Your fitness routine should change as you age, experts say.

Confession: I don't really stretch. Unless it's built into a class I'm taking, I pretty much skip the cooldown altogether (same with foam rolling). But working at Shape, it's pretty impossible to be completely unaware of the benefits of both: increased recovery time, decreased soreness after a workout, a decreased risk of injury, and better flexibility to name a few.

But whenever I mentioned that fact to a slightly-older-than-myself friend, I'd just get a knowing look. "Wait till you turn 30," they'd say. Suddenly, you'll be less able to bounce back from a hard workout, they'd tell me. In my 20s, I could work out hard one day, do nothing to recover, and still wake up feeling fine. In my 30s, they warned, my resilience will start to fade. Not stretching properly after a hard run would mean I'd wake up feeling sore and tight at best-in fact, even if I did stretch I might feel achier in the mornings that I was used to.

In my 20s, I admit that I smirked smugly at these warnings. But now I'm within spitting distance of 30 and I'm running scared-especially since a minor case of runner's knee I picked up while training for my last half marathon is still bothering me, six months later, despite visits to a doctor and a rigorous-for-me stretching and strength-building routine. It's the beginning of the end, I've been telling myself, just hoping that it wasn't too late to start righting my wrongs.

So I decided to ask celeb trainer Harley Pasternak what I should think about changing if I want to protect myself.

"As you age, your body becomes less resilient and recovers a little slower," he agreed, immediately dashing my hopes that all my aged friends were just being dramatic. "The aging process begins on a cellular level, and your body isn't as efficient at repairing damaged tissues." Worse: "All the little injuries you've had earlier in life start to accrue and create compensation issues," says Pasternak. "You could be a stretching superstar, and you'd still notice the aches and pains creeping up on you as you age."

But contrary to what I always assumed, Pasternak says that the answer doesn't lie in more stretching. "It's more about strengthening your weak muscles and creating proper muscle recruitment [meaning making sure you're using the right muscles and the right types of muscles at the right time]. So if you're doing a push-up and your shoulders are taking over all the work, you need to work on recruiting the right muscles and in the right order," he says. This will help minimize any muscle imbalances, which is important because muscle imbalances can lead to overuse injuries, inflexibility, and other issues.

While different people will have different muscle imbalances, based on factors like their posture and past injuries, Pasternak says certain ones are pretty much universal. "Most people tend to be anterior dominant, and have weaker posterior muscles relative to anterior muscles," he explains. Put simply, that means the muscles on the front side of your body are stronger than those on your backside. You'll know for sure you have this if you tend to have a sloped-forward posture. "I tell people to focus on strengthening the rhomboids, triceps, lower back, glutes, and hamstrings disproportionately more than the anterior muscles of the body," Pasternak says.

Another clue something's off is if you have an inward slant in your knees, which indicates weakness in the gluteus medius muscles-the ones that sit over each hipbone. The fix: Side-lying hip abduction, clam exercises, side plants, and single-leg squats.

It may also be worth working with a personal trainer and physical therapist to help you spot and correct those weak areas, Pasternak says. (These realignment exercises can also help.)

Luckily, it's not all bad news. After age 30 or so, you have stronger muscle memory and muscle maturity, he adds. "These two things are beneficial because it means you can resistance train for less time or at a lower intensity and your body should show results sooner," he says. Plus, since you know your body better, you'll likely be more in touch with certain movements and muscles; it'll be easier to notice if something feels off and then correct it, so you can focus a little less on form.

Bigger benefits from less exercise? That's something I can look forward to.