The Best Exercise for Your Age

What to do: Add intervals to your cardio workouts
Why: You know this is the decade when your metabolism starts to slow down, but you may not realize how important it is to counteract the decline. "Due to the slower metabolism, you could gain up to 2 pounds of fat every year," says Mike Siemens, director of exercise physiology at Canyon Ranch Health Resort, in Tucson, Arizona. Interval training helps your body keep burning calories after you stop exercising. The effect continues for 10 to 12 hours after an interval workout, compared with 4 or 6 hours after a regular one.
The Plan: Push yourself to an 8 or 9 on the exertion scale (you shouldn't be able to carry on a conversation), then lower to a 6 or 7 (you can say 10 words or so without being out of breath), and repeat. Alternate between those two levels for 20 to 40 minutes 2 to 3 days a week. Start with whatever ratio of work time to recovery time works best for you and progress from there. Here's a trick to make treadmill intervals a little easier.

What to do: Lift heavy weights
Why: "Around 30, you start to lose up to one-third of a pound of muscle a year—that's why most people get weaker as they age," says Siemens. "You might think anything that works your muscles, like Pilates or a boot camp class, is enough to build muscle back up, but those activities only maintain your muscle mass." Lifting weights can help rebuild what you've lost. You want a weight heavy enough that you can't do more than 8 to 12 reps in a row.
The Plan: The best moves are ones that work your largest muscle groups, so focus on your chest, legs, back and shoulders, doing 2 to 3 sets of moves that work those areas a few times a week. Grab your heavy dumbbell and try these 3 full-body exercises.