The Menstrual Cycle Workout – Dr. Rob, contributing author

This sounds crazy, but hear us out

If you ever used the I’m-having-my-period excuse to get out of gym class, you may have been onto something—even if you were exaggerating the cramps thing (we won’t tell). Not only do sex hormones like estrogen and progesterone govern your uterus, but your ligaments, tendons, muscles, and nerves all have receptors for those hormones, too—leading to some surprising consequences of your monthly cycle.

Most recently, a study in the journal Medicine & Science in Sports & Exercise found your muscle stretch reflex (the one your doctor tests by hitting your knee with a hammer) decreases just before you ovulate. Higher estrogen levels at this time may loosen your tendons or change the way your nerves respond to the environment around you. These shifts could help explain why women tear their ACL about 10 times as often as men, and why many of these injuries occur during the first half of women’s cycles, says study author Ellen Casey, MD, of the Rehabilitation Institute of Chicago.

Dr. Casey cautions that doctors are just beginning to understand the links, so you shouldn’t sit on the bench until your estrogen levels drop (besides, unless you live in a lab, you probably won’t know exactly when that occurs). But understanding how monthly hormonal shifts influence your fitness can help you take steps to avoid injury, as well as get the most out of your workout.

Week 1

What’s happening: The first part of your cycle starts with your period. A slight rise in follicle-stimulating hormones prompts your ovaries to start preparing an egg, but levels of other hormones (including luteinizing hormone and progesterone) stay flat. Your estrogen levels are lowest at this time, cuing your body to torch carbohydrates instead of fat for energy.

Your fitness move: You’re burning mostly high-octane fuel, so hard, fast workouts may feel easier on these days, provided bleeding isn’t bothersome for you. “This is the time to train hard and make gains in your overall fitness,” says Rob Kominiarek, DO, a family physician in Ohio who specializes in hormones and fitness. (Check out these 20-minute interval workouts or this intense 7-minute workout.)

Week 2

What’s happening: Most women ovulate around day 14, about halfway through their cycle. Shortly before your next egg begins its epic journey down the fallopian tube, estrogen levels surge. Besides your muscle stretch reflex, high estrogen levels have also been linked to more pliable hamstring muscles, which could predispose you to knee injuries by absorbing less force from your joints.

Your move: “Learn to listen to your body,” Dr. Casey says. If an exercise starts to bother your knees, back off. And don’t skip your warm-up—prepping your muscles and joints with about 15 minutes of squats, lunges, and other dynamic moves before a tough workout may protect your knees from injury, according to Swedish research.

Weeks 3 and 4

What’s happening: Now you’ve entered the luteal phase. Estrogen levels dip when you ovulate but increase again soon afterward, and progesterone begins to rise, too. The hormones peak midway through this phase and then drop again as your cycle nears completion. What’s this mean for your workout? This hormonal stew may cause you to feel sluggish, especially during high-intensity workouts, since your body turns to fat as a fuel source rather than carbohydrates. “This is also the time when many women have difficulty with motivation for following a diet and exercise plan,” Dr. Kominiarek says.

Your move: Plan ahead for drops in motivation. Schedule your workouts in advance, and consider keeping them more moderate and less stressful. At the very least, don’t get discouraged when the same class or routine suddenly feels more difficult (check out these 31 ways to overcome any workout motivation obstacle). Battle cravings by fueling your workouts with healthy carbs, such as sweet potatoes, Dr. Kominiarek advises.

If you’re on birth control

What’s happening: You’ll likely follow many of the same patterns, but perhaps to a lesser degree. “Women on birth control still have fluctuations, but the magnitude of change is less,” Dr. Casey says. In one recent study, Stanford University researchers found lower levels of relaxin—a hormone that may stretch out your tendons, increasing injury risk—in women taking pills.

Your move: Much more research is needed before recommending contraceptives as injury-prevention tools. But if you’re on them for other reasons, stick with the previous advice, recognizing that you may experience less dramatic swings than you would otherwise.

After menopause

What’s happening: Permanent drops in estrogen levels after your last period interfere with your body’s use of calcium, weakening your skeleton and increasing your risk for osteoporosis, says Wendy M. Kohrt, PhD, an exercise science researcher at the University of Colorado Denver. What’s more, some evidence suggests low estrogen levels also contribute to cardiovascular disease.

Your move: Maintaining a regular fitness habit can help offset some of menopause’s negative effects by strengthening your bones and your heart, Dr. Kohrt says. For best results, combine cardio and weights; women who did so lost about 3.2% less bone after menopause than those who stayed sedentary, according to a 2011 research review. (Follow these 12 tips to break-proof your bones for life.)


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