Indulge us for a moment by flexing your right arm.

Assuming you have an average build-and trust us, you do-your arm is packing about 5 pounds of muscle. It represents nearly 10 percent of the total muscle on your body.

Now, imagine that muscle gone. No biceps, no triceps-only a jiggly mass of skin and fat covering your bones from your shoulder down to your fingertips. That 5 pounds of muscle is about the same amount most men lose between the ages of 24 and 50. And that number doubles by the time they're 60. In fact, once a man passes the half-century mark, he can expect to lose 1 percent of his muscle each year for the rest of his life.

 

That is, unless he does something about it.

And there's good reason for intervention: The natural erosion of muscle and strength that comes with aging leads directly to weak bones, stiff joints, and a slumped posture, and increases your risk of developing heart disease, diabetes, and a host of other maladies. But there's no reason you can't maintain a healthy, strong musculature well into your 90s if you use man's most effective antiaging weapon: resistance training. "Lifting weights regularly signals your body to fight to keep your muscle," says Jeff Volek, Ph.D., R.D., an exercise and diet researcher at the University of Connecticut. That means a longer, healthier life. And we can prove it. Here's why every Men’s Health reader should be lifting weights even if he doesn't give a damn about the size of his biceps.

Lifting is Good for the Gray Matter
Researchers at the University of Michigan found that men who performed three total-body weight workouts per week for 2 months lowered their blood-pressure readings by an average of 8 points. That's enough to reduce the risk of a stroke by 40 percent.

It Strengthens Bones
As you age, you lose bone mass, increasing the likelihood that you'll one day suffer a debilitating fracture in your hips or vertebrae. That's even worse than it sounds, since Mayo Clinic researchers found that 30 percent of men die within 1 year of breaking a hip. In addition, significant bone loss in your spine can result in perpetually rounded shoulders and dowager's hump, eventually transforming you into a 21st-century Quasimodo. Resistance training can help you avoid this fate. Recent research in the Journal of Applied Physiology found that men who lifted weights for 16 weeks increased their hip-bone density by 3.8 percent and raised their blood levels of osteocalcin (a marker of bone growth) by 19 percent.

You'll Get More Years Out of Your Old Jeans
A study in The American Journal of Clinical Nutrition found that for every pound of muscle a man loses, he gains a pound of fat. In other words, that 5 pounds of muscle that most men lose by age 50 is typically replaced by 5 pounds of fat. Not only does that make you look flabby, but it also increases your pant size, even if your scale-weight remains the same. "One pound of fat takes up 18 percent more space on your body than 1 pound of muscle," says Volek. Bottom line: Keep your muscle, and you'll fend off fat.

And Maybe Touch Your Toes Again
Between the ages of 30 and 70, flexibility decreases 20 to 50 percent, making it harder for your joints to move through their full range of motion. For example, if you can't squat down until the backs of your thighs touch your calves (most men can't), you have tight hip flexors, which limits movement at the knees, setting you up for injury. In a study published in the International Journal of Sports Medicine, researchers found that three full-body workouts a week for 16 weeks increased flexibility of the hips and shoulders by more than 30 percent and improved sit-and-reach test scores by 11 percent. So there's still hope that someday you may once again be able to touch your toes. Weights can get you there.

It Negates the Danger of Eating Potatoes
Every time you eat fast-burning carbohydrates, such as white bread, rice, and potatoes, your level of insulin-a hormone that helps keep your blood sugar normal-rises dramatically. That's a problem because consistently elevated insulin increases your risk of diabetes and heart disease. But lifting can help: Researchers at the University of Massachusetts found that men who added two full-body weight workouts a week to their existing aerobic exercise program had insulin levels that were 25 percent lower after a meal that was high in carbohydrates than the levels of men who performed the same aerobic exercise program but didn't lift weights.

You'll Keep More Fast-Twitch Muscles
It's not just the quantity of the muscle you lose that's important to pay attention to but also the quality. Research shows that the aging process reduces the size of your fast-twitch muscle fibers by up to 50 percent but shrinks the size of slow-twitch fibers by less than 25 percent. That's significant because your fast-twitch fibers are the muscles largely responsible for generating strength and power (the key to peak sports performance when you're young) and helping you easily get out of a chair when you're old, says Alex Koch, Ph.D., an exercise researcher at Truman State University.

You'll Rev your Metabolism
Your body requires energy to digest food. So every time you eat, you actually burn some of the calories you've just consumed-typically, 15 to 20 percent. However, researchers at the University of Nevada found that you will burn 73 percent more calories when you eat right after you lift weights. Even better, scientists in the Netherlands calculated that men who lifted weights two times a week for 18 weeks burned an average of 9 percent more calories a day than non-lifters did. That's enough for the average man to lose 25 pounds in a year without making any changes to his diet.

Weight Lifting Will Make You Whistle
In a 2004 study at the University of Alabama, researchers found that older men who performed three weight workouts a week for 6 months improved their scores on measures of confusion, tension, anger, and overall mood. Although unsure of the mechanism, study author Gary Hunter, Ph.D., suggests, "It could simply be a feeling of accomplishment from having become fit and more confident in themselves." Makes sense: The study participants reversed a decade of age-related muscle loss and fat gain, by adding 4 pounds of muscle and dropping 3 pounds of fat, while increasing strength by an average of 42 percent. Those results would improve anyone's mood.