Starting as early as your 20s and throughout your 30s, you'll naturally start to lose muscle -- and gain fat at a rate of about 2 percent per decade, especially if you have a sedentary job or lifestyle.
This subtle muscle-to-fat ratio change can make it tougher to maintain your ideal weight as time goes on. But weight training can help -- no matter what your age.
"Each pound of muscle you add through weight training burns as much as 50 calories more a day, even when you're at rest," says James Orvis, a certified personal trainer and author of Weight Training Workouts That Work. "So if you put on 10 pounds of muscle through weight training, you're going to increase your metabolism by 500 calories a day, even if you're just sitting at your desk."
But don't think of those 10 pounds as 10 extra pounds. On the contrary, "If you're overweight, you'll probably lose weight through weight training," he says. "If you're of normal weight to begin with, the scale may not budge, but you'll be more compact as you gain muscle mass, since fat takes up to five times more volume than muscle."
Reaping the rewards of weight training doesn't take much time. Mr. Orvis recommends working out with weights two or three times a week, every other day, for 20 minutes.
"After about three months you should start to see dramatic results in your physical appearance and feel better," he says.
Where to begin? Talk to your doctor before starting any exercise program.
Mr. Orvis offers the following basic weight-training exercises that use only body weight or dumbbells to work the largest, calorie-incinerating muscle groups of the body.
This weight-training move uses body weight initially to target the muscles of the thigh, butt and hamstrings.
"As you get more comfortable with it, add a dumbbell (3 to 5 pounds) in your free hand," says Mr. Orvis.
Here's how: With one hand, hang onto a workout bench or a stable table. Take one step forward with your lead leg, like you're walking, your back leg should be comfortably flexed. Then slowly lower yourself down, keeping your back straight.
The ultimate goal is to bend the knee of your lead leg to about a 90-degree angle, but, initially, bend it only as far as you feel comfortable doing so. Always make sure your knee stays behind your foot to prevent injury to your knees.
Then, push straight up, stopping just before your front knee locks. After doing 5 to 10 repetitions on one leg, take a 30-second rest, then switch legs and work the other side.
Dumbbell shoulder press
This upper-body move targets shoulder and triceps (back of the arm) muscles. Start with 3- to 5-pound dumbbells, progressing by a pound as the exercise gets easier.
Here's how: Standing with your feet shoulder-width apart and dumbbells at shoulder height, grasp dumbbells with palms facing forward. Slowly push dumbbells toward the ceiling, stopping before your elbows lock, then slowly lower the dumbbells to shoulder height. Strive for one or two sets of 10 reps each.
Dumbbell bicep curls
This exercise targets the biceps (the upper-arm muscles).
Here's how: Sit in a chair with your back straight and feet firmly on the floor, dumbbells hanging by your sides. Curl both dumbbells simultaneously toward your shoulders, with your palms facing up. Then slowly lower the weight, rotating your palms so they're facing each other at the bottom of the exercise. Stop before your elbows lock. Do one or two sets of 8 to 12 reps each, using 3- to 5-pound dumbbells. Again, "increase the weight by a pound when the last few reps get easier," says Mr. Orvis.
This exercise targets the abdominals using body weight alone.
Here's how: Lie on your back with your feet on the floor. Place your hands across your chest and lift your torso 30 degrees. Try to avoid straining your neck muscles when you lift yourself upwards. Hold for a second, then slowly lower your torso to the ground, stopping just before your shoulders touch the floor. Repeat 15 times.