Even video fitness expert Denise Austin had to crank up her workout schedule when she turned the big 4-0.

“When I hit 40, I wanted to keep my energy level up and stay young at heart,” the 47-year-old fitness expert says. So she turned to more nutrient-dense foods and embraced joint-friendly exercises like yoga.

Austin, the host of dozens of exercise videos, says the body changes in fundamental ways when one enters the 40s.

“If you’re not working out, your muscles start to deteriorate and atrophy,” she says. “Your metabolism starts to slow down, your muscle cells slow down and your weight goes up.”

To fight Father Time, she recommends women weight train at least twice a week.

“It’s not only to look good but it helps keep your metabolism (high),” the married mother of two says. A few tricep extensions, for example, can mean the difference between a toned arm and one that keeps waving long after you stop.

“If your tricep muscles are strong, that will keep gravity from taking its course,” she says.

Lower-tummy troubles

Weight training conjures thoughts of cold gyms. Metal clanks against metal and beefy men grunt and grown on every exhale.

That doesn’t have to be the case.

“Sometimes, a good old-fashioned pushup is the best way to keep your chest firm,” she says.

One of the physical changes Austin saw personally after reaching 40 occurred just above her knees.

“You want to keep that nice and firm to look great in shorts and skirts,” she says of the portion of the leg just above the kneecap. For that, she recommends walking lunges, or even just taking the stairs instead of the elevator whenever possible. A weightless way to work the same area involves seated leg lifts.

The biggest complaint Austin hears from women over 40, though, involves their lower tummies. The best way to combat lower-belly fatigue, she says, is to try Pilates-style exercises.

The older we get, the more likely we pull muscles performing mundane chores, she says.

“As we age we lose elasticity,” Austin says. “When you reach for something, your back might go out.” Stretching as little as five minutes a day can prevent those nagging muscle pulls.

Working out does more than just shape up our outer bodies. According to the National Women’s Health Resource Center, women with heart disease or arthritis actually experience improved daily function from involvement in various modes of physical activity. And studies show physically active women have less intense and fewer menopausal symptoms like hot flashes than their sedentary counterparts.

Never too old to start

Yaz Boyum, a professional bodybuilder and fitness trainer based in Washington, D.C., says women entering menopause must deal with considerable body changes.

The adjustments can include dropping energy levels, weight gain and emotional instability, all of which can affect a fitness routine.

It’s also harder for both men and women as they age to build muscle, Boyum says. Older exercisers need to be more consistent with their, routine, she says. Boyum cautions those who may feel like they’re still 30 that their joints might tell them otherwise.

“You can’t do the heavy, over-exertive kind of workouts. The joints won’t be able to withstand that,” she says. Older women can still build lean mass by performing moderate to heavier workloads at lower repetition and taking days off between strenuous workouts. That will reduce the impact on the body’s joints, she says.

Boyum says another way to fight the aging process is to do aerobic exercises first thing in the morning. That jump-starts a body’s metabolism, she says, which can be kept at a higher rate by eating small meals throughout the day to stabilize insulin levels. Avoid fatty foods and processed carbohydrates like white bread whenever possible, she adds.

Austin, a Lifetime Channel regular whose 50th home video release (“Denise Austin Power Zone: The Ultimate Metabolism Boosting Workout”) came out in June, urges women over 40 try at least some form of exercise every day, even if it’s only a short walk.

“Move those muscles. You’re never too old to start,” she says in full motivational mode. “I think exercise is such a wonderful way to slow down the aging process.”