isn't the only thing keeping my arms from being Michelle Obama-tastic.

According to a new study, oral contraception may be limiting my lean muscle mass.

Exercise physiologist Chang Woock Lee from Texas A&M University found that women on birth control may develop 40 percent less muscle mass than women who are not on the pill.

Researchers studied lean muscle mass gains in 73 young women (18 to 34 years old), who completed an intense, individually-tailored workout regimen three times a week. After 10 weeks, the 34 recruits who were on oral contraceptives gained 2.1 percent muscle mass, 40 percent less than those not on the pill.

Lee found that women using oral contraception had lower blood levels of muscle-building anabolic hormones than those not on the pill. They also had higher concentrations of the stress hormone cortisol, which is associated with muscle breakdown. Both findings suggest that those hormone differences might be behind decreased muscle-building capacity.

Research also suggests that progestin, a synthetic form of the female sex hormone progesterone, in oral contraceptives may connect to androgen receptors in cells, which could prevent male sex steroids from binding. Basically, the progestin blocks male hormones that typically stimulate muscle growth.

"All the progestins are going to bind to these receptors to some degree," says Dr. Jill Rabin, spokeswoman for the American College of Obstetrics and Gynecology. But if you're an average woman, who isn't looking to be the next Arnold Schwarzenegger or (insert female equivalent here), Rabin says you don't need to worry.

You have to choose contraception based on what's best for you, she says. "Most of the time people are taking birth control pills to prevent conception. Others have acne and polycystic ovarian syndrome. If the downside is decreased muscle mass, I think it's fine."

If you are concerned about decreased muscle mass, Rabin suggests picking an oral contraceptive with a medium or lower hormone potency. Because birth control effects vary from person to person, you should consult with your doctor about your specific needs.