Jennifer Aniston, Courteney Cox, Halle Berry … these days, when a 40-something woman steps out in a bikini, it makes news.Turns out, there’s a scientific explanation for why we’re so fascinated by these lean, toned, impossibly supple bodies. In some ways, these apparent genetic wonders are defying the aging process.”Once a man or woman gets beyond their 30s, they’re in a situation where unless they are really proactive about what they do from an exercise standpoint as well as eating sensibly, they are going to lose some muscle tissue,” says Cedric Bryant, Ph.D., FACSM, Chief Science Officer for The American Council on Exercise.But that extra pudge is not all your fault — really. Research shows that about half of these changes in appearance can be attributed to the normal aging process, and the other half is often due to simply not having enough time to exercise.”One of the things that tends to happen for women — particularly because of the demands that women around that age tend to have from their kids and at work — time becomes a tremendous challenge for them to maintain whatever fitness habits they may have had until that point,” Bryant explains.MORE FROM SELF: Stop-The-Clock WorkoutIn fact, even the stars are watching the clock when they work out.”Celebrities, just like you and I, are always looking for what will yield the best results in the least amount of time, period,” reveals celebrity trainer Gunnar Peterson, who has worked with 40-somethings like Jennifer Lopez as well as younger stars like Kim Kardashian.In other words, rocking a six-pack like J-Lo at 42 has less to do with being genetically gifted and more to do with hitting the gym on a regular basis. Or more importantly, having the time to hit the gym on a regular basis. Which most women in this age group don’t have.”[Typical 40-something women] start to exercise less, they eat on the go more and the body composition of lean tissue and fat tissue gets flipped, meaning there’s greater fat and less muscle tissue, which doesn’t support the same level in terms of metabolism,” Bryant says. (Because muscle is more metabolically active than fat, if you maintain more normal body composition, you’re going to be able to maintain a more normal metabolism.)”All these factors combined lead to that creeping fat development known as middle-aged spread. That gradual subtle weight gain that happens where by the time you’re in your 50s, you’re like, ‘How did I become my mom?’MORE FROM SELF: Lose 8 Pounds with Boot Camp Or Yoga Moves!What You Can DoAccording to Bryant, we used to think getting flabbier and saggier was just a natural part of the aging process. But now we know that more than half of these changes are preventable, and can be significantly improved by lifestyle tweaks.”More than half of that is really just we sit on our fannies more and more as we get older,” Bryant says.But you don’t have to chalk up office butt to, “Well, I’m in my 40s, this is what should be expected.” Here’s what you can do to counteract and offset the aging process, Bryant says:1. Look for every possible opportunity to move.Take the stairs when possible. Invest in a little pedometer for $20-30 and track how many steps you take — make it a goal to get up to 10,000 steps a day. Just move consistently and regularly to help to tilt the odds.MORE FROM SELF: 3 CrossFit Workouts You’ll Love2. Do resistance training twice a week.Resistance training (free weights, machines, bands, even some forms of yoga and Pilates) is extremely important as you approach midlife to help to preserve and maintain lean muscle mass. Do some structured resistance training at least twice a week. You don’t have to spend an hour at gym — 20-30 minutes per session will offset physical decline and improve bone health.3. Get your heart pumping.Bryant recommends an interval training approach to cardio with periods of higher intensity followed by come-down periods. Do that for 20-30 minutes three to four times a week. But, he emphasizes, resistance training is as, if not more, important than cardio for the over-40 set.Head over to Self.com for the rest of this article.For daily fitness tips follow SELF on Facebook and Twitter.Get SELF on your iPad!