While pro athletes rarely complain aloud about competing on their periods—Olympic swimmer Fu Yuanhui really broke the seal when she spoke about being on her period during the 2016 Games—hormone fluctuations throughout your cycle really can affect your fitness. Your energy levels, strength, stamina, likelihood of injury, and how your body responds to exercise can change during your period, all of which can really suck for anyone working toward a fitness goal or simply trying to stay healthy.
While existing research on the topic has been critiqued as "woefully inadequate," any woman who's ever tried to break a sweat on day one of her period will tell you that hormones can strike out your best efforts to exercise. That said, everyone responds differently to their hormonal changes—you can't stop living because of them. If you want to max out on the benefits of exercise, listen to your body and sync your workouts to your cycle. You may just end up feeling better.
Your cycle begins on the first day of your period. Because your body kicks things off with cramping and bleeding, the first few days can be the hardest time to train.
There's no science-backed reason to skip a workout when you have your period, says gynecologist Lauren Streicher, an associate clinical professor of obstetrics and gynecology at Northwestern University and author of Sex Rx: Hormones, Health, and Your Best Sex Ever. Physically, "you can do the same thing you do every other day," she says — "unless you don't feel like working out." Which is a perfectly good excuse, especially because cramps and midnight tampon changes can mess with your sleep and make you extra tired.
If you feel perfectly fine, proceed with your scheduled workout. But if you're particularly uncomfortable, you might want to skip your cardio dance class or long-distance run. Research suggests your lungs work better later on in your cycle, when you have more stamina for endurance exercise.
Still, there's no reason to put your feet up until your period passes. (Take a weeklong break every month, and it'll take you that much longer to reach your fitness goals.) Push yourself to do some low-key yoga or a light-cardio workout, like walking or an easy bike ride. Surprisingly, swimming is another good option: Very little water gets into the vagina when you swim without a tampon, and the same goes for swimming with one, Dr. Streicher says.
If you're working out on your own, include some exercises that entail lying facedown, which can alleviate cramping and give you a gentle lower belly massage. (A disposable heating pad can also help—just apply it before you hit the gym.)
Another smart move: Drink a little extra water. Women with heavy periods lose extra fluids, which can make you feel light-headed when you stand up quickly, like during a yoga vinyasa. So take your time.
When your period lets up, your testosterone and estrogen levels begin to rise. Because testosterone can help you sculpt lean muscle mass and estrogen builds connective tissues that bind your muscles to your bones, this is the ideal time for you to tone up, according to Dr. Robert Kominiarek, an Ohio-based physician who specializes in hormone optimization.
Because oral contraceptives can interfere with testosterone, the effects might not be as pronounced if you're on the pill. But for everyone else: Science confirms that women gain more strength and muscle from strength-training during the first half of their cycle than the second half, so this is your chance to shine.
To that end, if there's ever a time to treat yo'self to a fancy boutique fitness class, now's the time. Try a hardcore workout like indoor cycling, high-intensity interval training, or boot camp. Rowing? CrossFit? Cardio kickbox? Anything is fair game.
After day 14, estrogen levels tank—and the same goes for your energy. This can make any workout feel even more strenuous, and could make you extra susceptible to ligament and tendon tears and other injuries, Dr. Kominiarek says. While there's conflicting research on whether your cycle can significantly increase your injury risk, it's smart to play it safe with supervised floor-based workouts like barre or Pilates instead of high-risk, high-impact sports like skiing or a first go at CrossFit.
If you typically hit up fitness classes, slow things down with a self-paced solo workout. Because stamina is on your side this week, slow-and-steady cardio, like a long run or elliptical session, is also ideal. Or you can always play it safe with a couple rest days. (You're welcome.)
Your body is revving up for your period with a surge in progesterone and, for some, crazy cramps, along with moodiness, tender breasts, and fluid retention, all which make you feel markedly un-hot—even in your most flattering yoga pants. It can also boost your body temperature up to 1 degree, Dr. Kominiarek says—which isn't enough to affect your performance but could help you break a sweat more quickly.
The best thing you can do is push through it—all the way to the gym. Because exercise gives you a natural endorphin high, it can elevate your mood and actually make you feel better. Another bonus: While PMS might make you feel bloaty, sweating can help you get rid of extra fluids.
Go for generally lighter exercises such as yoga, but take a more hardcore class like vinyasa or power yoga, which will get your heart rate up. And if you have the energy? An indoor cycling class can help you break a serious sweat. (If you get tired, you can always default to a time-tested excuse—PMS!—and plop down in the saddle.)
Follow Elizabeth on Twitter.