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"In Your 20s:
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The great thing about being in your 20s is that your body is so strong, you can get away with abusing it. The bad thing is that you often do, punishing it with late nights and bad eating habits. And you routinely fail to appreciate what you've got. This is the decade of anxiety—frantic exercise, fad diets, the mad pursuit of pinup perfection and self-hatred when you fail to meet it. The fitness challenge of these years: Get over it.
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"I tell my young clients, 'Forget looking like Jessica Simpson or Halle Berry, and forget weight; think health,'" says Jeanette Jenkins, a Los Angeles–based private trainer who has worked with rapper Queen Latifah and actress Taryn Manning. The mistake many 20-somethings make is simply opting for "endless cardio and crunches," adds Vanessa Carver, a personal trainer at Pillar Performance in Encinitas, whose clients include professional ice-skaters and dancers. Lots of cardio is great, she says, especially if you mix it up so you're really pushing the body. But it's weight training that builds muscle definition, not to mention bone density, which will be crucial for staying active later on and preventing osteoporosis. "You've got to lift more than just three or five pounds," she says. "If you can do 10 to 15 repetitions of a weight with no real effort, it's too light. The last 4 or 5 reps should be challenging enough that you feel your muscles getting fatigued." And put your mind into it, she says. "Lifting weights while chatting on the cell phone is a joke."
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As for killer abs, "it's about subcutaneous fat, not how many crunches you do," says Carver. "There's no secret here: Eat lean meat, lean fish, vegetables, and fruits." She suggests forgoing thousands of bouncy, quick sit-ups for focused core work, which strengthens not only the abs but also stabilization muscles and lower back.
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"In Your 30s:
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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."
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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.
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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 pliƩ, 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.
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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."