With the 30s, you start noticing that weight doesn't come off quite as easily as it used to. This is because after age 20, your basal metabolism drops by 1 to 2 percent every decade, and as lean muscle decreases and body fat increases, you don't need as many calories to sustain yourself. "Exercise is the number one form of preventive medicine," says Jillian Michaels, who is in her 9th season of NBC's The Biggest Loser and is the author of Winning by Losing: Drop the Weight, Change Your Life. "You won't see that big a difference between 31 and 39 if you've been living a healthy lifestyle, but if not, you'll see a huge difference in muscle tone, weight, and shape."
In this decade, experts agree, keeping fit means working harder. Jenkins favors circuit training—a series of resistance and cardio exercises done swiftly and back-to-back. But however you do it, Michaels advises strength training each muscle group twice a week with two days of rest between sessions. Don't stick with heavy weights/low reps or low weight/many reps, she says; switch it around to keep your body from getting used to the routine. One day of rest a week is crucial.
After pregnancy a program like Pilates can be invaluable in "pulling everything back in and up," says Brooke Siler, whose re:AB studio in New York City has attracted famous figures like Amber Valletta, Madonna, and Liv Tyler. "I especially like exercises that involve standing, because they teach you to fight what nature wants you to do, which is slump," says Siler, the author of The Pilates Body. One of the best antigravity moves, she says, is to stand with heels together, big toes two to three inches apart. Drawing your lower abs and inner thighs in and up, rise onto the balls of your feet, making sure the heels stay glued together. Now slowly bend the knees into a plié, keeping the tailbone straight. Lower your heels to the floor and slowly straighten legs, drawing together your inner thighs and pulling up deeper into your abdominals. Do five reps; then reverse the sequence for five more.
Now is the time to make good fitness habits a part of everyday life. "You always want to be standing instead of sitting, taking stairs instead of elevators," says Siler. "I'm constantly aware of how I sit and stand and walk down the street. I'm forever pulling in and up. These invisible workouts are really important for a woman in her 30s. It's how you start preparing your body for what's to come."
In Your 40s: One hour of weight training 3 days a week if you do your whole body at once (4 days for half an hour if you split it up), plus 45 minutes of cardio five days a week (it's more than in the 20s and 30s but with less impact and intensity). Take one day off.
This is the decade of the triple whammy: gravity, hormones, and yet more slowing of metabolism as lean muscle mass continues to decrease and body fat increases. Even women who don't put on a pound may expand, according to Pamela Peeke, MD, author of Body for Life for Women. "After 40 and certainly after 50, virtually all women find that they gain fat more easily in the torso—below the bra, through the triceps area, on the back, and in the belly," she says. "You're not doing anything wrong; your body composition is changing."
Cardio work at least three days a week is still important for keeping weight under control, but resistance training is crucial now. "Women should be doing more weight training—and really going for it," says New York City–based celebrity trainer Kacy Duke, who is in her 40s. "You have to find the time to do it consistently and train hard." If you're just starting, says Peeke, "you must learn proper form—take a class, get a trainer, make sure someone is there to correct you so you don't get hurt. And add intensity. If you're doing a biceps curl, tense the biceps—squeeze them—as you lift. Just when you think you're all the way up, push another 10 degrees."
Certain body parts may call out for extra attention. "Pilates can help some with the midsection," says fitness veteran Karen Voight, who teaches and writes a workout column for the Los Angeles Times. To tone the back of the upper arm, she instructs, "get on all fours in a bent-knee push-up position, with fingers facing forward and hands directly under your shoulders. Make sure your elbows point backward when they bend, and lower only halfway, which works the muscle but avoids strain on the joints." Then there's the butt. "For that," says Voight, who is in her 40s, "I'd try squats with weights or stair-climbing. Exercise is different at this age, because everyone has some aches or pains. I hold positions longer and do things more slowly and with more control. It's about precision and form, not quantity."
It's also about enjoyment. "I find exercise that's satisfying on a deeper level," says Donna Richardson Joyner, creator of the video Sweating in the Spirit and a recent appointee to the President's Council on Physical Fitness and Sports. "It's not just about moving my body—it's about strengthening my mind and my spirit."
Your 50s and 60s: Aches and pains shouldn't be an excuse for giving up on exercise